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[1] The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low-temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along-strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north-south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (<3 Ma) along both N-S transects in the high-relief zone of the Higher Himalaya and increase (4 to 6 Ma) toward the south in the Lesser Himalaya. We compare our new data to published low-temperature thermochronological data sets for Nepal and the Bhutan Himalaya. We use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the Pecube code in order to constrain potential along-strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in the geometry of the MHT (in particular the presence or absence of a major crustal-scale ramp) strongly control the kinematics and exhumation history of the orogen.Citation: Robert, X., P. van der Beek, J. Braun, C. Perry, and J.-L. Mugnier (2011), Control of detachment geometry on lateral variations in exhumation rates in the Himalaya: Insights from low-temperature thermochronology and numerical modeling,

Apatite fission track (AFT) thermochronology of Precambrian and Paleozoic basement samples from southern Norway reveals a post-Paleozoic exhumation history, related to offshore Mesozoic and Cenozoic extensional basin development. The data indicate two major phases of rapid exhumation. A first Mesozoic phase started in the Triassic (-220 Ma) in the east and south of the study area and migrated to the west where Jurassic (-160 Ma) ages of exhumation predominate. A second event is indicated by thermal history modeling of AFT ages and track length distributions. It is inferred to be Neogene in age, initiated at about 30 Ma, and it produced a domal pattern of AFT isochrons which follow present-day topographic elevation. Youngest AFT ages (-100 Ma) are encountered at sealevel in the inner fjords near the areas of highest topography; ages increase radially outward to the mountain peaks and the coastlines. Forward modeling of age-elevation patterns suggests that Mesozoic geothermal gradients were 10-15øC/km higher than the present value of 20øC/km. During the Triassic and Jurassic, a total of 1.3-3.5 km of overburden was removed from the study area, assuming a 30øC/km geothermal gradient for that period. We attribute this to rift margin erosion as a result of erosional base level lowering and flank uplift, as evidenced by thick continental clastic sequences deposited in Triassic-Jurassic half grabens in the North Sea basins. We propose that 1.5-2.5 km of Neogene exhumation were a result of late stage domal uplift. This is supported by basinward dipping pre-Neogene strata in the basins surrounding southern Norway and the infill of a 1-to 2-km-thick Neogene sediment wedge containing various internal unconformities. Domal uplift probably started in the Late Oligocene, may have been amplified in the Pliocene, and was overprinted by Plio-Pleistocene glacial erosion. Maximum Neogene tectonic uplift is estimated at approximately 1-1.5 km, radially decreasing outward to a value <500 m near the shoreline. Neogene domal uplift is coincident with Oligocene and Pliocene plate reorganizations in the North Atlantic; similar Neogene domes are found around the Norwegian-Greenland Sea (i.e., Svalbard and the Barents Sea, northern Norway, east Greenland), suggesting a regional tectonic cause. The onset of Neogene uplift postdates major volcanism and continental breakup by-25 m.y. and predates Plio-Pleistocene glaciations. Its origin is possibly a combina-Copyright 1995 by the American Geophysical Union. Paper number 95TC00088. 0278-7407/95/95TC-00088510.00 tion of induced mantle convection, resulting in thermal erosion of the lithosphere, and the operation of intraplate stresses. Introduction It has become apparent in recent years that rifted margins may record significant postrift (late stage) uplift events, in addition to better understood synrift flank uplift [e.g., Cloetingh and Kooi, 1992; Sales, 1992]. For instance, Cloetingh et al. [1990; 1992] stress the importance of accelerated postrift subsidence, in conjunction with margin upl...

We have undertaken 2D forward modelling across the northern North Sea, based on reprocessed, interpreted and depth-converted deep reflection seismic lines NSDP84-1 and −2 (15 s twt) and refraction data. Two separate stretching phases, Permo-Triassic and Jurassic, are recognized. The cumulative stretching is consistent with the observed crustal structure and the overall basin configuration, as reproduced by forward modelling. Good agreement between observed and modelled top basement level, and crustal thickness below the platform areas are particularly emphasized. Crustal-scale modelling indicates that crustal thickness varied across the northern North Sea at the onset of the Permo-Triassic rifting, from c. 35 km in the platform areas to less than 30 km in the interior of the basin. This may be ascribed to Devonian(-Carboniferous?) crustal stretching. Thinning of the crust has progressively been narrowed, from post-Caledonian extensional collapse, to less regional Permo-Triassic basins, and finally development of the Viking Graben area in the Jurassic-early Cretaceous time. Most of the Permo-Triassic stretching occurred between the Øygarden Fault Zone to the east and the Shetland Platform (southern transect) and the Hutton Fault alignment to the west. The width of the Permo-Triassic basin was c. 120–125 km, with calculated βmean between 1.38 and 1.40. Permo-Triassic βmean estimates across the present Horda Platform vary between 1.33 and 1.39. The Jurassic βmean estimates for the same area vary between 1.08 and 1.13. Across the Viking Graben, Permo-Triassic βmean varies between 1.28 (southern transect) and 1.41 (northern transect). This is lower than estimates for the Jurassic βmean, which amounts to 1.53 and 1.42. Permo-Triassic and Jurassic βmean estimates across the East Shetland Basin are 1.29 and 1.11, respectively. Lithospheric thermal evolution reflects the general differences between Permo-Triassic and Jurassic stretching, with a much wider thermal perturbation during the former and a focusing and lateral migration towards the east of the peak thermal elevation during the latter. There are still uncertainties related to the degree of (de)coupling between the upper crust and upper mantle during the Permo-Triassic and the Jurassic rift phases. These uncertainties are related to the interplay between age, strain rate, crustal rheology, crustal thickness and long-lived zones of weaknesses.

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[1] Recent studies integrating geomorphology, thermochronology, cosmogenic erosion rate estimates, and numerical modeling suggest that escarpment evolution may take place following two dramatically different modes: (1) parallel retreat from the escarpment's original position at the continent-ocean boundary to its present-day inland position and (2) formation-in-place by progressive downwearing of a plateau initially located between the coast and a preexisting inland drainage divide. Using a three-dimensional finite element model to solve the heat transfer equation, we show that the mode of migration of a passive margin escarpment can be constrained by low-temperature (apatite (U-Th)/He) thermochronology. We first couple the heat equation solver to a surface processes model that predicts the two different escarpment evolution modes from only slightly different initial conditions. We predict (U-Th)/He age distributions that are markedly different for the two scenarios. We perform a thorough investigation of the model behavior to determine under which circumstances thermochronological data can be used to constrain passive margin escarpment dynamics. These conditions include (a) a tall escarpment, (b) a high geothermal gradient, and/or (c) a low flexural rigidity of the lithosphere. We demonstrate that to determine the rate and mode of escarpment migration from low-temperature thermochronology, one needs to collect samples along transects perpendicular as well as parallel to the escarpment. Tightest constraints on escarpment development are provided by (in ascending order) the minimum (U-Th)/He age encountered seaward of the escarpment, the location of where the minimum age is found, the slope of the age-distance relationship (in a direction perpendicular to the coast), and the slope of the age-elevation relationship (from a transect parallel to the escarpment). We finally demonstrate that there are situations where thermochronological data sets do not provide constraints on the mode of escarpment migration, such as along the escarpment of southeastern Australia, where migration has possibly been very rapid. Using the Neighborhood Algorithm method, we are, however, able to extract from an existing apatite (U-Th)/He data set very useful constraints on the evolution of the southeastern Australian escarpment, including the duration of the migration event (<15 Myr), the local geothermal gradient (32°-40°C km À1 ), and the effective elastic thickness of the underlying lithosphere (6-8 km).