Thermokinematic Modeling Research Articles

Shear heating not a cause of inverted metamorphism

Research Article| August 01, 2013 Shear heating not a cause of inverted metamorphism Steven B. Kidder; Steven B. Kidder 1California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, USA *Current address: Department of Geology, University of Otago, Dunedin 9054, New Zealand. Search for other works by this author on: GSW Google Scholar Frédéric Herman; Frédéric Herman 2Faculté des Géosciences et de l’Environnement, Université de Lausanne, Geopolis Bureau 3232, 1015 Lausanne, Switzerland3Earth Sciences Department, ETH, Sonneggstrasse 5, 8092 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Jason Saleeby; Jason Saleeby 1California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Jean-Philippe Avouac; Jean-Philippe Avouac 1California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Mihai N. Ducea; Mihai N. Ducea 4Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA5Universitatea Bucuresti, Facultatea de Geologie Geofizica, Str. N. Balcescu Nr 1, Bucharest 010041, Romania Search for other works by this author on: GSW Google Scholar Alan Chapman Alan Chapman 4Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Steven B. Kidder *Current address: Department of Geology, University of Otago, Dunedin 9054, New Zealand. 1California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, USA Frédéric Herman 2Faculté des Géosciences et de l’Environnement, Université de Lausanne, Geopolis Bureau 3232, 1015 Lausanne, Switzerland3Earth Sciences Department, ETH, Sonneggstrasse 5, 8092 Zurich, Switzerland Jason Saleeby 1California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, USA Jean-Philippe Avouac 1California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, USA Mihai N. Ducea 4Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA5Universitatea Bucuresti, Facultatea de Geologie Geofizica, Str. N. Balcescu Nr 1, Bucharest 010041, Romania Alan Chapman 4Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA Publisher: Geological Society of America Received: 06 Dec 2012 Revision Received: 02 Apr 2013 Accepted: 03 Apr 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2013 Geological Society of America Geology (2013) 41 (8): 899–902. https://doi.org/10.1130/G34289.1 Article history Received: 06 Dec 2012 Revision Received: 02 Apr 2013 Accepted: 03 Apr 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Steven B. Kidder, Frédéric Herman, Jason Saleeby, Jean-Philippe Avouac, Mihai N. Ducea, Alan Chapman; Shear heating not a cause of inverted metamorphism. Geology 2013;; 41 (8): 899–902. doi: https://doi.org/10.1130/G34289.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract An archetypal example of inverted metamorphism purportedly resulting from shear heating is found in the Pelona Schist of southern California (United States). Recent studies demonstrate that the Pelona Schist was subducted and accreted at the onset of Laramide flat subduction under thermal and kinematic conditions not considered in earlier numerical models. To test the shear heating hypothesis under these conditions, we constructed a thermo-kinematic model of flat subduction initiation involving continuous accretion of the schist. A neighborhood algorithm inversion demonstrates that available metamorphic and thermochronologic constraints in the Sierra Pelona mountains are satisfied only if accretion rates were 0.2–3.6 km/m.y and shear heating was minimal (shear stress 0–19 MPa). Minimal shear heating is also consistent with an inversion of models constrained by thermochronology of the East Fork (of the San Gabriel River) exposure of the schist. Shear heating inhibits the formation of modeled inverted gradients during accretion and should not be considered an important factor in their generation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

