E-W extension and mid-crustal extrusion governed by strike-slip faulting in southern Tibet
Abstract
Unfortunately Tibet is flat and high and far away. The enduring models (e.g. England et Houseman 1989) of the India-Asia plate convergence zone require no fieldwork and typically consider the gravitational potential and low hypsometric variation to infer a viscous "weak" middle and or lower crust - there is no linear transduction of crustal failure stresses to the surface and no crustal scale localisation. It is thought that Tibet is currently flowing "downhill" to the east (e.g. Clark et Royden 2000) and it is generally accepted that a key temporal-spatial switch is the plateau reaching some critical elevation after which the upper brittle plateau cracks up, riding on the eastward flow. Locally high heat flow and geophysical data from project INDEPTH (Nelson et al. 1996) seem to corroborate, imaging possible melts or fluids in a N-S trending graben adjacent to a granitic-gneiss cored range (the NQTL) suggested to have been extruded (core-complex style) from a molten middle crust initiating at ca. 8 Ma and thereby timing initiation of eastward flow. This study presents mapping, structural surveying, palaeostress and geochronologic data from a hitherto largely unrecognised >200 km long, presently active, sinistral wrench fault (the DJSZ) that cuts the continental lithosphere. Gradual southwards migration (<15 km) has exhumed palaeostrands of the fault (to upper greenschist facies) allowing spectacular access to the deep interior of the fault zone (historic portions). The DJSZ was active pre-, syn and post-plutonism to the NQTL and was shearing at greenschist facies at ca. 8 Ma, suggesting the DJSZ has played a major role in the melt influx, growth and emergence of the NQTL. Moreover, extensive evidence for historic DJSZ fluid flow (range of T's) and sub-surface imaging of the present fault architecture suggests that much of the partial melt evidence in southern Tibet is engendered from fluids in shear splays from the DJSZ. This suggests that the tectonic setting in southern Tibet is more boring but more straightforward than previously postulated. Faults are globally known to be weaknesses in the continental crust that limit (govern) its strength and geometries of convergence (e.g. orogenesis) and extension (e.g. basin formation). Tibet is similarly regulated by transcurrent faults (cf. Tapponnier 2001) and, although the middle crust can flow, these dictate the weak creep realms of Tibet's rheological profile. The DJSZ and other fault zone locations are influenced by weaknesses in the pre-collisional terrane assemblage of the Eurasian margin. An overview model of fault growth, death and shear sense reversals related to continental escape indicate the dynamic space-time evolution of the continuing collision.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.V23B..06E
- Keywords:
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- 7205 Continental crust (1242);
- 8010 Fractures and faults;
- 8102 Continental contractional orogenic belts;
- 8159 Rheology: crust and lithosphere;
- 9604 Cenozoic