The Nanga Parbat-Haramosh massif (NPHM) is a unique structural
and topographic high in the northwestern corner of the Himalayan
convergence zone. Previously, the NPHM was thought to be bounded by
the MMT, a thrust along which the Kohistan-Ladakh island arc was
obducted onto the northern margin of fridia. This study presents field
evidence for the existence of the Raikot fault, an active dextral
reverse fault which has truncated the MMT and forms the western boundary
of the NPHM. The Raikot fault separates medium-grade Mesozoic to
mid-Cenozoic mafic metasediments and intrusive rocks of the Kohistan
sequence from high-grade Proterozoic metasediments and orthogneiss of
the Nanga Parbat group. The Kohistan sequence rocks have experienced
one tight to isoclinal folding event probably associated with the
obduction of the island arc, and a second folding event associated
with movement on the Raikot fault. The Nanga Parbat group rocks were
transposed by a pre-Himalayan isoclinal folding event and have subsequently
been folded around east-trending axes in the early Cenozoic by the obduction of Kohistan, then around north-trending axes in the late
Cenozoic, associated with the uplift of the NPHM and the initiation of
the Raikot fault. The Raikot fault consists of both mylonite zones and
numerous major and minor faults. Slickensides and mylonitic lineations
both indicate dextral reverse slip.
The Raikot fault and associated folding appear to have accommodated
15 to 25 km of uplift in the late Cenozoic. The concentration of
the uplift and involvement of the Moho suggests that the Raikot fault
follows a major crustal structure, possibly a pre-collision Indian
plate boundary. If this is the case, rotational underthrusting of
greater India along the MMT would require dextral slip along the
Raikot fault. It is proposed that the Raikot fault is a terminal tear
fault on the MCT.
Four ages of glaciation are recognized in the area, and correlated
with regional glacial chronologies. Numerous Holocene offsets of
glacial deposits and alluvial fans occur. A till surface attributed to
the most recent major advance has been offset approximately 160 m
vertically by strands of the Raikot fault. Analysis of the offsets in
combination with thermoluminessence dating yield a maximum Holocene
uplift rate of 3.9 mm /yr. This uplift rate is compatible with published
values derived from fission-track cooling rates.
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