On the origin of the ultradeep East Barents Sea basin

Very large subsidence, with up to 20 km thick sediment layers, is observed in the East Barents Sea basin. Subsidence started in early Paleozoic, accelerated in Permo‐Triassic times, finished during the middle Cretaceous, and was followed by moderate uplift in Cenozoic times. The observed gravity signal suggests that the East Barents Sea is at present in isostatic balance and indicates that a mass excess is required in the lithosphere to produce the observed large subsidence. Several origins have been proposed for the mass excess. We use 1‐D thermokinematic modeling and 2‐D isostatic density models of continental lithosphere to evaluate these competing hypotheses. The crustal density in 2‐D thermokinematic models resulting from pressure‐, temperature‐, and composition‐dependent phase change models is computed along transects crossing the East Barents Sea. The results indicate the following. (1) Extension can only explain the observed subsidence provided that a 10 km thick serpentinized mantle lens beneath the basin center is present. We conclude that this is unlikely given that this highly serpentinized layer should be formed below a sedimentary basin with more than 10 km of sediments and crust at least 10 km thick. (2) Phase changes in a compositionally homogeneous crust do not provide enough mass excess to explain the present‐day basin geometry. (3) Phase change induced densification of a preexisting lower crustal gabbroic body, interpreted as a mafic magmatic underplate, can explain the basin geometry and observed gravity anomalies. The following model is proposed for the formation of the East Barents Sea basin: (1) Devonian rifting and extension related magmatism resulted in moderate thinning of the crust and a mafic underplate below the central basin area explaining initial late Paleozoic subsidence. (2) East‐west shortening during the Permian and Triassic resulted in densification of the previously emplaced mafic underplated body and enhanced subsidence dramatically, explaining the present‐day deep basin geometry.

Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard

We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = − 1.34° ± 3.31°, latitude = 51.4° ± 0.3° with an angular velocity of Ω = 0.5029 ± 0.0072°/Myr. The pattern of coupling on the MHT is computed on a fault dipping 10° to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350°C. The convergence between India and South Tibet proceeds at a rate of 17.8 ± 0.5 mm/yr in central and eastern Nepal and 20.5 ± 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 ± 0.4 × 1019 Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 MW ≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 × 1019 Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT.

Limits to reconstructing paleotopography from thermochronometer data

Recent studies suggest that orogens can achieve a topographic steady state whereby equilibrium is reached between tectonics and erosion. However, steady state topography may not be the norm in many orogens experiencing large changes in climate or tectonics, which can produce topographic transients. The quantification of transient topography over geologic timescales requires reconstructing paleotopography, but this has proven difficult in many cases. This study investigates the utility of bedrock thermochronometer data to reconstruct orogen paleotopography over million year timescales. Apatite (U‐Th)/He and fission track ages are integrated with a thermokinematic model for a single‐parameter inversion of paleotopography. An iterative scheme is used that minimizes the misfit between predicted and observed cooling ages to identify the range of paleotopographies that could produce observed ages within sample uncertainty. Two approaches are considered. First, synthetic 2‐D topographies are used to test the robustness of the approach. The following topographic evolution scenarios are considered: (1) lateral ridge migration, (2) topographic relief change, and (3) valley widening and deepening from glaciation. Second, the method is applied in three dimensions to existing data from the Coast Mountains of British Columbia, Canada. Results from both applications of the model suggest that (1) paleotopographic reconstruction will typically underpredict the magnitude of topographic change, especially relief change; (2) paleotopography is most successfully reconstructed after lateral ridge migration in long‐wavelength topographies; and (3) reconstructed paleotopography from the Coast Mountains, British Columbia, suggests that glacial erosion may have the potential to remove drainage divides and laterally shift topographic ridges and peaks.

Post‐orogenic evolution of the southern Pyrenees: constraints from inverse thermo‐kinematic modelling of low‐temperature thermochronology data

