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This study visualizes, quantifies, and evaluates relationships between bedrock geology and topography through the use of GIS. The study area contains weakly consolidated Permian-aged sandstone and shale of slightly differing ages that have been dissected by the regional drainage. The erosion of these rocks has produced a subtle but well-defined topography. Data for this work were obtained from a 30 m resolution digital elevation model (DEM) and the bedrock geology map of Oklahoma. Numeric values of local slope angle and relief were extracted from the DEM and associations with geologic formations were summarized and compared. The maps reveal that local topographic variations are dependent on the relative abundance of sand to shale in the underlying bedrock. This finding is significant because the topographic expression is so subtle that associations between topography and bedrock geology would not be easily recognized or quantified with conventional field techniques. To identify controls on topography, sandstone thicknesses were measured in outcrop and the bulk density of field samples was measured in the laboratory. We find that sandstone thickness is greater in areas of higher local relief and thinner in areas of lower relief. |n contrast, bulk density, used as a proxy for susceptibility to erosion, is not significantly different between areas. These findings suggest that presence and thickness of sandstone, even if weakly consolidated, plays a role in determining topographic expression of bedrock. This GIS-based technique, when constrained by geology, can enhance the quality of multiuse mapping programs in the geosciences, agriculture, and civil engineering. Keywords: GIS, topography, sedimentology, stratigraphy, Oklahoma, Permian.

Biostratigraphical study of the early to mid-Ordovician conodont fauna from ribbon-banded radiolarian cherts of the middle Burubaital Formation in Central Kazakhstan reveals an almost complete succession of conodont biozones from the late Tremadocian to the early Darriwilian. During this interval, biosiliceous sediments were deposited in basinal environments, inhabited by lingulate brachiopods, sponges, pterobranchs and caryocaridids in conditions of high fertility and primary productivity of surface water. The community structure of taxonomically diverse conodont assemblages typifying open oceanic environments is not significantly different from that of epicratonic basins of the North Atlantic conodont province. The regional increase of oxygenated bottom waters at the base of the Oepikodus evae Biozone is possibly related to considerable changes in palaeo-oceanographical circulation patterns. The finds of three natural clusters of Prioniodus oepiki (McTavish) enable us to propose an emended diagnosis of this species. Keywords: radiolarites, Conodonta, Kazakhstan, Ordovician.

Hydrothermal alteration in volcanic arcs occurs in many settings and may involve magmatic, marine, lacustrine or groundwaters, driven by magmatic, tectonic or thermal events. King George Island, part of the South Shetland Island Cenozoic volcanic arc, contains an 80 km long zone of propylitized volcanic rocks, with numerous occurrences of quartz veining, silicic, sericitic, argillic and advanced-argillic alteration. On Barton Peninsula, a basaltic lava sequence (49-44 Ma) intruded by a small, high-level granodiorite pluton (~42 Ma), contains these alteration types, previously interpreted as a single porphyry-copper system. In this study, we report three, possibly four, distinct fossil hydrothermal episodes. (1) Banded chalcedonic quartz, quartz-sericite and propylitic alteration occurs along ESE faults and as reworked clasts in nearby tuffs. Drusy quartz + calcite veins with silicic/sericitic, argillic and propylitic wallrocks may represent feeders to the near-surface silicification. These characteristics, and anomalous Ag + Pb + Sb + Au plus Te + Se + Zn + As, suggest a neutral-pH geothermal system that was active during volcanism. (2) The lavas and banded-quartz rocks were brecciated, veined and replaced by alunite + native sulphur + pyrite, and pyrophyllite + quartz + pyrite + zunyite + diaspore assemblages with anomalous Hg + Se + As + Bi + Au + T1 + Sb + Cu. Such advanced-argillic alteration is diagnostic of degassing of a felsic magma into shallow (< 500 m) meteoric groundwaters. Rhyolite tuffs, previously not reported on King George Island, may represent leakage of this magma to the surface. (3) Subsequent burial to ~3 km was followed by emplacement of a granodiorite pluton and formation of a silicic contact-metasomatic aureole containing muscovite, biotite, actinolite, magnetite, K-feldspar and tourmaline. Disseminated andalusite + corundum also formed in areas previously affected by the advanced-argillic alteration. Iron/copper-sulphide veinlets are locally abundant, but a porphyry-style geochemical signature is not present. Early Cretaceous Ar-Ar ages near the intrusive contact indicate flow of an excess Ar-bearing hydrothermal plume up the contact. Finally, isolated areas of propylitic alteration in the lavas nearby may be related either to quartz veins of episode 1 at depth or to (4) continued circulation of heated groundwaters around the cooling pluton.

Weathering mantles (saprolites) are very widespread in central, western, and northern Europe, where they form thick near-surface residual deposits, often in excess of 50 m. They vary in the type of weathering, the bedrock lithology involved, the topographic setting, and the age and stratigraphic context. This review paper provides information about the distribution and major characteristics of weathering mantles from a number of 'key areas', including the British Isles, the Central European belt of mid-mountains and uplands, the Scandinavian Mountains, and the Fennoscandian Shield. It also examines the evidence for weathering (= etched) origin of various landforms, including boulders, tors, inselbergs, basins and some regional landforms assemblages. Major types of regionally important weathering mantles include ferrallitic (lateritic) saprolites, kaolinite-rich saprolites, sandy to grus mantles, and various residuals left after partial dissolution of carbonate rocks. Although dating of weathering mantles is often circumstantial, it appears sufficiently precise to argue for various ages of deep weathering. Mesozoic (pre-Late Cretaceous) mantles are particularly widespread, but there are also Early Tertiary and Miocene saprolites widely present, whilst most grus weathering is likely to be of Miocene to Pleistocene age. Individual upland areas have been subjected to multiple episodes of weathering, whose legacy is the present-day co-existence of different weathering products. Continental and shallow marine deposits around the uplands, particularly in the North Sea and intra-continental graben structures, are often dominated by quartz and kaolinite, pointing to deeply weathered land surfaces as the source areas and extending the record of weathering on the geological time scale. They demonstrate that weathering mantles repeatedly formed and were subjected to erosion, and new cycles of weathering were initiated. We suggest that deep unroofing of basement structures in many parts of Europe has been mainly accomplished by deep weathering and stripping of pre-weathered rock. The phenomenon of deep weathering has been very important for geomorphology. Etched landforms vary in size, from small residual boulders up to regional characteristic landscapes (etchsurfaces), such as joint-valley landscapes of southern Sweden or undulating multi-convex and multi-concave landscapes of many uplands in central Europe. Deep weathering has played a major part in the geomorphological evolution of Europe since at least the Late Triassic, and the concept involving long-term etching and stripping provides an appropriate framework to describe and explain both erosional landscapes and sediment features. Keywords: paleoweathering; weathering; saprolite; etching; Mesozoic; Cainozoic

