ALBERT

Description: We present a synthesis of recently conducted tectonic, global positioning system (GPS), geomorphological and seismic studies to describe the kinematics of the Zagros mountain belt, with a special focus on the transverse right-lateral strike-slip Kazerun Fault System (KFS). Both the seismicity and present-day deformation (as observed from tectonics, geomorphology and GPS) appear to concentrate near the 1000 m elevation contour, suggesting that basement and shallow deformation are related. This observation supports a thick-skinned model of southwestward propagation of deformation, starting from the Main Zagros Reverse Fault. The KFS distributes right-lateral strike-slip motion of the Main Recent Fault onto several segments located in an en echelon system to the east. We observe a marked difference in the kinematics of the Zagros across the Kazerun Fault System. To the NW, in the North Zagros, present-day deformation is partitioned between localized strike-slip motion on the Main Recent Fault and shortening located on the deformation front. To the SE, in the Central Zagros, strike-slip motion is distributed on several branches of the KFS. The decoupling of the Hormuz Salt layer, restricted to the east of the KFS and favouring the spreading of the sedimentary cover, cannot be the only cause of this distributed mechanism because seismicity (and therefore basement deformation) is associated with all active strike-slip faults, including those to the east of the Kazerun Fault System.

Description: Physical and numerical simulations of the development of mountain topography predict that asymmetric distributions of precipitation over a mountain range induce a migration of its drainage divide toward the driest flank in order to equilibrate erosion rates across the divide. Such migration is often inferred from existing asymmetries, but direct evidence for the migration is often lacking. New low-temperature apatite cooling ages from a transect across the northern North Cascades range (Washington, NW USA) and from two elevation profiles in the Skagit River valley record faster denudation on the western, wetter side of the range and lower denudation rates on the lee side of the range. This difference has already been documented further south along another transect across the range; however, in the south, the shift from young cooling ages to older ages occurs across the modern drainage divide. Here, further north, the shift occurs along a range-transverse valley within the Skagit Gorge. It has been proposed that the upper Skagit drainage was once a part of the leeward side of the range but started to drain toward the western side of the range across the Skagit Gorge in Quaternary time. Age-elevation profiles along the former drainage and in the Skagit Gorge restrict the onset of Skagit Gorge incision to the last 2 m.y., in agreement with 4 He/ 3 He data for the gorge floor. Breaching of the range drainage resulted in its displacement 40 km further east into the dry side of the range. In the 2000-m-deep, V-shaped Skagit Gorge, river stream power is still high, suggesting that incision of the gorge is still ongoing. Several other similar events have occurred along the range during the Pleistocene, supporting the proposed hypothesis that the repeated southward incursions of the Cordilleran ice sheet during this period triggered divide breaching and drainage reorganization by overflow of ice-dammed lakes at the front of the growing ice sheet. Since these events systematically rerouted streams toward the wet side of the range and resulted in leeward migration of the divide, we propose that in fact the Cordilleran ice sheet advance essentially catalyzed the adjustment of the mountain chain topography to the current orographic precipitation pattern.

Description: Author(s): T. van der Beek, P. Barthelemy, P. M. Johnson, D. S. Wiersma, and A. Lagendijk We present a study of optical transport properties of powder layers with submicrometer, strongly scattering gallium arsenide (GaAs) particles. Uniform, thin samples with well controlled thicknesses were created through the use of varying grinding times, sedimentation fractionation, annealing, and a ... [Phys. Rev. B 85, 115401] Published Thu Mar 01, 2012

Description: Fast uplift and exhumation of the Himalaya and Tibet and fast subsidence in the foreland basin portray the primary Neogene evolution of the Indian-Eurasian collision zone. We relate these events to the relative northward drift of India over its own slab. Our mantle-flow model derived from seismic tomography shows that dynamic topography over the southward-folded Indian slab explains the modern location of the foreland depocenter. Back in time, our model suggests that the stretched Indian slab detached from the Indian plate during the indentation of the Eurasian plate, and remained stationary underneath the northward-drifting Indian continent. We model the associated southward migration of the dynamic deflection of the topography and show that subsidence has amounted to ~6000 m in the foreland basin since 15 Ma, while the dynamic surface uplift of the Himalaya amounted to ~1000 m during the early Miocene. While competing with other processes, transient dynamic topography may thus explain, to a large extent, both the uplift history of the Himalaya and subsidence of its foreland basin, and should not be ignored.

