Orogenic Segmentation and Its Role in Himalayan Mountain Building

Abstract

The continental collision process has made a large contribution to continental growth and reconfiguration of cratons throughout Earth history. Many of the mountain belts present today are the product of continental collision such as the Appalachians, the Alps, the Cordillera, the Himalaya, the Zagros, and the Papuan Fold and Thrust Belt. Though collisional mountain belts are generally elongate and laterally continuous, close inspection reveals disruptions and variations in thrust geometry and kinematics along the strike of the range. These lateral variations typically coincide with cross structures and have been documented in thrust fault systems with a variety of geometries and kinematic interpretations. In the Himalaya, cross faults provide segment boundaries that, in some cases separate zones of differing thrust geometry and may even localize microseismicity or limit areas of active seismicity on adjacent thrust systems. By compiling data on structural segmentation along the length of the Himalayan range, we find lateral variations at all levels within the Himalaya. Along the Gish fault of the eastern Indian Himalaya, there is evidence in the foreland for changes in thrust-belt geometry across the fault. The Gish, the Ganga, and the Yamuna faults all mark boundaries of salients and recesses at the mountain front. The Benkar fault in the Greater Himalayan sequence of eastern Nepal exhibits a brittle-ductile style of deformation with fabric that crosscuts the older thrust-sense foliation. Microseismicity data from several regions in Nepal shows linear, northeast-striking clusters of epicenters sub-parallel to cross faults. The map pattern of aftershock data from the 2015 Nepal earthquakes has an abrupt northeast-trending termination on its eastern side suggesting the presence of a structure of that orientation that limited slip. The orientations of the recognized cross faults and seismic patterns also align with the extensional zones to the north on the Tibetan Plateau and the Indian basement structures to the south. Results from multiple studies are consistent with a link between cross faults and either of these structural trends to the north or south and suggest that cross faults may play a role in segmenting deformation style and seismic activity along the length of the Himalaya.

Evidence of structural segmentation of the Uttarakhand Himalaya and its implications for earthquake hazard

“…Other collisional mountain belts around the world, such as the Appalachians and Alps, have also demonstrated cross/transverse segmentation 60 . The segmentation of the MHT in the western Himalayas has already been described using low temperature thermochronometry data 61 and mapped geometry of duplex/ramp structures 19 , 62 , 63 .…”

Short communication: age2exhume – a MATLAB/Python script to calculate steady-state vertical exhumation rates from thermochronometric ages and application to the Himalaya

“…The lowest exhumation rates along the high Himalayan topographic front (< 0.8 km Myr −1 ) are found in Kashmir (west of ∼ 75 • E), western Nepal (∼ 81 • E), and from western Bhutan (∼ 90 • E) to the east. These lateral variations in exhumation rates have been interpreted as reflecting lateral variations in the presence/absence and geometry (location, height, and dip) of the mid-crustal ramp in the MHT, together with duplex formation and local out-of-sequence thrusting (Hubbard et al, 2021;Dal Zilio et al, 2021;van der Beek et al, 2023). In some of the more slowly exhuming regions, in particular in Bhutan, exhumation rates appear to be decreasing through time, with lower-temperature systems recording lower exhumation rates than higher-temperature systems.…”

Crustal Thickness Variations and their Tectonic Implications beneath the Uttarakhand Himalayas and the adjoining Tectonic Segments: Results from 3-D Tomographic Inversion of Local and Regional Earthquake Data

“…The great Himalayan-Tibetan orogen can be categorized and characterized by six orogen-parallel, fault-bounded litho-tectonic zones along its entire length [Heim and Gansser, 1939;Gansser, 1964;Le Fort, 1975;Hodges, 2000;Najman and Garzanti, 2000;Yin and Harrison 2000;Jayangondaperumal et al, 2018;Thakur et al, 2019]. These zones are longitudinally separated from successively deeper crustal levels towards north [Yin, 2006;Hubbard et al, 2021] by principal intra-continental, north-dipping, crustal scale thrust faults, and all the principal thrust faults root in a mid-crustal, gently northward dipping detachment or manuscript submitted to Tectonics decollement, the Main Himalayan Thrust (MHT) [Zhao et al, 1993;Nelson et al, 1996;Bilham et al, 1997;Hauck et al, 1998;Avouac, 2003;Nabelek et al, 2009;Stevens and Avouac, 2015;Thakur et al, 2019]. From south to north, major bounding faults and the classic litho-tectonic zones are: the Himalayan Frontal Thrust (HFT) also referred as Main Frontal Thrust (MFT), Sub-Himalaya (or outer Himalaya or Siwaliks), Main Boundary Thrust (MBT), Lesser Himalaya, Main Central Thrust (MCT), Greater (or Higher) Himalaya, South Tibetan Detachment System (STDS) also known as Tethyan Thrust (TT), Tethyan Himalaya (or Tibetan Himalaya), Indus-Tsangpo Suture Zone (ITSZ) delimiting the northern boundary of the Indian plate subducting/underthrusting under the Tibet, and the Trans Himalayan Zone (Figure 1) [Allégre et al, 1984;Bendick and Bilham, 2001;Burg and Chen, 1984;Chemenda et al, 2000;DeCelles et al, 2016;Gansser, 1964;He et al, 2015He et al, , 2016Heim and Gansser, 1939;Hodges, 2000;Kohn, 2014;Le Fort, 1975;Searle and Treloar, 2019;L...…”

“…Geological and geophysical observations demonstrate that the structural and seismic segmentation of the Himalayas is governed by lateral variations in geological structure, convergence, shortening rate, pre-orogenic sedimentary thickness, crustal thickness, erosion rates, thermal/exhumation patterns, stratigraphy, tectonic deformation pattern/style, lateral ramps along the main thrust faults, geometry of the MHT, cross-structures (e.g., DHR), and movements along the transverse faults/lineaments (e.g., RPMF, MGDF etc.) [Arora et al, 2012;Bai et al, 2019;Bollinger et al, 2004;Célérier et al, 2009;DiPietro and Pogue, 2004;Eugster et al, 2018;Gahalaut and Kundu, 2012;Gao et al, 2016;Gill et al, 2021;Gillian et al, 2015;Godin and Harris, 2014;Herman et manuscript submitted to Tectonics al., 2010;Heténeyi et al, 2016;Hubbard et al, 2021;Koulakov et al, 2015;Mandal et al, 2023;Murphy et al, 2014;Pandey et al, 1999;Prasad et al, 2011;Robert et al 2011;Stevens and Avouac, 2015;Thakur et al, 2019;Vance et al, 2003;Whipp et al, 2007;Wu et al, 1998;Yadav et al, 2019Yadav et al, , 2021Yin, 2006]. Valdiya [1976] was one of the first who put up the idea that pre-Himalayan heterogeneities in the underplated Indian basement may be the primary reason for these along-strike variations.…”