The Tien Shan are the quintessential intracontinental range, situated more than 1000 km north of the suture between India and Asia. Their initiation and growth in the Cenozoic, however, remain poorly understood. In this study we present stratigraphic, detrital fission-track, and magnetostratigraphic results that provide a basis for reconstructing the Cenozoic tectonic evolution of the Kyrgyz Range and adjacent Chu basin in the northwestern Tien Shan. Detrital fission-track thermochronology indicates that the northwestern Tien Shan was tectonically quiescent for much of the Cenozoic. Prior to uplift and exhumation in the late Miocene, the Kyrgyz Range was buried by sediments shed from highlands to the south and/or east. Paired bedrock fission-track and [U-Th]/He ages from a sampling transect of 2.4 km relief demonstrate that rapid exhumation commenced at ca. 11 Ma. Initial thrusting in the hinterland was followed by evaporite accumulation (~0.4 km/m.y.), which coincided with erosion of the pre-11 Ma strata that mantled the Kyrgyz Range. Between 10 and 3 Ma, bedrock-exhumation rates decreased to Keywords: Central Asia, erosion, fission-track, magnetostratigraphy, Tien Shan.

Nd isotopic composition has been determined for 16 igneous rocks, representing the wide geochemical, spatial and temporal range of post-collisional, late Cenozoic magmas in the Aegean area. Nd isotopes are used to further interpret previously published Pb and Sr isotope data. The overall pattern of late Cenozoic volcanism resulted from rapid extension, with thermal effects causing melting of hydrated, enriched, subcontinental lithosphere to produce widespread K-rich magmas. Slab break-off and intrusion of hot asthenosphere caused partial melting of rift-related continental margin basalts at the detachment point to generate adakitic magmas. Further outboard, mafic magma from enriched lithospheric mantle melted thickened lower crust to produce the granitoid plutons of the Cyclades. Nd isotopic variation in these varied rock types correlates with pre-Cenozoic palaeogeography. Proterozoic subduction-related enrichment in Th and U, together with other large-ion lithophile elements, produced distinctive Pb isotope composition. This was later modified where Mesozoic subduction of terrigenous sediment was important, whereas subduction of oceanic carbonate sediments produced enrichment in radiogenic Sr and low Ce/Sr ratios. Late Cenozoic magmas sourced in eastern Pelagonian zone sub-continental lithospheric mantle have Nd model ages of about 1.0 Ga, and generally high [sup.87]Sr/[sup.86]Sr and high [sup.207]Pb/[sup.204]Pb (~ 15.68) and [sup.208]Pb/[sup.204]Pb (~ 39.0) for low [sup.206]Pb/[sup.204]Pb (~ 18.6), but rocks to the west have more radiogenic Pb and higher Ce/Sr as a result of greater subduction of terrigenous sediment from the northern Pindos ocean. Magmas sourced from sub-continental lithosphere beneath the Apulian continental block were strongly influenced by subduction of oceanic crust and sediments north of the passive margin of north Africa. Subduction of Nile-derived terrigenous sediment in the east resulted in Nd model ages of 0.7 to 0.8 Ga and radiogenic Pb isotopes. Greater subduction of oceanic carbonate in the west resulted in magmas with higher [sup.87]Sr/[sup.86]Sr and lower Ce/Sr. The strongly negative [[epsilon].sub.Nd] for adakites in the central Aegean rules out a source from subducted oceanic basalt, and the adakite magma was probably derived from melting of hydrated Triassic sub-alkaline basalt of continental origin. Where trachytic rocks are succeeded by nepheline-normative basalts (e.g. Samos), Nd isotope data imply that early partial melting of the enriched subcontinental lithospheric mantle involved hydrous amphibole and phlogopite, but once these minerals were consumed, younger magmas were produced by partial melting dominated by olivine and orthopyroxene.

Eocene to Pleistocene paleogeography of the Mogollon Rim--southern Colorado Plateau region in central Arizona is interpreted by use of Tertiary sedimentary deposits and mafic lavas that have been correlated on the bases of provenance of pebbles, stratigraphic sequence, geomorphic positions of deposits and flows, published K-Ar ages, tracing of lava flows to vents, and lava textures and structures. Five categories of Tertiary sedimentary deposits are (1) Paleocene(?) to Oligocene(?) fluvial and lacustrine deposits on the margin of the Colo. rado Plateau that were derived from local and distant southern sources; (2) Oligocene(?) to Miocene deposits in valleys at the base of the Mogollon Rim that were derived from local and distant southern sources; (3) Oligocene(?) to Pliocene locally derived gravel on pediments and in canyons along the rim; (4) Miocene locally derived lag gravel beneath sheet flows on the plateau; and (5) Miocene to Pleistocene locally derived channel-filling gravel on the plateau. Mafic lavas used are (1) upper Oligocene to lower Miocene basalt and minette in the Transition Zone; (2) Miocene pahoehoe sheet flows on the Colorado Plateau and draped over the Mogollon Rim; (3) middle Miocene to Pleistocene cones and flows of basalt scattered across the region; and (4) Pliocene to Pleistocene basalts of the Mormon and San Francisco volcanic fields. Basalt sheet flows in ten middle (ca. 12 Ma) and upper (ca. 6 Ma) Miocene flow fields, traced to 12 low-profile vent structures, cover nearly 30% of a 9000 [km.sup.2] area of the central Mogollon Rim region and depict the paleogeography at the times of eruption. Principal conclusions are: (1) the Mogollon Rim developed near its present position after Precambrian rocks in central Arizona had been unroofed during the Paleocene to Eocene Epochs, and before late Oligocene to early Miocene mafic volcanism; (2) after erosional quiescence during the Paleogene, Triassic strata were stripped off the southern Colorado Plateau in a Miocene to Pleistocene erosional sweep from the southwest to the northeast; (3) incision of meanders on the Mogollon Slope occurred in the late Pliocene to Pleistocene Epochs as a result of integration of the Little Colorado River with the Colorado River in the Grand Canyon. Keywords: Arizona, basalt flows, Colorado Plateau, gravel deposits, Mogollon Rim, paleogeography.

