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

Abstract

The Tibetan plateau is host to numerous ~N‐S striking graben that have accommodated E‐W directed extension. The development of these structures has been interpreted to reflect a variety of different geological processes including plateau collapse, oroclinal bending or mid‐to‐lower crustal flow. New 40Ar/39Ar thermochronology and quartz c‐axis data from the Thakkhola graben of west‐central Nepal show that E‐W extension was ongoing at least locally by the early Miocene (ca. 17 Ma). Our new, and previously published chronologic information on the initiation of graben across the orogen shows that they typically developed immediately after cessation of the South Tibetan detachment system, a structural network that facilitated differential southward movement of the upper and middle crust. We interpret this fundamental switch in orogen kinematics to reflect recoupling of the middle and upper Himalayan crust such that the subsequent widespread flow of the mid‐to‐lower crust out of the system to the east forced brittle accommodation in the upper crust.

Contact metamorphism of the Tethyan Sedimentary Sequence, Upper Mustang region, west-central Nepal

“…Deformations recorded in the metamorphic rocks are unlikely to be related to movement on the STDS as there is no such fault documented in the area (Guillot et al 1995b;Hurtado, 2002;Larson et al 2019) and the closest documented exposure is nearly 50 km south of the present study area in the Annapurna region. There, Godin et al (2001) argue that final movement on the STDS occurred at c. 22.5 Ma based on the ages of weakly deformed dykes that intrude across the shear zone.…”

“…The graben extends from the STDS in the south almost to the Indus Tsangpo suture zone in the north (e.g. Hurtado et al 2001;Searle, 2010) and has accommodated E-W extension since at least middle Miocene time (Coleman & Hodges, 1995;Hurtado, 2002;Larson et al 2019). The Upper Mustang region hosts two adjacent plutonic bodies, the Mugu and the Mustang granites (Le Fort & France-Lanord, 1995).…”

“…The geometry of the Mugu pluton beneath the surface is unknown and therefore aerial distances to the nearest observed contact were used for data interpretation. At the contact, the Mugu pluton intrudes and crosscuts the schists with no evidence of a shear zone in the vicinity (Larson et al 2019).…”

“…Microtextural observations within the metapelites, such as the rotation and fragmentation of porphyroblasts, indicate that deformation in these rocks occurred after peak metamorphism, c. 19.4 Ma, for the youngest specimen. This deformation is interpreted to be related to the E-W extension across the adjacent Thakkhola graben (see Larson et al 2019), which initiated as early as c. 17 Ma (Guillot et al 1999;Hurtado, 2002;Larson et al 2019). However, the igneous specimens do not record extension-related deformation, which may reflect the strong rheological contrast between the graphite-rich pelitic schists and the leucogranite (e.g.…”

Timing and Source of Melting at the Eastern Edge of the Gurla Mandhata Core Complex, Nw Nepal Himalaya

“…The initiation of E-W extension generated extensional features across the Himalaya (e.g. Murphy et al, 2002;Murphy and Copeland, 2005;Hintersberger et al, 2010;Antolín et al, 2012;Nagy et al, 2015;Larson et al, 2020), coeval with the Karakoram fault system overprinting the South Tibetan detachment and ultimately kinematically linking with the Gurla Mandhata-Humla fault system between 14 Ma and 11 Ma (Fig. 5.9D) (Searle et al, 1998;Murphy et al, 2000;Murphy and Burgess, 2006;Leech, 2008).…”