The western margin of Mexico is ideally suited for testing two opposing models for the growth of continents along convergent margins: accretion of exotic island arcs by the consumption of entire ocean basins versus accretion of fringing terranes produced by protracted extensional processes in the upper plate of a single subduction zone. We present geologic and detrital zircon evidence that the Zihuatanejo terrane of the Guerrero composite terrane originated from the latter mechanism. The evolution of the Zihuatanejo terrane can be explained by extensional and compressional processes operating entirely within the upper plate of a long-lived subduction zone that dipped east under the Mexican margin. This process controlled crustal growth by continental margin rifting and addition of new igneous and volcaniclastic material during extension, followed by accretion and thickening of the crust during contraction. Prior to this study, all Mesozoic rocks in the western part of the Guerrero composite terrane were considered to be part of a single arc. However, we divide it into four distinctive tectonostratigraphic assemblages: (1) a Triassic-Early Jurassic accretionary complex (Arteaga complex); (2) a Jurassic to earliest Cretaceous extensional volcanic arc assemblage; (3) an Early Cretaceous extensional arc assemblage; and (4) a Santonian-Maastrichtian compressional arc assemblage. (1) The Arteaga subduction complex forms the basement to the Zihuatanejo terrane and includes Grenville, Pan-African, and Permiandetrital zircon suites that match the Potosi fan of the Mexican mainland. (2) The Jurassic to earliest Cretaceous extensional volcanic arc assemblage shows a Callovian-Tithonian (ca. 163-145 Ma) peak in magmatism; extensional unroofing began in this time frame and continued into through the next. (3) The Early Cretaceous extensional arc assemblage has two magmatic peaks: one in the Barremian- Aptian (ca. 129-123 Ma), and the other in the Albian (ca. 109 Ma). In some localities, rapid subsidence produced thick, mainly shallowmarine volcano-sedimentary sections, while at other localities, extensional unroofing of all older assemblages resulted in recycling of zircon from all older units (1, 2, 3). (4) For the Santonian-Maastrichtian compressional arc assemblage, our new detrital zircon dates show for the first time that arc volcanic rocks of this age are present in the coastal Zihuatanejo terrane. The contractional arc developed atop assemblages 1-3, which were shortened between Turonian and Santonian time (ca. 93 and 84 Ma). Taken together, the western Zihuatanejo terrane records a more protracted history of arc magmatism than has yet been dated in other terranes of western Mexico, but it closely matches the history of Baja California to the northwest.
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