Catálogo de publicaciones - revistas

<jats:p>Silcretes have long been recognized as modern and ancient duricrusts, but more recently also as silicified layers precipitated at groundwater tables, termed groundwater silcrete. However, the silica sources, transport mechanisms, and precipitation processes of groundwater silcrete are not well understood, and models are limited to the near-surface groundwater environment, where silica saturation is low. Here, an example of a groundwater silcrete from Upper Cambrian strata of the Potsdam Group is described and interpreted to be formed in a rift where Cambrian fault reactivation coincided with silcrete formation. Field relationships strongly support a connection between fault activity and silicification, including a systematic thickening and development of massive silcrete horizons above shear zones, brecciated silcrete near where faults intersect shear zones, and nodules along the margins of shear zones. Petrographic and cathodoluminescence microscopy of silcrete reveal early pre-compaction overgrowth cements with abundant primary fluid inclusions. Fluid inclusion microthermometry indicates that these fluids were high salinity (22.7–25.8 eq. wt% NaCl+CaCl2) brines with homogenization temperatures of ~120.2 °C–151.6 °C, which implies that silica precipitated from a hot, silica-saturated crustal brine from Grenville Province basement. A combination of weathering reactions and direct quartz dissolution explains the chemical evolution of the source fluid, which likely originated as infiltrated meteoric water that had chemically equilibrated with Grenville crust at depth. Later, this brine was mobilized upward along reactivated faults during the Late Cambrian, and ultimately to the water table, where a combination of reduced pH and temperature promoted quartz supersaturation and quartz overgrowths on detrital quartz. This case example, therefore, expands the definition of silcrete to include near-surface silicification from externally sourced crustal fluids, here termed brine silcrete, and provides a basis for interpreting silcrete as a feature of deformation and fluid migration along shear zones in fault-bounded continental basins.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Plutons offer an opportunity to study the extended history of magmas at depth. Fully exploiting this record requires the ability to track changes in magmatic plumbing systems as magma intrudes, crystallizes, and/or mixes through time. This task has been difficult in granitoid plutons because of low sampling density, poorly preserved or cryptic intrusive relationships, and the difficulty of identifying plutonic volumes that record the contemporaneous presence of melt. In particular, the difficulty in delineating fossil magma reservoirs has limited our ability to directly test whether or not high-SiO2 rhyolite is the result of crystal-melt segregation. We present new high-precision U-Pb zircon geochronologic and geochemical data that characterize the Miocene Searchlight pluton in southern Nevada, USA. The data indicate that the pluton was built incrementally over ~1.5 m.y. with some volumes of magma completely crystallizing before subsequent volumes arrived. The largest increment is an ~2.7-km-thick granitic sill that records contemporaneous zircon crystallization, which we interpret to represent a fossil silicic magma reservoir within the greater Searchlight pluton. Whole-rock geochemical data demonstrate that this unit is stratified relative to paleo-vertical, consistent with gravitationally driven separation of high-SiO2 melt from early-formed crystals at moderate crystallinity. Zircon trace-element compositions suggest that our geochronologic data from this unit record most of the relevant crystallization interval for differentiation and that this process occurred in &amp;lt;150 k.y.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Lithium is of great interest as a tracer of metamorphic reactions and related fluid-mineral interactions because of its potential to isotopically fractionate during inter- and intracrystalline diffusional processes. Study of its transfer through subduction zones, based on study of arc volcanic and metamorphic rocks, can yield insight regarding ocean-to-mantle chemical cycling.