Showing papers by "Paul W. Layer published in 2012"

Reconstruction of the volcanic history of the Tacámbaro-Puruarán area (Michoacán, México) reveals high frequency of Holocene monogenetic eruptions

Abstract: The 690 km2 Tacambaro-Puruaran area located at the arc-front part of the Michoacan-Guanajuato volcanic field in the Trans-Mexican Volcanic Belt (TMVB) records a protracted history of volcanism that culminated with intense monogenetic activity in the Holocene. Geologic mapping, 40Ar/39Ar and 14C radiometric dating, and whole-rock chemical analyses of volcanic products provide insights to that history. Eocene volcanics (55–40 Ma) exposed at uplifted blocks are related to a magmatic arc that preceded the TMVB. Early TMVB products are represented by poorly exposed Pliocene silicic domes (5–2 Ma). Quaternary ( 30 km) may be related to high rates of magma production at depth and a favorable tectonic setting.

Cenozoic tectono-thermal history of the Tordrillo Mountains, Alaska: Paleocene-Eocene ridge subduction, decreasing relief, and late Neogene faulting

Abstract: [1] Topographic development inboard of the continental margin is a predicted response to ridge subduction. New thermochronology results from the western Alaska Range document ridge subduction related orogenesis. K-feldspar thermochronology (KFAT) of bedrock samples from the Tordrillo Mountains in the western Alaska Range complement existing U-Pb, 40Ar/39Ar and AFT (apatite fission track) data to provide constraints on Paleocene pluton emplacement, and cooling as well as Late Eocene to Miocene vertical movements and exhumation along fault-bounded blocks. Based on the KFAT analysis we infer rapid exhumation-related cooling during the Eocene in the Tordrillo Mountains. Our KFAT cooling ages are coeval with deposition of clastic sediments in the Cook Inlet, Matanuska Valley and Tanana basins, which reflect high-energy depositional environments. The Tordrillo Mountains KFAT cooling ages are also the same as cooling ages in the Iliamna Lake region, the Kichatna Mountains of the western Alaska Range, and Mt. Logan in the Wrangell-St. Elias Mountains, thus rapid cooling at this time encompasses a broad region inboard of, and parallel to, the continental margin extending for several hundred kilometers. We infer these cooling events and deposition of clastic rocks are related to thermal effects that track the eastward passage of a slab window in Paleocene-Eocene time related to the subduction of the proposed Resurrection-Kula spreading ridge. In addition, we conclude that the reconstructed KFATmax negative age-elevation relationship is likely related to a long period of decreasing relief in the Tordrillo Mountains.

Geology of the epithermal Ag–Au Huevos Verdes vein system and San José district, Deseado massif, Patagonia, Argentina

Abstract: The San Jose district is located in the northwest part of the Deseado massif and hosts a number of epithermal Ag–Au quartz veins of intermediate sulfidation style, including the Huevos Verdes vein system. Veins are hosted by andesitic rocks of the Bajo Pobre Formation and locally by rhyodacitic pyroclastic rocks of the Chon Aike Formation. New 40Ar/39Ar constraints on the age of host rocks and mineralization define Late Jurassic ages of 151.3 ± 0.7 Ma to 144.7 ± 0.1 Ma for volcanic rocks of the Bajo Pobre Formation and of 147.6 ± 1.1 Ma for the Chon Aike Formation. Illite ages of the Huevos Verdes vein system of 140.8 ± 0.2 and 140.5 ± 0.3 Ma are 4 m.y. younger than the volcanic host rock unit. These age dates are among the youngest reported for Jurassic volcanism in the Deseado massif and correlate well with the regional context of magmatic and hydrothermal activity. The Huevos Verdes vein system has a strike length of 2,000 m, with several ore shoots along strike. The vein consists of a pre-ore stage and three main ore stages. Early barren quartz and chalcedony are followed by a mottled quartz stage of coarse saccharoidal quartz with irregular streaks and discontinuous bands of sulfide-rich material. The banded quartz–sulfide stage consists of sulfide-rich bands alternating with bands of quartz and bands of chlorite ± illite. Late-stage sulfide-rich veinlets are associated with kaolinite gangue. Ore minerals are argentite and electrum, together with pyrite, sphalerite, galena, chalcopyrite, minor bornite, covellite, and ruby silver. Wall rock alteration is characterized by narrow ( 220°C. Kaolinite occurring with the late sulfide-rich veinlet stage indicates pH 315°, whereas strike directions of <315° are predicted with an induced dextral strike–slip movement. The components of the structural model appear to be present on a regional scale and are not restricted to the San Jose district.

