Local Exhumation Research Articles

Lancang Fault Assists Block Extrusion in Southeastern Tibet During Early‐Middle Miocene

AbstractThe tectonic and topographic evolution of the southeastern Tibetan Plateau based on low‐temperature thermochronology data is controversial, especially whether it is tectonically‐ or climatically‐controlled, especially along the Lancang fault (LCF) that links the flat central plateau to the west with the high relief southeastern Tibetan Plateau to the east. To explore the tectonic evolution of the LCF and its role in the tectonic and topographic evolution of the southeastern Tibetan Plateau, we carried out detailed field investigation and low‐temperature thermochronology (AHe, AFT, and ZHe) analyses. Field evidence indicate that the northern LCF splits into two branches, the Yangda‐Yaxu and Baqing‐Leiwuqi faults, the latter striking N50°W and dipping to the SW at ∼55°, exposing >100 m‐wide fault rocks composed of a fault damage zone, breccia, and gouge. New thermochronology data and thermo‐kinematic modeling results suggest rapid exhumation of the region located between these two fault branches during ∼22–10 Ma at an exhumation rate of ∼1.57 km/Ma, compared to slow cooling prior to 22 Ma and since 10 Ma. We propose that internal anti‐clockwise block rotation triggered rapid local exhumation, and that the final merging of different parts of the LCF during the Early‐Middle Miocene assisted the southeastward escape of Sundaland, which profoundly affected the evolution of the regional geomorphology.

Structural Evolution, Exhumation Rates, and Rheology of the European Crust During Alpine Collision: Constraints From the Rotondo Granite—Gotthard Nappe

AbstractThe rheology of crystalline units controls the large‐scale deformation geometry and dynamics of collisional orogens. Defining a time‐constrained rheological evolution of such units may help unravel the details of collisional dynamics. Here, we integrate field analysis, pseudosection calculations and in situ garnet U–Pb and mica Rb–Sr geochronology to define the structural and rheological evolution of the Rotondo granite (Gotthard nappe, Central Alps). We identify a sequence of four (D1–D4) deformation stages. Pre‐collisional D1 brittle faults developed before Alpine peak metamorphism, which occurred at 34–20 Ma (U–Pb garnet ages) at 590 ± 25°C and 0.9 ± 0.1 GPa. The reactivation of D1 structures controlled the rheological evolution, from D2 reverse mylonitic shearing at amphibolite facies (520 ± 40°C and 0.8 ± 0.1 GPa) at 18–20 Ma (white mica Rb–Sr ages), to strike‐slip, brittle‐ductile shearing at greenschist‐facies D3 (395 ± 25°C and 0.4 ± 0.1 GPa) at 14–15 Ma (white mica and biotite Rb–Sr ages), and then to D4 strike‐slip faulting at shallow conditions. Although highly misoriented for the Alpine collisional stress orientation, D1 brittle structures controlled the localization of D2 ductile mylonites accommodating fast (∼3 mm/yr) exhumation rates due to their weak shear strength (<10 MPa). This structural and rheological evolution is common across External Crystalline Massifs (e.g., Aar, Mont Blanc), suggesting that the European upper crust was extremely weak during Alpine collision, its strength controlled by weak ductile shear zones localized on pre‐collisional deformation structures, that in turn controlled localized exhumation at the scale of the orogen.

Mesozoic exhumation of the northern West Junggar, NW China: Insights from low-temperature thermochronometers

As an important component of the Central Asian Orogenic Belt (CAOB), the West Junggar represents a natural laboratory to explore intracontinental deformation in a complex terrane. Although some low-temperature thermochronological studies have been conducted in this range, the number of exhumation stages and the corresponding driving forces behind exhumation remain highly debated. In particular, the exhumation history of the northern West Junggar remains poorly studied owing to a lack of data. Here, we present a study that integrates apatite fission track (AFT) and (UTh)/ He (AHe) dating from the igneous rock in the Sawur Mountains in northern West Junggar to further constrain the exhumation history of the region. AFT dating of the samples dominantly yields Late Triassic ages (200–227 Ma) with mean track lengths (MTLs) varying between 13.33 and 14.98 μm, while AHe dating produced widely dispersed Late Triassic to Late Cretaceous single grain ages (73–215 Ma), with sample mean ages between 100 and 195 Ma. Inverse thermal history modeling of our results and analysis of age-elevation relationships reveal that the Sawur Mountains experienced an extended period of rapid cooling related to local exhumation during the Middle–Late Triassic. A gradual reduction in surface relief following rapid erosion is related to peneplanation processes. The timing and patterns of this exhumation indicate it was caused by reverse oblique-lip reactivation of older strike-slip faults, and it can be explained as the response to the far-field effects of the final closure of the Paleo-Asian Ocean.