</jats:p> <jats:p>We investigated major- and trace-element concentrations and δ7Li in garnet in ultrahigh-pressure (UHP) Lago di Cignana metasedimentary rocks, relating these observations to reconstructed prograde devolatilization history. In all garnet crystals we studied, heavy rare earth elements (HREEs), Y, and Li showed strong zoning, with elevated concentrations in cores (15–50 ppm Li) and marked high-concentration anomalies (up to 117 ppm Li, 5500 ppm Y; little or no major-element shift) as growth annuli, in which some crystals showed subtle elevation in δ7Li greater than analytical error of ~3% (2σ). Rutile inclusions appeared abruptly at annuli and outward toward rims, accompanied by inclusions of a highly zoned, Ca- and rare earth element–rich phase and decreased Nb concentrations in garnet. These relationships are interpreted to reflect prograde garnet-forming reaction(s), in part involving titanite breakdown to stabilize rutile, which resulted in delivery of more abundant Y and HREEs at surfaces of growing garnet crystals to produce annuli. Co-enrichments in Li and Y + REEs are attributed to mutual incorporation via charge-coupled substitutions; thus, increased Li uptake was a passive consequence of elevated concentrations of Y + REEs. The small-scale fluctuations in δ7Li (overall range of ~9%) observed in some crystals may correlate with abrupt shifts in major- and trace-element concentrations, suggesting that changes in reactant phases exerted some control on the evolution of δ7Li. For one garnet crystal, late-stage growth following partial resorption produced deviation in major- and trace-element compositions, including Li concentration, accompanied by a 10%–15% negative shift in δ7Li, perhaps reflecting a change in the mechanism of incorporation or source of Li.</jats:p> <jats:p>These results highlight the value of measuring the major- and trace-element and isotope compositions of garnets in high-pressure and UHP metamorphic rocks in which matrix mineral assemblages are extensively overprinted by recrystallization during exhumation histories. Lithium concentrations and isotope compositions of the garnets can add valuable information regarding prograde (and retrograde) reaction history, kinetics of porphyroblast growth, intracrystalline diffusion, and fluid-rock interactions. This work, integrated with previous study of devolatilization in the Schistes Lustrés/Cignana metasedimentary suite, indicates retention of a large fraction of the initially subducted sedimentary Li budget to depths approaching those beneath volcanic fronts, despite the redistribution of this Li among mineral phases during complex mineral reaction histories.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Oblique convergence along strike-slip faults can lead to both distributed and localized deformation. How focused transpressive deformation is both localized and maintained along sub-vertical wrench structures to create high topography and deep exhumation warrants further investigation. The high peak region of the Hayes Range, central Alaska, USA, is bound by two lithospheric scale vertical faults: the Denali fault to the south and Hines Creek fault to the north. The high topography area has peaks over 4000 m and locally has experienced more than 14 km of Neogene exhumation, yet the mountain range is located on the convex side of the Denali fault Mount Hayes restraining bend, where slip partitioning alone cannot account for this zone of extreme exhumation. Through the application of U-Pb zircon, 40Ar/39Ar (hornblende, muscovite, biotite, and K-feldspar), apatite fission-track, and (U-Th)/He geo-thermochronology, we test whether these two parallel, reactivated suture zone structures are working in tandem to vertically extrude the Between the Hines Creek and Denali faults block on the convex side of the Mount Hayes restraining bend. We document that since at least 45 Ma, the Denali fault has been bent and localized in a narrow fault zone (&amp;lt;160 m) with a significant dip-slip component, the Mount Hayes restraining bend has been fixed to the north side of the Denali fault, and that the Between the Hines Creek and Denali faults block has been undergoing vertical extrusion as a relatively coherent block along the displacement “free faces” of two lithospheric scale suture zone faults. A bent Denali fault by ca. 45 Ma supports the long-standing Alaska orocline hypothesis that has Alaska bent by ca. 44 Ma. Southern Alaska is currently converging at ~4 mm/yr to the north against the Denali fault and driving vertical extrusion of the Between the Hines Creek and Denali faults block and deformation north of the Hines Creek fault. We apply insights ascertained from the Between the Hines Creek and Denali faults block to another region in southern Alaska, the Fairweather Range, where extreme topography and persistent exhumation is also located between two sub-parallel faults, and propose that this region has likely undergone vertical extrusion along the free faces of those faults.