New 40Ar/39Ar ages from the Central Part of the Chiapanecan Volcanic Arc, Chiapas, México

Abstract: El Arco Volcanico Chiapaneco (AVC; CVA = siglas en ingles) se localiza en la porcion central del estado de Chiapas, es una cadena de estructuras volcanicas alineadas en direccion NO-SE de aproximadamente 150 km de longitud. En su parte central el AVC esta compuesto de al menos 10 estructuras volcanicas alineadas al Noreste y concentradas en fallas con rumbo NNOSSE asociadas al Sistema de Fallas Motagua- Polochic. Las estructuras son siete Domos Volcanicos (Huitepec, Amahuitz, La Iglesia, Mispia, La Lanza, Venustiano Carranza y Santoton), un Crater de Explosion (Navenchauc), una Estructura de Colapso (Apas), y un Complejo de Domos Volcanico (Tzontehuitz). En este trabajo, reportamos 13 nuevas edades 40Ar/39Ar de rocas de los siete domos que incrementan las dataciones disponibles para la actividad volcanica durante el Cuaternario, en la parte central del Arco Volcanico Chiapaneco. Con las nuevas dataciones y las reportadas en bibliografia se tiene al Complejo de Domos Tzontehuitz como la estructura mas antigua 2.1 Ma y al Domo Volcanico Venustiano Carranza como el de actividad mas reciente de 0.225 Ma

Jurassic to Early Cretaceous postaccretional sinistral transpression in north-central Chile (latitudes 31–32°S)

Abstract: We describe the geometry and kinematics of a Jurassic to Early Cretaceous transpressive sinistral strike-slip system within a metamorphic basement inlier of the Mesozoic magmatic arc near Bahia Agua Dulce at latitudes 31–32°S in north-central Chile and discuss possible relations with the Atacama Fault System further north. Sinistral transpression overprints structures of an accretionary system that is represented by the metamorphic basement. Sub-vertical semi-ductile NNW-striking strike-slip shear zones are the most conspicuous structures. Chlorite and sericite grew, and white mica and quartz dynamically recrystallized, suggesting low-grade metamorphic conditions during semi-ductile deformation. Folds at the 10–100 metre scale developed before and during strike-slip shearing. The folds are deforming a former sub-horizontal transposition foliation that originated during prior accretion processes. The folds have axes sub-parallel to the strike-slip shear zones and sub-vertical axial surfaces indicating a component of shortening parallel to the shear-zone boundaries, suggesting an overall transpressive deformation regime. Transpressive strike-slip deformation also affects Middle Triassic (Anisian) basal breccias of the El Quereo Formation. 40Ar–39Ar laser ablation ages of synkinematically recrystallized white mica in one of the shear zones provide an age of 174–165 Ma for the waning stages of semi-ductile strike-slip shearing. The semi-ductile shear zones are cut by mafic and rhyolite dykes. Two rhyolite dykes yield 40Ar–39Ar ages of 160.5 ± 1.7 Ma and 131.9 ± 1.7 Ma, respectively. The latter dyke has been affected by brittle faulting. Fault-slip analysis shows that the kinematics of the faulting event is similar to the one of the semi-ductile shearing event, suggesting that sinistral transpression continued after ~130 Ma. Timing, kinematics and geographic position suggest that the shear zones at Bahia Agua Dulce represent a southern continuation of the prominent Atacama Fault System that affected the Jurassic/Early Cretaceous arc over its ~1400 km length.