RECONSTRUCTING REGIONAL DEFORMATIONAL EVENTS IN THE ZIHUATANEJO TERRANE FROM STRATIGRAPHY AND ZIRCON GEOCRONOLOGY

The Zihuatanejo terrane is the largest of all the terranes that form the Guerrero composite terrane. Its rocks cover a large time span, and include Triassic–Jurassic? basement rocks (Arteaga accretionary complex), Jurassic volcanic rocks and granitoids, as well as Lower to Upper Cretaceous volcanic and sedimentary rocks. Major angular unconformities that record periods of deformation are Early Jurassic, Late Jurassic, mid-Cretaceous (pre-Aptian), and Late Cretaceous (Turonian-Conacian) in age. The first is attributed to the formation of the Arteaga subduction accretionary prism, the next two are not well understood, but are preliminary interpreted as associated to intra-arc rifting. Extension and rifting led to formation of deep marine proto-oceanic basins that form within the arc and in a back-arc position, and are presently located toward the east and northeast of the Zihuatanejo terrane. In coastal Zihuatanejo terrane arc volcanic and sedimentary rocks overlie folded and thrusted Early Cretaceous to Cenomanian nonmarine and marine rocks in angular unconformity. Paleontologic and detrital zircon ages constrained this deformation to sometime between late Cenomanian-Turonian and Santonian. This deformation is interpreted to be related to the closing of the deep marine basins and amalgamation of the Guerrero terrane, event that predates Sevier or Laramide structures formed in the Sierra Madre Oriental Fold and Thrust belt (SMOFTB). After deformation and metamorphism of the Zihuatanejo terrane a Santonian-Maastrichtian contractional arc developed while inboard-migrating deformation was active in eastern Guerrero terrane and the SMOFTB. Thus, Santonian-Maastrichtian strata in the Zihuatanejo terrane represent the fill of an intra-arc basin within a compressional arc.