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Understanding the causes of intraplate earthquakes is challenging, as it requires extending plate tectonic theory to the dynamics of continental deformation. Seismicity in the western United States away from the plate boundary is clustered along a meandering, north-south trending 'intermountain' belt (1). This zone coincides with a transition from thin, actively deforming to thicker, less tectonically active crust and lithosphere. Although such structural gradients have been invoked to explain seismicity localization (2,3), the underlying cause of seismicity remains unclear. Here we show results from improved mantle flow models that reveal a relationship between seismicity and the rate change of 'dynamic topography' (that is, vertical normal stress from mantle flow). The associated predictive skill is greater than that of any of the other forcings we examined. We suggest that active mantle flow is a major contributor to seismogenic intraplate deformation, while gravitational potential energy variations have a minor role. Seismicity localization should occur where convective changes in vertical normal stress are modulated by lithospheric strength heterogeneities. Our results on deformation processes appear consistent with findings from other mobile belts (4), and imply that mantle flow plays a significant and quantifiable part in shaping topography, tectonics, and seismic hazard within intraplate settings., Interplate earthquakes are well described by plate tectonics as a consequence of horizontal motion gradients within the surface boundary layer of thermo-chemical mantle convection. For seismicity away from plate boundaries, [...]

The interplay between tectonics and surface processes defines the evolution of mountain belts. However, correlating these processes through the evolution of natural orogens represents a scientific challenge. Analog models can be used for analyzing and interpreting the effect of such interaction. To fulfill this purpose it is necessary to understand how the imposed boundary conditions affect analog models' evolution in time and space. We use nine analog models characterized by different combinations of imposed regional slope and rainfall rates to investigate how surface processes respond to the presence of tectonically built topography (imposed slope) under different climatic conditions (rainfall rate). We show how the combination of these parameters controls the development of drainage networks and erosional processes. We quantify the morphological differences between experimental landscapes in terms of a proposed Se/R ratio, accounting for both observables and boundary conditions. We find few differences between analog models and natural prototypes in terms of parametrization of the detachment-limited stream power law. We observe a threshold in the development of channelization, modulated by a tradeoff between applied boundary conditions. [ABSTRACT FROM AUTHOR]

The interplay between tectonics and surface processes defines the evolution of mountain belts. However, correlating these processes through the evolution of natural orogens represents a scientific challenge. Analogue models can be used for analyzing and interpreting the effect of such interaction. We use nine analog models characterized by different combinations of imposed regional slope and rainfall rates to investigate how a small scale orogen evolves in response to tectonics and surface processes. We show how the combination of these parameters control the development of drainage networks and erosional processes. We quantify the different morphological expression of analogue landscapes in terms of a proposed Ae number that accounts for both observables and boundary conditions. We find few differences between analogue models and natural prototypes, in terms of parametrization of the detachment-limited stream power law. We observe a threshold in the development of channelization, modulated by a tradeoff between applied boundary conditions. [ABSTRACT FROM AUTHOR]

Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry are applied to constraint the thermal history of the basement rock forming the Peloritani Mountains in northeast Sicily, Italy. AFT ages range between 29.0 [+ or -] 5.5 Ma and 5.5 [+ or -] 0.9 Ma while AHe ages vary from 19.4 Ma to 3.3 Ma. Most AFT ages are younger than the overlying terrigenous sequence that, in turn, postdates the main orogenic phase. Through the coupling of the thermal modeling with the stratigraphic record, a middle Miocene thermal event is revealed. This event affected an inner-intermediate portion of the Peloritani Mountains and is confined by the distribution of the AFT ages < 15 Ma. The U-shaped pattern of AFT ages along N-S transects across the mountain chain is consistent with burial below a thrust stack that is a few km thick, created by an out-of-sequence thrusting phase that affected the inner portion of the belt. The tectonic load was then removed initially by erosion enhanced by the high relief at 10-7 Ma. The difference between young AFT and AHe ages is used to infer a rate of exhumation of 0.3 mm/yr for this stage. The gap between the youngest AHe ages and the sedimentary record above indicate a final exhumation stage with increasing rates of denudation (1-3 mm/yr) since the Pliocene times due to extensional tectonics. doi: 10.1029/2009TC002659.

