Geology topics

Gabriele Casale

Publications and source records attributed to Gabriele Casale.

4 recordsLinked to original sources

USGS2026

Geochemical disequilibrium at the brittle-ductile transition

We investigate the microtextural, microchemical, and isotopic effects of late-stage ductile deformation in quartzite mylonites and kyanite–muscovite–quartz veins from the Raft River shear zone (Utah). Quartz microstructures record pervasive disequilibrium, expressed by unannealed features including undulatory extinction, deformation lamellae, and poorly defined fabrics, typical of waning deformation in shear zones. Microchemical disequilibrium is best preserved in kyanite quartzites, where CL-zoned kyanite records repeated fractures, overgrowth, and mineral precipitation, and in muscovite from muscovite-poor quartzite mylonites that shows minor-element zoning consistent with syn-deformational overgrowth on detrital cores. In contrast, muscovite from muscovite-rich kyanite quartzites exhibits minimal chemical zoning. These microchemical variations correlate with 40 Ar/ 39 Ar age systematics. Chemically zoned muscovite preserves variable single-step ages, including ∼150 Ma ages, reflecting retention of detrital cores. In contrast, syndeformational muscovite consistently yields Miocene ages, indicating recrystallization and new growth below argon closure temperatures that reset inherited isotopic signatures. Similar trends are observed in quartzite mylonites, where increasing quartz recrystallization and stronger crystallographic preferred orientations occur toward deeper structural levels. Together, these observations indicate increasing retrograde deformation and recrystallization with depth in the Raft River shear zone and demonstrate that strain-driven recrystallization exerts a first-order control on muscovite ages. We suggest that apparent thermochronologic gradients in retrograde shear zones may reflect recrystallization gradients rather than temperature gradients. Where cooling limits the thermal driving force for recrystallization, isotopic relics are preserved, and local deformation and fluid availability control re-equilibration. Consequently, isotopic disequilibrium—particularly in the 40 Ar/ 39 Ar system—may be the rule rather than the exception of retrograde tectonic environments.

Idaho, Utah↗

USGS2025

Timing of and pressure-temperature constraints on deformation in the Toxaway dome, eastern Blue Ridge: Evidence for continuous deformation from the Neoacadian orogeny to the Alleghanian orogeny

Many mountain belts are built through repeated collision, and in the case of orogenies closely spaced in time, determining when one orogeny ends and another begins can be challenging. The southern Appalachian mountains were formed by three mountain-building events closely spaced in time, including the Taconic (ca. 480–440 Ma), Neoacadian (ca. 375–340 Ma), and Alleghanian (ca. 330–265 Ma) orogenies. Notably, the end of the Neoacadian and the beginning of the Alleghanian are only separated by ~10 m.y., and some published dates record deformation and metamorphism in the eastern Blue Ridge during this interval, blurring the boundary between these two discrete events. The Toxaway dome, located along the North Carolina–South Carolina, USA, border at the eastern edge of the eastern Blue Ridge, is a structural dome cored by Mesoproterozoic Toxaway Gneiss and surrounded by the younger Tallulah Falls Formation. Previous ages constraints from the Toxaway dome (343 and 338 Ma U-Pb zircon ages) make it an ideal location to explore whether there was continuous deformation during this period of supposed quiescence between the Neoacadian and Alleghanian orogenies. We used optical microscopy and electron backscatter diffraction in quartz to determine deformation temperatures, thermobarometry to determine metamorphic pressure-temperature conditions, and monazite petrochronology to determine the timing of deformation. Quartz and feldspar recrystallization fabrics parallel to dome-defining fabrics indicate deformation occurred at amphibolite-facies conditions, which is corroborated by our pressure-temperature estimates of 0.67–0.8 ± 0.12 GPa and 661–689 ± 25 °C. Monazite grains that record the timing of reactions of garnet growth and breakdown range from 342 ± 4.8 Ma to 296 ± 10.8 Ma, bridging the interval between the Neoacadian and Alleghanian orogenies. Three samples from the nearby Tallulah Falls dome, which occupies a similar structural position along the edge of the eastern Blue Ridge in Georgia, record monazite dates of 334 ± 4.2 Ma to 304 ± 4.8 Ma, indicating there was tectonic activity in this region before the commonly defined beginning of the Alleghanian orogeny. We propose (1) there was no period of quiescence between the Neoacadian and Alleghanian orogenies in the eastern Blue Ridge, and (2) deformation during this time was at higher temperatures and pressures than previously reported.

Georgia, North Carolina, South Carolina↗

USGS2020

Exhumation of the Coyote Mountains metamorphic core complex (Arizona): Implications for orogenic collapse of the southern North American Cordillera.

A microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening and regional metamorphism, including the Late Cretaceous‐early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite‐biotite‐garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top‐to‐the‐north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of noncoaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 × 10 −11 to 5.0 × 10 −13 s −1 at depth of ~11–14 km. Detailed 40 Ar/ 39 Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation postdates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest.

British Columbia, Washington, Oregon, California, ↗

USGS2015

Thin‐ or thick‐skinned faulting in the Yakima fold and thrust belt (WA)? Constraints from kinematic modeling of the saddle mountains anticline

The Yakima fold and thrust belt (YFTB) deforms the Columbia River Basalt Group flows of Washington State. The YFTB fault geometries and slip rates are crucial parameters for seismic‐hazard assessments of nearby dams and nuclear facilities, yet there are competing models for the subsurface fault geometry involving shallowly rooted versus deeply rooted fault systems. The YFTB is also thought to be analogous to the evenly spaced wrinkle ridges found on other terrestrial planets. Using seismic reflection data, borehole logs, and surface geologic data, we tested two proposed kinematic end‐member thick‐ and thin‐skinned fault models beneath the Saddle Mountains anticline of the YFTB. Observed subsurface geometry can be produced by 600–800 m of heave along a single listric‐reverse fault or ∼3.5 km of slip along two superposed low‐angle thrust faults. Both models require decollement slip between 7 and 9 km depth, resulting in greater fault areas than sometimes assumed in hazard assessments. Both models require initial slip much earlier than previously thought and may provide insight into the subsurface geometry of analogous comparisons to wrinkle ridges observed on other planets.

Washington↗