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SUMMARY Receiver functions from teleseismic events, recorded by stations around the Gulf of California, are used to map the upper-mantle seismic discontinuities. We observe a mean transition zone thickness comparable to the global average for most of the region. A low-velocity layer is detected above the 410 discontinuity that varies in thickness along the Gulf of California. The 660 discontinuity shows complex waveforms south of latitude 30 ◦ N as a result of the phase change of garnet to perovskite. Within the transition zone, a complex behaviour of the receiver functions is observed mainly at the southern end of the Gulf. The north–south variations of this zone are likely associated with a slab window at the northern Gulf, resulting from the cessation of subduction of the Farallon plate 12 Ma, and the subduction of the Guadalupe and Magdalena microplates at the southern end, resulting in a hydrated upper mantle. Our results suggest that change in rifting styles occurring along the Gulf of California mirrors deeper processes in the upper mantle.

The Laramide Intrusive Arc constitutes a wide intrusive belt broadly parallel to the actual Sonora coastline. It was formed by the subduction of the Farallon Plate beneath the North-American Plate during the Late Cretaceous-Early Tertiary period. New isotopic data on rocks of this arc show initial 87Sr/86Sr and eNd isotopic values of 0.7066 to 0.7070 and -5 to -6, respectively, for two samples from Bacanora area; as well as 0.7074 to 0.7081 and -3 to -5.5 respectively, for five samples from Cananea, Mariquita and La Caridad areas. Isotopic ages were determined by U/Pb in zircons (95 Ma) and Ar/Ar in potassic feldspar (56 to 71 Ma) from a quartz monzonite porphyry, and by Ar/Ar in potassic feldspar (56 Ma) from another plutonic granodiorite, both from Bacanora. Initial 87Sr/86Sr and eNd values for samples reported in this study suggest that the Laramidic magmas had a large influence from the Proterozoic basement in northeastern Sonora. Four different isotopic zones are proposed for Sonora, according with Sr-Nd data of laramidic rocks and the substratum intruded.

[1] The volcanic basement of the Oregon and Washington Coast ranges has been proposed to represent a pair of tracks of the Yellowstone hotspot formed at a mid-ocean ridge during the early Cenozoic. This interpretation has been questioned on many grounds, especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and North America. Updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised. Forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55–49 Ma, and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments. This kinematic model indicates that there is no firm reason to reject the near-ridge hotspot hypothesis on the basis of plate motions. A novel element of the model is the Resurrection plate, previously proposed to exist between the Farallon and Kula plates. By including the defunct Resurrection plate in our reconstruction, we are able to model the Farallon hotspot track as docking against the Oregon subduction margin starting about 53 Ma, followed by docking of the Resurrection track to the north starting about 48 Ma. Accretion of the Farallon plate fragment and partial subduction of the Resurrection fragment complicates the three-dimensional structure of the modern Cascadia forearc. We interpret the so-called “E” layer beneath Vancouver Island to be part of the Resurrection fragment. Our new kinematic model of mobile terranes within the Paleogene North American plate boundary allows reinterpretation of the three-dimensional structure of the Cascadia forearc and its relationship to ongoing seismotectonic processes.

Whether the high seismic-velocity anomalies in the upper mantle under California represent cold downwelling mantle flow or fragments of the subducted Farallon plate has important implications for mantle dynamics related to the tectonic history of western North America. We investigate the former possibility by simulating the time-dependent thermal evolution in the asthenosphere beneath California, in the wake of the migrating Mendocino triple junction (MTJ), using a two-dimensional model that represents a transect parallel to the direction of the MTJ migration. For viscosity within the typical range for the asthenosphere beneath active tectonic regions (1×1018 – 2×1019 Pa s), our model predicts vigorous small-scale convection characterized by periodic formation of cold downwelling flows near the southern edge of the migrating Gorda plate. Within the relevant time frame, the predicted convective thermal structures are comparable to the major mantle features shown by seismic imaging.

We investigate here the ridge jump that led to abandonment of the Galapagos Rise and formation of the Bauer scarp during the initiation of the present day configuration of the East Pacific Rise since the lower Miocene. We use recently available high resolution satellite-derived gravity data to investigate in detail the tectonic structure of the eastern Pacific from the Equator to 20°S. With this data, we identify fracture zones, abandoned spreading ridges, scarps, and other seafloor features that provide evidence for discerning tectonic history. Based on our structural interpretation of the satellite-derived gravity field, we make the following conclusions: (1) The Galapagos Rise spreading center appears to have originated by opening of the Marquesas/Mendana transform complex as a result of the change in spreading direction following breakup of the Farallon Plate. (2) The Galapagos Rise was not the sole locus of spreading following plate reorganization at ∼ 20 Ma through to the initiation of the Bauer scarp at ∼ 8 Ma, as had been previously hypothesized. Rather, it and a second western spreading axis were likely active concurrently, forming a counterclockwise-rotating Bauer Microplate at a much earlier stage than thought previously. (3) The Bauer scarps are pseudofaults associated with northward rift propagation. Propagation proceeded in several stages. A first propagator emanating from the Garrett transform complex stalled at the future location of the Wilkes transform creating an area of complex morphology near its northern tip. A second propagator, also emanating from the Garrett complex followed in the first's wake and broke through the complex region. At this point the propagation proceeded very rapidly to the northern end of the Bauer Microplate (the Gallego fracture zone, later to become the Yaquina transform fault). Ridge propagation continued north in two more stages, creating the Gofar and Quebrada transforms at the terminus of each stage.

Lower Jurassic radiolarian chert sampled at two localities in the San Rafael Mountains of southern California (∼20 km north of Santa Barbara) contains four components of remanent magnetization. Components A, B′, and B are inferred to represent uplift, Miocene volcanism, and subduction/accretion overprint magnetizations, respectively. The fourth component (C), isolated between 580° and 680°C, shows a magnetic polarity stratigraphy and is interpreted as a primary magnetization acquired by the chert during, or soon after, deposition. Both sequences are late Pliensbachian to middle Toarcian in age, and an average paleolatitude calculated from all tilt-corrected C components is 1° ± 3° north or south. This result is consistent with deposition of the cherts beneath the equatorial zone of high biologic productivity and is similar to initial paleolatitudes determined for chert blocks in northern California and Mexico. This result supports our model, in which deep-water Franciscan-type cherts were deposited on the Farallon plate as it moved eastward beneath the equatorial productivity high, were accreted to the continental margin at low paleolatitudes, and were subsequently distributed northward by strike-slip faulting associated with movements of the Kula, Farallon, and Pacific plates. Upper Cretaceous turbidites of the Cachuma Formation were sampled at Agua Caliente Canyon to determine a constraining paleolatitude for accretion of the Jurassic chert sequences. These apparently unaltered rocks, however, were found to be completely overprinted by the A component of magnetization. Similar in situ directions and demagnetization behaviors observed in samples of other Upper Cretaceous turbidite sequences in southern and Baja California imply that these rocks might also give unreliable results.

The classic example of the not-well-understood rapid change of tectonic plate motion is the increase and then decrease of the convergence rate between the Nazca and South America plates during the past 25–20 m.y. that coincided with the growth of the Andes Mountains. Currently, the decrease in convergence rate is explained either by the increasing load of the Andes or by the appearance of flat slab segments beneath South America. Here, we present an alternative view based on a thermomechanical self-consistent (gravity driven) model of Nazca plate subduction. We explain the changes in the convergence rate as a natural consequence of the Nazca plate penetration into the transition zone and lower mantle after long-term oblique subduction of the Farallon plate. The model is consistent with seismic tomographic images of the Nazca plate beneath South America. Our model also shows that the presence of the Andes does not significantly affect the convergence rate between the Nazca and South America plates.

