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J.E. Wright

Publications and source records attributed to J.E. Wright.

5 recordsLinked to original sources
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↗
USGS1997

Mid-crustal flow during Tertiary extension in the Ruby Mountains core complex, Nevada

Structural analysis and geochronologic data indicate a nearly orthogonal, late Eocene–Oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer-thick, extensional, mylonitic shear zone of the Ruby Mountains metamorphic core complex, Nevada. New U-Pb radiometric dating indicates that the development of a northward-trending lineation in the infrastructure is partly coeval with the development of a pervasive, west-northwest–trending lineation in the mylonitic shear zone. U-Pb monazite data from the leucogranite orthogneiss of Thorpe Creek indicate a crystallization age of ca. 36–39 Ma. Zircon fractions from a biotite monzogranite dike yield an age of ca. 29 Ma. The three dated samples from these units exhibit a penetrative, approximately north-south–trending elongation lineation. This lineation is commonly defined by oriented bundles of sillimanite and/or elongated aggregates of quartz and feldspar, indicating a synmetamorphic and syndeformational origin. The elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic, nonmylonitic infrastructural core of the northern Ruby Mountains. A portion of this midcrustal flow is coeval with the well-documented, west-northwest sense of slip in the structurally overlying kilometer-thick, mid-Tertiary mylonitic shear zone. Lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure, suggesting fundamental plastic decoupling between structural levels in this core complex. Furthermore, the infrastructure is characterized by overlapping, oppositely verging fold nappes, which are rooted to the east and west. One of the nappes may be synkinematic with the intrusion of the late Eocene orthogneiss of Thorpe Creek. In addition, the penetrative, elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes, suggesting a kinematically related evolution.

Nevada, Utah↗
USGS1997

The grand tour of the Ruby-East Humboldt metamorphic core complex, northeastern Nevada: Part 1 - Introduction & road log

The purpose of this geological excursion is to provide an overview of the multiphase developmental history of the Ruby Mountains and East Humboldt Range, northeastern Nevada. Although these mountain ranges are commonly cited as a classic example of a Cordilleran metamorphic core complex developed through large-magnitude, mid-Tertiary crustal extension, a preceding polyphase Mesozoic contractional history is also well preserved in the ranges. An early phase of this history involved Late Jurassic two-mica granitic magmatism, high-temperature but relatively low-pressure metamorphism, and polyphase deformation in the central Ruby Mountains. In the northern Ruby Mountains and East Humboldt Range, a Late Cretaceous history of crustal shortening, metamorphism, and magmatism is manifested by fold-nappes (involving Archean basement rocks in the northern East Humboldt Range), widespread migmatization, injection of monzogranitic and leucogranitic magmas, all coupled with sillimanite-grade metamorphism. Following Late Cretaceous contraction, a protracted extensional deformation partially overprinted these areas during the Cenozoic. This extensional history may have begun as early as the Late Cretaceous or as late as the mid-Eocene. Late Eocene and Oligocene magmatism occurred at various levels in the crust yielding mafic to felsic orthogneisses in the deep crust, a composite granitic pluton in the upper crust, and volcanic rocks at the surface. Movement along a west-rooted, extensional shear zone in the Oligocene and early Miocene led to core-complex exhumation. The shear zone produced mylonitic rocks about 1 km thick at deep crustal levels, and an overprint of brittle detachment faulting at shallower levels as unroofing proceeded. Megabreccias and other synextensional sedimentary deposits are locally preserved in a tilted, upper Eocene through Miocene stratigraphic sequence. Neogene magmatism included the emplacement of basalt dikes and eruption of rhyolitic rocks. Subsequent Basin and Range normal faulting, as young as Holocene, records continued tectonic extension.

Brigham Young University Geology Studies↗
USGS1986

Late Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region

The Upper Triassic Chinle Formation of the Colorado Plateau contains voluminous volcanic detritus evidently derived from a source to the south. Volcanic rocks exposed in southern Arizona and northern Sonora have been assumed to represent this source terrane, but U-Pb isotopic geochronology and regional stratigraphic correlations indicate that these volcanic rocks are distinctly younger than the Chinle, and thus not a source for the volcanic detritus in the Chinle. Igneous rocks of known or possible Late Triassic age in Nevada, California, or northeastern Mexico are possible sources, but a clearly defined source terrane for the volcanic detritus in the Chinle has not been identified. Tectonic removal of the source terrane by rifting or strike-slip offset, though not proven, is a possibility.

Geology↗