Defining the Main Central Thrust in the Hazara-Kashmir Syntaxis,
Northern Pakistan: Evidence from New Isotopic Data
Abstract
Integrating detrital zircon U/Pb and whole-rock Nd data throughout the
Himalayan arc provides the means to distinguish between the Tethyan
(TH), Greater (GH), and Lesser Himalayan (LH) tectonostratigraphic zones
within the thrust belt. In the Kaghan valley of northern Pakistan,
debate exists over the existence and location of the Main Central thrust
(MCT) and the tectonostratigraphic affinity of the rocks in the
Hazara-Kashmir syntaxis. Three new detrital zircon U/Pb age spectra and
two new εNd(0) values from the footwall rocks of the
Batal fault yield 1.0-2.0 Ga age populations and an average
εNd(0) value of -13.6. Four new detrital zircon U/Pb age
spectra and two new εNd(0) values from the hanging wall
rocks have primarily <1.2 Ga age populations and an average
εNd(0) value of -16.7. Most detrital zircon analyses
exhibit Pb-loss from younger (<600 Ma) intrusive or
metamorphic events. The absence of zircon <1.0 Ga in the
footwall samples indicates that they are LH rocks, while
εNd(0) values indicate that they are not
Paleoproterozoic LH rocks, but are comparable to other Meso- and
Neoproterozoic LH rocks along the Himalayan arc. Neoproterozoic and
younger detrital zircon age populations from the hanging wall samples
and the presence of ~47 Ma intrusive leucogranite
indicate that these are either TH or GH rocks, with
εNd(0) values also consistent with TH or GH values.
Combining these data show that the hanging wall rocks are either TH or
GH rocks, and that the footwall rocks are Mesoproterozoic LH rocks. In
most places, the MCT is defined as GH rocks thrust over LH rocks.
However, in NW India, the GH/LH contact is buried in the subsurface, and
the MCT at the surface is a TH/LH contact. Therefore, these data define
the Batal fault in the Kaghan valley as equivalent to the MCT, with
TH/GH rocks thrust over LH rocks, and link it with the Indus River
valley to the west in Pakistan and NW India to the east.