CRUSTAL SCALE GEOMETRY OF THE ZAGROS FOLD-THRUST BELT: BASEMENT OR NO BASEMENT?

The Zagros orogen is known for its spectacular fold trains, detached on lower Cambrian salt, which control the characteristic valley and ridge topography of the region. Also characteristic of the orogen are widely distributed earthquakes that are similar in magnitude, (m_b~5-6), depth (11± 4), and geometry (45°-60° NE-dipping nodal planes) which suggest basement deformation. The overall morphology of the Zagros fold-thrust belt is two broad 200-300 km long salients (Lorestan in the NW and Fars to the SW) separated by the Dezful Embayment. While the Zagros deformation front is a linear feature, the sinuous morphology of the fold-thrust belt is a result of a 6 km structural step in the regional elevation of the Asmari Limestone that corresponds to a pronounced step in topography. This mountain front flexure (Falcon, 1961) is coincident with the southwestern limit of the fold-thrust belt in the Lorestan and Fars region but is 100-150 km north of the southern edge of deformation in the Dezful Embayment. Balanced cross sections across three segments of the Zagros fold-thrust belt are used to evaluate the geometry of deformation and the necessity of basement involved thrusting. Cross sections suggest that the sinuous morphology of the fold-thrust belt is controlled by the depth to detachment in each region. Depth to detachment in the frontal portion of the Lorestan and Fars salients is within the lower Cambrian salt (12 km) or at a depth of ~20 km within the basement. Within the Dezful Embayment, the depth to detachment is 6-7 km, which deepens northward to 10-15 km in the region of the mountain front flexure. The structural step is sufficient to permit basement fault-bend folds throughout the Lorestan and Fars regions. However, basement involvement as suggested by earthquake depth and focal mechanisms is not required. Mass balance constraints necessitate that detachment folds throughout the fold-thrust belt are cored by faults to maintain bed length. The steep dips of these faults and their depth within the lower Paleozoic sedimentary rocks are compatible with recorded earthquakes. This suggests that the ~11± 4 km deep earthquakes throughout the fold-thrust belt could be nucleating within sedimentary rocks. Total shortening in the Zagros fold-thrust belt is 70 ± 20 km, which corresponds to ~20% shortening of the Arabian shield.