Intraplate lithospheric extension revealed by seismic reflection profiling of South China

How lithospheric extension evolves in an intraplate setting remains uncertain due to the lack of reliable constraints on the lithospheric architecture. Here we present seismic reflection data across the Cretaceous extensional system of South China. Our results show that extension in magma-poor conditions was accommodated by localized normal faulting in the upper crust and distributed ductile stretching in the lower crust, followed by localized crustal necking and Moho uplift associated with mantle shear-zone formation. These vertically spaced crustal and mantle features appear to be kinematically linked as follows. First, lower-crustal stretching was accompanied by normal faulting and localized exhumation of extensional domes along low-angle detachments in the upper crust. Second, sub-horizontal lower-crustal stretching tends to compensate for upper-crustal heterogeneous thinning and distribute crustal strain evenly, enabling the crust to maintain an overall smooth and flat Moho. Third, mantle shear zones likely affected lithospheric extension by controlling localized Moho uplift and crustal necking. Our compilation of seismic observations suggests that the extensional modes vary laterally from magma-poor to magma-rich conditions, reflected in increased crustal melting, decreased crust-mantle decoupling, and the replacement of a two-layer (high-strength vs. low-strength) lithospheric mantle by a single-layer, low-strength lithospheric mantle. These findings reveal a first-order configuration of depth-dependent extension over ∼800 km, with vertical and lateral variations as a function of lithospheric strength, rheology, and temperature. This extension mechanism provides a basis for assessing modes of lithospheric extension in other tectonic settings.

Plate corner subduction and rapid localized exhumation: Insights from3Dcoupled geodynamic and geomorphological modelling

AbstractRapid, localized exhumation has been reported at many plate corners between adjacent subduction/collision segments. Here we use a fully‐coupled geodynamic and geomorphological modelling approach to investigate overriding plate deformation and resulting rock uplift patterns in these narrow, cuspate regions. In this study, we focus on the effects of internal deformation within a subducting convex‐upward‐shaped indenter and the strength of the interface between the upper and downgoing plate. The strongest localization of high rock uplift rates in the region above the indenter apex is predicted in experiments with a deformable lower plate, a weak interface layer and lateral shortening accommodated only by subduction (i.e., without an upper plate advance component). Our results suggest that bull’s eye shaped structures characterized by young thermochronological ages can, in principle, be reproduced numerically when taking into account a non‐rigid subducting plate together with complex brittle‐ductile rheology and stratification of the overriding lithosphere and realistically implemented fluvial erosion at its surface.

Inherited structure as a control on late Paleozoic and Mesozoic exhumation of the Tarbagatai Mountains, southeastern Kazakhstan

Central Asia hosts the Tianshan, the largest intracontinental mountain belt in the world, which experienced major reactivation and uplift since the Oligocene in response to the collision of India with Asia. This reactivation was focused around pre-existing structures inherited from the Paleozoic tectonic history of the region. The significant Cenozoic tectonic reworking of Central Asia complicates efforts to understand earlier phases of intracontinental tectonics during the late Paleozoic and Mesozoic. The Tarbagatai Mountains of eastern Kazakhstan record a thermotectonic history that provides insight into the timing and distribution of intracontinental tectonic activity in Central Asia prior to the India–Eurasia collision. Apatite fission-track and (U–Th–Sm)/He analysis of igneous samples from the Tarbagatai Mountains reveals two episodes of cooling as a result of exhumation following Paleozoic amalgamation. Initial intracontinental deformation during the Late Permian drove exhumation synchronous with activity along newly formed strike-slip faults spanning the Central Asian Orogenic Belt. The major Chingiz–Tarbagatai Fault was reactivated during the Early Cretaceous, driving localized exhumation along the fault. The relative lack of Cenozoic tectonic activity in the Tarbagatai Mountains means that they provide unique insight into the broader thermotectonic evolution of Central Asia during the late Paleozoic and Mesozoic. Supplementary material: Detailed thermochronological data, including plots and tables, are available at https://doi.org/10.6084/m9.figshare.c.5414555