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>This study documents seismic reflection evidence that two different bolide impacts significantly disrupted stratigraphic and depositional processes on the West Florida Platform (Gulf of Mexico). The first impact terminated the Late Cretaceous Epoch (Chicxulub impact, Mexico; ca. 66 Ma; end-Maastrichtian age). The second took place in the late Eocene (Chesapeake Bay impact, Virginia, USA, portion of the Chesapeake Bay; ca. 35 Ma; Priabonian age). Both impacts produced far-reaching seismic shaking and ground roll followed by an impact-generated tsunami, the effects of which are evident in the seismostratigraphic record. The Chicxulub seismic shaking caused collapse and shoreward retreat of the Florida Escarpment and widely disrupted (faulting, folding, slumping) normal flat-lying shelf beds. The associated tsunami currents redistributed these shelf deposits and mixed them together with collapse debris from the escarpment to form a thick wedge of sediments along the base of the escarpment. The Chesapeake Bay impact created a mounded sedimentary deposit near the outer edge of the late Eocene ramp slope. This deposit also has a bipartite origin. A lower layer is marked by en echelon faulting created in situ by seismic shaking, whereas an upper layer represents sediments redistributed from the late Eocene shelf and upper ramp slope by tsunami-driven bottom currents (debris flows, contour currents, slumps). This is the first report of seismic effects from the Chesapeake Bay impact in the Gulf of Mexico. These results further demonstrate that large-scale marine bolide impacts have widespread effects on the stratigraphic and depositional record of Earth.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The complex 2016 MW 7.8 Kaikōura earthquake ruptured &amp;gt;20 faults and caused highly variable uplift and subsidence of an ~110 km stretch of coastline. The earthquake raised questions about fault interactions in regions of oblique convergence and especially subduction to strike-slip transition zones like the Kaikōura region. We integrate 2016 coastal vertical deformation observations with new mapping and dating of Holocene marine terraces to: (1) compare spatial patterns of 2016 coseismic and longer-term vertical motions, (2) investigate possible past multi-fault ruptures or temporal clusters of earthquakes around Kaikōura, and (3) assess the relative contributions of crustal faults and the Hikurangi subduction interface to late Holocene coastal uplift. We identify possible multi-fault ruptures or loose clusters of earthquakes at ca. 850–550 yr B.P. and ca. 350–100 yr B.P. Most (and possibly all) of the Kaikōura coast has been uplifted over the late Holocene; the 25-km-long Parikawa section of coast subsided coseismically in 2016 but appears to be uplifted through reverse slip on an offshore fault. Late Holocene uplift everywhere along the coastline of interest can be attributed to slip on known upper-plate faults; slip on a shallow-dipping (&amp;lt;20°) subduction interface cannot be ruled out but is not required to explain uplift.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Longshou Shan of western China is the northern backstop of the Cenozoic Himalayan-Tibetan orogen and occupies a key linkage between the Tarim continent and North China craton which separate the pre-Cenozoic Tethyan orogenic system and Central Asian orogenic system. Therefore, the Paleoproterozoic–Paleozoic evolution of this region is critical to understanding the extent of overprinting Cenozoic deformation, construction of the Eurasian continent, and relationships between the pre-Cenozoic Tethyan orogenic system and Central Asian orogenic system. Here we present detailed field observations and results of geochronological and major and trace element and Sr-Nd isotope geochemical analyses of samples from the Longshou Shan to decipher its complex Paleoproterozoic–Paleozoic tectonic history. Our results show that the Paleoproterozoic basement rocks of the Longshou Shan were part of the North China craton and involved in Paleoproterozoic northern North China orogeny. A ca. 965 Ma granitoid in the Longshou Shan provides key evidence for a spatial linkage between northern Tibetan continents, the North Tarim continent, and the North China craton in the early Neoproterozoic. The presence of Early Ordovician granitoids and arc volcanic rocks in the Longshou Shan suggest that bivergent subduction of Qilian oceanic lithosphere occurred during the early Paleozoic. Crustal shortening and thickening during Ordovician–Carboniferous orogenesis are evidenced by the presence of several unconformities