Evidence for constriction and Pliocene acceleration of east‐west extension in the North Lunggar rift region of west central Tibet

Abstract

[1] The active north trending North Lunggar rift in west central southern Tibet exposes an extensional metamorphic core complex bounded by an east dipping low-angle normal fault. Apatite and zircon (U-Th)/He thermochronology and thermal modeling of the North Lunggar rift document a minimum timing for rift inception at >10 Ma and rapid footwall exhumation between 5 and 2 Ma. Miocene footwall cooling and exhumation rates were initially slow to moderate at <50°C Ma À1 and <1 mm a À1 , followed by increased Pliocene rates as high as >400°C Ma À1 and 4-10 mm a À1 . Footwall isotherms were significantly compressed during rapid exhumation resulting in an elevated transient geothermal gradient between 50 and 90°C km À1 . The minimum magnitude of horizontal extension for the North Lunggar rift is 8.1-12.8 km; maximum is 15-20 km, less in the south at~10 km. Mean Pliocene extension rate is 1.2-2.4 mm a À1 in the~120°direction. Results for the North Lunggar rift are similar in magnitude, rate, and orientation of slip to the kinematically linked Lamu Co dextral strike-slip fault to the north. This suggests a state of constrictional strain during Pliocene time along this stretch of the Bangong-Nujiang suture from which the Lamu Co fault emanates. The onset of extension in this region may be explained by crustal thickening and gravitational orogenic collapse, followed by accelerated rifting resulting from localized crustal stretching and increased magmatic activity, potentially driven by the position and northward extent of underthrusting Indian lithosphere.

Tibetan Magmatism Database

“…The 50 Ma event has been attributed to break‐off of the subducted Neo‐Tethyan oceanic slab after initial India‐Asia collision (Yin & Harrison, ; Zhu et al, ). The 15 Ma event has been attributed to delamination of Asian mantle lithosphere concomitant with and perhaps accelerated by underthrusting of Indian lithosphere (Chung et al, ; Sundell et al, ).…”

Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir

“…For example, northward subduction and underthrusting of India under Asia has contributed to Cenozoic crustal thickening and east-west extension in Tibet (DeCelles et al, 2002;Sundell et al, 2013) and most likely in the Pamir. If this is correct, the present-day location of the Indian slab under the Hindu Kush (Negredo et al, 2007) requires southward slab rollback following underthrusting, as proposed for Tibet during the Oligocene-Miocene .…”

Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet

“…7). A compilation of extension onset and acceleration ages (Sundell et al, 2013) indicates that the majority of extensional structures initiated between 15 and 11 Ma. Some structures may have initiated earlier, such as the ≥16 Ma Lopukangri fault, which crosscuts the Great Counter thrust system ~250 km to the west (Laskowski et al, 2017).…”

“…Orogen-parallel extension mainly initiated during Miocene time (16-12 Ma) along the suture zone in southern Tibet, providing a minimum age constraint for the Great Counter thrust system (Sanchez et al, 2013;Sundell et al, 2013;Laskowski et al, 2017). In addition, the ca.…”

“…7). It is possible that this switch was related to renewed Greater Indian underthrusting, which might have produced critical gravitational potential energy in the upper crust via simple-shear crustal thickening (e.g., Sundell et al, 2013;Styron et al, 2015).…”