GEOCHRONOLOGICAL CONSTRAINTS ON SYNTAXIAL DEVELOPMENT IN THE NANGA-PARBAT REGION, PAKISTAN

Ar-40/Ar-39 data Olornblende, biotite, muscovite, and K-feldspar) and U/Pb data (zircons) were obtained from the Nanga Parbat-Haramosh Massi f (NPHM), NW Pakistan, along three transects in the southern regions o f the NPHM. We have based our interpretations on our new data as well as geochronologic dates from previous studies in the northern regions of the massif. Geochronologic data show that the NPHM has experienced exceptionally high denudation and cooling rates over the past 10 m.y. U/Pb ages determined through sensitive high-resolution ion microprobe (SHRIMP) ''depth-profiling'' experiments on metamorphic zircons and co nventional U/Pb monazite dates suggest that the timing of metamorphism varied across the massif. In addition, we have documented that the ma ssif has experienced postmetamorphic, differential cooling both along and across strike. Thermochronologic data on currently exposed surface rocks suggest that cooling occurred more recently and at greater rate s in the south-central regions of the massif (representing deeper crus tal levels) than along the margins and northern regions of the massif. Within the Tate region, cooling following peak metamorphic temperatur es of 600 degrees-700 degrees C was as high as 140 degrees C/m.y. foll owing partial melting of pelitic units. Biotites from this area record plateau ages of 0.9 +/- 0.1 Ma. Along the Astor and Indus gorges, coo ling was less rapid (approximately 70 degrees-80 degrees C/m.y.) follo wing peak metamorphism as indicated by U/Pb monazite ages of 6-8 Ma an d Ar-40/Ar-39 muscovite cooling ages of 2.2-3.4 Ma. Cooling over the l ast 3 m.y. occurred at rates of 100 degrees-140 degrees C/m.y. The ove rall cooling age pattern within the massif is interpreted syntaxial gr owth through the development of north plunging antiforms prior to 3 Ma , followed by reverse faulting along east dipping fault zones. Along t he Raikot River transect the biotite cooling age pattern is consistent with the folding of isotherms during folding of the foliation surface s. The age pattern was disrupted at 1 Ma due to faulting along the Rai kot and Tate faults.