INVESTIGATING BASEMENT INVOLVEMENT IN THE ZAGROS FOLD-AND-THRUST BELT USING S-P DELAY TIME MAPPING OF SEISMICITY AND RECEIVER FUNCTIONS

The Zagros in Iran represents a relatively juvenile stage in the Himalaya-Alpine continental collision. Despite decades of studies, whether basement-involved thrusting is confined to the hinterland (High Zagros) or is also occurring in the foreland (Zagros Fold-and-Thrust belt) is still debated. In the foreland, a ~10-km-thick sediment package has taken up much of the surface shortening by faulting and mostly folding, and is separated from the basement by the Hormuz salt layer. Lateral variations of the thickness of the Hormuz salt and depth to basement are not well resolved and hold a key for determining the deformation style; shortening only in the sediments, in particular in the Zagros through extensive detachment folding, implies thickening of the salt layer at the core of the folds and thinning below synclines, while basement-involved shortening implies no salt overthickening and seismicity extending into the basement. Synthetic modeling shows that thinner versus overthickened Hormuz salt is possible to resolve using high-frequency P receiver functions, and preliminary receiver functions from broadband and short-period stations show possible signatures of the top of the Hormuz salt and basement interface. Receiver function arrivals are typically mapped to depth assuming a velocity profile, but the raw measurement is a delay time between the initial P and converted S pulse. We map local seismicity within the crustal structure by plotting events' S-P delay times on delay time cross sections of receiver functions to determine whether seismicity falls into the Phanerozoic sedimentary package or the basement.