New articles for Geosphere posted online in September - EurekAlert

Locations and topics studied include the Altiplano-Puna volcanic complex of the Central Andes; the Tornillo Group of Big Bend National Park, West Texas, USA; the San Gorgonio Pass, California, USA, region; and the central Cascadia arc, Oregon, USA.

Rhyolitic melt production in the midst of a continental arc flare-up—The heterogeneous Caspana ignimbrite of the Altiplano-Puna volcanic complex of the Central Andes.

Abstract: The ~5 km3, 4.54–4.09 Ma Caspana ignimbrite of the Altiplano-Puna volcanic complex (APVC) of the Central Andes records the eruption of an andesite and two distinct rhyolitic magmas.

The fall deposit and thin flow unit that record the first stage of the eruption (Phase 1) tapped a crystal-poor peraluminous rhyolite.

Phase 2 of the eruption records the emplacement of a more extensive flow unit with a crystal-poor, fayalite-bearing rhyolite and a porphyritic to glomeroporphyritic andesite containing abundant plagioclase-orthopyroxene-Fe-Ti oxide (norite) glomerocrysts.

The mineral assemblage of the noritic glomerocrysts and the observed trend between andesite and Phase 2 rhyolite are reproduced by rhyolite-MELTS–based models.

Rhyolite phase equilibria predict an estimated temperature of ~775 °C and ~5 wt% H2O.

Pressures derived from phase equilibria indicate that the rhyolite was extracted directly from the noritic cumulate at ~340 MPa and stored at slightly shallower pressures (200–300 MPa) prior to eruption.

Spikes in latent heat facilitated the segregation of the residual liquid, creating the pre-eruptive compositional gap of ~16 wt% SiO2 between the andesite and the Phase 2 rhyolite.

Unlike typical Altiplano-Puna volcanic complex (APVC) magmas, low ƒO2 conditions in the andesite promoted co-crystallization of orthopyroxene and ilmenite in lieu of clinopyroxene and magnetite.

This resulted in relatively high Fe concentrations in the rhyodacite and Phase 2 rhyolite.

Combined with the co-crystallization of plagioclase, this low oxidation state forced high Fe 2+/Mg and Fe/Ca in the Phase 2 rhyolite, which promoted fayalite stability.

Herein, new constraints on the timing, extent, and characteristics of deformation during mid-Cretaceous tectonism in the central Sierra Nevada (eastern California, USA) are synthesized with published geologic mapping, structural studies, and geochronology to create an updated reconstruction of one of the type examples of a hot, magma-rich orogen.

Deformation and magmatism show distinct and unrelated spatiotemporal patterns throughout this orogenic episode.

Contrary to previous models of direct tectonomagmatic links, many of which were developed in the central Sierra Nevada, arc activity did not control the location, intensity, or kinematics of intra-arc deformation, nor did shear zones control the location of magmatism.

In addition to changing plate-scale boundary conditions, lithospheric-scale rheological evolution likely played a key role in the patterns of Late Cretaceous deformation observed across strike of the entire Cordilleran margin.

The effects of pre-stress assumptions on dynamic rupture with complex fault geometry in the San Gorgonio Pass, California, region.

Abstract: We use three-dimensional (3-D) dynamic finite-element models to investigate potential rupture paths of earthquakes propagating along faults through the western San Gorgonio Pass, a structurally complex region along the San Andreas fault system in southern California (USA).

We focus on the right-lateral San Bernardino strand of the San Andreas fault system, the oblique thrust–right-lateral San Gorgonio Pass fault zone, and a portion of the right-lateral Garnet Hill strand of the San Andreas fault system.

We use the 3-D finite-element method to model rupture propagation along a fault geometry that reflects current understanding of the local geometrical complexity and is consistent with long-term loading and observed surface deformation.

We test three different types of pre-stress assumptions: (1) constant tractions (assuming pure right-lateral strike-slip motion on the San Bernardino and Garnet Hill strands and oblique thrust–right-lateral strike-slip motion on the San Gorgonio Pass fault zone), (2) a uniform regional stress regime, and (3) long-term (evolved) stress from quasi-static crustal deformation modeling.

The results also emphasize how fault geometry and stress patterns combine to influence rupture propagation on complex fault systems.

Abstract: Oligocene and early Miocene displacement on the Catalina–San Pedro detachment fault and its northern correlatives uncovered mylonitic fabrics that form the greater Catalina metamorphic core complex in southeastern Arizona, USA.

This is attributed to a stable sliding regime during the entire period of extension, with metamorphic core complex inflation by deep crustal flow leading to maintenance of wedge surface slope and detachment fault dip that favored stable sliding rather than internal wedge extension

Abstract: New cosmogenic 3He chronologies and geologic mapping of faulted glacial drift provide new constraints for the slip rates of active faulting in the central Cascade arc, Oregon, USA

The White Branch and Dilman Meadows fault zones cut deposits created by three distinct glacial advances, which provide timing, kinematics, and rate constraints for fault motion

Dip-slip displacement across fault scarps expressed by lidar data reveal similar magnitudes of extensional deformation for LGM and older glacial deposits on the White Branch fault zone, which implies a lack of earthquake ruptures between the oldest and LGM advances

In contrast, scarp profiles along the Dilman Meadows fault zone reveal progressive cumulative slip for surfaces of increasing age

The White Branch fault zone accommodates predominately fault-normal extension, whereas right-oblique slip characterizes the Dilman Meadows fault zone

Active deformation across the central Cascade Range thus reflects the combined effects of ongoing crustal block rotation and arc magmatism