We propose a major Eocene tectonic inversion related to an oblique convergence between Farallon (after Cocos plate) and Caribbean plates in the offshore of Colombian Caribbean. Between EoceneOligocene occurred an extension and another tectonic inversion since Oligocene to Pleistocene. By structural and stratigraphic evidence extracted from seismic, gravity and well data, we identified diverse diachronic Cenozoic deformations related to an important syndepositional wrenching event. Diachronic wrench system corresponds with Riedel structures and subordinated related-structures which have been growing progressively from north to south as result of convergence of Farallon (after Cocos plate) and Caribbean plates. Older basement-involved structures were inverted since Eocene. Tectonic inversions were interpreted from different strata geometries, relations of strata, growth strata and erosive truncations. A regional Eocene unconformity defined by erosional truncations reveals the timing of tectonic inversion and consequently closure of Colombian basin at northern boundary. The Eocene erosional unconformity becomes conformable toward the basin and should be an important correlative stratigraphic surface in the Caribbean offshore. Propõe-se que uma inversão tectónica maior tenha ocorrido durante o Eocénico associada à convergência oblíqua entre as placas Farallon (actual placa de Cocos) e Caribe no offshore do Caribe Colombiano. No período Eocénico-Oligocénico ocorreu extensão e do Oligocénico ao Pleistocénico ocorreu outra inversão tectónica. Foram ainda identificados diferentes eventos de deformação diacrónica no Cenozóico a partir de evidências estruturais e estratigráficas obtidas a partir de dados sísmicos, gravimétricos e de poço. As diferentes deformações estão associadas a um importante evento de transcorrência sin-deposicional. O sistema de transcorrência diacrónica é evidenciado em estruturas riedel e outras subordinadas que se desenvolveram progressivamente de norte para sul no seguimento da convergência das placas Farallon e Caribe. As estruturas antigas que afectaram o substrato foram invertidas a partir do Eocénico. As inversões tectónicas foram interpretadas a partir das diferentes relações geométricas dos estratos. A discordância regional eocénica, identificada a partir de superfícies erosivas, revela o período da inversão tectónica associada ao fecho da Bacia Colombiana no limite norte. A discordância erosiva do Eocénico passa a conformidade à medida que tende para o interior da bacia e deve ser uma importante superfície estratigráfica de correlação no offshore do Caribe. Fil: Alfaro, E.. Universidade Federal Da Bahia; Brasil Fil: Barrera, Daniel Florencio. Universidade Estadual de Campinas; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Rossello, Eduardo Antonio. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

The Chugach metamorphic complex of southern Alaska is an Eocene high-temperature ( T ), low-pressure ( P ) fore-arc metamorphic belt related to subduction of the Kula-Farallon spreading center beneath western North America. The Chugach metamorphic complex has a three-phase ductile deformational history that records major changes in kinematic axes during a short interval of geologic time (∼8 m.y.). The earliest deformation (D 1 ) is a regional event recognized throughout the flysch subterrane of the Chugach terrane. D 1 is a regional layer-parallel slaty/phyllitic cleavage developed during accretion and subsequent shortening. In the Chugach metamorphic complex, D 1 predates high-temperature metamorphism. During prograde metamorphism, there were two major structural events. D 2 records orogen-parallel extensional accompanied by vertical shortening with components of pure shear and top-to-the-east simple shear. D 2 is synchronous with melt injections (Ti 2 ) in the gneissic core of the complex and large plutons throughout the complex. D 3 records a return to subhorizontal contraction perpendicular to the margin and is interpreted as a dextral transpressional event. D 3 contraction produced a dramatic thickening of the complex in a regional-scale D 3 anticlinorium. In the gneissic core, the presence of melt (Ti 3 ) strongly influenced D 3 . Finite strain data and field observations indicate that both F 2 and F 3 have axes that are parallel to the stretching direction, yet these are not sheath folds because strains are too low. Instead the structures are examples of folds that developed with their axes parallel to the elongation axis. Together these observations provide further evidence for our previous interpretations that the Chugach metamorphic complex is a manifestation of an Eocene plate reorganization at ca. 56–52 Ma. Plate models predict that before 56 Ma the Kula–Farallon–North American triple junction migrated southward and is associated with a time-transgressive fore-arc plutonic belt. After plate reorganization, the triple junction either backtracked northward (Kula Plate model) or continued southward with intermittent northward motion (Pacific-Farallon model). We interpret the D 2 -to-D 3 progression as either a result of highly oblique subduction of the Kula plate followed by more orthogonal—but still dextral-oblique—convergence of the Farallon plate (Kula Plate model), or a special case of Pacific–Farallon–North American interaction.

We have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western United States. Fast polarization directions (ϕ) and delay times (δt) show spatial variations that are coherent within geologic provinces. Stations located near the San Andreas fault show clear evidence for fault-parallel anisotropy in the crust and upper mantle (115–125 km thickness). This can be explained by the finite strain associated with the relative plate motion between the North American and Pacific plates. The lateral extent of this strain field is probably narrow to the west, because stations 55 km west of the San Andreas fault do not show fault-parallel anisotropy in southern California. Station LAC located 80 km east of the San Andreas fault shows large fault-parallel anisotropy. This suggests that the Pacific-North American plate boundary in the mantle might be displaced to the east in southern California. A deeper E-W oriented fast direction of anisotropy underlies the fault-parallel anisotropic layer in the vicinity of the San Andreas fault. An E-W fast feature is also present beneath the western Basin and Range and the foothills of the Sierra-Nevada, although local variations are present. The magnitude of delay times suggests that this feature resides in the asthenosphere. We interpret this feature as the asthenospheric flow in the slabless window left behind the Farallon plate. The flow-induced anisotropy may partially be frozen-in at shallow depths. Station ORV is located near the southern edge of the Gorda slab where no anisotropy is detected. The absence of anisotropy at this location could therefore mark a boundary between Farallon associated flow and regions where E-W oriented asthenospheric flow did not occur. The lack of evidence for NE-SW fast orientation within the Walker Lane Shear Belt of western Nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the San Andreas fault. Stations located over the young subducting Gorda plate mark a change in the fast direction to nearly NE-SW. This direction aligns well with the maximum compressive stress direction in the overlying North American plate and the NE-SW directed internal shearing of the Gorda plate. The anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions. This implies that the anisotropic thickness may include some of the asthenosphere. Alternatively, using a higher anisotropy of 8% can bring thicknesses in line with other measures of lithospheric thicknesses. The correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method, and that asthenospheric flow may be a significant contributor to seismic anisotropy.

Subduction-zone magmatism became extensive along the west coast of South America during the Ordovician, soon after Gondwana was assembled. During the remainder of the Paleozoic and the early Mesozoic, eastward subduction of the Farallon plate led to emplacement of a succession of granitic and volcanic rocks. During the Cretaceous, when South America broke away from Africa and began moving independently toward the Pacific Basin, the resulting opposite motions of the South American and Farallon plates toward the subduction zone caused vigorous tectonic mountain building. But by the Oligocene, South America had advanced more than 2000 km beyond the position of the Cretaceous subduction zone's root in the lower mantle. The South American plate, moving westward over the subducting plate, pushed down and flattened the curved top of the subducting slab, as indicated by today's flattened earthquake zone under South America. I hypothesize that this flattening increased the subducting slab's resistance with the und...

The western quarter of North America consists of accreted terranes—crustal blocks added over the past 200 million years—but the reason for this is unclear. The widely accepted explanation posits that the oceanic Farallon plate acted as a conveyor belt, sweeping terranes into the continental margin while subducting under it. Here we show that this hypothesis, which fails to explain many terrane complexities, is also inconsistent with new tomographic images of lower-mantle slabs, and with their locations relative to plate reconstructions. We offer a reinterpretation of North American palaeogeography and test it quantitatively: collision events are clearly recorded by slab geometry, and can be time calibrated and reconciled with plate reconstructions and surface geology. The seas west of Cretaceous North America must have resembled today’s western Pacific, strung with island arcs. All proto-Pacific plates initially subducted into almost stationary, intra-oceanic trenches, and accumulated below as massive vertical slab walls. Above the slabs, long-lived volcanic archipelagos and subduction complexes grew. Crustal accretion occurred when North America overrode the archipelagos, causing major episodes of Cordilleran mountain building. A new explanation for the origin of the accreted terranes that form the mountainous Cordillera of western North America is proposed and tested: stationary, intra-oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface, which were overridden by and accreted to North America during Cretaceous times. The generally accepted account of how the geology of western North America developed involves the oceanic Farallon plate acting as a conveyor belt, sweeping crustal terranes onto the continental margin as it moves beneath the North American plate. This model fails to explain many aspects of this accreted terrane, and here Karin Sigloch and Mitchell Mihalynuk present an alternative model based on new tomographic images of lower-mantle slabs and modelling of deep-mantle structure in the region. They propose that, during the Cretaceous period, there were a series of island arcs west of North America, beneath which proto-Pacific plates subducted into almost stationary trenches. Long-lived volcanic archipelagos and subduction complexes grew above the slabs, and then accreted onto North America as it overrode the archipelagos, causing episodes of Cordilleran mountain building.

[1] Subduction of the Farallon plate beneath northwestern Mexico stalled by ∼12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (∼24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ∼40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ∼8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes.

Introduction: The Pacific and North America plates are among the world’s largest, and most of the active tectonics in North America takes place on their mutual plate boundary, extending from the Aleutians down the west coast of British Columbia through the Pacific Northwest to California and Baja California. The diffuse plate boundary extends north across the Denali fault into central Alaska, northeast into the Canadian Yukon Territory, east through the Basin and Range Province of the United States and northwestern Mexico to the Wasatch fault of Utah and the Rio Grande Rift of New Mexico and adjacent regions, and southwest across the California Continental Borderland. Earlier in the Cenozoic, nearly all the western margin of North America was a subduction zone, with the offshore East Pacific Rise separating the Pacific Plate from the subducting Farallon Plate on the east and the Kula Plate on the north (Atwater, 1989). (For a recent synthesis, see Madsen et al ., 2006.) The North America Plate moved westward as sea-floor spreading increased the width of the Atlantic Ocean. Some time after 30 Ma, parts of the East Pacific Rise reached the margin of the continent and were subducted so that the Pacific Plate came into direct contact with the North America Plate (Figure 2.1). Pacific Plate motion was northwest relative to North America, parallel to the boundary at the base of the continental slope, so that the resulting boundary was strike-slip, the beginning of the San Andreas transform-fault system in California. The remnants of the Farallon Plate, the Juan de Fuca Plate in the north and the Cocos Plate in the south, continue to subduct beneath North America, the former as the Cascadia subduction zone, and the latter as the Middle America subduction zone. The Juan de Fuca Plate developed two satellite microplates at its northern and southern ends, the Explorer and Gorda plates, respectively, and the Cocos Plate spun off the Rivera Plate in the mouth of the Gulf of California. An oceanic plateau within the Pacific Plate became entrained near the plate boundary, colliding with southern Alaska as the Yakutat Microplate.

