New paleomagnetic results from the northern margin of the Kohistan Island Arc

Abstract

Different phases of remanent magnetizations have been identified in the Cretaceous–Tertiary rocks collected from the northern margin of the Kohistan Island Arc, northern Pakistan. Among them, a magnetite-related component is recognized as the most useful one because of its relatively stable behaviour and widespread presence in the volcanics and red beds. Calculation of mean direction for this component reveal two visible groupings in terms of paleomagnetic directions (Yasin + Baris Group: D = 341.6o, I = 10.6o, α 95 = 23.3 o, k = 11.7, N = 5; Sandhi Group: D = 28.4o, I = −27.4o, α 95 = 32.5 o, k = 8.96, N = 4). The fold tests applied to both these groups are inconclusive, indicating a syn-folding to post-folding origin for this component. A set of inclinations from this study provide new insight into post-collision northward displacement of the Kohistan Arc with respect to its surrounding terranes. Reliability of the paleomagnetic declinations from this study is not fully guaranteed, but when compared with previously reported paleomagnetic directions, a systematic increase in counter-clockwise deflections towards west has been detected along this paleo-island arc. This trend of declinations is well matched with the extrusion model of Asia, where counter-clockwise rotation has been suggested for the tectonic terranes around Western Himalaya. Another important observation is a divergence in paleomagnetic declinations across the rivers, which may indicate the presence of faulted zones under the cover of flowing water. This aspect can be compared to recent geological interpretations that Kohistan may not have acted as a rigid block following its collision with India but may have been deformed through localized shears and faults.