CHANGES IN TETONICS AND CLIMATE ACROSS THE HIMALAYAN-TIBETAN OROGEN, NORTHERN INDIA REFLECTED IN CONTEMPORANEOUS FAST AND SLOW EXHUMATION (CAN WE FIND THIS CONTRAST IN NEW ENGLAND?)

There is a pronounced precipitation gradient across the Himalayan-Tibetan orogen and major crustal-scale thrust faults have episodically propagated southwards from the High Himalaya towards the foreland since late Miocene-Pliocene time; out-of-sequence reactivation of faults occurred in Plio-Pleistocene time. In northern India, the High Himalaya of Lahul lie in the transition zone between the monsoon-influenced southern flank of the orogen and semi-arid to arid ranges to the north, and in Lahul, coupling of tectonic and surface processes has produced rapid erosion and uplift. Apatite helium ages record exhumation of amphibolite- and greenschist facies rocks from shallow crustal levels to the surface at 1-2 km/m.y. during the past 2.5 m.y. The mean rate of bedrock incision of the Chandra River (a major trunk stream) and its tributaries (n=15) is 3.5±1.3 km/m.y., with some reaches of the Chandra incising as fast as 12 km/m.y. In contrast, the Ladakh Range to the north has been tectonically quiescent since the Early Miocene and mean annual rainfall across the Range is < 500 mm/yr. Erosion rates determined using cosmogenic ¹⁰Be across the southern slope of the Ladakh Range from the range divide towards the Indus Valley are extremely slow: erosion rates for bedrock tors along the range divide, bedrock along interfluvial ridge crests, and in unglaciated tributary catchments are 5-13, < 1, and ≤ 2 m/m.y., respectively. Trunk streams here are eroding at 20-40 m/m.y. The absence of active tectonics and reduced geomorphic activity within the orographic rain shadow account for this extremely slow denudation; metamorphic rocks have remained at depth since the Miocene. As the crow flies, Lahul and Ladakh are less than 200 km apart.

In the Late Devonian following Acadian prograde metamorphism, deep crustal levels exposed in domes in the Rowe-Hawley Zone of western New England were exhumed from depths of 30-35 to 16-21 km at a rate of 1-2 km/m.y. (derived from petrology and zircon U-Pb and hornblende argon ages). Similar post-Acadian exhumation occurs in amphibolite facies rocks southwestwards to New York City. South of this belt towards Long Island Sound, amphibolite facies rocks along-strike record very slow cooling and amphibole ages are Middle Mississippian. Can this difference be explained in terms of climate and surface processes?