We compare 12 recent three-dimensional (3D) seismic imaging results that made extensive use of data from the Earthscope Transportable Array (TA). Our goal is to sort out what can be said about the geometry of the Farallon plate. Our main approach is 3D visualization using a kinematic plate motion model as a framework. Comparison of results from all 12 image volumes indicates that the results are most consistent with a single, coherent Farallon slab overridden by North American. The Farallon can be tracked from the trench in the Pacific Northwest to its remnants in the lower mantle under eastern North America. From the trench the lithosphere has a low dip to the volcanic arc. Immediately east of the arc the slab steepens sharply before undergoing a decrease in dip above the 410. km discontinuity. The gently dipping section varies along strike. Under Washington the deflection is minor but to the south the slab flattens to become nearly horizontal beneath southern Idaho. There is a strong agreement that the high velocity anomaly associated with the slab vanishes under eastern Oregon. Scattered wave imaging results, however, suggest the top of the anomaly is continuous. These can be reconciled if one assumes the wavespeed anomaly has been neutralized by processes linked to the Yellowstone system. We find that all results are consistent with a 4D kinematic model of the Mendocino slab window under Nevada and Utah. In the eastern US the larger scale models all show a lower mantle anomaly related to the older history of Farallon subduction. The link between the lower mantle and new results in the U.S. Cordillera lies under the High Plains where the required USArray coverage is not yet complete. Image volumes in a unified format are supplied in an electronic supplement. © 2012 Elsevier B.V.

We present a detailed analysis of the 1921 and 1922 deep events in Northern Peru, which we identify as the only reliably located earthquakes in the vicinity of the great 1970 deep Colombian shock. All other reported seismicity in a three-dimensional radius of 550 km centered on the 1970 hypocenter is erroneous, and can be relocated to other seismic provinces. The 1921–1922 earthquakes relocate approximately 230 km to the south of the 1970 event, at depths of 630 km and 660 km, respectively. A combination of centroid moment tensor inversion and forward modeling using ray theory and reflectivity synthetics yields focal mechanisms indicating down-dip compression and seismic moments of 1.2 × 1027 dyn cm and 6 × 1027 dyn cm, respectively. The 1921, 1922 and 1970 events take place on a piece of subducted Farallon plate aged approximately 55 Ma, i.e., anomalously young to support deep seismicity. We suggest that this unusual seismic activity may be due to a local heterogeneity on the slab, whose nature, however, remains unknown.

K. Piotrowska’s paper (Piotrowska, 1993) addresses some of the fundamental problems of Cuban and western Caribbean geology. These include the definition, original position and later displacement of suspect terranes in the area. Problems that have been controversial for many years. The model presented by K. Piotrowska (her Fig. 4) is extremely simple. In her model, tectonic terranes such as Los Organos and Escambray originally formed part of the Yucatan borderland, from which they were detached and transported northward. During the Cretaceous, the Farallon plate pushed the Escambray Terrane until it reached its present position. Finally, the Escambray Terrane was overthrust and pressed into the Zaza zone (ophiolites and Cretaceous volcanic arc suites). Later, during the Middle Eocene, the Los Organos Terrane detached from the Yucatan and overthrust the Farallon plate (Zaza zone in this case). In order to support this scenario, Piotrowska (1993) relies on opinions rather than observa

Jurassic rocks of the Caribbean are a sampling of 100 million years of Farallon Plate history with fragments originating at diverse paleolatitudes and from varied tectonic settings. Fragments with clear paleogeographic signatures are components of the basement complexes of Duarte in Hispaniola, Bermeja in Puerto Rico and La Desirade off Guadeloupe. Paleolatitudinally sensitive radiolarian faunas document origination of Duarte as equatorial, La Desirade as higher latitude, and various Bermeja cherts as both equatorial and higher latitude. Red ribbon chert of Duarte and Bermeja of the same age, physical appearance, and lithological association are probably dismembered components of the same slab of Pacific crust. La Desirade red ribbon chert is slightly younger than the Duarte and Bermeja red ribbon chert and was deposited at higher latitude. Bermeja tuffaceous chert is also of higher latitude and probably had an arc-proximal origin. On the basis of modeled plate trajectories in the Pacific, the origin of various cherts from different paleolatitudes that end up in the same location requires different arrival times at the trench between North and South America. Based on radiolarian paleobiogeography plus indications of origin at a spreading ridge and ignoring the poorly constrained, modeled trajectories for the Late Jurassic, at least one of the higher latitude fragments may have originated in the southern hemisphere. The accumulation of multifarious chert, greenstone, and other ocean floor components was accomplished by offscraping strata transported to the subduction zone along the eastern Pacific margin and warehousing this material in an accretionary complex prior to entry of the Caribbean Plate into the gap between North and South America.

The geochemical characteristics of volcaniclasts from Punta Sámara are used to infer magmaticprocesses that generated Late Cretaceous – Middle Paleocene arc lavas in the western margin of the Caribbeanplate. The Ba/La ratios of the volcaniclasts are higher than the ratios for any of the modern lavas from Costa Rica, and the La/Yb ratios are lower; suggesting that the lavas from the primitive arc were generated by greatercontribution of slab material than the modern arc lavas. Furthermore, the lower La/Yb ratios imply that thedegree of melting was significantly higher and/or that the mantle source was more depleted than in modernCosta Rica. The low Zr/Nb ratios for the modern lavas from central Costa Rica imply a more enriched mantlesource (OIB-like), whereas the higher ratios for the primitive arc volcaniclasts imply a more depleted mantlesource (MORB-like). We use the geochemical information to infer the conditions of subduction during the earlystages of arc magmatism in the southern part of the Central American arc. We infer that the primitive arc wasassociated with a subducting slab that dip at a steeper angle than the present. The steeper angle would imply thatthe subducting Farallon plate was older, colder and denser than the present Cocos plate. Se usan las características geoquímicas de los volcaniclastos de Punta Sámara, para inferir los procesos magmáticos que generaron las lavas de arco en el Cretácico tardío – Paleoceno Medio. Las relaciones Ba/La sonmás altas que las de las lavas jóvenes de Costa Rica y las de La/Yb son más bajas, lo que sugiere que las lavas delarco primitivo se generaron por una mayor contribución del material de la placa subducida que las lavas del arcomoderno. Además, las relaciones más bajas de La/Yb implican que el grado de fusión era significativamente más al-to y/o la fuente mantélica estaba más empobrecida que en la actualidad. La baja razón de Zr/Nb para las lavas jóve-nes de Costa Rica central implica una fuente mantélica más enriquecida (tipo OIB), mientras que las razones mayo-res para los volcaniclastos del arco primitivo implican una fuente mantélica más empobrecida (tipo MORB). Usa-mos la información geoquímica para inferir las condiciones de la subducción durante las etapas tempranas del mag-matismo en la parte sur del arco de América Central. Inferimos que el arco primitivo estaba asociado con una placaque se subducía a un ángulo mayor que el presente, lo que implicaría que la subducida placa Farallón era más vieja, fría y densa que la actual placa del Coco.

