REGIONAL GRAVITY AND MAGNETIC DATA HELP MAP SUBSURFACE GEOLOGY OF AFGHANISTAN

The U.S. Geological Survey, U.S. Naval Research Laboratory and Islamic Republic of Afghanistan Ministry of Mines and Industries conducted a regional airborne geophysical survey over much of Afghanistan during the summer of 2006. These data were merged with higher resolution existing data sets. The resulting gravity and magnetic data provide a new view of the subsurface geology of Afghanistan that can be used to aid resource and hazard assessments of the country as well as help unravel its tectonic history. The gravity data can be used to identify basins critical for petroleum and hydrologic studies. The magnetic data can detect accreted arc terranes and Precambrian crystalline basement of interest to mineral resource studies. Both data sets map faults critical to hazards assessments. Because the Bouguer gravity anomaly map is dominated by a gravity low associated with the low density root of the Hindu Kush Range, we applied an isostatic correction in order to isolate upper-crustal sources. The magnetic data were reduced to the pole to center anomalies over their geologic sources. Depths to magnetization sources were calculated with the Euler method.

The most striking observation in the gravity data is the lack of an expected large gravity low over the Helmand Basin. Instead are a few 30-60 km diameter, 10-30 mGal isostatic gravity lows that may be interpreted as small basins or as batholiths. This suggests that the oil and gas potential could be lower than previously thought. Instead, shallow crystalline basement indicated by the magnetic data suggests the possibility of a continuation of arc volcanic rocks associated with copper deposits across the southern border with Pakistan. In addition, magnetic highs associated with arc rocks in Iran that host copper deposits seem to continue into Afghanistan. Prominent faults in Afghanistan, notably the Chaman, Helmand and Hari-Rud, separate highly magnetic from non-magnetic rocks which will allow modeling of fault dip. Most of Afghanistan, with the exception of Northern Afghanistan which is part of the Eurasian plate, is composed of accreted Gondwanan terranes. The pseudo-gravity map enhances the long-wavelength component of the magnetic data and appears to show these major terranes.