Constraints on the post-orogenic tectonic history along the Salmon River suture zone from low-temperature thermochronology, western Idaho and eastern Oregon

ABSTRACTThe abrupt boundary between accreted terranes and cratonic North America is well exposed along the Salmon River suture zone in western Idaho and eastern Oregon. To constrain the post-suturing deformation of this boundary, we assess the cooling history using zircon and apatite (U–Th)/He thermochronology. Pre-Miocene granitic rocks, along a regional transect, were sampled from accreted terranes of the Blue Mountains Province to cratonic North America (Idaho batholith). Each sample was taken from a known structural position relative to a paleotopographic surface represented by the basal unit of the Miocene Columbia River basalts. An isopach map constructed for the Imnaha Basalt, the basal member of the Columbia River Basalt Group (CRBG), confirms the presence of a Miocene paleocanyon parallel to the northern part of Hells Canyon. The (U–Th)/He zircon dates indicate mostly Cretaceous cooling below 200°C, with the ages getting generally younger from west to east. The (U–Th)/He apatite dates indicate Late Cretaceous–Paleogene cooling, which post-dates tectonism associated with the western Idaho shear zone (WISZ). However, (U–Th)/He apatite dates younger than the Imnaha Basalt, with one date of 3.4 ± 0.6 Ma, occur at the bottom of Hells Canyon. These young (U–Th)/He apatite dates occur along the trend of the Miocene paleocanyon, and cannot be attributed to local exhumation related to faults. We propose that burial of Mesozoic basement rocks by the Columbia River basalts occurred regionally. However, the only samples currently exposed at the Earth’s surface that were thermally reset by this burial were at the bottom of the Miocene paleocanyon. If so, exhumation of these samples must have occurred by river incision in the last 4 million years. Thus, the low-temperature thermochronology data record a combination of Late Cretaceous–Paleogene cooling after deformation along the WISZ that structurally overprinted the suture zone and Neogene cooling associated with rapid river incision.

Climate and relief‐induced controls on the temporal variability of denudation rates in a granitic upland

AbstractHow soil erosion rates evolved over the last about 100 ka and how they relate to environmental and climate variability is largely unknown. This is due to a lack of suitable archives that help to trace this evolution. We determined in situ cosmogenic beryllium‐10 (10Be) along vertical landforms (tors, boulders and scarps) on the Sila Massif to unravel their local exhumation patterns to develop a surface denudation model over millennia.Due to the physical resistance of tors, their rate of exhumation may be used to derive surface and, thus, soil denudation rates over time. We derived soil denudation rates that varied in the range 0–0.40 mm yr‐1. The investigated boulders, however, appear to have experienced repositioning processes about ~20–25 ka bp and were therefore a less reliable archive. The scarps of the Sila upland showed a rapid bedrock exposure within the last 8–15 ka. Overall, the denudation rates increased steadily after 75 ka bp but remained low until about 17 ka bp. The exhumation rates indicate a denudation pulse that occurred about 17–5 ka bp. Since then the rates have continuously decreased.We identify three key factors for these developments – climate, topography and vegetation. Between 75 and 17 ka bp, climate was colder and drier than today. The rapid changes towards warmer and humid conditions at the Pleistocene–Holocene transition apparently increased denudation rates. A denser vegetation cover with time counteracted denudation.Topography also determined the extent of denudation rates in the upland regime. On slopes, denudation rates were generally higher than on planar surfaces. By determining the exhumation rates of tors and scarps, soil erosion rates could be determined over long timescales and be related to topography and particularly to climate. This is key for understanding geomorphic dynamics under current environmental settings and future climate change. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterised by a complex pattern of faulting, thin continental fault blocks, and the serpentinisation, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Travel-time tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here we demonstrate the application of two-dimensional (2D) time-domain acoustic full-waveform inversion to deep water seismic data collected at the Deep Galicia Margin, in order to attain a high resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modelled waveforms, checkerboard tests, testing of parameter and inversion strategy, and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinisation. This study supports the hypothesis that normal faulting acts to hydrate the upper mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinisation. Our results confirm the feasibility of applying the full-waveform inversion method to sparse, deep water crustal datasets.