Description: We studied the relationships among present-day relief, precipitation, stream power, seismic energy, seismic strain rate, and long-term exhumation rates for the Venezuelan Andes. Average long-term exhumation rates were determined for seven large catchments in the Venezuelan Andes from fission-track analysis of detrital apatite. A quantitative comparison between eight new detrital apatite fission-track (AFT) age distributions presented here and previously published bedrock AFT age patterns shows that detrital AFT ages can be used for predicting exhumation patterns across the mountain belt. Catchment-averaged exhumation rates estimated from the raw data range from 0.48 ± 0.02 km m.y. –1 to 0.80 ± 0.26 km m.y. –1 Accounting for variable sediment yield and assuming that short-term sediment production rates scale with long-term exhumation rates, these rates vary from 0.33 ± 0.07 km m.y. –1 to 0.48 ± 0.08 km m.y. –1 No variation in rates is observed between the northwestern and southeastern flanks of the mountain belt, despite a threefold increase in precipitation from the northwest to the southeast. Long-term exhumation rates are strongly correlated with relief in the different catchments, weak or negative correlations exist with precipitation data or present-day erosion indexes, while the correlation with seismic energy released by earthquakes is weak to moderate. This lack of correlation may be caused by the insufficient temporal range of the available precipitation and seismicity data, and the different time scales involved in the comparison. Long-term exhumation rates are, however, strongly correlated with seismic strain rates (which take the temporal earthquake magnitude-frequency scaling into account), suggesting that the moderate correlation with seismic energy is indeed related to the different time scales and that tectonic control on exhumation is significant. In contrast, given that precipitation patterns in the Venezuelan Andes should have been installed during Miocene times, we suggest that decoupling of relief and exhumation from present-day climate explains the lack of correlation between exhumation and precipitation.

Description: The central Pyrenees experienced an episode of rapid exhumation in Late Eocene–Early Oligocene times. Erosional products shed from the range during this time were deposited in large palaeovalleys of the southern flank of the Axial Zone, leading to significant sediment accumulation. A recent numerical modelling study of the post-orogenic exhumation and relief history of the central Axial Zone allowed us to constrain this valley-filling episode in terms of timing and thickness of conglomeratic deposits. This paper aims to test these results for the southern fold-and-thrust belt using apatite fission-track and (U–Th)/He analysis on detrital samples from the Tremp–Graus and Ager basins. Inverse thermal-history modelling of the low-temperature thermochronology data indicates that the fold-and-thrust belt was covered during the Late Eocene to Miocene by 0.7–1.6 km of sediments and confirms the timing of re-excavation of the valleys during the Miocene. A detailed analysis of the apatite (U–Th)/He results shows that the significant scatter in grain ages can be explained by the influence of alpha-recoil damage with varying effective uranium content together with distinct pre-depositional thermal histories; the age scatter is consistent with initial exhumation of the sediment sources during the Triassic and Early Cretaceous. Supplementary materials: Modelling results obtained using the Gautheron et al . (2009) diffusion algorithm are available at www.geolsoc.org.uk/SUP18560 .

Description: We use two-dimensional mechanical models to investigate the effects of syntectonic sedimentation on fold-and-thrust belt development, testing variable syntectonic (wedge-top and foredeep) sediment thicknesses and flexural rigidities. Model results indicate a first-order control of syntectonic sedimentation on thrust-sheet length and thrust spacing. Thrust sheets are longer when syntectonic sediment thickness and/or flexural rigidity increase. Comparison with observations from several fold-and-thrust belts confirms this first-order control of syntectonic sedimentation.

Description: The exhumation history of the central Himalaya is well documented, but lateral variations in exhumation remain poorly constrained. In this study, we identify sediment source areas and examine the late Neogene exhumation history of the eastern Himalaya from the synorogenic sedimentary record of its foreland basin. We present Nd and Hf isotopic data as well as apatite and zircon fission-track analyses from the Miocene–Pliocene Siwalik Group along the recently dated Kameng River section in Arunachal Pradesh, northeastern India. Our isotopic data show that Siwalik Group sediments deposited between 13–7 and 〈2.6 Ma in Arunachal Pradesh were mainly derived from Higher Himalayan source rocks. In contrast, sediments deposited between ca. 7 and 3 Ma have far less negative Nd and Hf values that require involvement of the Gangdese Batholith and Yarlung suture zone source areas via the Brahmaputra River system. Consequently, these sediments should also record incision of the Namche Barwa massif by this river. Source-area exhumation rates of Himalayan-derived sediments, determined from detrital zircon fission-track data, were on the order of 1.8 km/m.y. in the fastest-exhuming areas. These rates are very similar to those calculated for the central Himalaya and have been relatively constant since ca. 13 Ma. Our results do not support the hypothesis of a major change in exhumation rate linked to either local or regional climate change or to Shillong Plateau uplift during the Miocene, as reported elsewhere. The zircon fission-track data further suggest that exhumation of the Namche Barwa massif between 7 and 3 Ma was much slower than the ~10 km/m.y. rate recorded in the recent past. Detrital apatite fission-track data indicate deformation of the Siwaliks due to forward propagation of the frontal thrust since around 1 Ma.