Dr. Adolph Yonkee Research Projects

We have begun studies (funded by the National Science Foundation collaborative grant with Arlo Weil at Bryn Mawr College and colleagues with the Argentina CONICET) in the south-central Andes to evaluate structural evolution of the Precordillera fold-thrust belt and Sierras Pampeanas foreland uplifts. These actively deforming belts have been proposed as analogues to the ancient Sevier fold-thrust belt and Laramide foreland. By studying the active Andean orogenic system, including changes in tectonic styles related to along-strike variations in subduction geometry from normal to flat-slab that are imaged geophysically, we can better understand relations between mountain building processes and subduction dynamics. The study region includes multiple active faults, thick synorogenic strata, as well as the highest peak in the western hemisphere- Aconcagua. The photo above shows highly deformed Miocene strata thrust over young gravels along an active Las Penas fault north of Mendoza. This city was largely destroyed by a M~7 earthquake on a related fault in 1861. Better understanding of active faults in the region may lead to improved planning and hazard mitigation. We are also collaborating with researchers at the University of Arizona, University of Texas, and University of Wisconsin Eau Claire, who are studying patterns of synorogenic sedimentation and cooling histories related to exhumation.

Interrelations between fluid flow, alteration, and deformation control crustal rheology. Aspects of rock deformation, from behavior of individual faults to regional deformation along plate boundaries, depend on rheology of rocks that evolve over geological time scales and in presence of fluids. A previous project (funded by a National Science Foundation collaborative grant with Dyanna Czeck at University of Wisconsin- Milwaukee) combined detailed strain, geochemical and microtextural analysis of diamictite that was deformed in the footwall and hanging wall of a major thrust (Willard thrust) in northern Utah. The diamictite provided a natural laboratory to study how various rock types (represented by granitic gneiss, schist, and quartzite clasts in the diamictite) responded during progressive deformation and fluid-rock interaction. Relations between measured clast shapes/ strain, evolving microtextures, geochemical changes, and water contents of quartz measured by infrared spectroscopy have shown linkages between fluid pathways, reaction softening by alteration of feldspar to mica, hydrolytic weakening, and concentration of strain, which has important implications for understanding rheology during progressive deformation. A new project (funded by a National Science Foundation collaborative grant with Gautam Mitra at University of Rochester and Mark Evans at Eastern Connecticut) builds on this work, by integrating structural, fluid-inclusion, isotopic, and geochronologic studies to better understand feedbacks between fluid pathways, softening mechanisms, and propagation of fold-thrust wedges in frontal parts of the Sevier belt.

Quantifying fault slip history, strain rates, and exhumation patterns within thrust systems is important for understanding rates of mountain building and erosion, and for potential feedbacks. This ongoing project (funded by a National Science Foundation collaborator research grant Michael Wells at the University of Nevada Las Vegas, with support from Shari Kelley at New Mexico Tech and Danny Stockli at the University of Texas at Austin) focuses on laserprobe 40Ar/39Ar geochronology of mica grains from various microstructrual settings (strain shadows, veins, fault zones), zircon U/Pb-He and zircon fission-track thermochronology on vertical and horizontal transects across thrust sheets in northern Utah, and detrital zircon geochronology of associated synorogenic deposits to constrain sediment sources and unroofing histories. Initial results indicate the hanging wall of the Willard thrust experienced low-grade metamorphism and early internal strain from ~140 to 120 Ma, with large-scale thrust slip, exhumation, and cooling from ~120 to 90 Ma, consistent with the general unroofing pattern recorded by synorogenic strata deposited from 120 to 90 Ma. The footwall experienced low-grade metamorphism and internal strain from ~110 to 90 Ma as the Willard thrust sheet was emplaced and tectonically buried rocks below, followed by large-scale slip on the underlying Ogden and Crawford thrusts as the fold-thrust wedge propagated toward the foreland.

Well exposed sections of Neoproterozoic to Cambrian sedimentary and volcanic rocks from Utah to Idaho comprise a key part of the rifted margin of western Laurentia and subsequent North American Cordilleran mountain belt. Importantly, these sections also include diamictites and associated cap carbonates that record Cryogenian (635–850 Ma) glaciations (possibly of global extent), disruptions to geochemical cycles, and subsequent rapid deglaciations. Although a general stratigraphic framework had been established for these rocks, depositional ages, spatial and temporal variations in provenance, and petro -tectonic setting of volcanic rocks were incompletely understood. Work to better understand these rocks (funded by a National Science Foundation collaborative grant with Carol Dehler at Utah State University and Paul Link at Idaho State University) has focused on geochronologic, geochemical, and sedimentalogic analyses of strata from Utah to SE Idaho. This work has revealed two episodes of rifting and igneous activity leading to final development of a passive margin with subsequent thermal subsidence, a protracted period of glaciation likely spanned ~700 to 670 Ma, and that “Marinoan-style” cap carbonates in Utah with distinctive isotopic geochemical signatures may have formed at different times. Ongoing work is focused on integrating additional detailed CA-ID-TIMS geochronologic analysis, additional sampling for geochemical changes, and analysis of preserved microfossils to evaluate changes in the biosphere during major shifts in Earth global climate and ocean water chemistry.