Meteoric water circulation in a rolling-hinge detachment system (northern snake range core complex, Nevada)

Combined petrofabric, microstructural, stable isotopic, and 40Ar/39Ar geochronologic data provide a new perspective on the Cenozoic evolution of the northern Snake Range metamorphic core complex in east-central Nevada. This core complex is bounded by the northern Snake Range detachment, interpreted as a rolling-hinge detachment, and by an underlying shear zone that is dominated by muscovite-bearing quartzite mylonite and interlayered micaschist. In addition to petrofabric, microstructural analysis, and 40Ar/39Ar geochronology, we use hydrogen isotope ratios (δD) in synkinematic white mica to characterize fluid-rock interaction across the rolling-hinge detachment. Results indicate that the western flank of the range preserves mostly Eocene deformation (49-45 Ma), characterized by coaxial quartz fabrics and the dominant presence of metamorphic fluids, although the imprint of meteoric fluids increases structurally downward and culminates in a shear zone with a white mica 40Ar/39Ar plateau age of ca. 27 Ma. In contrast, the eastern flank of the range displays pervasive noncoaxial (top-tothe-east) fabrics defined by white mica that formed in the presence of meteoric fluids and yield Oligo cene-Miocene 40Ar/39Ar ages (27-21 Ma). Evolution of the Oligocene-Miocene rolling-hinge detachment controlled where and when faulting was active or became inactive owing to rotation, and therefore where fluids were able to circulate from the surface to the brittle-ductile transition. On the western flank (rotated detachment), faulting became inactive early, while continued active faulting on the eastern flank of the detachment allowed surface fluids to reach the mylonitic quartzite. The combined effects of synkinematic recrystallization and fluid inter action reset argon and hydrogen isotope ratios in white mica until the early Miocene (ca. 21 Ma), when the brittle-ductile transition was exhumed beneath the detachment

Coupled basin-detachment systems as paleoaltimetry archives of the western North American Cordillera

Stable isotope paleoaltimetry data from the Snake Range metamorphic core complex (MCC) and Sacramento Pass Basin (NV, USA) document that extensional mylonite zones and kinematically linked syntectonic basins reliably record paleotopography in the continental interior of western North America when compared to a sea-level reference. Here we show that this basin-MCC pair tracks meteoric fluid flow at different levels of actively extending crust in a high-topography region during Oligo-Miocene extension of the Basin and Range Province. For paleoaltimetry purposes we compare multi-proxy oxygen (δ 18O) and hydrogen (δD) isotope data as well as geochronological information from the Snake Range MCC to a time-equivalent (ca. 20Ma) stable isotopic proxy record from the Buckskin Mountains MCC (AZ, USA), which developed next to the Pacific Coast near Miocene sea level. We complement this paleoaltimetry study by comparing the Buckskin Mountains MCC data with older (~35Ma) lacustrine stable isotope and paleofloral records from the nearby House Range (UT, USA), whose paleoelevation has been determined independently through paleobotanical analysis. Each of the investigated compartments of the paleohydrologic system within the Snake Range MCC depicts a coherent scenario of low Oligo-Miocene δ 18O and δD values of meteoric water that reflect precipitation sourced at high elevation. A 77‰ difference in δD water between the Snake Range (δD water~-113‰) and the Buckskin Mountains (δD water~-36‰) is consistent with minimum mean paleoelevation of the Snake Range of about 3850±650m above Miocene sea level. Additional support for such elevations comes from a comparison between the Buckskin Mountains MCC and the Eocene House Range basin (UT, USA) where differences in δ 18O water values are consistent with 2300±500m minimum paleoelevation of the House Range. Based on the presence of brecciated rock-avalanche deposits within the Sacramento Pass Basin, we conclude that the Snake Range was a topographic high and locus of significant relief during regional scale extension within the Cordilleran hinterland. © 2012 Elsevier B.V

Oligo-Miocene extensional tectonics and fluid flow across the Northern Snake Range detachment system, Nevada