AbstractThe late‐stage evolution of the southern central Pyrenees has been well documented but controversies remain concerning potential Neogene acceleration of exhumation rates and the influence of tectonic and/or climatic processes. A popular model suggests that the Pyrenees and their southern foreland were buried below a thick succession of conglomerates during the Oligocene, when the basin was endorheic. However, both the amount of post‐orogenic fill and the timing of re‐excavation remain controversial. We address this question by revisiting extensive thermochronological datasets of the Axial Zone. We use an inverse approach that couples the thermo‐kinematic model Pecube and the Neighbourhood inversion algorithm to constrain the history of exhumation and topographic changes since 40 Ma. By comparison with independent geological data, we identified a most probable scenario involving rapid exhumation (>2.5 km Myr−1) between 37 and 30 Ma followed by a strong decrease to very slow rates (0.02 km Myr−1) that remain constant until the present. Therefore, the inversion does not require a previously inferred Pliocene acceleration in regional exhumation rates. A clear topographic signal emerges, however: the topography has to be infilled by conglomerates to an elevation of 2.6 km between 40 and 29 Ma and then to remain stable until ca. 9 Ma. We interpret the last stage of the topographic history as recording major incision of the southern Pyrenean wedge, due to the Ebro basin connection to the Mediterranean, well before previously suggested Messinian ages. These results thus demonstrate temporally varying controls of different processes on exhumation: rapid rock uplift in an active orogen during late Eocene, whereas base‐level changes in the foreland basin control the post‐orogenic evolution of topography and exhumation in the central Pyrenees. In contrast, climate changes appear to play a lesser role in the post‐orogenic topographic and erosional evolution of this mountain belt.

Denudation of the Namche Barwa antiform, eastern Himalaya

The Namche Barwa massif has long been identified as a region of highly localized rapid exhumation. Previous studies suggest that this region contributes ~ 50% of the total sediment load of the Brahmaputra River, which cuts through the eastern end of the high Himalayan Range and exits into the Indian plain. This study presents new detrital zircon cooling ages from 19 sand samples collected along the Brahmaputra River and tributaries with catchments covering the Namche Barwa massif, and its surroundings, including the so-far unexplored regions in the south. The new results confirm that the Namche Barwa massif is a major source of sediment for the Brahmaputra River composing 60–70% of the entire load. Furthermore, the data from southern regions much better constrains the extent of young cooling ages at Namche Barwa, modestly extending this zone to the south. Our more robust and higher estimates of sediment yield from Namche Barwa together with the increased source area give decadal timescale denudation rates of 5–17 mm/yr. Results from thermokinematic modeling of the ages suggest million-year timescale denudation rates as high as 7–9 mm/yr.

The role of footwall deformation and denudation in controlling cooling age patterns of detachment systems: An application to the Kongur Shan extensional system in the Eastern Pamir, China

Determining the relationship between deformation and exhumation history in the footwall rocks of major low-angle normal faults (i.e., detachment faults) provides a way to quantify different models of continental extension. Although pressure–temperature–time ( P–T–t) datasets are commonly used to infer magnitude and rates of slip across detachment faults, correct use of these datasets requires a sound understanding of coupling effects between footwall deformation and advection of isotherms on the distribution of cooling ages. In this study we present thermo-kinematic modeling results which examine the relationship between different footwall kinematic models, erosion, and the spatial distribution of muscovite and biotite 40Ar/ 39Ar cooling ages. Our results show that large magnitudes of footwall erosion during normal faulting significantly affect cooling age patterns as footwall rocks at different distances from the fault surface pass through different thermal gradients. We apply our modeling results to simulating muscovite and biotite cooling-age patterns from the footwall of the Cenozoic Kongur Shan normal fault system in the Pamir. Previous studies had interpreted a documented ~5-fold increase in cooling rate at ~ 2 Ma to indicate an increase in exhumation rate during the Quaternary. However, our results show that the observed cooling age patterns and increase in cooling rate can be produced by a constant exhumation rate over the last 7 Ma due to the effect of changes in the rate of isotherm advection during exhumation.

Two‐ and three‐dimensional thermal modeling of a low‐angle detachment: Exhumation history of the Simplon Fault Zone, central Alps