in the Longshou Shan. Late Carboniferous arc granites exposed in the study area are likely associated with the southward subduction of the Paleo-Asian Ocean to the north and with Permian siliciclastic strata sourced from a proximal arc-subduction system, based on detrital zircon ages. Although the tectonic history of the Longshou Shan can be traced back to Neoproterozoic time, most of the recorded deformation and uplift of the region occurred during the early Paleozoic Qilian orogeny and late Paleozoic Central Asian orogeny. Furthermore, we interpret that the several orogenic events recorded in the Longshou Shan (i.e., northern North China, Qilian, and Central Asian orogenies) are spatially and temporally correlative along strike with those recorded in the Tarim and North China cratons.</jats:p>

<jats:p>A series of large earthquakes in 1899 affected southeastern Alaska near Yakutat and Disenchantment Bays. The largest of the series, a MW 8.2 event on 10 September 1899, generated an ~12-m-high tsunami and as much as 14.4 m of coseismic uplift in Yakutat Bay, the largest coseismic uplift ever measured. Several complex fault systems in the area are associated with the Yakutat terrane collision with North America and the termination of the Fairweather strike-slip system, but because faults local to Yakutat Bay have been incompletely or poorly mapped, it is unclear which fault system(s) ruptured during the 10 September 1899 event. Using marine geophysical data collected in August 2012, we provide an improved tectonic framework for the Yakutat area, which advances our understanding of earthquake hazards. We combined 153 line km of 2012 high-resolution multichannel seismic (MCS) reflection data with compressed high-intensity radar pulse (Chirp) profiles, basin-scale MCS data, 2018 seafloor bathymetry, published geodetic models and thermochronology data, and previous measurements of coseismic uplift to better constrain fault geometry and subsurface structure in the Yakutat Bay area. We did not observe any active or concealed faults crossing Yakutat Bay in our high-resolution data, requiring faults to be located entirely onshore or nearshore. We interpreted onshore faults east of Yakutat Bay to be associated with the transpressional termination of the Fairweather fault system, forming a series of splay faults that exhibit a horsetail geometry. Thrust and reverse faults on the west side of the bay are related to Yakutat terrane underthrusting and collision with North America. Our results include an updated fault map, structural model of Yakutat Bay, and quantitative assessment of uncertainties for legacy geologic coseismic uplift measurements. Additionally, our results indicate the 10 September 1899 rupture was possibly related to stress loading from the earlier Yakutat terrane underthrusting event of 4 September 1899, with the majority of 10 September coseismic slip occurring on the Esker Creek system on the northwest side of Yakutat Bay. Limited (~2 m) coseismic or postseismic slip associated with the 1899 events occurred on faults located east of Yakutat Bay.</jats:p>

<jats:p>The Kodiak Islands lie near the southern terminus of the 1964 Great Alaska earthquake rupture area and within the Kodiak subduction zone segment. Both local and trans-Pacific tsunamis were generated during this devastating megathrust event, but the local tsunami source region and the causative faults are poorly understood. We provide an updated view of the tsunami and earthquake hazard for the Kodiak Islands region through tsunami modeling and geophysical data analysis. Using seismic and bathymetric data, we characterize a regionally extensive seafloor lineament related to the Kodiak shelf fault zone, with focused uplift along a 50-km-long portion of the newly named Ugak fault as the most likely source of the local Kodiak Islands tsunami in 1964. We present evidence of Holocene motion along the Albatross Banks fault zone, but we suggest that this fault did not produce a tsunami in 1964. We relate major structural boundaries to active forearc splay faults, where tectonic uplift is collocated with gravity lineations. Differences in interseismic locking, seismicity rates, and potential field signatures argue for different stress conditions at depth near presumed segment boundaries. We find that the Kodiak segment boundaries have a clear geophysical expression and are linked to upper-plate structure and splay faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf fault zone when compared to the nearby Albatross Banks fault zone, suggesting short wave travel paths and little tsunami warning time for nearby communities.</jats:p>