U-PB AGE CONSTRAINTS ON THE ORIGIN OF THE ALPINE BOUNDARY FAULTS IN CENTRAL-EAST IRAN

The Alpine compressional deformation within the Central Iranian Terrane (CIT) has been accommodated by combined strike-slip and thrust displacements along a complex system of intersecting faults and rotating crustal domains, in sharp contrast to rather simple crustal shortening in the surrounding Alborz, Zagros and Makran fold/thrust belts. These intersecting faults outline four internal crustal domains of the CIT, distinguished by differences in stratigraphy and overall style of deformation. Although debate exists over the origin of the CIT boundary faults, many have interpreted them as Precambrian structures reactivated during episodes of Alpine (Cenozoic) collisional tectonism.

New high-precision U-Pb isotopic ages demonstrate that nearly all of the documented magmatic and metamorphic rocks that were once believed to make up the "Precambrian basement" of CIT fall in one of two geologically contrasting groups: a) Orogenic rocks of predominantly Early Cambrian age (ca. 547-526 Ma) formed in a magmatic arc setting and, b) Alpine metamorphic complexes and associated post-peak metamorphic granitoid intrusions of Eocene age (ca. 47-43 Ma). The former group includes the Kushk Series rocks, early reported Pb-Pb isotopic ages from which (595 to 715 Ma) have long been referred to as evidence for the late Precambrian age of basement in Iran. The above two rock groups occur in disparate lithotectonic domains with dissimilar tectonothermal histories, separated by the CIT boundary faults. No effects of the Cenozoic thermal event have been observed in the rocks of the Early Cambrian suite. These age-structural relationships suggest that the boundary fault movements responsible for the juxtaposition of the intra-CIT domains post-dated the Alpine peak metamorphism, with no confirmed evidence for pre-existing Precambrian structures.