Two alternative models have been proposed to explain footwall exhumation along major low‐angle detachments: (1) crustal‐scale exhumation along a detachment fault that maintained a low dip angle or (2) exhumation along a high‐angle fault passively rotated by isostatic rebound (“rolling hinge model”). These proposed models were tested against a well‐documented example of a low‐angle detachment fault in the European central Alps, the Simplon Fault Zone (SFZ). An extensive thermochronological data set provides the basis for 2‐ and 3‐D thermokinematic models (Pecube), coupled with a stochastic inversion algorithm (the Neighbourhood Algorithm). Model results establish that the thermochronological pattern is better reproduced by a low‐angle detachment that maintained a 30° dip, rather than by a rolling hinge model. Although a range of histories involving either steady state or variable exhumation rates is possible, the preferred model of highest probability is for a variable rate, with the fault zone initiated at 18.5 ± 2.5 Ma and active until the present day. Footwall exhumation was relatively fast until 14.5 ± 1.5 Ma (∼1.4 mm yr−1). This enhanced SFZ footwall exhumation is similar in timing and kinematics to orogen‐parallel extension reported throughout the Alpine orogen. After 14.5 Ma, SFZ footwall exhumation continued at a reduced rate (∼0.7 mm yr−1) until 4 Ma. The subsequent increase (to ∼1 mm yr−1) reflects enhanced regional erosion rates across both footwall and hanging wall after circa 4 Ma (from 0.35 ± 0.15 mm yr−1 to 0.70 ± 0.15 mm yr−1), probably in response to climate changes during the Pliocene.

Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

Two end‐member kinematic models of crustal shortening across the Himalaya are currently debated: one assumes localized thrusting along a single major thrust fault, the Main Himalayan Thrust (MHT) with nonuniform underplating due to duplexing, and the other advocates for out‐of‐sequence (OOS) thrusting in addition to thrusting along the MHT and underplating. We assess these two models based on the modeling of thermochronological, thermometric, and thermobarometric data from the central Nepal Himalaya. We complement a data set compiled from the literature with 114 40Ar/39Ar, 10 apatite fission track, and 5 zircon (U‐Th)/He thermochronological data. The data are predicted using a thermokinematic model (PECUBE), and the model parameters are constrained using an inverse approach based on the Neighborhood Algorithm. The model parameters include geometric characteristics as well as overthrusting rates, radiogenic heat production in the High Himalayan Crystalline (HHC) sequence, the age of initiation of the duplex or of out‐of‐sequence thrusting. Both models can provide a satisfactory fit to the inverted data. However, the model with out‐of‐sequence thrusting implies an unrealistic convergence rate ≥30 mm yr−1. The out‐of‐sequence thrust model can be adjusted to fit the convergence rate and the thermochronological data if the Main Central Thrust zone is assigned a constant geometry and a dip angle of about 30° and a slip rate of <1 mm yr−1. In the duplex model, the 20 mm yr−1 convergence rate is partitioned between an overthrusting rate of 5.8 ± 1.4 mm yr−1 and an underthrusting rate of 14.2 ± 1.8 mm yr−1. Modern rock uplift rates are estimated to increase from about 0.9 ± 0.31 mm yr−1 in the Lesser Himalaya to 3.0 ± 0.9 mm yr−1 at the front of the high range, 86 ± 13 km from the Main Frontal Thrust. The effective friction coefficient is estimated to be 0.07 or smaller, and the radiogenic heat production of HHC units is estimated to be 2.2 ± 0.1 μW m−3. The midcrustal duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr−1. We also run 3‐D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effect of the evolving topography can explain a fraction of the scatter observed in the data but not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumation are likely. It has been argued that the steep physiographic transition at the foot of the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well.

Thermal and tectonic evolution of the southern Alps (northern Italy) rifting: Coupled organic matter maturity analysis and thermokinematic modeling