Carboniferous basin in Holm Land records local exhumation of the North-East Greenland Caledonides: Implications for the detrital zircon signature of a collisional orogen

Carboniferous strata of the Wandel Sea Basin unconformably overlie the Laurentian Precambrian crystalline rocks of the Caledonian hinterland at its northernmost exposures in Holm Land (∼80°N). Complex zircon from an intermediate gneiss gives an upper-intercept age of 1878 ± 71 Ma, a protolith age which fits with the regional 1.8–2.0 Ga calc-alkaline arc. A strongly deformed pegmatite was intruded at 435 ± 17 Ma, and it is a rare example of Caledonian magmatism in the northern sector of the orogen. Omphacite confirms the presence of eclogite (sensu stricto) lenses in the basement complex, thus documenting the northern extent of the North-East Greenland eclogite province formed during the Caledonian collision with Baltica. Holm Land lies in the eastern block of the eclogite province, where an ultrahigh-pressure (UHP) metamorphic event took place at 365–350 Ma. Zircon from a Holm Land eclogite lacks a Eu anomaly, has a flat heavy rare earth element pattern, and gives a sensitive high-resolution ion microprobe U-Pb age of 423 ± 7 Ma, and it is thus interpreted as the time of high-pressure (HP) metamorphism. This age overlaps with the established age of widespread HP metamorphism for the eclogite province (i.e., 415–395 Ma), rather than the younger UHP metamorphism. Westward thrusting of the North-East Greenland eclogite province onto the Laurentian margin after 395 Ma and subsequent exhumation of this uppermost thrust sheet provided a substrate for Carboniferous deposition. Detrital zircon age spectra from arkosic sandstones of the late Visean (ca. 330–340 Ma) Sortebakker and early Moscovian (ca. 310–315 Ma) Kap Jungersen Formations record the progressive unroofing of the North-East Greenland Caledonides. All seven samples have a major peak at 1.8–2.0 Ga, and five also have a 1.75 Ga peak, matching the Paleoproterozoic arc and later anorogenic granitoids that comprise the crystalline basement. Paleozoic grains are sparse in the Sortebakker sandstones, but they constitute a pronounced 400 Ma peak in the younger Kap Jungersen Formation. The composition of the detritus—including garnet clasts, the high amount of discordant zircon (40%), and the large numbers of grains with metamorphic rims that cluster around 410 Ma—reflects a local provenance sourced in the North-East Greenland eclogite province, with some input from the overlying thrust sheets. Other Devonian and Carboniferous basins within and peripheral to the Caledonides also show distinct signatures, demonstrating that there is not a simple, representative detrital zircon signature for the Caledonian orogen.

The Role of Surface Processes in Partitioning of Strain and Uplift, St. Elias Orogen, Southern Alaska

<span style="font-size: 10.5pt; font-family: 'Times New Roman','serif'; mso-bidi-font-size: 12.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA;" lang="EN-US">Three-dimensional thermo-mechanical numerical simulations of the ongoing Yakutat–North America collision are used to identify the role of surface processes in triggering localized rapid uplift, exhumation, and strain observed within the St. Elias orogen of southern Alaska. Thermochronological data reveal localized rapid exhumation associated with the Seward-Malaspina and Hubbard Glaciers within a tectonic corner structure where transpressional motion to the south along the Fairweather Fault system transitions to shortening to the north and west within the active fold-and-thrust belt of the St. Elias orogen. The modeled deformation patterns are characteristic of oblique convergence within a tectonic corner, recording the transition from simple shear to contractional strain within a zone spatially consistent with the highest exhumation rates suggesting the corner geometry is the primary control of strain partitioning.</span><span style="font-size: 10.5pt; font-family: 'Times New Roman','serif'; mso-bidi-font-size: 12.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA;" lang="EN-US">The relative roles of surface-related processes versus tectonics-related processes in the development of this pattern of deformation were tested with the inclusion of an erosional surface model. The presence of surface processes enhanced the uplift and development of a localized rapid exhumation. When spatially and temporally erosion models are employed, the location of maxima is shifted in response. This indicates that efficient erosion, and resultant deposition and material advection can influence the localization of strain and uplift.</span>

Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir

The Pamir is the western continuation of Tibet and the site of some of the highest mountains on Earth, yet comparatively little is known about its crustal and tectonic evolution and erosional history. Both Tibet and the Pamir are characterized by similar terranes and sutures that can be correlated along strike, although the details of such correlations remain controversial. The erosional history of the Pamir with respect to Tibet is significantly different as well: Most of Tibet has been characterized by internal drainage and low erosion rates since the early Cenozoic; in contrast, the Pamir is externally drained and topographically more rugged, and it has a strongly asymmetric drainage pattern. Here, we report 700 new U-Pb and Lu-Hf isotope determinations and >300 40Ar/39Ar ages from detrital minerals derived from rivers in China draining the northeastern Pamir and >1000 apatite fission-track (AFT) ages from 12 rivers in Tajikistan and China draining the northeastern, central, and southern Pamir. U-Pb ages from rivers draining the northeastern Pamir are Mesozoic to Proterozoic and show affinity with the Songpan-Ganzi terrane of northern Tibet, whereas rivers draining the central and southern Pamir are mainly Mesozoic and show some affinity with the Qiangtang terrane of central Tibet. The εHf values are juvenile, between 15 and −5, for the northeastern Pamir and juvenile to moderately evolved, between 10 and −40, for the central and southern Pamir. Detrital mica 40Ar/39Ar ages for the northeastern Pamir (eastern drainages) are generally older than ages from the central and southern Pamir (western drainages), indicating younger or lower-magnitude exhumation of the northeastern Pamir compared to the central and southern Pamir. AFT data show strong Miocene–Pliocene signals at the orogen scale, indicating rapid erosion at the regional scale. Despite localized exhumation of the Mustagh-Ata and Kongur-Shan domes, average erosion rates for the northeastern Pamir are up to one order of magnitude lower than erosion rates recorded by the central and southern Pamir. Deeper exhumation of the central and southern Pamir is associated with tectonic exhumation of central Pamir domes. Deeper exhumation coincides with western and asymmetric drainages and with higher precipitation today, suggesting an orographic effect on exhumation. A younging-southward trend of cooling ages may reflect tectonic processes. Overall, cooling ages derived from the Pamir are younger than ages recorded in Tibet, indicating younger and higher magnitudes of erosion in the Pamir.

Extreme localized exhumation at syntaxes initiated by subduction geometry

AbstractSome of the highest and most localized rates of lithospheric deformation in the world are observed at the transition between adjacent plate boundary subduction segments. The initiating perturbation of this deformation has long been attributed to vigorous erosional processes as observed at Nanga Parbat and Namche Barwa in the Himalaya and at Mount St. Elias in Alaska. However, an erosion‐dominated mechanism ignores the 3‐D geometry of curved subducting plates. Here we present an alternative explanation for rapid exhumation at these locations based on the 3‐D thermomechanical evolution of collisions between plates with nonplanar geometries. Comparison of model predictions with existing data reproduces the defining characteristics of these mountains and offers an explanation for their spatial correlation with arc termini. These results demonstrate a “bottom‐up” tectonic rather than “top‐down” erosional initiation of feedbacks between erosion and tectonic deformation; hence, the importance of 3‐D subduction geometry.

Rapid long-term erosion in the rain shadow of the Shillong Plateau, Eastern Himalaya

Geodynamic models of collisional orogens suggest that precipitation gradients profoundly influence spatial patterns of exhumation and deformation in active collisional mountain ranges. A basic tenet of this hypothesis is that in unglaciated areas, spatial patterns of long-term precipitation, erosion and exhumation should be correlated. A correlation of this type has been observed in the Eastern Himalaya, where uplift of the Shillong Plateau by Pliocene time drastically reduced monsoonal rainfall in the Himalayan range downwind. Existing apatite fission-track data suggest that the resulting precipitation gradient caused a twofold gradient in long-term erosion rates across an area with similar geology, suggesting a strong influence of climate on the region's geomorphic and tectonic evolution. We extend this dataset by presenting 53 new bedrock apatite and zircon fission-track ages from deeper within the rain shadow. We expected latest Miocene to Pliocene apatite ages, similar to previously published ages from neighboring areas in the rain shadow. Instead, apatites as young as 1.3±0.2Ma and zircons as young as 4.5±1.0Ma (2σ) demonstrate that spatial gradients in precipitation do not correlate with variations in long-term erosion and crustal strain as predicted by geodynamic models. Thermal–kinematic modeling of these data suggests that local exhumation patterns reflect gradients in rock uplift dictated by fault kinematics in this rapidly deforming area, despite a dramatic precipitation gradient. These findings both highlight the need to better understand how erosive processes scale with precipitation amount and intensity in such settings, and suggest a disconnect between the predictions of orogen-scale geodynamic models and the relationship between erosion and tectonics at the regional scale.