A detailed description, history and analysis are presented on the Basin and Range, or the Great Basin, region between Sierra Nevada and the Colorado Plateau, focusing on plate tectonic movement during the Cenozoic era. Findings support the theory of a large, eastward flow of deep crust from the Sierra to the Plateau during extension

The elevation of the Andean Cordillera is a crucial boundary condition for both climatic and tectonic studies. The Andes affect climate because they form the only barrier to atmospheric circulation in the Southern Hemisphere, and they intrigue geologists because they have the highest plateau on Earth formed at a noncollisional plate margin, the Altiplano-Puna. Yet, until recently, few quantitative studies of their uplift history existed. This study presents both (1) a review of the quantitative paleoelevation estimates that have been made for the Central and Colombian Andes and (2) an examination of the source and magnitude of error for each estimate. In the Central Andes, paleobotanical evidence suggests that the Altiplano-Puna had attained no more than a third of its modern elevation of 3700 m by 20 Ma and no more than half its modern elevation by 10.7 Ma. These data imply surface uplift on the order of 2300-3400 m since the late Miocene at uplift rates of 0.2-0.3 mm/yr. Paleobotanical and geomorphological data suggest a similar uplift history for the Eastern Cordillera - namely no more than half the modern elevation present by 10 Ma. No evidence exists for an exponential increase in uplift rate, as has been interpreted from fission-track data. These uplift rates mostly reflect mean surface uplift rather than rock uplift - that is, uplift of material points - because little dissection of the western Eastern Cordillera has occurred south of lat 19 degrees S and of the Altiplano-Puna. Thus, the Central Andean Plateau appears to be young. In the Colombian Andes, paleobotanical data imply rapid uplift of the Eastern Cordillera between 2 and 5 Ma at rates on the order of 0.6-3 mm/yr. However, some of this uplift is likely rock uplift due to erosion-driven isostatic rebound rather than mean surface uplift. Keywords: Andes, Cenozoic, mountain building, paleoclimate, paleogeology, tectonics.

The Ventura basin lies within the east-west-trending active fold-and-thrust belt of the western Transverse Ranges, California. This basin has been the site of significant earthquakes on structures within it and bordering it. The purpose of our study is to identify the main structures in the basin and its borders and to quantify their rate of deformation. Our study includes the onshore and offshore Ventura basin, the arcuate basin-bounding Oak Ridge reverse fault, and the Oxnard shelf to the south. Shortening, fault-slip, and crustal-block motions were studied using a three-dimensional map-restoration technique. Structure-contour maps on the 6 Ma surface and other horizons were digitized and restored to the initial horizontal state by unfolding them using the computer program UNFOLD and then fitting the unfolded surfaces across faults. Comparing the restored and present configuration allows us to estimate total net finite displacements relative to a fixed horizontal reference line. Average post-5 Ma shortening rates estimated from our restoration are slower than both post-1 Ma rates and present rates determined by global positioning systems. Most shortening due to folding in the onshore basin is post-1 Ma, although slip on the Oak Ridge fault has occurred both before and after 1 Ma. Displacement due to faulting and folding includes left-lateral strike-slip motion on the northeast-southwest coastal segment of the Oak Ridge fault and associated clockwise rotation of the adjacent Ventura basin. The Oxnard shelf is bordered to the south by mountains and islands that have been previously interpreted as folds above thrust-fault ramps. This onshore-offshore block moves as one continuous thrust sheet. Similarly, beyond the well-studied onshore fault, a kinematically continuous offshore Oak Ridge-Mid-Channel left-oblique fault system is interpreted to continue at least an additional 100 km westward beneath the Santa Barbara Channel. Keywords: faulting, kinematics, restoration, Santa Barbara Channel, Ventura basin.

From at least 2-4 Ma to present, crust in the southeastern part of the Tibetan Plateau west of the convex-east Xianshuihe-Xiaojiang fault system has deformed internally and rotated clockwise around the eastern Himalayan syntaxis. The northwest-striking Ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78-100 km, of which approximately 60 km is transferred to the Xianshuihe fault zone across a diffuse transfer zone, and approximately 22-40 km is absorbed by bending of older structures and crustal shortening. Crustal shortening is expressed along and east of the eastern end of the Ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the Tibetan Plateau. A similar transfer of left slip into shortening occurs farther south across the Xianshuihe fault in the high mountains around and east of Gongga Shan (7556 m). The northwest-striking, convex-east, left-lateral Litang fault zone lies southwest of the Ganzi-Xianshuihe-Xiaojiang fault zone and appears to be less well developed but otherwise similar to the Ganzi fault zone. The Batang, Chenzhi, and other northeast-striking right-lateral faults of small displacement occur within the rotating crustal fragment. Together with the left-slip faults, they accommodate east-west shortening northeast of the eastern Himalayan syntaxis. South of this region of shortening, the crust is extending to form grabens within the Dali and southern Xiaojiang fault systems and in the Tengchong volcanic province. The progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern Himalayan syntaxis. The assemblage of structures in southwestern Sichuan geometrically resembles structures of Eocene to Miocene age in southern Yunnan that were positioned northeast of the eastern Himalayan syntaxis, similar to present-day southwestern Sichuan, at the time of their development. The similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past approximately 50 m.y. Structures in Sichuan are less fully developed than older structures in southwestern Yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis. Deformation in these areas indicates that plateau formation has been complex, inhomogeneous, and diachronous at scales from 1000 km to less than 100 km. Keywords: Tibet, Sichuan, tectonics, late Cenozoic, syntaxis.

An overview is presented on the shift to specialized browsing and grazing in herbivorous mammals of the Cenozoic era. Research includes dental adaptations, data on extant and extinct eutherian New World mammals, and the link between adaptation and the increase in grasslands.

Paleomagnetic declination data were collected from Cenozoic strata of the southern rim of the Tarim basin to address whether the Altyn Tagh fault has undergone significant rotation during the Indian-Asian collision. Results from the eastern and central Altyn Tagh fault zone suggest that it has undergone no significant rotation since the Oligocene. This implies that the boundary between the Tarim and Tibet has remained relatively stationary during most of the Cenozoic. In contrast, declination data from the western Kunlun Shan on the eastern limb of the Pamir orocline, where the Karakax segment of the Altyn Tagh fault system terminates, suggest that the range has undergone clockwise rotations of between 19.3 [degrees] [+ or -] 8.6 [degrees] and 27.8 [degrees] [+ or -] 5.8 [degrees]. Such rotation is in mirror image with the documented counterclockwise rotation of 20 [degrees] - 50 [degrees] in the western Pamir orocline and implies relatively small displacements on the Karakorum fault. Our results also suggest that the Karakax fault may have formed as an accommodation zone between the western Kunlun Shan and the Karakorum Mountains.

Average concentrations of incompatible elements and isotopes of radiogenic Sr, Nd and Pb in a 350 km long belt of Central European volcanics (CECV) resemble those in OIB in general and in OIB from the N Atlantic in particular. This similarity allows to infer comparable sources for CECV and OIB which might have been located in the lower mantle according to seismic tomography and chemistry, with the latter unlike a MORB source. The incompatible element contribution of lower mantle origin can be modelled from primitive mantle minus continental crust and upper mantle inventories. Alkali basaltic magmas from the modelled source are close in composition to CECV and OIB. The continental crust contains almost half of the silicate Earth's content of Rb, K, Pb, Ba, Th and U, which were mobilized to a major extent through dehydration of subducted ocean crust. Related losses from the lower mantle had to be replaced by deep subductions of oceanic lithosphere recognized from their isotopic imprint. From a balance based on Nd isotopes it can be concluded that average CECV contains 60% matter from residual primitive mantle and 40% from deeply subducted lithosphere (including some young upper mantle materials). Plume products from separate CECV regions developed, within 45 Ma, from rather depleted to more primitive isotopic signatures. Four periods of volcanism from Eocene to late Quaternary time are explained as four pulses of an almost stationary ultrafast plume uprise as modelled by Larsen and Yuen (1997). Magma production has increased from the first to the third pulse with the peak during Miocene time in the Vogelsberg region. The final pulse produced the Quaternary Eifel volcanoes. Tectonism from the Alpine orogen has probably triggered the synchronous volcanism of CECV, Massif Central etc. The European lithospheric plate has moved under the control of the opening Atlantic almost in an eastern direction with a velocity of 1 cm per year and has shifted extinct volcanoes off their source channels. Received: 10 July 1998 / Accepted: 10 February 1999