The southern Alps were characterized by strong variations, both in space and time, of heat flow during Mesozoic rifting. The regional thermal history was reconstructed using organic matter (OM) maturity data from outcropping sediments. One-dimensional (1-D) thermal modeling performed on selected successions suggests that OM maturity was mainly controlled by high geothermal gradients (heat flow peaks of 85 to 105 mW/m2 in the Middle Jurassic) and differential burial during Norian–Early Jurassic extensional phases. The results of 1-D modeling show an eastward increase of heat flow peak values. These results were compared with those obtained with two-dimensional (2-D) thermokinematic models. The models show a time shift (ca. 10 Ma) in the heat-flow peak (Aalenian-Bajocian for 2-D and Bajocian for 1-D modeling). However, the Bajocian age was a priori imposed on 1-D models. Available geochemical data could be fitted assuming Aalenian-Bajocian peak ages. Consequently, this misfit is not alarming. The eastward increase in heat-flow peak values is tentatively explained with an eastward increase of radiogenic heat production in the crust instead of with differential stretching. The comparison of paleothermal data and numerical modeling was done to gain knowledge on the potentials and limitations of numerical modeling in frontier areas. Although some differences do exist in the results of geochemical and thermokinematic models, we can conclude that if a reasonable knowledge of the thermal parameters of both covers and basement is available, thermokinematic modeling can provide useful first-order estimates in frontier areas of heat flow and temperature evolution through time.

Low‐temperature thermochronology and thermokinematic modeling of deformation, exhumation, and development of topography in the central Southern Alps, New Zealand

Apatite and zircon (U‐Th)/He and fission track ages were obtained from ridge transects across the central Southern Alps, New Zealand. Interpretation of local profiles is difficult because relationships between ages and topography or local faults are complex and the data contain large uncertainties, with poor reproducibility between sample duplicates. Data do form regional patterns, however, consistent with theoretical systematics and corroborating previous observations: young Neogene ages occur immediately southeast of the Alpine Fault (the main plate boundary structure on which rocks are exhumed); partially reset ages occur in the central Southern Alps; and older Mesozoic ages occur further toward the southeast. Zircon apparent ages are older than apatite apparent ages for the equivalent method. Three‐dimensional thermokinematic modeling of plate convergence incorporates advection of the upper Pacific plate along a low‐angle detachment then up an Alpine Fault ramp, adopting a generally accepted tectonic scenario for the Southern Alps. The modeling incorporates heat flow, evolving topography, and the detailed kinetics of different thermochronometric systems and explains both complex local variations and regional patterns. Inclusion of the effects of radiation damage on He diffusion in detrital apatite is shown to have dramatic effects on results. Geometric and velocity parameters are tuned to fit model ages to observed data. Best fit is achieved at 9 mm a−1 plate convergence, with Pacific plate delamination on a gentle 10°SE dipping detachment and more rapid uplift on a 45–60° dipping Alpine Fault ramp from 15 km depth. Thermokinematic modeling suggests dip‐slip motion on reverse faults within the Southern Alps should be highest ∼22 km from the Alpine Fault and much lower toward the southeast.

Burial and temperature evolution in thrust belt systems: Sedimentary and thrust sheet loading in the SE Canadian Cordillera

The southern Canadian foreland fold‐and‐thrust belt (FFTB) (SW Alberta–SE British Columbia) records the interplay between foreland basin evolution with the deforming wedge and thus controls the regional‐scale overburden and exhumation history. Overburden estimates are typically based on the assumption that peak burials were reached by sedimentary burial prior to the emplacement of thrust sheets. This study combines organic maturity ranks from a newly compiled catalog with forward thermokinematic modeling to examine this assumption. The organic maturity rank trends correlate not only to sedimentary but also to tectonic burial histories. The forward thermokinematic modeling builds on this combined burial history scenario and shows how required peak burial and temperatures can be achieved with reduced sedimentary overburden when combined with tectonic loading. We thus consider that the overburden was reached during instead of prior to the contraction of the FFTB as result of an integrated sedimentary and tectonic burial history. Thermokinematic modeling also suggests that first‐order steady state temperature conditions prevail during the development of the FFTB. Differently from earlier findings that considered a drop in paleotemperature gradient during the belts growth, steady state conditions reduce the amount of regional overburden required and, consequently, of eroded sediments. Besides tracing the regional evolution, organic maturity ranks are also affected by more local phenomena such as thrust‐scale denudation patterns and fluid flow.

