Structural styles, deformation, and uplift of the Olympic Mountains, Washington: Implications for accretionary wedge deformation

The Olympic subduction complex is the exposed subaerial Cascadia accretionary wedge in the Olympic Mountains of Washington State. Uplift of the mountains has been attributed to two competing models: margin-normal deformation from frontal accretion and underplating, and margin-parallel deformation from the clockwise rotation and northward movement of the Oregon Coast Range block compressing the Olympic Mountains block against the Canadian Coast Range. East-northeast−oriented folds and Quaternary thrust faults and paleostress analysis of faults in the Coastal Olympic subduction complex, west of the subduction complex massif, provide new evidence for north-south shortening in the Coastal Olympic subduction complex that fills a large spatial gap in the north-south shortening documented in prior studies, substantially strengthening the block rotation model. These new data, together with previous studies that document north-south shortening in the subduction complex and at numerous locations in the Coast Range terrane peripheral to the complex, indicate that margin-parallel deformation of the Cascadia forearc has contributed significantly to uplift of the Olympic Mountains. Coastal Olympic subduction complex shallow-level fold structural style and deformation mechanisms provide a template for analyzing folding processes in other accretionary wedges. Similar-shaped folds in shallow-level Miocene turbidite sediments of the Coastal Olympic subduction complex formed in two shortening phases not previously recognized in accretionary wedges. Folds began forming by bed-parallel flow of sediment into developing hinges. When the strata could no longer accommodate shortening by flexural flow, further shortening was taken up by flexural slip. Similar-shaped folds in the deeper accretionary wedge rocks of the subduction complex massif have a well-developed axial-surface cleavage that facilitated shear folding with sediment moving parallel to the axial surface into the hinges, a structural style that is common to accretionary wedges. The pressure-temperature conditions and depth at which the formation of similar folds transitions from bed-parallel to axial-surface−parallel deformation are bracketed.

Double dating (U–Pb and (U–Th)/He) of detrital zircon from the Gonfolite Group (European Alps) and implications for the lag-time approach to detrital thermochronology

<p>Detrital thermochronologic analyses are increasingly employed to develop quantitative models of landscape evolution and constrain rates of exhumation due to erosion. Crucial for this kind of application is a correct discrimination between thermochronologic ages that record cooling due to exhumation, i.e., the motion of parent rocks towards Earth’s surface, and thermochronologic ages that record cooling independent from exhumation, as expected for example in volcanic and shallow-level plutonic rocks. A suitable approach for the identification of magmatic crystallization ages is provided by double dating, which combines for example U–Pb and (U–Th)/He analyses of the same mineral grain. Magmatic zircon crystallized from volcanic or shallow-level plutonic rocks should display identical U–Pb and (U–Th)/He (ZHe) ages within error, because of rapid magma crystallization in the upper crust where country rocks are at temperatures cooler than the partial retention zone of the ZHe system. Conversely, zircon grains crystallized at greater depth and recording cooling during exhumation should display ZHe ages younger than the corresponding U–Pb ages. These latter ZHe ages may constrain the long-term exhumation history of the source rocks according to the lag-time approach, provided that a range of assumptions are properly evaluated (e.g., Malusà and Fitzgerald 2020). Here, we explore the possibility that detrital zircon grains yielding ZHe ages younger than the corresponding U–Pb ages may record country-rock cooling within a contact aureole rather than exhumation. To tackle this issue, we applied a double-dating approach including U-Pb and ZHe analyses to samples of the Gonfolite Group exposed south of the European Alps. The Gonfolite Group largely derives from erosion of the Bergell volcano-plutonic complex and adjacent country rocks, and its mineral-age stratigraphy is extremely well constrained (Malusà et al. 2011, 2016). Analyses were performed in the UTChron Geochronology Facility at University of Texas at Austin. For U-Pb LA-ICPMS depth-profile analysis, all detrital zircon grains were mounted without polishing, which allowed for subsequent ZHe analysis on the same grains. Zircon for ZHe analyses were selected among those not derived from the Bergell complex or other Periadriatic magmatic rocks, as constrained by their U-Pb age. We found that ca 40% of double-dated grains, despite yielding a ZHe age younger than their U-Pb age, likely record cooling within the Bergell contact aureole, not exhumation. These findings have major implications for a correct application of the lag-time approach to detrital thermochronology and underline the importance of a well-constrained mineral-age stratigraphy for a reliable geologic interpretation.</p><p>Malusà MG, Villa IM, Vezzoli G, Garzanti E (2011) Earth Planet Sci Lett 301(1-2), 324-336</p><p>Malusà MG, Anfinson OA, Dafov LN, Stockli DF (2016) Geology 44(2), 155-158</p><p>Malusà MG, Fitzgerald, PG (2020) Earth-Sci Rev 201, 103074</p>

Mineralogy of a High-Temperature Skarn, in High CO2 Activity Conditions: The Occurrence from Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania)

A shallow-level monzodioritic to quartz-monzodioritic pluton of the Upper Cretaceous age caused contact metamorphism of Tithonic–Kimmeridgian reef limestones at Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania). The preserved peak metamorphic assemblage includes gehlenite (Ak 33.64–38.13), monticellite, wollastonite-2M, Ti-poor calcic garnet, and Ca-Tschermak diopside (with up to 11.15 mol.% Ca-Tschermak molecule). From the monzodioritic body to the calcitic marble, the periplutonic zoning can be described as: monzodiorite/agpaitic syenite-like inner endoskarn/wollastonite + perovskite + Ti-poor grossular + Al-rich diopside/wollastonite + Ti-poor grossular + diopside + vesuvianite/gehlenite + wollastonite + Ti poor grossular + Ti-rich grossular (outer endoskarn)/wollastonite + vesuvianite + garnet (inner exoskarn)/wollastonite + Ti-rich garnet + vesuvianite + diopside (outer exoskarn)/calcitic marble. Three generations of Ca garnets could be identified, as follows: (1) Ti-poor grossular (Grs 53.51–81.03 mol.%) in equilibrium with gehlenite; (2) Ti-rich grossular (Grs 51.13–53.47 mol.%, with up to 19.97 mol.% morimotoite in solid solution); and (3) titanian andradite (Grs 32.70–45.85 mol.%), with up to 29.15 mol.% morimotoite in solid solution. An early hydrothermal stage produced retrogression of the peak paragenesis toward vesuvianite, hydroxylellestadite (or Si-substituted apatite), clinochlore, “hibschite” (H4O4-substituted grossular). A late hydrothermal event induced the formation of lizardite, chrysotile, dickite, thaumasite, okenite and tobermorite. A weathering paragenesis includes allophane, C-S-H gels and probably portlandite, unpreserved because of its transformation in aragonite then calcite. Overprints of these late events on the primary zoning are quite limited.

A hydrothermally synthesized MoS2(1−x)Se2x alloy with deep-shallow level conversion for enhanced performance of photodetectors

Schematic diagram of MoS2(1−x)Se2x synthesis and schematic diagram of device preparation.