Episodic exhumation and relief growth in the Mont Blanc massif, Western Alps from numerical modelling of thermochronology data

The Pliocene–Quaternary exhumational and topographic evolution of the European Alps and its potential climatic and tectonic controls remain a subject of controversy. Here, we apply inverse numerical thermal–kinematic modelling to a spatially dense thermochronological dataset (apatite fission-track and (U–Th)/He) of both tunnel and surface samples across the Mont Blanc massif in the Western Alps, complemented by new zircon fission-track data, in order to better quantify its Neogene exhumation and relief history. Age–elevation relationships and modelling results show that an episodic exhumation scenario best fits the data. Initiation of exhumation in the Mont Blanc massif at 22 ± 2 Ma with a rate of 0.8 ± 0.15 km/Myr is probably related to NW-directed thrusting during nappe emplacement. Exhumation rates decrease at 6 ± 2 Ma to values of 0.15 ± 0.65 km/Myr, which we interpret to be the result of a general decrease in convergence rates and/or extensive exposure of less erodible crystalline basement rocks from below more easily erodible Mesozoic sediments. Finally, local exhumation rates increase up to 2.0 ± 0.6 km/Myr at 1.7 ± 0.8 Ma. Modelling shows that this recent increase in local exhumation can be explained by valley incision and the associated increase in relief at 0.9 ± 0.8 Ma, leading to erosional unloading, isostatic rebound and additional rock uplift and exhumation. Given the lack of tectonic activity as evidenced by constant thermochronological ages along the tunnel transect, we suggest that the final increase in exhumation and relief in the Mont Blanc massif is the result of climate change, with the initiation of mid-Pleistocene glaciations leading to rapid valley incision and related local exhumation.

Correction to “Spatial and temporal patterns of exhumation across the Venezuelan Andes: Implications for Cenozoic Caribbean geodynamics”

[1] In the paper “Spatial and temporal patterns of exhumation across the Venezuelan Andes: Implications for Cenozoic Caribbean geodynamics” by Mauricio A. Bermúdez, Barry P. Kohn, Peter A. van der Beek, Matthias Bernet, Paul B. O'Sullivan, and Reginald Shagam (Tectonics, 29, TC5009, doi:10.1029/2009TC002635, 2010), there are corrections to Figure 9 caption and an additional reference. Figure 9. Possible kinematic configuration for the South American-Caribbean plate boundary zone during earliest Oligocene (Figure 9a), late Miocene (Figure 9b), and Pliocene times (Figure 9c). (a) Earliest Oligocene, widespread but relatively slow exhumation in the Venezuelan Andes, Eastern Cordillera of Colombia, and Perija range results from inversion of preexisting rift structures during regional compression, possibly related to initial subduction of Panama arc. (b) Late Miocene, rotation of the Maracaibo block (MB) is almost completed, left-lateral strike-slip displacement between the Maracaibo block and the South American plate is concentrated on the BFS, producing translation and minor rotation in the Trujillo block (TB) during its tectonic escape. Major exhumation of the SNB and SLCB occurs during this phase. (c) Pliocene, final closure of Panama arc is expressed by rapid localized exhumation in parts of the Venezuelan Andes (ECB) and Eastern Cordillera (Quetame Massif [Parra et al., 2009]). See text for more detailed discussion. Figures are modified from http://www.tectonicanalysis.com/site/caribbean/default.htm [see also Pindell and Kennan, 2009] with minor inputs from Montes et al. [2005], Corredor [2003], and Hoorn et al. [1995].

Intense localized rock uplift and erosion in the St Elias orogen of Alaska

The St Elias orogen in Alaska is one of the world’s highest coastal mountain ranges. An age analysis of detrital material in active sediment systems in the inaccessible ice-covered valley bottoms reveals localized exhumation that is driven by coupling between erosion and active tectonic rock uplift.