Taiwan mountain building: insights from 2-D thermomechanical modelling of a rheologically stratified lithosphere

SUMMARY The Taiwan orogen has long been regarded as a case example for studying mountain building in association with subduction processes. In this paper, we present a fully coupled thermomechanical modelling of the Taiwan collision based on a realistic viscous-elastic–plastic rheology. It satisfactorily reproduces available thermochronometric data, long-/short-term deformation patterns, heat flux and erosion/sedimentation distribution across the Taiwan orogeny. We found that a deep seated flux of Asian crustal material into the orogenic wedge should be invoked to counter-balance observed exhumation and erosion in the Central Range. However, in contrast with recent thermokinematic models of exhumation and deformation suggesting that underplating plays a significant role, we show that most constraints on exhumation and deformation can be more straightforwardly interpreted by the frontal accretion of the rheologically layered Asian crust. We finally infer that such a model is in better agreement with the basic expectation that the hot/young and buoyant Chinese continental margin should hardly be subducted beneath the cold/old and dense oceanic plate of the Philippines Sea.

Quaternary tectonic response to intensified glacial erosion in an orogenic wedge

Active orogens are thought to behave as internally deforming critical-taper wedges that are in rough long-term equilibrium with tectonic influx and erosional outflux. Spatial and temporal variations in climate are therefore hypothesized to have a significant influence on denudation, topography and deformation of orogens, thereby affecting wedge taper. However, the impact of the most severe transition in Northern Hemisphere climate during the Cenozoic era—the onset of glaciation—has hitherto not been empirically documented. Here we analyse the spatial patterns of denudation and deformation, and their temporal variations, in the heavily glaciated St Elias orogen in southern Alaska. Low-temperature thermochronometry, thermokinematic modelling and offshore seismic reflection and borehole data suggest that the global-scale intensification of glaciation in the middle Pleistocene epoch enhanced glacier growth and caused ice streams to advance to the edge of the continental shelf. This led to focused denudation across the subaerial reaches of the orogen and burial of the actively deforming wedge toe by the eroded sediment. We propose that this climatically driven mass redistribution forced a structural reorganization of the orogen to maintain critical taper. Our empirical results thus support decades of numerical model predictions of orogenesis and provide compelling field evidence for the significant impact of climate change on tectonics. Intense glaciation during the middle Pleistocene epoch led to focused denudation and mass redistribution within the St Elias orogen in southern Alaska, and resulted in structural reorganization of the orogen. The tectonic response of this orogen to climate change is consistent with predictions of numerical models.

Mountain building in Taiwan: A thermokinematic model

The Taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening. However, a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge. We propose here a new thermokinematic model of the Taiwan mountain belt reconciling existing kinematic, thermometric and thermochronological constraints. In this model, shortening across the orogen is absorbed by slip on the most frontal faults of the foothills. Crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge (Tananao Complex, TC), where the uplift rate is estimated to ∼6.3 mm a−1, and beneath the westernmost internal region of the orogen (Hsueshan Range units, HR), where the uplift rate is estimated to ∼4.2 mm a−1. Our model suggests that the TC units experienced a synchronous evolution along strike despite the southward propagation of the collision. It also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures. Exhumation of the HR units increases northward but has not yet reached an exhumational steady state. Presently, frontal accretion accounts for less than ∼10% of the incoming flux of material into the orogen, although there is indication that it was contributing substantially more (∼80%) before 4 Ma. The incoming flux of material accreted beneath the TC significantly increased 1.5 Ma ago. Our results also suggest that the flux of material accreted to the orogen corresponds to the top ∼7 km of the upper crust of the underthrust Chinese margin. This indicates that a significant amount (∼76%) of the underthrust material has been subducted into the mantle, probably because of the increase in density associated with metamorphism. We also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field. By combining available geological data on the thermal and kinematic evolution of the wedge, our study sheds new light onto mountain building processes in Taiwan and allows for reappraising the initial structural architecture of the passive margin.