The Upper Cretaceous/Cenozoic basins of Costa Ricaare underlain by oceanic crustand located along the western margin of the Caribeean Plate. Since Upper Cretaceous time, an intraoceanic arc was formed along the western margin of the Caribeean Plate. This arc marked the beggining of the subdution of the Farallon Plate in the region and also the creationof the first land bridge joining North and South America. Continued subdution of the Farallon Plate from Campanian until the present, created a convergent marginal tectonic setting; with, from SW to NE: a trench, an outer arc, a forearc region, a volcanic arc and a backarcr egion. Localized platforms of shallow-water carbonates, overlain by extensive cyclical turbidites system, characterized the sediments of most of the basis until the Eocene, when a major tectonic compressive event affected the area. As well, about this time, a major sinistral east-west fault (COSTA RICA TRANSCURRENT FAULT SYSTEM) divided the region into north and south segments and transtensional or pull apart basins developed along this major fault system. In the Oligocene time, the north segment was emerged and subject to erosion, whereas in the south segment the sedimentation continued. From Miocene on, shallow marine deposition was widespread in both sements. In the north segment, and extensional stage in the backarc region, created a structural depression filled with Neogene sediments. About the same time, the south segment experienced compressional tectonicis due to noth-directed movementof the Panama block. Later in the Pleistocene, northeasterly-directed shallow subdution of the thick Cocos Ridge, resulted in adiotional thrusting and the developmentof a "foreland" basin, both, in the back arc and fore arc regions. Organic-rich, deep water, marginally to termally mature source rock, from Cretaceous age, occur in several basins. Also, source rocks of moderate to high TOC values are present in all basins, in the Tertiary. Potential carbonate reservoirs in some of these basins include shallow water carbonates, rudistic reef buildups and grainstone accumulations. Potential clastic reservoirs include turbidite sandstones, regressive deltaic deposits and near-shore porous sandstones. The presence of potential structural and stratigrafic traps in all basins, confer an important as far as the pretoleum potential is concern. Las cuencas sedimentarias de Costa Rica, de edad Cretácico Superio-Cenozoico están subyacidas por corteza oceánica y se localizan a lo largo del margen oeste de la Placa Caribe. Su historia se inicia con la formación de un arco volcánico primitivo durante el Cretácico Superior. Este arco marcó el inicio de la subducción de la Placa Farallón bajo la Placa Caribe, y representa los primeros ragos del puente ístmico que ulteriormente uniría a Norte y Sur América. El desarrollo de la subducción desde el Campaniano hasta el presente, ha creado los rasgos típicos de un margen convergente intraoceánico, que comprende de SW a NE: una fosa, un arco externo, una región antearco y una región tras-arco. Plataformas locales de caliza de aguas someras sobreyacidas por extensas y cíclicas turbiditas, caracterizan los sedimentos de la mayoría de los rellenos de las cuencas hasta el Eoceno, cuando un evento tectónico compresivo afectó el arco. Por esta misma época, una falla siniestral, de tendencia E-W dividió la región en dos segmentos: Norte y Sur. Cuencas transtensivas (de tipo "pull-apart") se desarrollaron a lo largo de este sistema de falla (SISTEMA DE FALLA TRASCURRENTE DE COSTA RICA). Durante este tiempo, gran parte del segmento Norte estuvo emergido y sujeto a erosión, mientras en el segmento Sur continuó la sedimentación volcaniclástica y carbonatada. Desde el Mioceno la sedimentación somera marina predominó en ambos segmentos. En el segmento Norte, extensión en la región tras-arco creó una depresión estructural que fue rellenada por sedimentos neógenos. El segmento Sur experimentó una compresión tectónica debido al movimiento hacia el norte del bloque de Panamá. Posteriormente en el Pleiostoceno, la subducción somera de la dorsal asísmica de Cocos resultó en compresiones adicionales y el desarrollo de cuencas tipo "foreland intermontane" y "foredeep", en ambas regiones antearco y trasarco. Rocas fuentes del Cretácico, ricas en materia orgánica, marginal a termalmente maduras, acurren en varias cuencas. Asímismo, otras rocas fuentes del Terciario, con valores de TOC moderados a altos están presentes en todas las cuencas. Potenciales rocas reservorio en carbonatos, incluyen calizasde aguas someras, construcciones arrecifales de rudistas y calcarenitas. En depósitos clásicos incluyen areniscas turbidíticas, depósitos deltaicos regresivos y areniscas de fajas costeras. La presencia de trampas estructurales y estratigráficas le confieren un importante potencial petrolero a las cuencas sedimentarias del país.

International audience; The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ''regular'' calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondu' and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calcalkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ''no-slab'' regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calcalkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.

This is a survey of mantle provinces (large-scale seismic anomalies) under North America, from the surface down to 1500–1800 km depth. The underlying P velocity model was obtained by multifrequency tomography, a waveform-based method that systematically measures and models the frequency-dependence of teleseismic body waves. A novel kind of three-dimensional rendering technique is used to make the considerable structural complexities under North America accessible. In the transition zone and below, the North American mantle is dominated by seismically fast provinces, which represent distinct subduction episodes of the Farallon plate. I attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes. Differences in vertical sinking velocity have led to large vertical offsets across adjacent, coeval slabs. Some of the mantle provinces have not been discussed much previously, including (1) a seismically slow blanket overlying the oldest Farallon subduction along the eastern continental margin, (2) a transition zone slab coeval with the Laramide orogeny (ca. 80–60 Myr), which I discuss in analogy to the “stagnant slab” subduction style commonly found in the western Pacific today, (3) the lower mantle root of present-day Cascadia subduction, which may have started out as intraoceanic subduction,(4) a lower mantle slab under Arizona and New Mexico, the last material to subduct before strike-slip motion developed along the San Andreas boundary, and (5) two narrow plate tears thousands of kilometers long, one of which is the subducted conjugate of the Mendocino Fracture Zone.

The southern and central Andes reflect significant along-strike differences of tectonic activity, including shortening, alternating flat-to-normal subduction styles and magmatism. In northern Patagonia, the subduction/supra-subduction system, fore arc, arc and back arc basins developed in an extensional setting during the Paleogene. This was accompanied by landward migration of calc-alkalic magmatism which changed to synextensional bimodal volcanism of rhyolitic ignimbrites and interbedded tholeiitic and alkalic basalts. These Paleogene events occurred during a time when the Farallon–Aluk active ridge reached the South American plate, and the Farallon plate subduction was interrupted. They represent a new tectonic regime, characterized by a transcurrent plate margin. The presence in the back arc of a rigid lithospheric block of 100,000 km 2 represented by the North Patagonian Massif focused the rotation of the coastal blocks. This resulted in the development of two Paleogene extensional regions to the north and south, respectively, of the Massif and replaced the former back arc. Plate rearrangement caused by the inauguration of the Nazca plate and its regime of orthogonal subduction at the beginning of the Miocene, re-established typical calc-alkaline arc magmatism at the former upper Cretaceous arc locus. Present seismic activity in the subducted plate and tomographic modeling of p-wave velocity anomalies in the upper mantle also suggest the presence of a subduction gap that lasted for most of the Paleogene in northern Patagonia.

Present-day Cuba is a complex of terranes. In the western and central parts of the island the Sierra de los Organos and the Escambray terranes, which are genetically related, and the volcanosedimentary Cretaceous sequences of the Greater Antilles arc are distinguished. The Escambray and Sierra de los Organos massifs are terranes that were detached from their source areas near the Yucatan. A rift separating the Yucatan from the Florida-Bahama plate was involved. Separation of the Escambray sequence from that of the Sierra de los Organos took place in the Early Cretaceous along a NE-SW-trending transform fault. The Escambray sequence was transported together with the Greater Antilles island arc in front of the Farallon plate. About 80 Ma ago the volcanosedimentary Cretaceous sequence of the Greater Antilles island arc was thrust over the ophiolitic association. In this way the Zaza zone was formed. At that time some overthrusting and metamorphism took place in the Escambray massif, which was then overthrust and pressed into the Zaza zone. At about 45–50 Ma ago detachment and nappe overthrusting took place in the Sierra de los Organos and overthrust units reached the margin of the margin of the Florida-Bahama plate. Various structures are involved in the Zaza zone, including: (1) the volcanosedimentary Cretaceous sequences of the Greater Antilles arc (which is a tectonized and overthrust terrane); and (2) the ophiolitic association (the Greater Antilles arc terrane is thrust over this association). Both of these tectonic units were folded together. The Sierra de los Organos and the Escambray terranes were thrust onto the Zaza zone and then deformed together with it during later phases.

Cenozoic Tectonics The Basin and Range Province of western North America—exemplified by the alternating mountain ridge and valley landscapes across nearly the entire U.S. state of Nevada—started to form ∼50 million years ago through a series of extensions of the continental crust. Prior to that, massive mountain-building collisions at the boundary between North America and the Pacific Ocean formed a subduction zone and compressed the continent. By combining observations of global subduction zone velocities with numerical modeling, Schellart et al. (p. 316 ) demonstrate that the thinning of the subducting portion of the oceanic plate controlled how and when the transition from compression to extension occurred. Even today, where the much smaller remnant oceanic plate continues to slowly subduct below North America in the Pacific Northwest, the width of the slab and not its age controls the velocity of subduction. Indeed, this relation may explain the dynamics of other modern subduction zones, from South America to Japan.

The cause of the Laramide phase of mountain building remains uncertain. Conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant Hess or Shatsky oceanic plateaux. Independent verification of these models has remained elusive, because the putative ridges or plateaux are no longer at the Earth’s surface. Inverse convection models have identified two prominent seismic anomalies on the recovered Farallon plate. Here we combine inverse convection models with reconstructions of plate motions, to show that these seismic anomalies coincide palaeogeographically with the restored positions of the Shatsky and Hess conjugate plateaux as they subducted beneath North America. Specifically, the distribution of Laramide crustal shortening events tracked the passage of the Shatsky conjugate beneath North America, whereas the effects of the Hess conjugate subduction were restricted to the northern Mexico foreland belt. We propose that continued subduction caused the oceanic crust to undergo the basalt–eclogite phase transformation, during which the Shatsky conjugate lost its extra buoyancy and was effectively removed. Increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward, followed by regional-scale surface rebound. We conclude that Laramide uplift resulted from the removal, rather than emplacement, of the Shatsky conjugate.