The Northern Snake Range (Nevada) represents a spectacular example of a metamorphic core complex and exposes a complete section from the mylonitic footwall into the hanging wall of a fossil detachment system. Paired geochronological and stable isotopic data of mylonitic quartzite within the detachment footwall reveal that ductile deformation and infiltration of meteoric fluids occurred between 27 and 23 Ma. 40Ar/39Ar ages display complex recrystallization-cooling relationships but decrease systematically from 26.9 ± 0.2 Ma at the top to 21.3 ± 0.2 Ma at the bottom of footwall mylonite. Hydrogen isotope (δD) values in white mica are very low (-150 to-145 ‰) within the top 80-90 m of detachment footwall, in contrast to values obtained from the deeper part of the section where values range from-77 to-64 ‰, suggesting that time-integrated interaction between rock and meteoric fluid was restricted to the uppermost part of the mylonitic footwall. Pervasive mica-water hydrogen isotope exchange is difficult to reconcile with models of 40Ar loss during mylonitization solely by volume diffusion. Rather, we interpret the 40Ar/ 39Ar ages of white mica with low-δD values to date syn-mylonitic hydrogen and argon isotope exchange, and we conclude that the hydrothermal system of the Northern Snake Range was active during late Oligocene (27-23 Ma) and has been exhumed by the combined effects of ductile strain, extensional detachment faulting, and erosion. Copyright 2011 by the American Geophysical Union

Eocene and Miocene extension, meteoric fluid infiltration, and core complex formation in the Great Basin (Raft River Mountains, Utah)

Metamorphic core complexes (MCCs) in the North American Cordillera reflect the effects of lithospheric extension and contribute to crustal adjustments both during and after a protracted subduction history along the Pacific plate margin. While the Miocene-to-recent history of most MCCs in the Great Basin, including the Raft River-Albion-Grouse Creek MCC, is well documented, early Cenozoic tectonic fabrics are commonly severely overprinted. We present stable isotope, geochronological (40Ar/39Ar), and microstructural data from the Raft River detachment shear zone. Hydrogen isotope ratios of syntectonic white mica (δ2Hms) from mylonitic quartzite within the shear zone are very low (-90‰ to -154‰, Vienna SMOW) and result from multiphase synkinematic interaction with surface-derived fluids. 40Ar/39Ar geochronology reveals Eocene (re)crystallization of white mica with δ2Hms ≥ -154‰ in quartzite mylonite of the western segment of the detachment system. These δ2Hms values are distinctively lower than in localities farther east (δ2Hms ≥ -125‰), where 40Ar/39Ar geochronological data indicate Miocene (18-15 Ma) extensional shearing and mylonitic fabric formation. These data indicate that very low δ2H surface-derived fluids penetrated the brittle-ductile transition as early as the mid-Eocene during a first phase of exhumation along a detachment rooted to the east. In the eastern part of the core complex, prominent top-to-the-east ductile shearing, mid-Miocene 40Ar/39Ar ages, and higher δ2H values of recrystallized white mica, indicate Miocene structural and isotopic overprinting of Eocene fabrics

Miocene paleoaltimetry of the Mt. Everest region

Abstract HKT-ISTP 2013 A

Impact of the Southern Ecuadorian Andes on Oxygen and Hydrogen Isotopes in Precipitation

Determining how the elevation of the Northern Andes has evolved over time is of paramount importance for understanding the response of the Northern Andes to deformational and geodynamic processes and its role as an orographic barrier for atmospheric vapor transport over geologic time. However, a fundamental requirement when using stable isotope data for paleoaltimetry reconstructions is knowledge about the present-day changes of δ18O and δD with elevation (isotopic lapse rate). This study defines the present-day river isotopic lapse rate near the Equator (∼3°S) based on analysis of δ18O and δD of surface waters collected from streams across the Western Cordillera and the Inter-Andean depression in Southern Ecuador. The results for the two domains show a decrease of δ18O with elevation which fits a linear regression with a slope of −0.18‰/100 m (R2 = 0.73, n = 83). However, we establish a present-day lapse rate of −0.15‰/100 m for δ18O (R2 = 0.88, n = 19) and -1.4‰/100 m for δD (R2 = 0.93, n = 19) from water samples collected along the west facing slopes of the Western Ecuadorian Cordillera which is mainly subject to moisture transport from the Pacific. We argue that this empirical relationship, consistent with those obtained in different tropical areas of the world, can inform stable isotope paleoaltimetry reconstructions in tropical latitudes.</jats:p

Synergistic effects of diachronous surface uplift and global climate change on the isotopic composition of meteoric waters: implications on paleoelevation estimates across the European Alps