Plio‐Quaternary exhumation history of the central Nepalese Himalaya: 2. Thermokinematic and thermochronometer age prediction model

In the Himalaya and other active convergent orogens, linear relationships between thermochronometer sample age and elevation are often used to estimate long‐term exhumation rates. In these regions, high‐relief topography and nonvertical exhumation pathways may invalidate such one‐dimensional (1‐D) interpretations and lead to significant errors. To quantify these errors, we integrate apatite fission track (AFT) ages from the central Himalaya with a 3‐D coupled thermokinematic model, from which sample cooling ages are predicted using a cooling‐rate‐dependent algorithm. By changing the slip partitioning between faults near the Main Central thrust and the Main Frontal thrust system at the Himalayan range front, we are able to explore the significance of different tectonic scenarios on predicted thermochronometer ages. We find that the predicted AFT cooling ages are not sensitive to the different slip partitioning scenarios but depend strongly on erosion rate: Predicted ages are most consistent with kinematic models that produce erosion rates of 1.8–5.0 mm/yr. This range is considerably smaller than that derived from regression lines through the data (−2.6–12.2 mm/yr). The pattern of observed AFT ages shows more complexity than our models predict. None of the kinematic scenarios are able to fit >80% of all of the AFT data, most likely because erosion is spatially variable. Such complexities notwithstanding, we conclude that the use of thermokinematic modeling and thermochronologic data sets to explore detailed fault kinematics in rapidly eroding active orogens has great promise but requires integration of higher‐temperature (>∼350°C) data sets to maximize effectiveness.

Plio‐Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U‐Th]/He analyses

New apatite and zircon fission track and (U‐Th)/He analyses serve to document the bedrock cooling history of the central Nepalese Himalaya near the Annapurna Range. We have obtained 82 apatite fission track (AFT), 7 zircon fission track (ZFT), and 7 apatite (U‐Th)/He (AHe) ages from samples collected along the Marsyandi drainage, including eight vertical relief profiles from ridges on either side of the river averaging more than 2 km in elevation range. In addition, three profiles were sampled along ridge crests that also lie ∼2 km above the adjacent valleys, and a transect of >20 valley bottom samples spans from the Lesser Himalaya across the Greater Himalaya and into the Tethyan strata. As a consequence, these data provide one of the more comprehensive low‐temperature thermochronologic studies within the Himalaya. Conversely, the youthfulness of this orogen is pushing the limits of these dating techniques. AFT ages range from >3.8 to 0 Ma, ZFT ages from 1.9 to 0.8 Ma, and AHe ages from 0.9 to 0.3 Ma. Most ridges have maximum ages of 1.3–0.8 Ma at 2 km above the valley bottom. Only one ridge crest (in the south central zone of the field area) yielded significantly older ZFT and AFT ages of ∼2 Ma; we infer that a splay of the Main Central Thrust separates this ridge from the rest of the Greater Himalaya. ZFT and AFT ages from a vertical transect along this ridge indicate exhumation rates of ∼1.5 km Myr−1 (r2 > 0.7) from ∼2 to 0.6–0.8 Ma, whereas AHe ages indicate a faster exhumation rate of ∼2.6 km Myr−1 (r2 = 0.9) over the last 0.8 Myr. Exhumation rates calculated for six of the remaining seven vertical profiles ranged from 1.5 to 12 km Myr−1 (all with low r2 values of <0.6) for the time period from ∼1.2 to 0.3 Ma, with no discernible patterns in south to north exhumation rates evident. The absence of a trend in exhumation rates, despite a strong spatial gradient in rainfall, argues against a correlation of long‐term exhumation rates with modern patterns of rainfall. AFT ages in the Tethyan strata are, on average, older than in the Greater Himalaya and may be a response to a drier climate, slip on the South Tibetan Detachment, or a gentler dip of the underlying thrust ramp. These data are further evaluated with thermokinematic modeling in the companion paper by Whipp et al.