Exhumation of UHP/LT rocks due to the local reduction of the interplate pressure: Thermo-mechanical physical modelling

Spatial distribution of UHP/LT terrains suggests that their exhumation is essentially a three-dimensional process that occurs only locally in specific sites along mountain belts. On the other hand, the continental subduction resulting in the formation of UHP/LT rocks takes place along the whole belt. The previously performed by the authors 2-D thermo-mechanical laboratory modelling of continental subduction has shown that exhumation of deeply subducted continental crust is possible only when the effective interplate pressure p n is lower than the lithostatic pressure (low compression subduction regime). At the same time, this modelling showed that for the deeply subducted continental crust to be preserved at low temperature at great depth, the continental subduction should be accompanied by the subduction of the fore-arc block or the arc plate. The latter process occurs only when p n is high (high compression regime). To reconcile both processes we suggest that within the background of a generally high compression regime the interplate pressure can be locally reduced in some specific situations which would then allow the local exhumation of UHP/LT material. Using physical modelling technique we investigate one of such situations that occurs when the frontal part of the overriding plate undergoes (subduction induced or not) extension parallel to the plate boundary with activation or formation of a strike–slip transform fault oblique to the plate boundary (to the interplate zone). The displacement along this fault results in a local reduction of the interplate pressure at the intersection of the fault with the interplate zone. This pressure reduction permits the rise of the deeply subducted low-density continental crust and sediments submitted to UHP/LT conditions under buoyancy force. A 10 km-thick slice of crust detaches at ~ 150 km-depth and moves up along the interplate zone with a starting rate of ca. 3 cm/yr. The ascent rate reduces when the unit reaches crustal depth. The deformation of this unit is small when it first detaches and starts rising, but increases when reaching the base of the overriding plate. The exhuming material is sheared and stretched in a narrow passage at ~ 70 km-depth. Therefore in real conditions the deformation is mainly recorded when the unit is submitted to HP and not UHP conditions.

Crustal thinning, mantle exhumation and serpentinization in the Porcupine Basin, offshore Ireland: evidence from wide-angle seismic data

New wide-angle seismic data were gathered along a 230 km long profile that runs east–west across a deep structural feature in the Porcupine Basin, offshore Ireland, known as the Porcupine Arch. Ocean bottom seismometers were deployed at 3–4 km intervals and seismic sources fired every 120 m along it. Prominent primary and secondary arrivals indicate that the continental crust is extremely thin (locally less than 2 km) across the basin centre. The sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers. The two uppermost layers are interpreted as mostly a post-rift succession of Cretaceous and Cenozoic strata. The lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly Jurassic synrift sediments. A strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced. Crustal thinning is far greater than in the adjacent Rockall Basin and local exhumation of continental mantle lithosphere may have occurred in parts of the Porcupine Basin. Low P n velocities beneath the Porcupine Arch are compatible with larger amounts of mantle serpentinization than in the Rockall Basin.

Magma emplacement during exhumation of the lower- to mid-crustal orogenic root: The Jihlava syenitoid pluton, Moldanubian Unit, Bohemian Massif

In this study, we present structural and AMS data from the ∼335 Ma ultrapotassic Jihlava syenitoid pluton, which intruded the lower- to mid-crustal orogenic root (Moldanubian Unit) in the Bohemian Massif, Central European Variscides. The emplacement of the pluton was accommodated by multiple processes, such as ductile host-rock shortening, formation of sheeted zones by magma wedging, magmatic stoping, and possibly host-rock displacement within a wide transtensional zone. Magmatic fabrics preserved in the pluton reflect both intrusive processes and regional strain. Margin-parallel and ∼ENE–WSW foliations, which probably formed by strain during emplacement of inner magma pulses, were overprinted by tectonic strain within a zone of distributed wrench-dominated dextral transtension. This zone probably accommodated exhumation of different segments in the eastern part of the Moldanubian Unit during pluton emplacement. In contrast to existing models, we emphasize that the Jihlava pluton, as well as other ultrapotassic plutons widespread in the Moldanubian Unit, are structurally highly variable bodies emplaced by multiple intrusive processes. Our case study illustrates how careful documentation of structural relations around these ultrapotassic plutons may constrain the kinematic framework and local exhumation histories in different segments of the orogenic root during and shortly after the ∼340 Ma mechanical event in the Central European Variscides.