Stable isotope paleoaltimetry is widely used to infer past elevations of orogens due to the robust systematic inverse relationships between elevation and oxygen (δ18O) and hydrogen (δD) isotopic composition of meteoric waters recorded in geologic archives, such as paleosol carbonates or hydrous silicates. This δ18O-elevation relationship (or isotopic lapse rate) is commonly attributed to the preferential rainout of heavy water isotopologues from air masses ascending over topography. However, numerous non-linear climatic processes, such as surface recycling, vapor mixing, variability in moisture source, and precipitation dynamics, can also influence the isotopic lapse rate and thus complicate stable isotope paleoaltimetry estimates. This highlights the need for a better quantitative understanding of topographic and regional climatic effects on the isotopic composition of ancient waters. Through topographic sensitivity experiments, Boateng et al. (2023) suggested plausible changes in isotopic lapse rates across the Alps in response to different diachronous surface uplift scenarios and validated that the expected isotopic signal difference due to elevation changes is significant enough to be reflected in geologic archives. Recent paleoelevation reconstructions across the Alps estimate the mean elevation of >4000 m in the Central Alps during the Middle Miocene (Krsnik et al., 2021). These high elevation estimates have been attributed to the complicated transition from pre- to mid-Miocene Central Alps with a diverse landscape and a complex topography, mainly driven by the rapid exhumation of deep-seated core complexes, followed by a rearrangement of the drainage system. However, the paleoelevation estimate is based on the assumptions that the isotopic lapse rate (1) is similar to the modern lapse rate (~2.0 ‰/km), which is lower than the global average, (2) did not change during the deposition of the paleoaltimetry proxies compared to the present day, and (3) remained constant across the entire Alps. Here, we use a high-resolution isotope-tracking ECHAM5-wiso General Circulation Model to simulate the Middle Miocene climate and δ18Op responses to different surface uplift scenarios of the Alps. More specifically, we performed topographic sensitivity experiments by varying the height of the Western/Central Alps and Eastern Alps under two atmospheric CO2 concentration scenarios for Middle Miocene paleoenvironmental conditions. The simulated δ18Op values are consistent with the proxy reconstructions across the low- and high-elevation sites in the Alps. The topographic scenarios indicated δ18Op values differences of up to -10 ‰ between the low- and high-elevation sites, primarily due to changes in orographic precipitation and local near-surface temperature. Even though the differences across the low-elevation sites showed minor changes compared to the present-day climate, the high-elevation sites indicated significant changes mainly due to differences in moisture transport and moisture redistribution. These changes resulted in different isotopic lapse rates across the different transects around the Alps, contradicting the assumption of a regionally similar isotopic lapse rate. Using the simulated Middle Miocene isotopic lapse rates with the reconstructed Δδ18Op signal between the low-elevation Northern Alpine Foreland Basin and high-elevation Simplon fault gouge reveals an overestimation of paleoelevation estimates by 2 km when compared to the constant isotopic lapse rate of -2.0 ‰/km across the Alps. These uncertainty estimates are an improvement of the previous paleoelevation reconstruction across the Alps and support the integration of paleoaltimetry and paleoclimate modelling to reconstruct past surface elevations accurately

The Alps Paleoelevation and Paleoclimate Experiment: Reconstructing Eastward Propagation of Surface Uplift in the ALps (REAL)

Geological observations, geodynamic models, and seismic studies suggest Neogene eastward propagating surface uplift of the European Alps. Whereas 4DMB Phase I project APE focused on reconstructing surface uplift of the Central Alps, 4DMB Phase II project REAL aims at testing the predicted west-to-east surface uplift of the Alps by combining stable isotope paleoaltimetry and paleoclimate modeling. Stable isotope paleoaltimetry is based on the inverse relationship between elevation and the stable isotopic composition of meteoric water and provides a tool to reconstruct the elevation of mountain belts in the geological past. First, REAL explores applications of the δ-δ method (see Poster Phase I APE), which requires that various recorders of past rainfall are available in the rock record: soil carbonates from low-elevation (foreland) basins and hydrous minerals from high-elevation fault gouges/shear zones. Paleoelevation estimates are obtained by contrasting time-equivalent low- and high-elevation proxy data sets, provided that the isotopic composition of the fluids during mineral formation is estimated accurately. Whereas formation temperatures of fault gouge minerals (such as illite and syntectonic micas) can be readily estimated, we apply clumped isotope paleothermometry to provide robust estimates of meteoric water δ18O from the low-elevation foreland basin carbonate record. Second, meteoric water δ18O values are not only sensitive to local elevation, but also to the complex climatic changes resulting from different paleoenvironmental boundary conditions and regional topographic configuration. To isolate the contribution of each of these components δ-δ stable isotope paleoaltimetry is applied in combination with ECHAM5-wiso paleoclimate simulations for a number of topographic scenarios of diachronous surface uplift. This unique combination allows for the removal of climate change effects on the stable isotope data, and therefore improves the accuracy of paleoelevation reconstructions. Results from our ongoing Phase II project (spring 2021 - spring 2024): 1. Reveal that diachronous surface uplift would produce patterns of climate, δ18O in precipitation values, and isotopic lapse rates that are distinctly different from those of today and those produced by bulk surface uplift scenarios. Importantly, this signal would be detectable in stable isotope paleoaltimetry results (Boateng et al., in revision). 2. Present a Miocene (23–13 Ma) continental paleotemperature record from the northern Mediterranean region (Digne-Valensole basin, SE France), which indicates near-constant temperatures from 23.0-18.8 Ma, followed by a highly variable and warm climate during the Middle Miocene and rapid cooling after 14 Ma (Ballian et al., 2023). 3. Together with new and existing paleotemperature records, preliminary results of the δ-δ method show for the first time that (a) the Central Alps were already high during the Early Miocene and (b) the Eastern Alps were appreciably lower than the Central Alps during the Middle Miocene (Ballian et al., 2022)