The causal relationship between the Cenozoic sequence development in the southeastern North Sea Basin and sea-level changes, climatic fluctuations and tectonic events is unravelled by relating variations in the relative sea level and base level, based on interpretations of seismic surveys, to published [Delta]18O variations and eustatic changes. The latter curve is based on the Earth's orbital forcing, and here informally termed as the GSI curve. The analysis shows that the Cenozoic sequence development in the southeastern North Sea was influenced by climatically and tectonically induced sea-level changes. The major Cenozoic sequence stratigraphic boundaries (lower order) are highly influenced by tectonic events, e.g. uplift of Fennoscandia and increased subsidence rates in the basin centre. Reactivation of Mesozoic fault zones controlled the deposition of minor sand bodies transported to the centre of the basin during the Late Palaeocene by mass flows. The location of an Oligocene mound structure, which constitutes part of a sequence, is controlled by the overall palaeotopography of the basin and local fault-related depressions. Correlation between (i) the ages of our sequences and the [Delta]18O variations in the Oligocene succession, and (ii) the GSI curve and the base-level fluctuations of the late Miocene and younger sequences, show that the generation of the higher order sequence boundaries were driven by glacio-eustatic sea-level changes. A climatic control of the sequence formation due to glacio-eustatic sea level changes is therefore suggested for the Oligocene and Pliocene sequences, and probably also for the Upper Miocene sequences. Keywords: Cenozoic, North Sea, glaciotectonics, eustasy, deposition.

A long-lasting Cenozoic record ([approximately]50 m.y.) of alkaline igneous rocks characterizes northern Victoria Land, Antarctica. Landscape analysis allows distinction between older volcanic and intrusive rocks with well-developed alpine topography sculptured by wet-based glaciers, and younger volcanic cones lacking these features. Many K-Ar and Rb-Sr dates testify that the erosion that formed the alpine landscape ceased between ca. 8.2 and 7.5 Ma. Since ca. 8 Ma, morphological evolution has been driven by cold-based glaciers; warm-based glaciers were no longer active. That this change affected a 300-km-long coastal area suggests a persistent cause of global significance. Glacier dynamics control landscape shaping as a function of ice thickness and temperature, which are driven by climatic conditions. In this view, a significant climatic change occurred in northern Victoria Land between 8.2 and 7.5 Ma. The perfectly preserved serrated alpine ridges, with their delicate spires, testify that no warm-based ice sheets overrode the region after 7.5 Ma and that polar conditions held sway in the Pliocene and Pleistocene Epochs.

Paleoproterozoic high-Mg andesites of the Birch Lake assemblage in the Trans-Hudson orogen display trace element enrichments characteristic of Cenozoic boninites, occur in an analogous stratigraphic setting, and share a metallogenic association with the younger rocks. Their geochemical compositions indicate substantial prior depletion and a selective reenrichment of the mantle sources consistent with metasomatism by siliceous melts. These requirements are best accounted for in an island-arc setting and indicate that early extensional stages of subduction-zone development have not changed substantially over the past 2000 m.y.

A histogram of 778 isotopic ages of magmatic rocks younger than Eocene in central Mexico shows a multimodal distribution with peaks at about 30 Ma, 23 Ma, 10 Ma, and 5 Ma. The sample suite displays systematic spatial variations with age that likely reflect the protracted transition from the north-northwest - trending arc of the Sierra Madre Occidental to the east-west - trending Mexican Volcanic Belt. The reorientation of the arc is accompanied by a change in the dominant composition of the products from silicic ignimbrites and rhyolites to andesitic and basaltic lavas. The observed transition is related to the Miocene reorganization of the subduction system following the cessation of subduction off Baja California and the eastward motion of the Caribbean - Farallon - North America triple junction along the southeastern margin of Mexico. Our data support an early - middle Miocene age for the initiation of subhorizontal subduction in southern Mexico and confirm that the locus of arc volcanism was primarily controlled by the geometry of plate boundaries and the thermal structure of the subducting slab.

Pb, Sr, and Nd isotope data for Cenozoic basalts in eastern Australia indicate that mantle isotope signatures of Pacific Ocean mid-oceanic ridge basalts (MORB) type characterize the lava-field basalts (55-14 Ma) in southeastern Australia, whereas mantle isotope signatures of Indian Ocean MORB type characterize younger basalts (6-0 Ma) from northeastern Australia. This discovery further constrains the secular distribution of major asthenospheric mantle reservoirs represented by the Pacific and Indian Oceans MORB sources during and following the breakup of eastern Gondwana and tracks, for the first time, the locus of the boundary of the two reservoirs beneath the Australian continent. These new data fill the gap between previous Indian Ocean MORB-Pacific Ocean MORB boundary locations determined from backarc basin basalts in the southwestern Pacific Ocean and ocean-floor basalts in the Southern Ocean. We propose that the Indian Ocean MORB source is a long-term asthenospheric reservoir beneath most of eastern Gondwana and that the westward migration of the Pacific Ocean MORB source may have been associated with the opening of the Tasman Sea (ca. 85-60 Ma) along a broad front southeast of the Australian continent.

Northern and central African geology is characterized by wide plateaux, volcanism and narrow swells. Largest magma volumes occur on the greater than 1000-km-wide Ethiopian and east African plateaux. A model of a single large plume beneath the Ethiopian plateau is presented, taking into account lateral flow and ponding of plume material. It may explain distributions and timings of magmatism and uplift throughout east Africa.

John Alroy's conclusions based on his observations of substantial diachrony in mammalian appearance events of the Cenozoic period in North America are not altogether valid because of some defects in his sampling methodology. The methodology produces skewed results because of the choice of nonrepresentative samples. By correcting these sampling errors, Alroy's suggestion that the fossil mammal taxa being studied is useless for biochronology loses weight. Alroy, on the other hand, defends his method.

