Is there any evidence for tectonic denudation of the Nanga Parbat massif?

No part of the eastern [from Chhichi Nullah to Stak-la] and western [from Diamari to Sassi] margins of the NPHM shows any strong or widespread evidence of ductile strain that would permit extensional motion of the Kohistan/Ladakh cover relative to the Nanga Parbat massif core. Either the mylonites mostly show sense-of-shear indicators for an opposite [thrust] component of motion [Sassi, Liachar] or the motion is near strike-slip [eastern margin]. While some of the brittle faults near and on the western margin near Sassi do have dip-slip displacement appropriate for denudation of the massif, there is no reason to suppose that they have been rotated significantly from their present steep attitude. Additionally, 1) they do not have a systematic relation to an immediately underlying mylonitic shear zone, 2) do not have the same sense or direction of shear as nearby mylonites, 3) nor do they accompany an abrupt and major decrease in metamorphic grade across the brittle fault like well-known core complex denudational faults. West of Diamiri [Bunar Gah], the MMT zone possesses a prominent NW-directed normal shear sense ductile component of strain, which would permit denudation of the Indian cover metasediments and basement to the southeast [Hubbard et al, 1996]. Dating suggests that this motion is too old [>20Ma] to explain the denudation of the NPHM, the bulk of which has to be <10Ma [ Zeitler , and others, numerous refs]. We have not observed this fabric where the MMT mylonites are exposed farther north around the western margin of the NPHM massif, nor along the eastern margin. Perhaps it has been overprinted by ductile strain in the mylonites related to initial rise of the NPHM; dating of granitoid rocks with various structural relations to these MMT mylonites, now in progress, may resolve this question. In lower Rupal Valley, and extending into Chhichi Nullah, a newly recognised major ductile shear zone has SE-directed thrust sense of shear [Nanga Parbat core up], along with a component of dextral shear [ Photos 5, and 6]. If it has been strongly rotated [overturned] by subsequent folding, it could have originally allowed significant denudation of the core of the NPHM. However, here too, the shear zone does not pass “up” into a brittle capping fault, and there is no decrease in apparent metamorphic grade at the margin of the shear zone, and we therefore conclude that it probably does not have denudational significance. Dating of the granitoid protolith of this shear zone, and of a granitic dike cutting the shear fabric, may permit a firmer conclusion about this issue. Near the upper end of the Rupal Glacier, some evidence of SW-directed extensional ductile strain has been outlined by reconnaissance observations. Again, dating of the deformed rocks, and more extensive mapping, may help to focus the significance of this occurrence. In general, at present, we find the lack of convincing and extensive evidence for tectonic denudation of the NPHM to be the first-order conclusion.

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