Among convergent margins, Mexico is an excellent site for studying along-strike variation in plate tectonics at the trench and its effect on the accompanying volcanism and tectonics in the overriding plate. The progressive fragmentation and consumption of the Farallon plate and the interaction between the Pacific and North America plates during the Cenozoic produced some unique geologic features. These include the Sierra Madre Occidental, one of the largest silicic igneous provinces on Earth, and the Trans-Mexican Volcanic Belt, a complex continental volcanic arc underlain by two of the youngest slabs in the world. In addition, extension occurred concurrently in much of the region affected by volcanism, producing a significant part of the southern Basin and Range province, and the reactivation of older basement structures which controlled the emplacement of volcanism.

Received 30 June 2008; revised 30 November 2008; accepted 12 December 2008; published 5 March 2009. [1] Low-angle subduction of oceanic lithosphere may be an important process in modifying continental lithosphere. A classic example is the underthrusting of the Farallon plate beneath North America during the Laramide orogeny. To assess the relevance of this process to the evolution and composition of continental lithosphere, the mantle stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted in Plio-Pleistocene cinder cones. Whole-rock chemistry, clinopyroxene trace element and Nd isotope data, in combination with geothermometry and surface heat flow, indicate kilometer-scale compositional layering. The shallow parts are depleted in radiogenic Nd (eNd = 13 to 6.4) and are interpreted to be ancient continental mantle that escaped tectonic erosion by low-angle subduction. The deeper samples are enriched in radiogenic Nd (eNd = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within each slice, compositions range from fertile lherzolites at the top to harzburgites at the bottom: the latter formed by 25–28% low-pressure melt depletion and the former formed by refertilization of harzburgites by mid-ocean-ridge-basalt-like liquids. The superposition and internal compositional zonation of the slices preclude recent fertilization by Cenozoic extension-related magmas. The above observations imply that the lower Mojavian lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic protolith. If so, the lherzolitic domains may be related to melting and refertilization beneath mid-ocean ridges. The present Mojavian lithosphere is thus a composite of a shallow section of the original North American lithosphere underlain by Farallon oceanic lithosphere accreted during low-angle subduction.

An animated reconstruction shows South Pacific plate kinematics, in the reference frame of West Antarctica, between 55 Ma and the present-day. The ocean floor in the region formed due to seafloor spreading between the Antarctic, Pacific, Phoenix and Nazca plates (a plate formed by fragmentation of the Farallon plate early in Oligocene times). The Pacific–Antarctic Ridge remained fairly stable throughout this time, migrating relatively northwestwards, by various mechanisms, behind the rapidly-moving Pacific plate. The Nazca and Phoenix plates also moved quickly, but relatively towards the east or southeast, and were subducted in these directions beneath the South American and Antarctic plates. Segments of spreading centres forming at the trailing edges of the Nazca and Phoenix plates periodically collided with these subduction zones, resulting in the total destruction of the Nazca–Phoenix spreading centre and the partial destruction of the Nazca–Antarctica spreading centre (the Chile Ridge) and Antarctic–Phoenix Ridge, which ceased to operate shortly before its northeasternmost three segments could collide with the Antarctic margin. Following collision of segments of the Chile Ridge, parts of the Antarctic plate underwent subduction at the Chile Trench. After these collisions, slab windows should have formed beneath both the South American and Antarctic convergent margins, and the animation shows occurrences of alkaline volcanism that have been, or can newly be, related to them. Further occurrences of alkali basalts, at the margins of the Powell Basin and, more speculatively, James Ross Island, can be related to the formation of a slab window beneath them following the collision of segments of the South America–Antarctica spreading centre in the northwest Weddell Sea.

Flat subduction of the Farallon plate beneath the western United States during the Laramide orogeny was caused by the combined effects of oceanic plateau subduction and unusually great suction in the mantle wedge, the latter of which was caused by the shallowing slab approaching the North American craton root. Once in contact with basal North America, the slab cooled and hydrated the lithosphere. Upon removal, asthenospheric contact with lithosphere resulted in magma production that was especially intense where the basal lithosphere was fertile (in what now is the Basin and Range Province), and this heating weakened the lithosphere and made it convectively unstable. Small-scale convection has since affected many areas. With slab sinking and unloading, the western United States elevated into a broad plateau, and the weak portion gravitationally collapsed. With development of a transform plate boundary, the western part of the weak zone became entrained with the Pacifi c plate, and deformation there is dominated by shear. 2 Humphreys mwr204-04 1st pages E. Surface heat flow P-wave velocity D. Seismic velocity at 100 km Background: S-wave velocity/2 100-175 km C. S-wave velocity

in press; The western part of Ecuador is made of several oceanic terranes, which comprise two oceanic plateaus, of Early (≈ 120 Ma), and Late Cretaceous age (≈ 90 Ma), respectively. The older oceanic plateau was accreted to the andean margin in the Late Campanian (≈ 75 Ma). Fragments of the Turonian-Coniacian plateau were accreted to the ecuadorian margin in the Late Maastrichtian (≈ 68 Ma, Guaranda terrane) and Late Paleocene (≈ 58 Ma, Piñón-Naranjal terrane). The Guaranda terrane received either fine-grained oceanic sediments of Coniacian-Maastrichtian age, or island arc / back-arc volcanic suites of Middle Campanian-Middle Maastrichtian age. The Piñón-Naranjal terrane recorded a comparable history, completed in the Maastrichtian-Paleocene, either by pelagic cherts, or by island arc products (Macuchi arc). The Late Cretaceous plateau of Ecuador is interpreted as part of the Caribbean oceanic plateau (COP), because their evolutions are comparable. If so, the COP was not formed by the Galápagos hotspot, but on the Farallón oceanic plate, south of Ecuador and close to the South American margin. The COP belonged to the Farallón plate, until a subduction zone separated both plates in the Middle Campanian, giving way to a well developed Mid Campanian-Mid Maastrichtian island arc. Accretion in the Late Maastrichtian triggered a change in the subduction system, and the development of a new arc system of Late Maastrichtian-Late Paleocene age, which crosscut the South America-COP plate boundary. The last accretion occurred in the Late Paleocene.

Deep-penetration seismic reflection profile RU-3 crosses the central California continental margin revealing a subducted oceanic plate, a modified accretionary prism, and complex structures of the overlying sedimentary basins. This structural framework was established by subduction processes during Paleogene and earlier time and subsequently was modified by Neogene transform motion combined with apparent components of extension and compression. Subducted rocks are indicated by deep, gently dipping reflectors that extend beneath the continental margin for at least 38 km at a depth of about 15 km. We interpret the subducted crust as either a part of the Pacific plate or, more likely, a subducted fragment derived from the Farallon plate. A set of more steeply dipping, deep events may indicate faulting within the subducted plate or its boundary with a no-slab zone. The overlying, largely nonreflective layer of accreted material rapidly reaches 10 km in thickness landward of the paleotrench and increases to 15 km in thickness near the coast. The Santa Lucia Basin, landward of the steep continental slope, originated as a slope basin during Paleogene subduction. The lower strata of this basin were deposited onto and partially incorporated into the accretionary complex. The offshore Santa Maria Basin exhibits a variety of compressional structures that formed in the last 3–5 m.y. and whose locations correspond to an earlier framework of extensional faults. Structural inversion has occurred in Miocene depocenters adjacent to the Santa Lucia Bank fault and at the Queenie structure. Miocene and lower Pliocene strata also thicken toward the Hosgri fault zone where subsequent compression is characterized by low-angle thrusts and folding.

[1] The convergence between the Pacific, Farallon, and Kula oceanic plates and the North American plate has provided a driving force for tectonic processes that have shaped the western margin of North America. Yet reconstructions of plate motion in the Pacific basin have traditionally relied on the assumption of fixed hot spots. In the last decade, substantial Cretaceous to Paleogene motion between the Pacific and Atlantic hot spots has been recognized, providing motivation to reevaluate the history of relative motion between Pacific oceanic plates and North America by means of plate circuit reconstructions. This paper presents new kinematic models based on two alternative plate circuits linking the Pacific plate to North America since Late Cretaceous time. When compared to models relying on fixity of Pacific and Indo-Atlantic hot spots, our reconstructions suggest that the Pacific-Kula and Pacific-Farallon ridges were ∼600–1000 km closer to the western margin of North America and hence that the Farallon and Kula plates were smaller in Late Cretaceous to middle Eocene time. These findings cast significant doubt on the viability of a configuration of the Farallon-Kula ridge that juxtaposes the Kula plate with the southern part of the paleo-North American margin during this time interval. The results of our new reconstructions also suggest more oblique convergence between the Farallon (or Kula) plate and North America from the Late Cretaceous to the middle Eocene related to the combination of dominantly northwest motion of the Pacific plate and steady southwest motion of North America. This provides an efficient mechanism for the coast-parallel translation of some accreted terranes of North America. It may also explain the structure of geological units in western North America, such as the Central Belt of the Franciscan Complex, which show a dominant coast-parallel fabric formed during an interval coeval with the episode of oblique convergence.