The opening of the Tyrrhenian Sea has been punctuated by short-lived episodes of oceanic accretion on separate small backarc basins during early Pliocene (Vavilov basin) and early Pleistocene (Marsili basin) time. These spreading pulses are related to slab rollback and are synchronous with the reduction of the subduction zone width during the formation of the narrow Calabrian arc. Using laboratory models, we investigated the long-term and transient effects of the reduction of slab width on the subduction kinematics. We found that the abrupt reduction in slab width results in a pulse of acceleration of the trench retreat velocity, as the balance between driving and resisting forces acting on the slab is temporarily modified. Our findings also show that the time scale and amplitude of spreading observed in the Tyrrhenian Sea can be experimentally fitted if the scaled viscosity of the uppermost part of the mantle ranges between [10.sup.19] and [10.sup.20] Pa s. doi: 10.1130/G31038.1

[1] Recent geodetic data are compatible with NNE-SSW tectonic extension at a rate of ~5 mm/yr in Sicily, southern Italy, within a broader region of net active compression along the Nubian plate margin (northern Africa). The structures that accommodate such extensional regime and its cause are still unknown. From field structural surveys and seismological analyses, the geometry, kinematics, structural architecture, and seismic potential of an extensional seismic zone linking Cefalu and Mount Etna in central eastern Sicily are defined. The zone includes high-angle WNW striking normal and right-lateral strike-slip faults and subordinate north and NNE striking strike-slip faults either right or left lateral. The occurrence of small discontinuous faults and the absence of related depressions and sedimentary basins suggest that the extensional regime is still in an incipient stage. The ongoing seismic activity possibly reactivates preexisting faults. Instrumentally and historically recorded earthquakes are lower than about 6 in magnitude, and destructive events are historically unknown since at least 1300 A.D. This apparent upper bound of earthquake magnitudes is consistent with the maximum magnitude values estimated from the length of the longest mapped faults and sources of seismic swarms, which all together suggest a value between 6 and 6.5 as the maximum expected magnitude that can be proposed at the present stage of investigation for earthquakes in the study area. Lateral extension on preexisting faults and upwelling of melt mantle material beneath Mount Etna are considered viable processes to explain, at least in part, the active extensional tectonics along the Cefalu-Etna seismic zone. Strike-slip seismic faulting beneath Mount Etna may be part of a previously proposed diffuse transfer zone affecting northeastern Sicily and including the Tindari Fault. Citation: Billi, A., D. Presti, B. Orecchio, C. Faccenna, and G. Neri (2010), Incipient extension along the active convergent margin of Nubia in Sicily, Italy: Cefalu-Etna seismic zone, Tectonics, 29, TC4026, doi: 10.1029/2009TC002559.

[1] The Neogene tectonics of the central Mediterranean are related to the subduction and trench rollback of the Ionian basin under Eurasia, causing the opening of the Liguro-Provencal and Tyrrhenian back-arc basins and the formation of the Calabrian accretionary wedge. The Calabrian accretionary wedge is a partially submerged accretionary complex located in the Ionian offshore and laterally bounded by the Apulia and Malta escarpments. While the history of the back-arc extension process is fairly well defined, the structure and evolution of the wedge are still poorly known. We have analyzed and interpreted the available reflection multichannel and single-channel seismic profiles in the Ionian offshore and integrated them with other geological and geophysical data acquired in the last 40 years. Here we present unpublished seismic profiles to provide a new map of the tectonic structure of the Ionian offshore and to define the structure of the wedge and its evolution during the last 15 Myr. Our reconstruction points out that the Messinian salinity crisis represents an important break in the evolution of the wedge, as the basal decollement ramps up onto the Messinian salt deposits, producing a dramatic and fast forward propagation of the frontal thrust and resulting underplating of the underlying crustal Ionian sequence during progressive trench rollback. Our results provide new insight into the style of accretion in a weakly converging setting, which is typical for the Mediterranean region. Citation: Minelli, L., and C. Faccenna (2010), Evolution of the Calabrian accretionary wedge (central Mediterranean), Tectonics, 29, TC4004, doi: 10.1029/2009TC002562.

