AmericanChemicalSociety.com

We are interested in active deformation within the interior of Tibet because it is thought that this region represents a modern analogue to Basin and Range style deformation, and serves as an excellent natural laboratory to test many aspects of extensional tectonics, and locally, basin formation.

The North Lunggar Rift is one of seven north-trending extensional structures located within the Tibetan plateau, and has been characterized as a youthful, active metamorphic core-complex in western Tibet (Kapp et al., 2008) (Fig. 1).  The Lunggar Shan is ~70 km long north-south and 30 km wide east-west.  The adjacent rift valley to the east is ~60 km long north-south and 10-20 km wide east-west.  The range is bounded on its east side by a low-angle normal fault that is mapped as inactive at the surface as it is unconformably overlain by cobble-boulder conglomerates and glacial moraines.  Active extension is located within the rift valley approximately 5-6 km east of the range front – high angle antithetic and synthetic faults strike subparallel to the inactive range-bounding low-angle detachment, and exhibit up to 50 meters of vertical displacement.  Located midway north – south in the rift valley is a drainage divide, locally forming an externally drained system with active depocenters at the rift tips to the north and south.  This is atypical for youthful internally drained half-graben rift systems.  Within the central portion of the footwall there is a large package of fine-grained sedimentary rocks of lacustrine facies with growth strata geometry with stratal dips steepening toward the rift valley, which is consistent with coeval deposition and fault slip along a listric normal fault. 

Observations from the North Lunggar Rift are consistent with many aspects of the rolling-hinge model described by Wernicke and Axen (1988).  Aspects of this model were tested using apatite and zircon (U-Th)/He thermochronology using the sampling strategy of Stockli (2005).  Bedrock samples were collected every kilometer horizontally, or 100 meters vertically.  Apatite and zircon thermochronology analyses were conducted at the University of Kansas.  Figure 2 shows cooling age-elevation plots from the North Lunggar footwall.  Apatites from this area have been problematic yielding anomalously old ages because they are prone to zircon inclusions, typical of apatites from Tibetan rifts.  However, zircons are of excellent quality.  There is no correlation between age and elevation for any of the vertical transects – elevation invariant cooling ages are observed for all transects.  Zircons yield reproducible ages consistent with rapid exhumation between 3-5 Ma, with youngest ages in the central part of the range.  Zircon’s thermal sensitivity window is between 5-7 kilometers, resulting in rapid uplift rates between 1.2 and 2.1 mm/yr.  There is no correlation between age and grain size, which shows the rift footwall was rapidly cooled. Detrital cooling ages obtained from the rift basin are between 7-10 Ma.  Since these rocks had to pass be exhumed, eroded, and deposited, this places a minimum age for the onset of extension for the North Lunggar Rift.          

For structural evolution our preliminary interpretation begins with an internally drained half graben (Kapp et. al., 2008).  As the system develops with further tectonic unloading of the footwall, isostatic rebound of the footwall at depth results in a change from an internally to externally drained rift valley.  High angle normal faults are back rotated and deactivated in response to the isostatic rebound of the footwall at depth, consistent with lacustrine sediments stranded in the footwall and overlain by cobble-boulder conglomerates.  The North Lunggar Rift is not consistent with the Gawthorpe and Leeder (2000) style of extension and rift basin development because the youngest cooling ages are located within the central portion of the range in the area of maximum inferred displacement.  Our estimate for the timing of the onset of extension is not precise, as it is only constrained by 6 multiple aliquot detrital samples.  A large detrital dataset is currently being analyzed by our collaborators at Austin.  We plan to model the higher temperature thermal history using Ar/Ar combined with our existing apatite and zircon low-temperature data to yield a precise estimate for the timing of the onset of extension of the North Lunggar Rift.  In summary, the North Lunggar Rift is an active, youthful metamorphic core-complex at an early stage in its development displaying many characteristics of a rolling-hinge evolution.  Apatite and zircon (U-Th)/He data indicate rapid cooling between 3 and 5 million years, with an exhumation rate between 1.2 and 2.1 mm/yr.  The timing of onset of extension of the North Lunggar Rift is roughly bracketed between 7 and 10 Ma.  The next step is to conduct thermal modeling of (U-Th)/He results with the addition of biotite ⁴⁰Ar/³⁹Ar and K-feldspar MDD data to determine the entire thermal history of the North Lunggar Rift, which we can then compare with previous estimates for the timing of the onset of extension in southern Tibet.  Our results are further applicable to understanding the development of supradetachment basins (Friedman and Burbank, 1995), which is currently based on the records from Tertiary structures which are fragmented by the eastern California shear zone.