[1] Teleseismic traveltime data are inverted for mantle Vp and Vs variations beneath a 1400 km long line of broadband seismometers extending from eastern New Mexico to western Utah. The model spans 600 km beneath the moho with resolution of ∼50 km. Inversions show a sharp, large-magnitude velocity contrast across the Colorado Plateau-Great Basin transition extending ∼200 km below the crust. Also imaged is a fast anomaly 300 to 600 km beneath the NW portion of the array. Very slow velocities beneath the Great Basin imply partial melting and/or anomalously wet mantle. We propose that the sharp contrast in mantle velocities across the western edge of the Plateau corresponds to differential lithospheric modification, during and following Farallon subduction, across a boundary defining the western extent of unmodified Proterozoic mantle lithosphere. The deep fast anomaly corresponds to thickened Farallon plate or detached continental lithosphere at transition zone depths.

International audience; New field, major and trace element and whole rock K–Ar data on the Neogene volcanic fields of northern Baja California (Jaraguay and San Borja) show that the calc-alkaline activity of the Comondú arc vanished around 11 Ma, following the end of the subduction of the young oceanic Farallon plate. It was immediately replaced by magnesian andesite volcanism, showing paroxysmal activities between 8.5 and 3.8 Ma in Jaraguay, and 3.7 to 1.8 Ma in San Borja. Our modelling results show that the characteristic geochemical signature of the magnesian andesite suite (high MgO, Cr, Co and Ni contents; highly fractionated rare earth element (REE) patterns with strong depletion in heavy REE and Y; exceptional enrichments in Sr and Ba and relative depletion in Rb and Th) is consistent with an origin through dehydration melting of pargasite-rich lithospheric mantle at depths of ca. 80 km. The temporal geochemical evolution of the Baja California magnesian andesite suite is studied using a set of 98 K–Ar dated whole rock samples ranging in age from Upper Miocene to Pleistocene. It is marked by an increase of incompatible elements concentrated in pargasitic amphibole (Sr, Ba, K, light REE), which were released in large amounts during dehydration melting, and conversely by the progressive exhaustion in Rb and Th hosted in small quantities by this mineral. The progressive temporal depletion in Y and heavy REE of the suite is consistent with the increase of the amount of residual garnet in the lithospheric mantle as a product of the dehydration melting reaction. This melting occurred at minimal temperatures of 1050–1075 °C, consistent with a high thermal flux in the mantle wedge. This flux was linked to the opening of an asthenospheric window following ridge–trench collision, and later to the “no-slab” regime which followed the sinking of the Farallon plate into the deep mantle.

[1] We invert phase and amplitude data of Rayleigh waves for attenuation (Q � 1 ) and shear wave velocities beneath southern California using teleseismic sources recorded by the TriNet/USArray network. Fundamental mode surface wave studies from 25 to 143 s period allow us to constrain the vertical variation of shear quality factor Qm in the upper mantle. We use 2-D sensitivity kernels for surface waves based on single-scattering (Born) approximation to account for the effects of scattering on amplitude. A onedimensional shear velocity model reveals a pronounced low velocity zone (LVZ) from � 80 km to � 200 km underlying a high velocity lid. Qm shows a similar pattern; large Qm at depths shallower than 80 km and much smaller Qm at depths greater than 100 km. Models that attribute the variations of attenuation and shear velocities with depth solely to temperature and pressure effects predict too low Qm values if they match the shear velocities. Alternative models considering the presence of partial melt can explain the observed very low Vs velocities in the asthenosphere. Partial melt in the asthenosphere could be generated due to decompression and the reduced solidus for damp mantle when the asthenosphere rose to fill the space left by the subducted Farallon plate.

[1] We present anisotropic models for the shallow lithosphere in the northern and central Gulf of California obtained from receiver function (RF) analysis. This region has undergone intense plate reorganization over the past 25 Ma that led to the transfer of the plate boundary east of the modern Baja California Peninsula posterior to the cease of the subduction of the Farallon plate beneath the North American plate. Our main interest in modeling the seismic anisotropy is to characterize rock fabric inherited from past and recent tectonism and to provide new information for understanding the tectonic processes involved in the evolution of the local plate boundary. We calculate RFs from teleseismic P waves recorded by the Network of Autonomously Recording Seismographs(NARS)-Baja array, and we stack them in 20° wide back azimuthal bins to enhance coherent information. These RFs yield significant transverse energy whose azimuthal variations are not consistent with that expected for a crust with dipping Moho. Therefore we interpret the conspicuous RFs energy of the transverse component as convincing evidence of anisotropy. We present forward models obtained by fitting the radial and transverse RFs with synthetic seismograms calculated with a reflectivity code. Our analysis suggests substantial variations in the structure of the shallow lithosphere along the Baja California Peninsula and the presence of stalled fragments of the extinct Farallon plate beneath its central part. The results at station NE81 in Sonora are interpreted as frozen crustal anisotropy inherited from Miocene Basin and Range extension, consistent with previous observations of the upper mantle anisotropy beneath this station.

Eastward subduction of oceanic tectonic plates has shaped the geologic history of western North America over the past 150 million years. The mountain-building and volcanism that brought forth the spectacular landscapes of the West are credited to the vast ancient Farallon plate, which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle. Here, we use finite-frequency travel-time and amplitude measurements of teleseismic P-waves in seven frequency bands to obtain a high-resolution tomographic image to 1,800 km depth. We discover several large, previously unknown pieces of the plate which show that two distinct stages of whole-mantle subduction are present under North America. The currently active one descends from the Pacific northwest coast to 1,500 km depth beneath the Great Plains, whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent. We argue that the separation between them is linked to the Laramide era 70-50 Myr ago, a time of unusual volcanism and mountain-building far inland generally explained by an episode of extremely flat subduction. © 2008 Macmillan Publishers Limited.

Se cuantificó la anisotropía sísmica del manto superior usando estaciones que rodean la parte sur del Golfo de California. El movimiento absoluto de la placa de América del Norte puede explicar las mediciones en la parte continental estudiada e implica que la litosfera arrastra al material de la astenosfera que se encuentra por debajo. Estas mediciones también son consistentes con la dirección de la extensión que ocurrió en la región de Cuencas y Sierras durante el Mioceno. Además las observaciones concuerdan con trabajos anteriores realizados en la región de Cuencas y Sierras, pero más al norte. El flujo astenosférico producido por la placa de Farallón subducida puede explicar la dirección anisotrópica rápida, orientada E-O, en el extremo sur de la península. Dicha explicación fue propuesta anteriormente para las estaciones de la mitad norte de la península. En la mitad sur de la península predominan valores de retraso pequeños y mediciones nulas. Estas mediciones difieren de las realizadas en el norte de la península. La existencia de fragmentos subducidos de las microplacas de Guadalupe y Magdalena podría explicar los pequeños valores de anisotropía observados. Una explicación alternativa podría ser el flujo ascendente de material caliente producido en la desaparecida dorsal de Magdalena ya que la placa subducida de Magdalena se había roto. La estación NE75, en la parte central de la península, parece registrar la transición entre los regímenes predominantes en el norte y el sur.

Understanding the relative motion between the Pacific plate and its neighboring plates in the Paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the Pacific Ocean basin, the history of circum-Pacific subduction, and the cause of the Hawaiian-Emperor bend (HEB). We quantitatively model the Farallon/Vancouver-Pacific-Antarctic seafloor spreading history from 67 to 33 Ma based on a comprehensive synthesis of magnetic anomaly and fracture identifications. We find a well-constrained increase from 75 ± 5 mm/yr to 101 ± 5 mm/yr in Pacific-Farallon full spreading rates between 57.6 Ma and 55.9 Ma, followed by a stepwise increase to 182 ± 2 mm/yr from 49.7 to 40.1 Ma. The increases in Pacific-Farallon spreading rates are not accompanied by any statistically significant change in spreading direction. The 57.6–55.9 Ma surge of Pacific-Farallon spreading reflects an eastward acceleration in Farallon plate motion, as it precedes west Pacific subduction initiation and is not associated with any significant change in Pacific-Antarctic spreading. We interpret the increase in Pacific-Farallon spreading rates after ca. 50 Ma as a consequence of further acceleration in Farallon plate motion. We find no indication of a major change in Pacific plate absolute motion at this time. Our model suggests that changes in relative motion direction between the Pacific and Farallon and Pacific and Antarctic plates were insignificant around the formation time of the HEB (ca. 47.5 Ma), and the bend is largely a consequence of Hawaiian hotspot motion, which ceased rapid motion after 47 Ma.

