R.A. Donelick | USGS Science

USGS2005

Overview of radiometric ages in three allochthonous belts of northern Venezuela: Old ones, new ones, and their impact on regional geology

The margin of northern Venezuela is a complex zone representing the orogenic events from basement formation to subsequent subduction and exhumation during transpressional collision. This boundary zone has six east-west–trending belts that each record a different segment of its development. This geologic complexity requires radiometric ages to unravel, and we herein provide 48 new ages including U-Pb (4), Rb-Sr (2), 40 Ar/ 39 Ar (24), zircon and apatite fission-track (17), and 14 C (1) ages to constrain the evolution of three of these belts. These three belts are the Cordillera de la Costa, Caucagua–El Tinaco, and Serranía del Interior belts. In the Cordillera de la Costa belt, U-Pb geochronologic data indicate portions of the basement igneous and metaigneous rocks formed in the Cambro-Ordovician (513–471 Ma). New 40 Ar/ 39 Ar data from Margarita Island indicate that some of the subduction complex was rapidly cooled and exhumed, whereas other portions indicate slower cooling. This contrasts with new 40 Ar/ 39 Ar data from the Puerto Cabello portion of the subduction complex that has Eocene to Oligocene (42–28 Ma) cooling ages. New fission-track data imply the entire Cordillera de la Costa belt from Puerto Cabello to La Guaira (∼150 km) was uplifted at the same time. In the Caucagua–El Tinaco belt, the oldest 40 Ar/ 39 Ar amphibole ages from the Tinaquillo ultramafic complex are Jurassic (190 Ma). Additional amphibole 40 Ar/ 39 Ar cooling ages are older than previously recorded in either the Tinaco or Tinaquillo complex. One amphibole 40 Ar/ 39 Ar cooling age for the Tinaco complex is similar to previous U-Pb results. New apatite fission-track results from the Serranía del Interior foreland fold and thrust belt are synchronous with exhumation in the Cordillera de la Costa belt. In addition, several zircon fission-track ages in the Serranía del Interior belt are older than their fossil ages, indicating a Cretaceous minimum provenance age for Miocene beds. Significant new findings from these geochronologic studies include (1) several igneous and metaigneous bodies that may be correlated with orogenic events in the Appalachians occur within the subduction mélange; (2) the Tinaquillo complex may record Jurassic rifting; (3) Cretaceous source rocks for the Serranía del Interior sedimentary strata; (4) exhumation of the subduction complex is segmented because two regions have significantly different cooling histories, with Margarita Island exhumed in the Cretaceous, whereas to the west, the Puerto Cabello region has widespread Paleogene cooling and exhumation ages; and (5) earthquake activity in 1812 caused uplift as recorded by exposure of Recent corals.

northern Venezuela↗

USGS2002

Tertiary cooling and tectonic history of the White River uplift, Gore Range, and western Front Range, central Colorado: Evidence from fission-track and 39Ar/ 40Ar ages

Apatite fission-track (AFT) data from Proterozoic and Paleozoic rocks in the mountains of north central Colorado (White River Uplift, Gore Range, and western Front Range) record significant cooling that began with uplift and erosion related to the Laramide Orogeny and continued through the Tertiary to Pliocene time. The mountains immediately flanking the Blue River half graben (Williams Fork Mountains to the east and the Gore Range to the west) cooled significantly during the Neogene. The AFT ages along the flanks of the Blue River half graben are significantly younger than AFT ages farther to the east in the central and eastern Front Range and to the west in the White River uplift. In both of these areas, the apatite ages suggest Laramide cooling. The Williams Fork Mountains–Gore Range zone of young AFT ages extends southward adjacent to the axis of the Rio Grande rift through southern Colorado and New Mexico. These young ages result from a combination of elevated heat flow, uplift, and erosion along the axis of the Rio Grande rift during Neogene time. Zircons from Proterozoic rocks yield Proterozoic fission-track ages, indicating that this part of the Colorado basement has not been heated to temperatures 200 C since Middle Proterozoic time. A sanidine 40 Ar/ 39 Ar age of 27 Ma from a rhyolite tuff just above a basal boulder conglomerate of the Troublesome Formation in a tilted fault block within the Blue River half graben shows that Tertiary deposition started there in middle Oligocene time. Xenocrystic sanidine from a basalt stratigraphically higher than the rhyolite tuff has an age of 24 Ma. Thus, the basalt is significantly younger than its postulated source, the 32 Ma laccolithic complex at Green Mountain.

Colorado↗