The Rhodope metamorphic core complex, exposed beneath the Strymon Valley detachment in northeastern Greece, comprises a platform carbonate sequence >5000 m thick intruded by Tertiary calc-alkaline plutons. The final thickening of the north Aegean Alpine collisional orogen in Paleogene time and its Neogene-Quaternary extensional dismemberment in the backarc of the Hellenic subduction zone produced structures within and above the Rhodope core complex that are related here to four successive deformations. In early-middle Eocene time (D1), the Falakron marble series was subducted northeastward beneath the Serbo-Macedonian-West Thracian gneiss complex, a heterogeneous ophiolite-bearing highgrade metamorphic terrane that was accreted to southeastern Europe in Cretaceous time. The orogen began to extend on a northeast-southwest axis in earliest Miocene time, evidenced in the Rhodope core complex by the emplacement of the Symvolon granodiorite ca. 21 Ma within a northwest-trending midcrustal coaxial rupture of the Falakron slab (D2). The Strymon Valley detachment system succeeded the D2 Symvolon rupture and related structures, facilitating unroofing of the core complex and a transition from ductile to brittle deformation ca. 16-3.5 Ma (D3). The Serbo-Macedonian gneiss complex, the island of Thasos, and a supradetachment basin were translated relatively southwestward as much as 80 km in the D3 hanging wall. Balanced reconstructions of D2 and D3 displacements predicate two new hypotheses concerning the Cenozoic tectonic evolution of northeastern Greece. First, the southwest-vergent Thasos detachment and northeast-vergent low-angle normal faults in the Olympos region may both have rooted in the coaxial Symvolon rupture zone to form a bivergent early Miocene extensional system. Second, the Olympos and Falakron carbonate platforms may be correlative remnants of the subducted eastern margin of the Apulian microcontinent, implying that the 'Vardar zone,' the putative Alpine suture, is a roofless ophiolite belt, and that segments of the Alpine suture are actually exposed over a zone as wide as 200 km, including the Rhodope province. Cumulative D2 and D3 stretching of [greater than or equal to]100% created the north Aegean basin. The North Anatolian fault, which accommodates the westward escape of Anatolia from the Pontide suture, propagated into the north Aegean region in late Pliocene time. Its offshore continuation, the North Aegean Trough, transfers dextral strike-slip displacement into extension to the north, principally expressed in northern Greece by the Thermaikos, Strymon, and Drama half grabens (D4).

The apatite fission-track method is used to determine the exhumation history of the Olympic subduction complex, an uplifted part of the modern Cascadia accretionary wedge. Fission-track ages are reported for 35 sandstones from the Olympic subduction complex, and 7 sandstones and 1 diabase from the Coast Range terrane, which structurally overlies the Olympic subduction complex. Most sandstone samples give discordant results, which means that the variance in grains ages is much greater than would be expected for radioactive decay alone. Discordance in an unreset sample is caused by a mix of detrital ages, and in a reset sample is caused by a mix of annealing properties among the detrital apatites and perhaps by U loss from some apatites. Discordant grainage distributions can be successfully interpreted by using the minimum age, which is the pooled age of the youngest group of concordant fission-track grain ages in a dated sample. The inference is that this fraction of apatites has the lowest thermal stability, and will be the first to reset on heating and the last to close on cooling. Comparison of the minimum age with depositional age provides a simple distinction between reset samples (minimum age younger than deposition) and unreset samples (minimum age older than deposition). The success of the minimum-age approach is demonstrated by its ability to resolve a well-defined age-elevation trend for reset samples from the Olympic subduction complex. Microprobe data suggest that the apatites that make up the minimum-age fraction are mostly fluorapatite, which has the lowest thermal stability for fission tracks among the common apatites. Reset minimum ages are all younger than 15 Ma, and show a concentric age pattern; the youngest ages are centered on the central massif of the Olympic Mountains and progressively older ages in the surrounding lowlands. Unreset localities are generally found in coastal areas, indicating relatively little exhumation there. Using a stratigraphically coordinated suite of apatite fission-track ages, we estimate that prior to the start of exhumation, the base of the fluorapatite partial annealing zone was located at [approximately] 100 [degrees] C and [approximately] 4.7 km depth. The temperature gradient at that time was 19.6 [+ or -] 4.4 [degrees] C/km, similar to the modern gradient in adjacent parts of the Cascadia forearc high. Apatite and previously published zircon fission-track data are used to determine the exhumation history of the central massif. Sedimentary rocks exposed there were initially accreted during late Oligocene and early Miocene time at depths of 12.1-14.5 km and temperatures of [approximately]242-289 [degrees] C. Exhumation began at ca. 18 Ma. A rock currently at the local mean elevation of the central massif (1204 m) would have moved through the [Alpha]-damaged zircon closure temperature at about 13.7 Ma and [approximately]10.0 km depth, and through the fluorapatite closure temperature at about 6.7 Ma and [approximately]4.4 km depth. On the basis of age-elevation trends and paired cooling ages, we find that the exhumation rate in the central massif has remained fairly constant, [approximately]0.75 km/m.y., since at least 14 Ma. Apatite fission-track data are used to construct a contour map of long-term exhumation rates for the Olympic Peninsula. The average rate for the entire peninsula is [approximately]0.28 km/m.y., which is comparable with modern erosion rates (0.18 to 0.32 km/m.y.) estimated from sediment yield data for two major rivers of the Olympic Mountains. We show that exhumation of this part of the Cascadia forearc high has been dominated by erosion and not by extensional faulting. Topography and erosion appear to have been sustained by continued accretion and thickening within the underlying Cascadia accretionary wedge. The rivers that drain the modern Olympic Mountains indicate that most of the eroded sediment is transported into the Pacific Ocean, where it is recycled back into the accretionary wedge, either by tectonic accretion or by sedimentary accumulation in shelf and slope basins. The influx of accreted sediments is shown to be similar to the outflux of eroded sediment, indicating that the Olympic segment of the Cascadia margin is currently close to a topographic steady state. The record provided by our fission-track data, of a steady exhumation rate for the central massif area since 14 Ma, suggests that this topographic steady state developed within several million years after initial emergence of the forearc high.

In the Golden Triangle region of southeast Asia (northern Thailand, Laos and Burma, southern Yunnan), the Mekong, Salween, and neighboring rivers show hairpin geometries where they cross active strike-slip faults. Restoration of young, left-lateral offsets of these rivers leaves residual right-lateral bends of many kilometers. We interpret these hairpins as evidence of late Cenozoic slip-sense inversion on these faults, about 5 to 20 Ma. Near the Red River fault, stress field and slip-sense inversion occurred ca. 5 Ma. This implies that the present course of these large rivers has existed for at least several million years. Pliocene-Quaternary slip rates, possibly on the order of 1 mm/yr, are inferred on each of the strike-slip faults of the Golden Triangle.

The Cenozoic basaltic rocks in Sikhote-Alin and Sakhalin exhibit a change over 55 m.y. from subduction-zone-related to continental-rift-related volcanism as the Japan Sea opened between eastern Sikhote-Alin and the Japan arc. The temporal geochemical trends in volcanic rocks erupted before and during the opening of the Japan Sea, suggesting that there was a change in magma source from the lithosphere to the depleted asthenosphere as the Japan Sea opening progressed. Two geochemical groups can he identified within the basalts that postdate the Japan Sea opening: tholeiitic basalts and alkalic basalts. As the tholeiitic basalts have EMI-type signatures, we propose that the tholeiites had a significant contribution from an EMI-type Precambrian subcontinental lithospheric mantle. The alkalic basalts, yielding geochemical characteristics similar to enriched oceanic island basalts, may have been derived from the subcontinental asthenospheric mantle. The opening of the Japan Sea was triggered by lateral migration of asthenospheric mantle from beneath northeast China toward the Japan arc.

