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The Lunggar Rift

The Lunggar Rift (Kapp et al., 2008) is approximately 70 km long in the north-south direction and about 40 km wide east-west; maximum elevations exceed 6500 m. Along its central region, the Lunggar Range is incised by numerous glaciers. The NE-trending rift valley is an externally drained system that exhibits a drainage divide in its central part with active depocenters located at the north and south tips of the rift valley. The eastern margin of the Lunggar Range is bounded by a <40° east dipping normal fault that juxtaposes mylonitic gneiss and variably deformed leucogranites in its footwall against Paleozoic-Neogene strata in its hanging wall. Foliations in footwall mylonitic gneisses are east-dipping and range between 30-40° with east plunging stretching lineations. Kapp et al., (2008) refer to this fault zone as the Lunggar detachment fault. Locally, the fault zone is exposed and the detachment dips about 30° degrees with top to the east displacement. Based on the map pattern of the exposed detachment footwall, the magnitude of slip is estimated to be about 15 km, perhaps greater. The range-bounding Lunggar detachment continues for approximately 30 km along strike of the central part of the range and branches into higher angle east–dipping normal faults to the north and to the south.

Zircons from footwall mylonitic gneisses and variably deformed leucogranites yield weighted mean U-Pb ages of 9 and 15 Ma. Associated apatite U-Th/He cooling ages range between 0.4 Ma and 0.7 Ma. These data are consistent with the Lunggar detachment footwall experiencing Miocene magmatism followed by rapid cooling in Plio-Pleistocene time. Currently, the range-bounding Lunggar detachment fault is inactive, as indicated by undisturbed moraines and alluvial fans that unconformably overlie it. Younger brittle synthetic and antithetic normal faults are common in the hanging wall basin, where lacustrine rift sediments are cut by moderately dipping normal faults. Active faulting has migrated basinward with time, as indicated by scarp-forming faults located up to 6 km from the inactive range front. The fault scarps trend parallel to the range front, consistent with the active scarp-forming faults linking to active slip on a low-angle detachment fault at depth. This geometric relationship is found in other regions believed to be undergoing active low-angle normal faulting (Axen et al., 1999).

The oldest sediments of the basin are locally exposed and are generally east dipping at about 10°. Basin fill generally consists of interbedded medium- to fine-grained sandstone. Up section, grain size increases to pebbly sandstone and conglomerate that locally displays a matrix-supported texture. Unconformably overlying this sedimentary package are interbedded medium- to coarse-grained pebbly sandstones and cobble to boulder conglomerates that are dominantly clast supported. Clasts are poorly sorted, subangular to subrounded, and clast compositions are dominantly composed of granite and variably deformed gneisses. The relative abundance of cobbles and boulders increases upsection. We interpret this sedimentary succession to represent a transition from lacustrine to locally debris-flow dominated processes, and finally to fluvial dominated depositional environments.

Common Observations of Evolved Rifts: While our understanding of Tibetan rifts remains limited, it appears that the more evolved Tibetan rifts share several common features. (1) Extension is highly localized along or in the proximal hanging walls (<6 km distance in map view) of range-bounding normal faults. This contrasts with Cordilleran metamorphic core complexes, where the hanging walls of the detachments have been extensively attenuated by normal faulting (e.g., Coney, 1980; Wernicke, 1981; Lister and Davis, 1989). (2) The rifts include range-bounding low-angle normal faults that have not been observed to be cut by younger high-angle normal faults.  Geologic and neotectonic relations are not conclusive, but suggest that active slip is occurring along the detachments at the surface or on high-angle normal faults within the rift basin that sole into the down-dip extension of the detachments at shallow structural levels. (3) The more evolved Tibetan rift basins show intrabasinal drainage divides and maximum incision in areas that correspond broadly to the inferred location of maximum extension. This is in contrast to the more nascent rifts in Tibet that are bounded by high angle normal faults and have active, internally drained depocenters located in the central part of the rift, typical of half-graben systems bounded by high-angle faults (e.g., Leeder and Gawthorpe, 1987; Friedmann and Burbank, 1995). 

Our working hypothesis for Tibetan rift evolution is as follows, and is based on the premise that rifts of variable extension magnitude provide different snapshots of a regionally applicable extension process and that slip is actively occurring along the detachment faults at shallow structural levels. In this hypothesis, the rifts initiate as half-graben basins, bounded by a high-angle normal fault that becomes listric and soles into a subhorizontal mylonitic shear zone. With increasing extension magnitude, tectonic unloading results in isostatic rebound of the footwall and back-tilting and eventual abandonment of the up-dip portion of the range-bounding high angle normal fault ("breakaway 1"), as well as upwarping of the normal fault in the mid to upper crust. Progressive development of breakaway normal faults in the proximal hanging wall and upwarping of the master normal fault at depth during continued slip result in a detachment fault that is actively slipping in the brittle upper crust. In the process of breakaway development, portions of the rift basin are captured into the footwall, uplifted, and eroded. Furthermore, footwall isostatic rebound in areas of maximum slip results in uplift and incision of the overlying rift basin and development of an intrabasinal drainage divide. Isostatic rebound of the footwall at depth leading to basinward migration of faulting has been observed elsewhere in regions of thinner crust including the Basin and Range province (Horton and Schmitt, 1998) and western Turkey (Dart et al., 1995).