Mobile belts are long-lived deformation zones composed of an ensemble of crustal fragments, distributed over hundreds of kilometres inside continental convergent margins (1,2). The Mediterranean represents a remarkable example of this tectonic setting (3):the region hosts a diffuse boundary between the Nubia and Eurasia plates comprised of a mosaic of microplates that move and deform independently from the overall plate convergence (4). Surface expressions of Mediterranean tectonics include deep, subsiding backarc basins, intraplate plateaux and uplifting orogenic belts. Although the kinematics of the area are now fairly well defined, the dynamical origins of many of these active features are controversial and usually attributed to crustal and lithospheric interactions. However, the effects of mantle convection, well established for continental interiors (5-7), should be particularly relevant in a mobile belt, and modelling may constrain important parameters such as slab coherence and lithospheric strength. Here we compute global mantle flow on the basis of recent, high-resolution seismic tomography to investigate the role of buoyancy-driven and plate-motion-induced mantle circulation for the Mediterranean. We show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region. More generally, vigorous small-scale convection in the uppermost mantle may also underpin other complex mobile belts such as the North American Cordillera or the Himalayan-Tibetan collision zone., Deep to intermediate seismicity in the Mediterranean is restricted to the Hellenic and Calabria Wadati--Benioff zones and locally beneath the Alboran region (8), although fast seismic velocity anomalies (9-11) run [...]

[1] We report new paleomagnetic and structural data from late Cretaceous to Mio-Pliocene continental sandy/silty sedimentary rocks from the Eastern Cordillera (central Andes). Here, N-S to NNE-SSW ridges hosting Paleozoic basement and upper Cretaceous continental red beds overthrust thick adjacent Cenozoic basins. Pretilting (and likely primary) reliable directions gathered at 15 sites document 45.9 [degrees] [+ or -] 9.4, 30.1 [degrees] [+ or -] 23.9 [degrees], and 15.4[degrees] [+ or -] 19.3 [degrees] clockwise (CW) rotations with respect to South America occurring after the late Cretaceous (~80 Ma), Oligo-Miocene (20-30 Ma), and late Miocene-Pliocene (5-10 Ma), respectively. Conversely, four upper Cretaceous sites from the walls of a N-S left-lateral strike-slip fault (Yavi-Abra Pampa fault) yield a null rotation. About 20 km to the west, flower structures and subvertical syntectonic strata dated at 14.26 [+ or -] 0.19 Ma are exposed along the subparallel Abra Moreta left-lateral strike-slip fault. Relying on data from the literature on the period when deformation began, we suggest that since Eo-Oligocene times (30-40 Ma) the Eastern Cordillera has undergone a regional CW rotation of 40[degrees]-50[degrees], synchronous with crustal shortening and large-scale bending of the Andean salient. The CW rotation is possibly still active today, as documented by regional GPS data from the Andes. Since ~15 Ma ago, the activity of N-S left-lateral strike-slip faults induced counterclockwise rotations along the fault zone, locally annulling the regional CW rotation. In agreement with a previous model, we speculate that mid-Miocene strike-slip activity accommodated the progressive southward spreading of the Altiplano-Puna high-altitude plateau, laterally migrating from the overthickened crustal region of the salient apex. Citation: Maffione, M., F. Speranza, and C. Faccenna (2009), Bending of the Bolivian orocline and growth of the central Andean plateau: Paleomagnetic and structural constraints from the Eastern Cordillera (22-24[degrees]S, NW Argentina), Tectonics, 28, TC4006, doi: 10.1029/2008TC002402.

Tectonic plate motions predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere and slab–upper plate interface. A wide range of observations from active subduction zones and exhumed rocks suggest that subduction interface shear zone rheology is sensitive to the composition of subducting crustal material—for example, sediments versus mafic igneous oceanic crust. Here we use 2-D numerical models of dynamically consistent subduction to systematically investigate how subduction interface viscosity influences large-scale subduction kinematics and dynamics. Our model consists of an oceanic slab subducting beneath an overriding continental plate. The slab includes an oceanic crustal/weak layer that controls the rheology of the interface. We implement a range of slab and interface strengths and explore how the kinematics respond for an initial upper mantle slab stage, and subsequent quasi-steady-state ponding near a viscosity jump at the 660-km-discontinuity. If material properties are suitably averaged, our results confirm the effect of interface strength on plate motions as based on simplified viscous dissipation analysis: a ∼2 order of magnitude increase in interface viscosity can decrease convergence speeds by ∼1 order of magnitude. However, the full dynamic solutions show a range of interesting behaviour including an interplay between interface strength and overriding plate topography and an end-member weak interface-weak slab case that results in slab break-off/tearing. Additionally, for models with a spatially limited, weak sediment strip embedded in regular interface material, as might be expected for the subduction of different types of oceanic materials through Earth's history, the transient response of enhanced rollback and subduction velocity is different for strong and weak slabs. Our work substantiates earlier suggestions as to the importance of the plate interface, and expands the range of quantifiable links between plate reorganizations, the nature of the incoming and overriding plate and the potential geological record. [ABSTRACT FROM AUTHOR]