The effects of diachronous surface uplift of the European Alps on regional climate and the oxygen isotopic composition of precipitation

The European Alps are hypothesized to have experienced diachronous surface uplift in response to post-collisional processes such as, e.g., slab break-off. Therefore, understanding the geodynamic and geomorphic evolution of the Alps requires knowledge of its surface uplift history. This study presents the simulated response of regional climate and oxygen isotopic composition of precipitation (&delta;18Op) to different along-strike topographic evolution scenarios. These responses are modeled to determine if diachronous surface uplift in the Western and Eastern Alps would produce &delta;18Op signals in the geologic record that are sufficiently large and distinct for stable isotope paleoaltimetry. This is tested with a series of sensitivity experiments conducted with the water isotope tracking atmospheric General Circulation Model (GCM) ECHAM5-wiso. The topographic scenarios are created from the variation of two free parameters, (1) the elevation of the West-Central Alps and (2) the elevation of the Eastern Alps. Results suggest significant changes in the spatial patterns of &delta;18Op, the elevation-dependent rate of change in &delta;18Op (&ldquo;isotopic lapse rate&rdquo;), near-surface temperatures, precipitation amounts, and atmospheric circulation patterns in response to the different scenarios. The predictions for the diachronous surface uplift experiments are distinctly different from simulations forced with present-day topography and for simulations where the entire Alps experience synchronous surface uplift. Topographic scenarios with higher elevations in the West-Central Alps produce higher magnitude changes and an expansion of the affected geographical domain surrounding the Alps when compared to present-day topography. Furthermore, differences in &delta;18Op values of up to &minus;2 to &minus;8 &permil; are predicted along the strike of the Alps for the diachronous uplift scenarios, suggesting that the signal can be preserved and measured in geologic archives. Lastly, the results highlight the importance of sampling far-field and low-elevation sites using the &delta;-&delta; paleoaltimetry approach to discern between different surface uplift histories.</p

Phase equilibrium modelling of the amphibolite to granulite facies transition in metabasic rocks (Ivrea Zone, NW Italy)

The development of thermodynamic models for tonalitic melt and the updated clinopyroxene and amphibole models now allow the use of phase equilibrium modelling to estimate P–T conditions and melt production for anatectic mafic and intermediate rock types at high‐temperature conditions. The Permian mid‐lower crustal section of the Ivrea Zone preserves a metamorphic field gradient from mid amphibolite facies to granulite facies, and thus records the onset of partial melting in metabasic rocks. Interlayered metabasic and metapelitic rocks allows the direct comparison of P–T estimates and partial melting between both rock types with the same metamorphic evolution. Pseudosections for metabasic compositions calculated in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) system are presented and compared with those of metapelitic rocks calculated with consistent endmember data and a–x models. The results presented in this study show that P–T conditions obtained by phase equilibria modelling of both metabasic and metapelitic rocks give consistent results within uncertainties, allowing integration of results obtained for both rock types. In combination, the calculations for both metabasic and metapelitic rocks allows an updated and more precisely constrained metamorphic field gradient for Val Strona di Omegna to be defined. The new field gradient has a slightly lower dP/dT which is in better agreement with the onset of crustal thinning of the Adriatic margin during the Permian inferred in recent studies