Andrew J. Parsons

Thermo‐kinematic evolution of the Annapurna‐Dhaulagiri Himalaya, central Nepal: The Composite Orogenic System

The Himalayan orogen represents a ‘‘Composite Orogenic System’’ in which channel flow, wedge extrusion, and thrust stacking operate in separate ‘‘Orogenic Domains’’ with distinct rheologies and crustal positions. We analyze 104 samples from the metamorphic core (Greater Himalayan Sequence, GHS) and bounding units of the Annapurna-Dhaulagiri Himalaya, central Nepal. Optical microscopy and electron backscatter diffraction (EBSD) analyses provide a record of deformation microstructures and an indication of active crystal slip systems, strain geometries, and deformation temperatures. These data, combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the GHS. The profiles define a three-stage thermokinematic evolution from midcrustal channel flow (Stage 1, >7008C to 550–6508C), to rigid wedge extrusion (Stage 2, 400–6008C) and duplexing (Stage 3, <280–4008C). These tectonic processes are not mutually exclusive, but are confined to separate rheologically distinct Orogenic Domains that form the modular components of a Composite Orogenic System. These Orogenic Domains may be active at the same time at different depths/positions within the orogen. The thermokinematic evolution of the Annapurna-Dhaulagiri Himalaya describes the migration of the GHS through these Orogenic Domains and reflects the spatial and temporal variability in rheological boundary conditions that govern orogenic systems.

Grain-size trends reveal the late orogenic tectonic and erosional history of the south–central Pyrenees, Spain

Stratigraphic grain-size trends record tectonic and climatic signals. Here, we show how measurements of sediment calibre and clast lithology can be used to identify changes in accommodation space and sediment budget, using examples from Palaeogene syntectonic clastic deposits in the southern Pyrenees. We identify a mid Eocene interval of rapid grain-size fining, driven by local tectonic subsidence; a late Eocene interval of diminished local accommodation generation; and an Oligocene interval showing order-of-magnitude lower grain-size fining rates, driven by increased sediment supply. Our results demonstrate that grain-size trends provide a powerful means to explore the tectonic and climatic boundary conditions governing sediment routing systems.

Geology of the Dhaulagiri-Annapurna-Manaslu Himalaya, Western Region, Nepal. 1:200,000

Geological mapping of mountains belts is fundamental to understanding their structure and evolution. Here, a 1:200,000 scale geological map of the central Himalaya of Western Region, Nepal is presented. This map represents a compilation of previously published maps, integrated with new geological field data. The wide spatial coverage of the map and the accompanying cross sections reveal the detailed structure of the Dhaulagiri-Annapurna-Manaslu Himalaya. The addition of modern topographic and infrastructure data makes this map suitable for navigation through the region.

Orogen-parallel deformation of the Himalayan mid-crust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna-Dhaulagiri Himalaya, central Nepal

The metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna-Dhaulagiri region, central Nepal recorded orogen-parallel stretching during mid-crustal evolution. Anisotropy of magnetic susceptibility (AMS) and field-based structural analyses suggest that mid-crustal deformation of the amphibolite facies core of the GHS occurred under an oblate/sub-oblate strain regime with associated formation of low-angle northward-dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right-lateral orogen-parallel stretching. We propose that oblate strain within a mid-crustal flow accommodated oblique convergence between India and the arcuate orogenic front without need for strain partitioning in the upper-crust. Oblate flattening may have also promoted orogen-parallel melt migration and development of melt-depleted regions between km3-scale leucogranite culminations at ~50-100 km intervals along orogen-strike. Following cessation of flow, continued oblique convergence led to upper-crustal strain partitioning between orogen-perpendicular convergence on thrust faults and orogen-parallel extension on normal and strike-slip faults. In the Annapurna-Dhaulagiri Himalaya, orogen-parallel stretching lineations are interpreted as a record of transition from mid-crustal orogen-perpendicular extrusion to upper-crustal orogen-parallel stretching. Our findings suggest that mid-crustal flow and upper-crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen-wide transition from mid-crustal orogen-perpendicular extrusion to upper-crustal orogen-parallel extension during the mid-Miocene. The 3D nature of oblate strain and orogen-parallel stretching cannot be replicated by 2D numerical simulations of the Himalayan orogen.

Spatial variation in exhumation rates across Ladakh and the Karakoram: New apatite fission track data from the Eastern Karakoram, NW India

Characterization of low-temperature cooling histories and associated exhumation rates is critical for deciphering the recent evolution of orogenic regions. However, these may vary significantly over relatively short distances within orogens. It is pertinent therefore to constrain cooling histories and hence exhumation rates across major tectonic boundaries. We report the first apatite fission track ages from the Karakoram Fault Zone in the Eastern Karakoram range, which forms part of the western margin of the Tibetan Plateau. Ten samples, from elevations of 3477–4875m, have apatite fission track dates from 3.3 ± 0.3 Ma to 7.4± 1.1Ma. The ages correspond to modeled average erosional exhumation rates of 0.67+ 0.27-0.18mm/yr across the Eastern Karakoram. The results are consistent with a trend northward from the Indus suture zone, across the Ladakh terrane and into the Karakoram, in which tectonic uplift associated with crustal thickening increases toward the north, raising elevation and promoting glaciation and generation of extreme relief. As a result, erosion and exhumation rates increase south to north. Present-day precipitation on the other hand varies little within the study area and on a larger scale decreases southwest to northeast across this portion of the orogen. The Eastern Karakoram results highlight the diverse patterns of exhumation driven by regional variations in tectonic response to collision along the western margin of Tibet.