Active intraplate strike-slip faulting and transpressional uplift in the Mongolian Altai

Abstract

The Mongolian Altai is a Late Cenozoic intraplate strike-slip deformation belt which formed as a distant strain response to the Indo-Eurasian collision over 2000 km to the south. We report results from 5 weeks of detailed fieldwork carried out during summer 2000 in northwestern Mongolia investigating the crustal architecture of the Altai at latitude 48°N. The region can be divided into discrete Cenozoic structural domains each dominated by a major dextral strike-slip fault system or range-bounding thrust fault. Gentle bends along the major strike-slip faults are marked by transpressional uplifts including asymmetric thrust ridges, restraining bends, and triangular thrust-bounded massifs. These transpressional uplifts (Tsambagarav Massif, Altun Huhey Uul, Sair Uul, Hoh Serhiyn Nuruu, Omno Hayrhan Uula, Mengildyk Nuruu) comprise the highest mountains in the Mongolian Altai and are structural and metamorphic culminations exposing polydeformed greenschist-amphibolite grade basement recording at least two phases of Palaeozoic ductile deformation overprinted by Cenozoic brittle structures. Cenozoic thrust faults with the greatest amounts of displacement bound the W and SW sides of ranges throughout the region and consistently verge WSW. Each major range is essentially a NE-tilted block and this is reflected by asymmetric internal drainage patterns. Many faults are considered active because they deform surficial deposits, form prominent scarps, and define range fronts with low sinuosity where active alluvial fan deposition takes place. Reactivation of the prevailing NW-striking, NE-dipping Palaeozoic basement anisotropy is a regionally important control on the orientation and kinematics of Cenozoic faults. At first order, the Altai is spatially partitioned into a low-angle thrust belt that overthrusts the Junggar Basin on the Chinese side and a high-angle SW-vergent dextral transpressional belt on the Mongolian side. The mechanically rigid Hangay craton and Junggar basement block which bound the Altai on either side have played a major role in focusing Late Cenozoic deformation along their boundaries and within the Altai. The geometric relationship between rigid block boundaries, Palaeozoic basement structural anisotropy, and the dominantly NE SHmax (derived from India’s continued NE indentation) has dictated the kinematics of Late Cenozoic deformation in the Altai, Gobi Altai, and Sayan regions.

How did the Alxa Block respond to the Indo-Eurasian collision?

“…Because southern Mongolia and northwestern China belong to the Paleozoic Central Asian Orogenic Belt (Sengör and Natal'in 1996;Li 2006;Windley et al 2007), many early structures were easily reactivated, and our fieldwork shows that almost all faults active during the Cenozoic around the Alxa Block were superimposed on earlier ductile shear zones. Similar fault reactivation occurred in Mongolia (Cunningham et al 2003;Lamb et al 1999;Webb and Johnson 2006;Graham et al 2001;Johnson 2004); therefore although the compression was in a roughly north-south direction, the angle between the ''conjugate'' faults (direction of compression) was not acute but obtuse (Fig. 18).…”

“…Due to the coeval important tectonic activities in the northern QinghaiTibetan Plateau, the Alxa Block inevitably underwent strong compression from the plateau, and the Altyn Tagh fault did not go into the Alxa Block. Due to the obstacle of the rigid Hangay Block in central Mongolia at the same time (Cunningham et al 2003) and the compression of the northern plateau northwards, a large-scale set of ''conjugate ''shear faults developed along the southern Alxa Block and to the south of Hangay Block (Fig. 18); they are the right-lateral strike-slip faults along the Mongolian Altai mountain, the NE-trending left-lateral strike-slip faults in central Mongolia to the north (Cunningham et al 2003(Cunningham et al , 2005, the right-lateral strike-slip southern Alxa fault and the left-lateral Altyn Tagh fault to the south, and the JintaHuahai basin, and in consequence the Beishan area underwent north-south compression as mentioned above (Figs.…”

Late Cenozoic Basin Opening in Relation to Major Strike‐Slip Faulting Along the Porto–Coimbra–Tomar Fault Zone (Northern Portugal)

“…In general, large-displacement faults produce wide deformation zones either by extension or by compression. Wide damaged zones develop a complex internal geometry, which may influence the intrinsic behaviour of major faults (Cunningham et al, 2003). These structures comprise upward-diverging faults, typically cutting antiformal push-ups or synformal pull-aparts, that normally form an anastomosed network of faults, where strike-slip faulting is one of the most important deformation mechanisms (Cabral, 1989;Woodcock & Schubert, 1994;Dooley & McClay, 1997;Rahe et al, 1998).…”

“…The basement of intra and/or interplate settings consists mainly of exposures of highly deformed crystalline rocks, often with a smooth topography (Cabral, 1989;Ribeiro et al, 1996;Bonnet et al, 2000;Cunningham et al, 2003). According to recent data from studies on surface processes and topographic relief of the formation of basement rocks in a collisional framework, the gravity collapse during the orogenic late stage of evolution is an important mechanism by which the elevation of mountain chains is strongly reduced (Burbank & Anderson, 2000;Summerfield, 2000).…”

Geological evolution of Central Asian Basins and the western Tien Shan Range

“…The Jurassic -Cretaceous was characterized by a general planation of the previously formed relief (e.g. Makarov 1977;Chediya 1986;Burbank et al 1999;Allen et al 2001;Cunningham et al 2003;Jolivet et al 2010Jolivet et al , 2013Jolivet et al , 2015, providing sediments to these newly forming extensional basins (Brookfield & Hashmat 2001;Klett et al 2006;Fü rsich et al 2015;Brunet et al 2017).…”