According to literature summarized by Wright et al. (2015), it is generally accepted that hotspots have moved relative to each other since 80 Ma, and that the Hawaiian hotspot moved south relative to the spin axis during formation of the Emperor seamounts prior to 48 Ma. Koivisto et al. (2014) advanced the alternative view that the minimum relative motion between hotspots is much smaller than generally accepted, implying errors in the global plate circuit are larger than generally accepted. In particular, they suggest that the global circuits are incorrect for motion prior to 48 Ma between East and West Antarctica. In this view, southward motion of the Hawaiian hotspot is evidence for motion of the entire lower mantle relative to the spin axis. Wright et al. defend conventional wisdom using freshly calculated relative motion solutions for plates in the Pacific basin. They conclude that the Hawaiian-Emperor bend (HEB), formed at ca. 50-42 Ma and sharpest at 47.5 Ma, is a result of the cessation of southward motion of the Hawaiian hotspot and is not a result of change of absolute motion of the Pacific plate. The purpose of this Comment is to point out that their results are at least as consistent with a large change in motion of the Pacific plate that was responsible for a significant part of the HEB. Wright et al. calculate Pacific-Farallon finite half-rotations from the Pacific-plate record at more frequent intervals than previous workers. They recognize an acceleration from ~75 mm/yr full rate prior to 60 Ma to ~180 mm/yr after 40 Ma. According to their preferred Cande and Kent (1995) time scale, this acceleration is spread out over 57–40 Ma, but according to the Ogg (2012) time scale, the acceleration is contained within the interval of 50–43 Ma, a time interval entirely within the age span of the HEB. Their statement that their “results are not strongly dependent on the time scale used” is hard to understand. In the discussion associated with Wright et al.’s figure 3, they suggest that this acceleration is dominated by eastward acceleration of the Farallon plate in an absolute reference frame. Any conclusion that Pacific absolute motion didn’t change much is circular because they assume that one of three published models for Pacific plate absolute motion is correct, and for all of these published models, changes in Pacific plate absolute motion are small. This discussion does not address the possibility that all three of these models for absolute reference frames share common errors in the global plate circuit. Wright et al. briefly mention their analysis of Pacific-Antarctic motion, presented in their figure 2, in terms of rate and direction of motion at the Pitman Fracture Zone in the far south Pacific. In discussion of Northern Hemisphere events possibly related to Pacific-Farallon acceleration, they suggest that westward acceleration of the Pacific plate would correspond to increased Pacific-Antarctic spreading rates. Because they observe a rate decrease at the Pitman Fracture Zone, they suggest that change in Pacific absolute motion was “relatively minor.” Here they make a serious error in failing to consider spherical geometry. The Pitman transform fault active at 48 Ma was 90–100° of arc away from the Molokai and Murray Fracture Zones, and the HEB. Their discussion frames the most relevant issue as how the motion of the Pacific plate changes at the location of the Hawaiian hotpsot. The velocity of the Pacific plate relative to Antarctica when calculated at relevant Northern Hemisphere locations does increase sharply near 48 Ma. Calculated at the location of Hawaii, the velocity of the Pacific plate changes from slow (35 mm/yr) northwest motion to fast (83 mm/yr) west-northwest motion

[1] Differential traveltimes of SS precursors have been widely used to map large-scale mantle structure and depths of discontinuities. Measurements are commonly made by stacking individual records to increase the signal-to-noise ratio pertaining to these mild reflections. However, ray parameters of the SS precursors are typically less well constrained and undesired seismic arrivals with vastly different slownesses (for example, scattered waves) could potentially contaminate the time domain stacks. To overcome these pitfalls, we introduce a processing scheme based on the Radon transform and well-constrained inversions. Our method is particularly effective in suppressing background noise, constraining differential ray parameter and traveltime, and detecting weak reflected or converted phases. We apply the Radon-based method to delineate the discontinuity structure beneath the northeastern Pacific Ocean and the northwestern Canada. An elevated 410-km discontinuity and a thickened transition zone (252 km) are observed beneath the northern British Columbia, which may be caused by remnant oceanic lithosphere from the subduction of Kula-Farallon plate under North America. A thin transition zone is identified beneath the northern Pacific Ocean and its presence is supported by a low shear-velocity anomaly from recent tomographic models. The improved accuracy and resolution using the least squares Radon transform also offer clear evidence for a regional 520-km discontinuity and several mild reflectors in the depth ranges of 250–330 km and 900–1200 km. We do not observe a 220-km discontinuity beneath the study region.

The Sierra Madre Occidental is the result of Cretaceous-Cenozoic magmatic and tectonic episodes related to the subduction of the Farallon plate beneath North America and to the opening of the Gulf of California. The stratigraphy of the Sierra Madre Occidental consists of fi ve main igneous complexes: (1) Late Cretaceous to Paleocene plutonic and volcanic rocks; (2) Eocene andesites and lesser rhyolites, traditionally grouped into the so-called Lower Volcanic Complex; (3) silicic ignimbrites mainly emplaced during two pulses in the Oligocene (ca. 32–28 Ma) and Early Miocene (ca. 24–20 Ma), and grouped into the “Upper Volcanic Supergroup”; (4) transitional basaltic-andesitic lavas that erupted toward the end of, and after, each ignimbrite pulse, which have been correlated with the Southern Cordillera Basaltic Andesite Province of the southwestern United States; and (5) postsubduction volcanism consisting of alkaline basalts and ignimbrites emplaced in the Late Miocene, Pliocene, and Pleistocene, directly related to the separation of Baja California from the Mexican mainland. The products of all these magmatic episodes, partially overlapping in space and time, cover a poorly exposed, heterogeneous basement with Precambrian to Paleozoic ages in the northern part (Sonora and Chihuahua) and Mesozoic ages beneath the rest of the Sierra Madre Occidental. The oldest intrusive rocks of the Lower Volcanic Complex (ca. 101 to ca. 89 Ma) in Sinaloa, and Maastrichtian volcanics of the Lower Volcanic Complex in central Chihuahua, were affected by moderate contractile deformation during the Laramide orogeny. In the fi nal stages of this deformation cycle, during the Paleocene and Early Eocene, ~E-W to ENE-WSW–trending extensional structures formed within the Lower Volcanic Complex, along which the world-class porphyry copper deposits of the Sierra Madre Occidental were emplaced. Extensional tectonics began as early as the Oligocene along the entire eastern half of the Sierra Madre Occidental, forming grabens bounded by high-angle normal faults, which have traditionally been referred to as the southern (or Mexican) Basin and Range Province. In the Early to Middle Miocene, extension migrated westward. In northern Sonora, the deformation was sufficiently intense to exhume lower crustal rocks, whereas in the rest of the Sierra Madre Occidental, crustal extension did not exceed 20%. By the Late Miocene, extension became focused in the westernmost part of the Sierra Madre Occidental, adjacent to the Gulf of California, where NNW-striking normal fault systems produced both ENE and WSW tilt domains separated by transverse accommodation zones. It is worth noting that most of the extension occurred when subduction of the Farallon plate was still active off Baja California. Geochemical data show that the Sierra Madre Occidental rocks form a typical calc-alkaline rhyolite suite with intermediate to high K and relatively low Fe contents. Late Eocene to Miocene volcanism is clearly bimodal, but silicic compositions are volumetrically dominant. Initial 87Sr/86Sr ratios mostly range between 0.7041 and 0.7070, and initial eNd values are generally intermediate between crust and mantle values (+2.3 and −3.2). Based on isotopic data of volcanic rocks and crustal xenoliths from a few sites in the Sierra Madre Occidental, contrasting models for the genesis of the silicic volcanism have been proposed. A considerable body of work led by Ken Cameron and others considered the mid-Tertiary Sierra Madre Occidental silicic magmas to have formed by fractional crystallization of mantle-derived mafi c magmas with little (

International audience; We combine results from seismic tomography and plate motion history to investigate slabs of subducted lithosphere in the lower mantle beneath the Americas. Using broadband waveform cross correlation, we measured 37,000 differential P and S traveltimes, 2000 PcP-P and ScS-S times along a wide corridor from Alaska to South America. We invert the data simultaneously to obtain P and S wave velocity models. We interpret slab structures and unravel subduction history by comparing our V S tomographic images with reconstructed plate motion from present-day up to 120 Myr. Convergence of the Pacific with respect to the Americas is computed using either (1) the Pacific and Indo-Atlantic hot spot reference frames or (2) the plate circuit passing through Antarctica. Around 800 km depth, four distinctive fast anomalies can be associated with subduction of the Nazca, Cocos, and Juan de Fuca plates beneath South, Central, and North America, respectively, and of the Pacific plate beneath the Aleutian island arc. The large fast anomalies in the lowermost mantle, which are most pronounced in the S wave models, can be associated with Late Cretaceous subduction of the Farallon plate beneath the Americas. Near 2000 km depth, the images record the post-80 Myr fragmentation of the proto-Farallon plate into the Kula plate in the north and the Farallon plate in the northeast. Near 1000 km depth, we infer separate fast anomalies interpreted as the Kula-Pacific, Juan de Fuca, and Farallon slabs. This interpretation is consistent with the volume and length of slabs estimated from the tomographic images and the plate history reconstruction.