The central and southern Coast Ranges of California coincide with the broad Pacific-North American plate boundary. The ranges formed during the transform regime, but show little direct mechanical relation to strike-slip faulting. After late Miocene deformation, two recent generations of range building occurred: (1) folding and thrusting, beginning ca. 3.5 Ma and increasing at 0.4 Ma, and (2) subsequent late Quaternary uplift of the ranges. The ranges rose synchronously along the central California margin and are still rising; their long axes are quasiparallel to the plate boundary and strike-slip faults. The upper crustal internal and marginal structures of the ranges are contractional, dominated by folds and thrusts resulting from the convergent component of plate motion. Newly constructed transects using seismic reflection and refraction, plus gravity and magnetic studies, reveal lower crustal basement(s) at depths of 10-22 km. The upper surface of the basement and Moho show no effect of the folding and thrusting observed in the upper crust. We conclude that horizontal shortening is accommodated at depth by slip on subhorizontal detachments, and by ductile shear and thickening. The ranges are marked by high heat flow; weak rocks of the Franciscan subduction complex; high fluid pressure; bounding high-angle reverse, strike-slip, or thrust faults; and uplift at a rate of 1 mm/yr beginning about 0.40 Ma. Transverse compression manifested in folding within the Coast Ranges is ascribed in large part to the well-established change in plate motions at about 3.5 Ma.

A 4.3-km-thick section of clastic sediments of the Surma basin, northeastern Bangladesh, that is exposed along the Shari River was sampled for a magnetostratigraphic study. Miocene through Pleistocene ages have been discussed previously; however, there has been no firm dating through biostratigraphy, or radiometric or other methods. Primary paleomagnetic remanent magnetizations of normal or reversed polarity were determined for most of the 300 collected samples. On the basis of assumptions of (1) constant sedimentation rates, on average, throughout the deposition of the sedimentary sequence, (2) no major gaps in sedimentation, and (3) a fairly young (< 2 Ma) age for the topmost Dupi Tila deposits, we find good agreement between the measured profile and the geomagnetic polarity time scale if the entire sedimentary sequence is between 4.9 and 1.4 Ma in age. The Upper Marine Shales, an important seismic marker horizon, are dated as 3.5 Ma and are therefore [approximately]1.5 m.y. younger than previously assigned by biostratigraphic results. The Upper Marine Shales presumably correspond to the last pre-Pleistocene sea-level highstand. The dating of rock units below the Upper Marine Shales is tentative because of a large gap in exposure. The inferred sedimentation rate - and thus the rate of delta subsidence - of 1.2 m/k.y. is much larger than previously assumed; it appears to be one of the highest sedimentation rates in Earth history that was sustained for millions of years.

The western Koryak fold and thrust belt consists of a set of tectonostratigraphic terranes that contain units ranging from Lower Palaeozoic to Cenozoic. Three deformational events have been identified in the study area. The first event structures are folds, domes and shear zones with related high-pressure/low-temperature metamorphism. These structures are early Carboniferous and are only recognized in the metamorphic terranes. The second event structures are imbricate fans of thrusts and folds with southeast vergence, broken formation and serpentinite melange. These are latest Jurassic to early Cretaceous (early Albian) and occur throughout the study area. During this event, thrusting was accompanied by dextral strike-slip faulting. The second event structures are overlapped by the Upper Albian sedimentary rocks with an angular unconformity at the base. During metamorphism associated with the first and second deformational events, some of the rocks were metamorphosed to blueschist grade and were affected by strain with axial ratios of up to 15:1. The third deformational event is characterized by significant sinistral strike-slip displacement at higher crustal levels. This resulted in a new set of structures and rotation of pre-existing structures. The age of the sinistral strike-slip faults is interpreted to be late Cretaceous to Cenozoic. The kinematics of the second and third deformational events correspond to assumed proto-Pacific plate motions based on palaeomagnetic data.

Backstripping analysis of three continuously cored, well-dated boreholes from the New Jersey Coastal Plain (Ocean Drilling Program [ODP] Leg 150x) indicates a long-term ([10.sup.8]-[10.sup.7] yr) eustatic fall of [approximately equal to]100 m since 55 Ma (early Eocene) and suggests short-term (0.5-3 m.y.) eustatic falls of less than [approximately equal to]70 m. Eustatic estimates are calculated from residuals between the decompacted, unloaded, and paleodepth-corrected records and tectonic subsidence (assuming a cooling lithospheric plate). Because the residuals are similar among the three sites, we interpret them as an approximation of the eustatic signal.

Recent geophysical studies in the southern Sierra Nevada found no significant Airy-type crustal root; the mountain range seems largely supported by the buoyant asthenosphere beneath an abnormally thin mantle lithosphere. We suggest that the late Cenozoic uplift of the Sierra Nevada may have resulted from mantle upwelling under the Basin and Range province, which tends to push ductile material within the surrounding lithosphere, causing it to flow away and downward. Numerical modeling indicates that such ductile flow could lead to pronounced lithospheric thinning under the High Sierra and lithospheric thickening under the western Sierra Nevada, comparable to the observed structure.

Stratigraphic sequences preserved in the Bengal basin provide detrital information that documents a significantly older history of the eastern Himalaya than that available from Ocean Drilling Program and Deep Sea Drilling Project cores recovered from the Bengal fan. The Bengal basin, formed as a result of the Himalayan collision, is located at the juncture of the Indian craton to the west, the Shillong massif and the Himalayan belt to the north, and the Indo-Burman ranges to the east. Modal analyses of Eocene and Oligocene sandstones of the Cherra, Kopili, and Barail Formations document compositions that are dominated by subangular monocrystalline quartz grains with scarce to no feldspar grains, and few lithic fragments (Cherra and Kopili: [Qt.sub.99][F.sub.1][L.sub.0]; Barail: [Qt.sub.90][F.sub.3][L.sub.7]; Qt - total quartz, F - feldspar, L - lithic fragments). These compositions are similar to sands derived from the Indian craton, suggesting that they underwent intense chemical weathering, likely in a source with low relief and considerable transport. Himalayan tectonism during this time was probably significantly more distant from the Bengal basin than at present. Sandstones of the Miocene Surma Group (Bhuban and Boka Bil Formations) are rich in feldspar grains, argillite, and very low-grade metamorphic lithic fragments (Bhuban: [Qt.sub.66][F.sub.15][L.sub.19]; Boka Bill: [Qt.sub.57][F.sub.23][L.sub.20]) relative to older sandstones, suggesting onset of uplift and erosional unroofing in the eastern Himalaya, and initiation of stream systems supplying orogenic detritus to the Bengal basin. Sands of the upper Miocene to Pliocene Tipam Group and the Pliocene-Pleistocene Dupi Tila Sandstone contain abundant argillitic and low- to medium-grade metamorphic lithic fragments and feldspar grains (Tipam: [Qt.sub.61][F.sub.19][L.sub.19]; Dupi Tila: [Qt.sub.70][F.sub.13][L.sub.17]), suggesting continued orogenic unroofing. These younger sands are rich in potassium feldspar (P/F = 0.30, 0.20) relative to plagioclase (P)-rich Bhuban and Boka Bil sandstones (P/F = 0.66 and 0.48), suggesting a granitic source, probably the Miocene leucogranites of the High Himalayan Crystalline terrane. These results document for the first time contrasts in orogenic history recorded in the Bengal system vs. western Himalayan foreland basins. Sands deposited in the Bengal basin are generally more quartzose and less lithic than those from the western Himalayan foreland basins, and pre-Miocene strata in the Bengal system show little to no evidence of orogenic activity. In part, this probably reflects west to east progression of the Himalayan collision, but it probably also results from sedimentary systems propagating southward, ahead of the advancing mountain belt as it has been consuming the remnant ocean basin trapped between the Indian craton and the Burmese block.