Neotectonics of the central Nepalese Himalaya: Constraints from geomorphology, detrital 40Ar/39Ar thermochronology, and thermal modeling

The southern flanks of the central Nepalese Himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate. This transition can be explained by either (1) accretion of footwall material to the hanging wall across a ramp in the décollement separating India from Eurasia, thereby enhancing rock uplift rates above the zone of accretion or (2) out‐of‐sequence surface thrust faulting at the physiographic transition. Here we use geomorphic data, 649 new detrital 40Ar/39Ar cooling ages, and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central Nepalese Himalaya. We first define and delineate the physiographic transition in central Nepal using maps of knickpoints, river steepness indices, local relief, and the distribution of thick alluvial fill deposits. We then report new detrital 40Ar/39Ar data from two trans‐Himalayan transects, each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition. Thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions. We contend that the physiographic and thermochronologic data in our study area are most simply explained by recent out‐of‐sequence surface thrusting within the Lesser Himalayan metasedimentary sequence, approximately 15–30 km south of the mapped surface trace of the Main Central Thrust system. An important finding of this work is that there are substantial along‐strike variations in physiography and thermal history that reflect along‐strike changes in the degree and location of out‐of‐sequence surface thrusting.

Exhumation of the central Wasatch Mountains, Utah: 2. Thermokinematic model of exhumation, erosion, and thermochronometer interpretation

The Wasatch fault is a ∼370 km long normal fault in Utah that marks the boundary between the stable Colorado Plateau to the east and the extending Basin and Range to the west. Understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones (e.g., East Africa Rift, Transantarctic Mountains). We explore the thermokinematics of footwall exhumation and erosion in the Cottonwood Intrusive Belt of the central Wasatch Mountains. Emphasis is placed on using low‐temperature thermochronometers to quantify (1) the spatial and temporal variability of exhumation and erosion rates, (2) the geometry of footwall tilt, (3) the fault dip angle, and (4) the magnitude and duration of exhumation. These processes are investigated using two‐dimensional (2‐D) thermal‐kinematic models coupled with cooling‐rate‐dependent kinetic models which predict exhumed apatite fission track (AFT) and (U‐Th)/He ages. The range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0.2 and 2.0 mm yr−1, footwall tilt hinge positions between 15 and 40 km distance from the fault, a single planar normal fault with dip angles of 45° and 60°, and exhumation magnitudes of up to 15 km at the fault. Simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat‐producing layers. Erosion maintains a constant topographic profile. The kinematic and exhumation history of the Wasatch Mountains is investigated by comparing model predicted thermochronometer ages to observed AFT, ZFT, and (U‐Th)/He ages. Predicted and observed ages are compared using a reduced chi‐square analysis to determine a best fit kinematic model for the Wasatch Mountains. The preferred model includes exhumation occurring on either a 45° or 60° dipping fault, a footwall hinge located a minimum of 20–25 km from the fault, and a step decrease (deceleration) in the footwall exhumation rate at the fault from 1.2 to 0.8 mm yr−1 at around 5 Ma. The model also suggests an exhumation duration of ∼12 Myr ± 2 Myr).

A site for deep ice coring in West Antarctica: results from aerogeophysical surveys and thermo-kinematic modeling

AbstractThe U.S. Science Plan for Deep Ice Coring in West Antarctica calls for two ice cores to be collected. the first of these cores, from Siple Dome, was completed during the 1997/98 field season. the second core is to be collected from a site near the divide that separates ice flowing to the Ross Sea and to the Amundsen Sea.Using high-resolution, grid-based aerogeophysical surveys of the Ross/Amundsen ice-divide region, we identify seven candidate sites and assess their suitability for deep coring. We apply ice-flow and temperature calculations to predict time-scales and annual-layer resolution, and to assess the potential for basal melting for several selected sites. We conclude that basal melting is likely for sites with very thick ice, as was observed at the Byrd core site. Nevertheless, these sites are most attractive for coring since they promise recovery of a long climate record with comparatively high time resolution during the last glacial period.