Middle to late Miocene extremely rapid exhumation and thermal reequilibration in the Kung Co rift, southern Tibet

Abstract

The Kung Co rift is an approximately NNW striking, WSW dipping normal fault exposed in southern Tibet and is part of an extensive network of active approximately NS striking normal faults exposed across the Tibetan Plateau. Detailed new and published (U‐Th)/He zircon and apatite thermochronometric data from the footwall of the early Miocene Kung Co granite provide constraints on the middle Miocene to present‐day exhumation history of the footwall to the Kung Co fault. Inverse modeling of thermochronometric data yield age patterns that are interpreted as indicating (1) initiation of normal fault slip at ∼12–13 Ma and rapid exhumation of the footwall between ∼13 and 10 Ma, (2) acceleration of normal fault slip at rates of 21.9–6.9 mm/yr at ∼10 Ma, (3) rapid thermal reequilibration between 10 and 9 Ma, and (4) slow exhumation and/or quiescence from ∼9 Ma to the present day. Hanging glacial valleys in the footwall and fault scarps that cut late Quaternary till and moraine deposits indicate that fault slip continues today. Middle to late Miocene initiation of extension across the Kung Co rift is broadly the same as the documented initiation of EW extension across the south central Tibetan Plateau. Eastward flow of middle or lower crust from beneath Tibet accommodated by northward underthrusting of Indian crust beneath Tibet provides a plausible explanation for the onset of EW extension across the Tibetan Plateau.

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“…15 Ma. The timing of extension in this region is broadly the same as that documented farther east in southcentral Tibet (Lee et al, 2011), and therefore we interpret this to be a regional event, rather than a local event associated with transtensional deformation along the Karakoram fault.…”

Present‐day crustal thinning in the southern and northern Tibetan Plateau revealed by GPS measurements

“…Dating of grabens throughout Tibet, but mostly in southern Tibet, has yielded onsets of such normal faulting since 15 Ma, and in some cases closer to 10 Ma. For southernmost Tibet near the Himalaya, dates for different grabens from west to east are 9 Ma [Murphy et al, 2002;Saylor et al, 2010], 14-13 Ma [McCallister et al, 2014,~11 Ma [Garzione et al, 2003],~13-12 Ma [Lee et al, 2011], 13-12 Ma [Kali et al, 2010], and <12.5 Ma [Edwards and Harrison, 1997]. Farther north but still in southern Tibet, similar ages include 11-8 Ma [Woodruff et al, 2013], 16-12 Ma [Styron et al, 2013], and~8 Ma [Harrison et al, 1995;Pan and Kidd, 1992].…”

Investigation of Coulomb stress changes in south Tibet (central Himalayas) due to the 25th April 2015 M W 7.8 Nepal earthquake using a Coulomb stress transfer model

“…A normal fault near the Gyirong basin (NGF) dips 50°-70°to the west (Yang et al 2009). The Kung Co fault (KCF) refers to the Kung Co rift normal fault, dipping *70°roughly to the west (Lee et al, 2011). Another two normal faults, the Kharta fault (KF) and the Dinggye fault (DgF), bound the Ama Drime Massif.…”

“…Dips and depth data here are set according to Baltz (2012), Yang et al (2009), Lee et al (2011), and Kali et al (2010). Strikes and rakes here are set according to this model and normal fault are associated with heightened seismicity rates for aftershocks following the Nepal earthquake.…”