Cenozoic volcanic rocks of coastal California were erupted west of the magmatic arc trend related to subduction along the continental margin. Two assemblages, representing discrete pulses of mid-Tertiary (Oligocene-Miocene) and mid-Miocene volcanism, occupied relatively compact tracts, as restored palinspastically prior to disruption and dispersal by tectonic displacements within the Neogene San Andreas transform system. A third assemblage records migratory post-mid-Miocene volcanism at centers located progressively farther north. The distribution and petrologic character of all three assemblages reflect slab-window magmatism triggered by decompression melting of upwelling mantle. Paleogeographic reconstructions, based on analysis of magnetic anomaly patterns offshore and restoration of on-land features prior to San Andreas transform slip and associated transrotational deformation, indicate the paleotectonic positions of the Cenozoic volcanic fields before structural disruption. The pulses of mid-Tertiary and mid-Miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise-trench encounters associated with subduction of the Vancouver-Farallon and Monterey-Arguello plates, respectively, along the continental margin off southern California. Migratory post-mid-Miocene volcanism in central California accompanied the incremental expansion of a growing slab window as the Mendocino triple junction migrated northward along the continental margin. Mid-Miocene Columbia River basalt volcanism in the Pacific Northwest was coeval with the mid-Miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the San Andreas transform as a coherent plate boundary. Columbia River volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the Pacific plate.

The origin of the high plateau in the Central Andes in South America was studied to determine the origin of the high plateaus in mountain belts. The study described the Cenozoic geological evolution of the region by utilizing a revised chronostratigraphy and analyzing the crustal and lithospheric structure. Results present geomorphological and geochronological evidence for the surface uplift. The present relief in the Altiplano is a consequence of ductile flow in the lower crust, sedimentation and erosion in an internal drainage basin.

Time-varying global dynamic topography has been computed for the entire Cenozoic. The spherical viscous-flow model is driven by a distribution of density heterogeneity based on Mesozoic and Cenozoic subduction. By using the calculated topography, the uplift and subsidence histories of five regions fixed to the North American, Indian, Australian, Indonesian, and South American continents are computed. Predicted vertical motions are in qualitative agreement with observed values except for India and South America. For Australia and North America, the predicted periods of uplift and subsidence are out of phase with respect to observations, but display the correct overall trend. The latest phase of uplift for North America is directly related to the cessation of Kula plate subduction after 48 Ma. Dynamic topography depends on the viscosity structure of the mantle; a lower mantle that is 50 times more viscous than the upper mantle yields the best agreement between modeled and predicted vertical motions. The presence of a low-viscosity channel beneath the lithosphere has only a small influence on our results. Compared to dynamic topography, the time-varying geoid is insignificant in controlling relative sea level.

It is widely believed that the Tibetan plateau is a late Cenozoic feature produced by the Indo-Asian collision. However, because Tibet was the locus of continental accretion and subduction throughout the Mesozoic, crustal thickening during that time may also have contributed to growth of the plateau. This portion of the geologic history was investigated in a traverse through the central Lhasa block, southern Tibet. Together with earlier studies, our mapping and geochronological results show that the Lhasa block underwent little north-south shortening during the Cenozoic. Rather, our mapping shows that [approximately]60% crustal shortening, perhaps due to the collision between the Lhasa and Qiangtang blocks, occurred during the Early Cretaceous. This observation implies that a significant portion of southern Tibet was raised to perhaps 3-4 km elevation prior to the Indo-Asian collision.

The taxonomy of the predominantly Australian fossil dipnoan genus, Neoceratodus, is revised and the Recent Australian lungfish, Neoceratodus forsteri, and two fossil species, Neoceratodus eyrensis and Neoceratodus nargun, are redefined. Two new species of the related Tertiary genus, Mioceratodus, are described on the basis of tooth plates from central and northern localities in Australia. These are Mioceratodus diaphorus and Mioceratodus poastrus. A new genus, Archaeoceratodus, is erected to include three rare Tertiary species and one Mesozoic species. The Tertiary members of this genus are the type species, Archaeoceratodus djelleh, described originally as Neoceratodus djelleh, and two new species, Archaeoceratodus rowleyi and Archaeoceratodus theganus. The Mesozoic species is Archaeoceratodus avus from Triassic and Cretaceous deposits in southeastern Australia, described originally as Ceratodus avus. All three genera belong in the family Neoceratodontidae.

The present morphological expression of the continental margin off NW Britain is a mid- to late Cenozoic phenomenon, initiated by a major phase of rapid subsidence in the Rockall Trough and Hatton - Rockall Basin during the early late Eocene. This led to a deepening of the basins and the onset of bottom-current activity in this region. On seismic-reflection profiles, this basin-subsidence event is manifest in the form of a widespread deep-water unconformity caused by bottom-current erosion. This boundary is particularly enhanced at the basin margins and adjacent to the axial seamounts of Rosemary Bank and Anton Dohrn, where the downwarped and eroded surface of lower upper Eocene and older strata is onlapped by middle to upper Cenozoic sediments. The latter comprise two megasequences of late Eocene to mid-Miocene and mid-Miocene to Holocene age, which consist predominantly of deep-marine contourites both in the Rockall Trough and the Hatton-Rockall Basin, although a clastic wedge has built out along the Hebridean shelf-margin since the mid-Miocene. These megasequences reflect a gross, two-stage, depositional history; a response to intra-plate tectonism which modified sedimentation patterns and palaeoceanographic circulation. The development of the bounding unconformities (early late Eocene and mid-Miocene) was coincident with major phases of regional tectonism. Keywords: Cenozoic, Rockall Trough, sedimentation, bottom currents, intra-plate processes.

