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

Abstract

The 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.

Mesozoic Crustal Thickening of the Longmenshan Belt (NE Tibet, China) by Imbrication of Basement Slices: Insights From Structural Analysis, Petrofabric and Magnetic Fabric Studies, and Gravity Modeling

“…The magnetic susceptibility of the Pengguan granitic rocks mostly arises from paramagnetic biotite and partly ferromagnetic magnetite (PSD or MD) based on Km , thermomagnetic, hysteresis loop, and isothermal remanent magnetization measurements. Numerous previous studies have demonstrated that paramagnetic minerals and ferromagnetic minerals carry comparable magnetic fabrics with the mineral petrofabrics in terms of orientation (Borradaile & Jackson, ; Bouchez & Gleizes, ; Bouchez et al, ; Parsons, Ferré, et al, ; Rochette et al, ]. Stereonet projection and microscopic characteristics indicate that the well‐clustered magnetic fabrics of the mylonitic samples within the Pengguan complex are consistent with the field observed fabrics (Figures b, S2a, and S2b).…”

“…Magnetic susceptibility ( K ) is the ratio of the induced magnetization ( M ) of a specimen to the externally applied magnetic field ( H ) (Borradaile & Jackson, , ). The anisotropy of magnetic susceptibility (AMS) is a second rank tensor that describes directional variation of a specimen's magnetic susceptibility, which can be used as deformation proxy if the contributing factors (mineral content, mineral shape fabric, and crystallographic fabric) are determined (Borradaile & Jackson, , ; Kruckenberg et al, ; Parsons, Ferré, et al, ). The AMS is geometrically represented by an ellipsoid defined by three orthogonal principal axes, K 1 (maximum), K 2 (intermediate), and K 3 (minimum) (Tarling & Hrouda, ).…”

Preservation of the Early Evolution of the Himalayan Middle Crust in Foreland Klippen: Insights from the Karnali Klippe, West Nepal

“…At a few localities in the middle of the GHS, the quartz and biotite aggregate lineation is subparallel to the strike of the foliation and trends east‐west. Elsewhere in Nepal Himalaya, east‐west trending mineral lineations have been interpreted to reflect orogen‐parallel flow (e.g., Parsons et al, ; Pêcher, ). Top‐to‐the‐SW shear‐sense indicators are abundant in the MCT high‐strain zone and sparse above it.…”

Mid‐Miocene initiation of E‐W extension and recoupling of the Himalaya

“…Ruddiman, Raymo, Prell, & Kutzbach, ; Zhisheng, Kutzbach, Prell, & Porter, ). Formation of the graben has alternatively been described as a consequence of expansion of the Himalayan arc (Klootwijk, Conaghan, & Powell, ; Seeber & Armbruster, ), oblique convergence between India and Asia (McCaffrey & Nabelek, ) possibly with an oblate strain component (Parsons et al, ), oroclinal bending (Ratschbacher, Frisch, Liu, & Chen, ), or a far field manifestation of wide‐scale Neogene extension that affects much of SE Asia (Yin et al, ). Regardless of the specific mechanism, the cessation of the South Tibetan detachment system and subsequent initiation of E‐W striking graben represents a first order shift in orogen‐scale kinematics.…”