GENESIS OF THE KAMPA DOME AND IMPLICATIONS FOR MIOCENE-RECENT TECTONICS OF SOUTHERN TIBET

The Kampa Dome, one of the North Himalayan gneiss domes, provides an exceptional view into the relationship between Miocene-Recent upper and middle crustal processes in southern Tibet. This doubly-plunging, fault bounded antiform is cored by a moderately deformed orthogneiss, which is intruded by a suite of leucogranite intrusions of varying compositions, geometries, and finite strain states, ranging from strongly deformed to undeformed. Overlying the granitic core is a carapace of deformed, Carboniferous to Triassic metasedimentary strata, decreasing in metamorphic grade up-section from upper amphibolite facies, garnet + staurolite + kyanite-bearing mylontic paragneisses directly above the contact zone, to lower greenschist facies metasediments roughly 1200m above the contact. The interleaved contact zone is mylontic and contains a consistently outward-verging sense of shear. Mica 40Ar / 39Ar thermochronology yields ~15 to 10 Ma cooling ages which temporally overlap with estimates of the onset of E-W extension in the region. Evidence that this extension has continued through the Quaternary is present along the western range front, where the Kampa Fault truncates glacial valleys, has well preserved fault scarps, and appears to be associated with isoclinal folding of fluvio-lacustrine deposits in the adjacent Kung Ma Basin.

Early N-S contractional structures related to crustal thickening were succeeded by symmetric extensional fabrics at depth and E-W brittle extension in the upper crust. The coeval development of an apparently radial geometry in the shear zone fabrics and intrusion of leucogranites may reflect a complex middle crustal response to top-to-north normal slip along the STDS, top-to-south thrusting on the overlying Gyirong-Kangmar thrust, and E-W extension in the G-K thrust hanging wall during the middle Miocene. This “lateral spreading” and associated crustal thinning promoted decompression melting and diapirism, a process confirmed by the abundance of syn- to post-tectonic plutonism. North-dipping thrusts and west-dipping normal faults with Quaternary movement, and seismic evidence for a partially molten south Tibetan middle crust suggest that the processes that formed the North Himalayan gneiss domes are presently active.