Age data and estimates on upper-crustal shortening together with observations on the present-day deep-seismic structure, magmatic activity, and geomorphology indicate that the central Andean arc and back arc (lat 20 [degrees]-22 [degrees] S) reflect two major thickening mechanisms active during crustal convergence. The first process (from ca. 27 to 5 Ma) involved crustal under-thrusting and pervasive lower crustal shortening and thickening. This caused thickening and uplift in the Altiplano, and shortening in the Eastern Cordillera and Interandean zone. The second process (from ca. 5 Ma to the present) involved crustal stacking and caused thickening and uplift in the Eastern Cordillera and shortening in the Subandes. This concept of two thickening mechanisms is synthesized in an area-balanced model describing the crustal evolution from the late Oligocene to the present along a profile from the Western Cordillera to the Brazilian Shield.

Several well-dated stratigraphic markers permit detailed assessment of the temporal and spatial variation in slip rates along the interconnected faults of the Fish Lake Valley, Emigrant Peak, and Deep Springs fault zones in west-central Nevada and east-central California. Right-lateral motion on the Fish Lake Valley fault zone apparently began ca. 10 Ma (11.9-8.2 Ma). Associated extensional faulting probably began ca. 5 Ma (6.9-4 Ma) and resulted in the opening of Fish Lake Valley and Deep Springs Valley. The long-term lateral-slip rate for the Fish Lake Valley fault zone since about 10 Ma is 5 mm/yr (3-12 mm/yr). Our preferred lateral-slip rate for the central, most active part of the Fish Lake Valley fault zone decreased from about 6 to 3 mm/yr from the late Miocene to the early Pleistocene, increased to about 11 mm/yr during the middle Pleistocene, and decreased to about 4 mm/yr during the late Pleistocene. Extension may account for some of the change in lateral-slip rate during the Pliocene. The large increase in lateral-slip rate during the middle Pleistocene is circumstantially linked to an increase in vertical-slip rates on the Fish Lake Valley and Deep Springs fault zones at about the time of the eruption of the Bishop ash (0.76 Ma). Vertical-slip rates along the three fault zones are also related to fault strike; vertical rates are highest on north-striking faults and approach zero on northwest-striking faults. The long-lived slip history of the Fish Lake Valley fault zone fits a tectonic model in which the Death Valley-Furnace Creek-Fish Lake Valley fault system is integrated with right-lateral shear on faults of the central Walker Lane and the Eastern California shear zone to accommodate part of the Pacific-North American relative plate motion. Our research demonstrates that the Fish Lake Valley fault zone accounts for about half the rate of 10-12 mm/yr of Pacific-North American plate-boundary shear accommodated within the Basin and Range Province between about lat 37 [degrees] and 38 [degrees] N.

This paper expands the K-Ar dating and palynologically controlled stratigraphical data base reported in earlier papers to the north trending Selendi and Usak-Gure basins located to the north of east-west trending Alasehir graben in western Turkey. These north trending basins began to form during Early Miocene times and most of their basin fills accumulated before 14 Ma, except for the youngest Asartepe formation. Recent studies of both east-west grabens and north trending basins show that they started to develop simultaneously during Early Miocene times under the north-south extensional regime, and the classification of the structures as 'replacement' and 'revolutionary' has no meaning for the Alasehir graben and the basins located to its north.

Paleogeographic reconstructions facilitate three-dimensional modeling of the kinematics of late Cenozoic deformation in the Death Valley region, southeastern California. The model suggests that deformation started at 6 Ma as a localized pull-apart system between two conjugate strike-slip faults. This was followed by a phase of distributed dextral transtension associated with the eastern California shear zone. Rotations and translations dominated the displacement field and the actual crustal thinning was small. The model is able to explain the complex evolution of the Black Mountains.

Geographic mapping and structural and paleomagnetic analysis of the Northeast Mojave Domain's Fort Irwin region were studied to address plate motion partitioning across and within the plate boundary zone and to determine the kinematics, geometry, faulting timing and the distribution and role of vertical-axis rotations. The fault slips determined from geologic data do not match the rotations inferred from paleomagnetic declination anomalies because of the fault blocks' nonrigidity and the domain deformation's three-dimensional nature.

Although Cenozoic crustal shortening and thickening by thrust faulting have built the present Tien Shan, active right-lateral shear on the northwest-trending Talas-Ferghana fault appears to be the most rapid localized deformation in the belt. Ephemeral stream valleys have been offset right-laterally tens of metres. New and published radiocarbon dates of organic material deposited in depressions blocked by offset ridges place upper bounds on the average Holocene slip rate at 18 localities. Uncertainties allow 14 upper bounds to overlap the range of 8-16 mm/yr, and 95% confidence limits on such bounds at 11 sites are entirely within this range. We infer that the rate of [approximately equal to]10 mm/yr is not simply an upper bound, but applies to the late Holocene Epoch. Although the bounds on rates permit more rapid slip in the northwest than the southeast, they do not place a useful constraint on variations in slip rate along the fault. Offsets of Paleozoic facies boundaries imply a total right-lateral shear of 180-250 km, but Early Cretaceous sedimentary rock appears to have been offset only 60 [+ or -] 10 km. Published paleomagnetic declinations of Cretaceous-Miocene rock demonstrate 20 [degrees]-30 [degrees] of counterclockwise rotation of the Ferghana Valley, which lies just west of the Talas-Ferghana fault, with respect to stable parts of Eurasia and 20 [degrees] [+ or -] 11 [degrees] with respect to the central Tien Shan east of the fault. These declinations are consistent with a maximum northwest-ward translation of 70-210 km of the Ferghana Valley at the Talas-Ferghana fault and, therefore, with a similar maximum horizontal shortening across the Chatkal Ranges, which lie between the Ferghana Valley and the Kazakh platform. Estimates of crustal thickness beneath the Chatkal Ranges, however, permit only 60-100 km of Cenozoic shortening. If < 100 km of slip on the Talas-Ferghana fault accumulated at a constant rate of 10 mm/yr, it would imply an initiation of slip more recently than ca. 10 Ma, long after India collided with the rest of Eurasia.

Magnetic anomalies over large ice- and sea-covered areas of the West Antarctic rift system are interpreted as proof of late Cenozoic volcanism. Aeromagnetic profiles and detailed surveys indicate that the late Cenozoic magmatic rock volume is much more than that of exposed volcanos. A theory suggests greater lower lithosphere stretching, resulting in enhanced decompression melting. However, the presence of spreading centers around the Atlantic Plate introduces a space problem in this theory.

The upper ignimbrite that represents the voluminous mid-Cenozoic rhyolites in La Olivina region, north central Mexico, has been formed by assimilation-fractional crystallization of mantle-derived basaltic magmas. The isotopic content of the magmas is similar to that of pyroxenites obtained from the mantle. The lower ignimbrite was formed from basaltic magma before the formation of the upper ignimbrite. Analysis of granulite xenoliths shows that large volumes of cumulates form at crustal depths greater than those exposed by the orogenic uplift.