[1] Northern Victoria Land is located at the boundary between an extended, presumably hot, region (West Antarctic Rift System) and the thick, possibly cold, East Antarctic craton. The style and timing of Tertiary deformation along with relationships with the magmatic activity are still unclear, and contrasting models have been proposed. We performed structural and morphotectonic analyses at the NE termination of northern Victoria Land in the Admiralty Mountains area, where the relationship between topography, tectonics, and magmatism is expected to be well pronounced. We found evidence of two subsequent episodes of faulting, occurring concurrently with the Neogene McMurdo volcanism. The first episode is associated with dextral transtension, and it is overprinted by extensional tectonics during the emplacement of large shield alkaline volcanoes. Upper mantle seismic tomography shows that the extensional regime is limited to regions overlying a low-velocity anomaly. We interpret this anomaly to be of thermal origin, and have tested the role of large-scale upwelling on lithosphere deformation in the area. The results of this integrated analysis suggest that the morphotectonic setting of the region and the magmatism is likely the result of upwelling flow at the boundary between the cold cratonic and the hot stretched province (WARS), at work until recent time in this portion of the northern Victoria Land.

The Peloritani Mountains constitute the Sicilian portion of the Calabria--Peloritani Arc (Italy), a tectono-metamorphic edifice recording the history of the subduction--exhumation cycle during Tertiary convergence between the African and European plates. Here, we describe the kinematic and the petrological characteristics of the major shear zones bounding the lowermost continental-derived metamorphic units cropping out in the eastern portion of the Peloritani Mountains. Both meso-and micro-scale shear sense criteria indicate a top-to-the-SSE tectonic transport, during a general evolution from ductile to brittle deformation conditions. Quantitative thermobarometry on texturally equilibrated phengite-chlorite pairs crystallized along the shear bands indicates pressure of 6-8 kbar at temperatures of 360-440 [degrees]C for the structurally highest units and 3-4 kbar at 380-440 [degrees]C for the lowest ones. This documents an overall inverse-type nappe arrangement within the tectonic edifice and a transition from an Alpine-(13-18 [degrees]C [km.sup.-1]) to a Barrovian-type (28-36 [degrees]C [km-.sup.1]) geothermal gradient during the progress of the Alpine orogenic metamorphism in the Peloritani Mountains. The integration of these results allows the Peloritani Mountains to be considered as a constituent element of the Apennine orogenic domain formed during the progressive space--time transition from oceanic to continental subduction at the active convergent margin. Keywords: Alpine metamorphism, metamorphic petrology, continental subduction, Peloritani Mountains, Calabria--Peloritani Arc.

[1] New estimates of trench migration rates allow us to address the dynamics of trench migration and backarc strain. We show that trench migration is primarily controlled by the subducting plate velocity [V.sub.sub], which largely depends on its age at the trench. Using the hot and weak arc to back-arc region as a strain sensor, we define neutral arcs characterized by the absence of significant strain, meaning places where the forces (slab pull, bending, and anchoring) almost balance along the interface between the plates. We show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions: [V.sub.t] = 0.5[V.sub.sub] - 2.3 (in cm [a.sup.-1] ) in the HS3 reference frame. Deformation occurs when the velocity combination deviates from kinematic equilibrium. Balancing the torque components of the forces acting at the trench indicates that stiff (old) subducting plates facilitate trench advance by resisting bending. Citation: Lallemand, S., A. Heuret, C. Faccenna, and F. Funiciello (2008), Subduction dynamics as revealed by trench migration, Tectonics, 27, TC3014, doi:10.1029/ 2007TC002212.