Abstract
AbstractThe metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna‐Dhaulagiri region, central Nepal, recorded orogen‐parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field‐based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate 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 midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the 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 the 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 midcrustal orogen‐perpendicular extrusion to upper crustal orogen‐parallel stretching. Our findings suggest that midcrustal 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 midcrustal orogen‐perpendicular extrusion to upper crustal orogen‐parallel extension during the mid‐Miocene. The 3‐D nature of oblate strain and orogen‐parallel stretching cannot be replicated by 2‐D numerical simulations of the Himalayan orogen.
Highlights
Understanding tectonic evolution of continental-collision zones requires consideration of deformation in three dimensions
Integrated Anisotropy of magnetic susceptibility (AMS) and structural analyses have been conducted across the Greater Himalayan Sequence (GHS) and bounding units in the Kali Gandaki Valley of the Annapurna-Dhaulagiri Himalaya, central Nepal
Correlation with previously published constraints indicates that AMS fabrics from the Upper Greater Himalayan Sequence (UGHS) and base of the South Tibetan Detachment System (STDS) provide a record of high-temperature, synmigmatitic to postmigmatitic deformation (>550–650°C)
Summary
Understanding tectonic evolution of continental-collision zones requires consideration of deformation in three dimensions. Current models of Himalayan orogenesis are largely based on midcrustal kinematic evolution of the orogenic core: the Greater Himalayan Sequence (GHS, Figure 1) [e.g., Searle et al, 2006; He et al, 2014; Montomoli et al, 2015; Cottle et al, 2015; Parsons et al, 2016a, 2016b] These models describe one of the three generalized end-member processes, (1) channel flow [e.g., Beaumont et al, 2001; Searle et al, 2006], (2) wedge extrusion [e.g., Burchfiel et al, 1992], and (3) underplating/duplexing [e.g., Herman et al, 2010; Montomoli et al, 2015; Carosi et al, 2016], or describe a composite model typically involving channel flow followed by wedge extrusion and/or underplating/duplexing [e.g., Larson et al, 2010; Mukherjee, 2013; Cottle et al, 2015; Parsons et al, 2016a]. Present-day and recent upper crustal deformation of the Himalayan orogen is a three-dimensional process, with hinterland orogen-parallel extensional and strike-slip faulting occurring simultaneously to orogen-perpendicular thrust faulting along the frontal thrust system [e.g., Styron et al, 2011].
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