Recent advances in the knowledge of the Cenozoic structure and stratigraphy of southern Mexico reveal a geological evolution characterized by Upper Cretaceous orogenic deformation, followed by truncation of the continental margin and gradual extinction of arc magmatism in the Sierra Madre del Sur, prior to the onset of magmatism in the Trans-Mexican Volcanic Belt. Orogenic deformation began in the Late Cretaceous and was coeval with the Laramide orogeny with structures of similar orienta tion. Deformation consisted of E-W shortening that migrated to the east with time and with a general easterly vergence. Models that relate the Laramide deformation to a decrease in the angle of subduction of the Farallon plate, which was converging in western Mexico, cannot be applied in southern Mexico because Paleocene to upper Eocene arc magmatism occurs near the inferred paleo-trench. An alternative possible origin due to collision of an insular arc against the western margin of Mexico suffers from an absence of features and petrogenetic associations indicating the closure of an oceanic basin. In light of recent geochronological data, the general pattern of magmatic extinction from Upper Cretaceous–Paleocene in Colima and Jalisco to the middle Miocene in central and southeastern Oaxaca presents some variations inconsistent with a simple pattern of extinction toward the E-SE. Maastrichtian to lower Paleocene plutonism recognized in the Jalisco block and Manzanillo areas is contemporaneous with a magmatic episode that has some documented adakitic affi nities in the central part of the Sierra Madre del Sur. Magmatism from the Paleocene to middle Eocene seems to be concentrated in the Presa del Infi ernillo area, although isolated centers existed in areas such as Taxco or the eastern Jalisco block. Finally, the main axis of magmatism between the middle Eocene and Oligocene developed along what is the present-day continental margin and extends

The Tehachapi complex lies at the southern end of the Sierra Nevada batholith adjacent to the Neogene-Quaternary Garlock fault. The complex is composed principally of high-pressure (8–10 kbar) Cretaceous batholithic rocks, and it represents the deepest exposed levels of a continuous oblique crustal section through the southern Sierra Nevada batholith. Over the southern ~100 km of this section, structural/petrologic continuity and geochronological data indicate that ≥35 km of felsic to intermediate-composition crust was generated by copious arc magmatism primarily between 105 and 99 Ma. In the Tehachapi complex, these batholithic rocks intrude and are bounded to the west by similar-composition gneissic-textured high-pressure batholithic rocks emplaced at ca. 115–110 Ma. This lower crustal complex is bounded below by a regional thrust system, which in Late Cretaceous time tectonically eroded the underlying mantle lithosphere, and in series displaced and underplated the Rand Schist subduction assemblage by low-angle slip from the outboard Franciscan trench. Geophysical and mantle xenolith studies indicate that the remnants of this shallow subduction thrust descend northward through the crust and into the mantle, leaving the mantle lithosphere intact beneath the greater Sierra Nevada batholith. This north-dipping regional structure records an inflection in the Farallon plate, which was segmented into a shallow subduction trajectory to the south and a normal steeper trajectory to the north. We combine new and published data from a broad spectrum of thermochronometers that together form a coherent data array constraining the thermal evolution of the complex. Integration of these data with published thermobarometric and petrogenetic data also constrains the tectonically driven decompression and exhumation history of the complex. The timing of arc magmatic construction of the complex, as denoted above, is resolved by a large body of U/Pb zircon ages. High-confidence thermochronometric data track a single retrogressing path commencing from widely established solidus conditions at ca. 100 Ma, and traversing through time-temperature space as follows: (1) Sm/Nd garnet ~770–680 °C at ca. 102–95 Ma, (2) U/Pb titanite ~750–600 °C at ca. 102–95 Ma, (3) Ar/Ar hornblende ~570–490 °C at ca. 94–91 Ma, (4) Rb/Sr biotite ~390–260 °C at ca. 90–86 Ma, (5) Ar/Ar biotite ~320–240 °C at ca. 88–85 Ma, and (6) (U-Th)/He zircon ~230–170 °C at ca. 88–83 Ma. Additional stratigraphic constraints place the complex at surface conditions in Paleocene–early Eocene time (ca. 66–55 Ma). Integration of these results with thermobarometric and structural data, including published data on the underlying Rand Schist, reveals a profound tectonic event whereby rapid cooling and exhumation at rates potentially as high as 100s °C/m.y. and >5 mm/yr initiated at ca. 98 Ma and peaked between 96 and 94 Ma. Between 93 and 85 Ma, cooling rates remained high, but decelerated with or without significant exhumation. Subsequent cooling and exhumation rates are poorly constrained but were much slower and ultimately resulted in Paleocene-Eocene surface exposure. Initial rapid exhumation and cooling are hypothesized to have been driven by abrupt flattening in the corresponding segment of the Farallon plate and the resulting tectonic erosion of the underlying mantle lithosphere. Protolith as well as metamorphic pressure-temperature and age constraints on the Rand Schist indicate its rapid low-angle subduction between 93 and 88 Ma. Comparison of the Rand Schist and Tehachapi complex pressure-temperature-time paths in conjunction with structural relations strongly suggest that the schist ascended the equivalent of ~4 kbar relative to the Tehachapi complex by low-angle normal displacement along the Rand fault between 88 and 80 Ma to attain its current underplated structural position. Such extensional tectonism is hypothesized to have been driven by slab rollback during the demise of the southern California region shallow slab segment.

Current interpretations of Cretaceous tectonic evolution of the Northwest Pacific trace interactions between the Pacific plate and three other plates, the Farallon, Izanagi and Kula plates. The Farallon plate moved generally eastward relative to the Pacific plate. The Izanagi and Kula plates moved generally northwards relative to the Pacific plate, with the Izanagi being the name given to the northward-moving plate prior to the Cretaceous Normal Polarity Superchron and the name Kula applied to the post-Superchron plate. In this paper I suggest that these names apply to the same plate and that there was only one plate moving northwards throughout the Cretaceous. I suggest that the tectonic reorganization that has previously been interpreted as formation of a new plate, the Kula plate, at the end of the Superchron was actually a plate boundary reorganization that involved a 2,000 km jump of the Pacific-Farallon-Kula/Izanagi triple junction. Because this jump occurred during a time of no magnetic reversals it is not possible to map or date it precisely, but evidence suggests mid Cretaceous timing. The Emperor Trough formed as a transform fault linking the locations of the triple junction before and after the jump. The triple junction jump can be compared with an earlier jump of the triple junction of 800 km that has been accurately mapped because it occurred during Late Jurassic formation of the Msequence magnetic lineations. The Northwest Pacific also contains several volcanic features, such as Hawaii, that display every characteristic of a hotspot, although whether deep mantle plumes are a necessary component of hotspot volcanism is debatable. Hawaiian volcanism today is apparently independent of plate tectonics, i.e. it is a center of anomalous volcanism not tied to any plate boundary processes. The oldest seamounts preserved in the Hawaii-Emperor Chain are located on Obruchev Rise at the north end of the Emperor Chain, close to the junction of the Aleutian and Kamchatka Trenches. These seamounts formed in the mid Cretaceous close to the spreading ridge abandoned by the 2,000-km triple junction jump. Assuming that Obruchev Rise is the oldest volcanic edifice of the Hawaiian hotspot and thus the site of its initiation, the spatial and temporal coincidence between these events suggests that the Hawaii hotspot initiated at the spreading ridge that was abandoned by the 2,000-km jump of the triple junction. This implies a tectonic origin for the hotspot. Other volcanic features in the Northwest Pacific also appear to have tectonic origins. Shatsky Rise is known to have formed on the migrating Pacific-FarallonIzanagi triple junction during the Late Jurassic-Early Cretaceous, not necessarily involving a plume-derived hotspot. Models for formation of Hess Rise have included hotspot track and anomalous spreading ridge volcanism. The latter model is favored in this paper, with Hess Rise forming on a ridge axis possibly abandoned as a result of a ridge jump during the Superchron. Thus although a hotspot like Hawaii could be associated with a deep mantle plume today, it would appear that it and other NW Pacific volcanic features originally formed as consequences of shallow plate tectonic processes.

[1] Recent installation of 3-component broadband digital stations around the Gulf of California allowed us to make shear wave splitting observation from records of 73 SKS and SKKS phases from 30 events recorded at 12 stations since January of 2001. Stations in the southern Basin and Range province yield upper mantle fast shear wave direction from NE-SW to E-W, consistent with both local direction of Miocene extension and North American absolute plate motion. The shear at the Pacific-North American plate limit seems to control tightly the anisotropy pattern of the closest station NE70. The anisotropic pattern along the Peninsular Range, east of the former trench between the North American plate and the now subducted Farallon plate, is almost uniform and is interpreted as asthenospheric flow induced by the sinking fragment of the Farallon plate although frozen anisotropy associated with in situ captured fragment of this plate cannot be ruled out.