Cretaceous to Middle Cenozoic Exhumation History of the Cordillera de Domeyko and Salar de Atacama Basin, Northern Chile

Spatiotemporal patterns of deformation and exhumation in the central Andes are key parameters for reconstructing the kinematic history of the orogenic belt. Previous studies of the retroarc thrust belt document overall eastward propagation of deformation since the late Eocene, but the amount and timing of exhumation during the early phase of Andean orogeny remains largely unconstrained, particularly in the modern forearc region. In order to determine the timing and amount of exhumation prior to the late Eocene, we employed a multidating approach combining zircon U-Pb geochronology with apatite fission track and apatite (U-Th)/He thermochronology. We focus on the low-temperature cooling history of the Cordillera de Domeyko thrust belt and synorogenic deposits in the Salar de Atacama basin. Our results show Late Cretaceous to Oligocene cooling and exhumation in the Cordillera de Domeyko. The distribution of cooling ages in the forearc indicates three periods of exhumation: 86-65, 65-50, and 50-28Ma. The amount of cooling was variable in space and time but requires total exhumation of 2.5-3.3km of rocks above major structures in the thrust belt. Regional unconformities in the Salar de Atacama basin correlate with periods of eastward migration of the orogenic front at 65Ma and 50-40Ma. Pulses of deformation at the front of the thrust belt alternated with periods of out-of-sequence hinterland deformation and exhumation. Overall, our data show that shortening in the central Andes commenced during the Late Cretaceous (as early as 86Ma) and that deformation (shortening) and exhumation were coupled in space and time.GSA Research Grant; U.S. National Science Foundation-Tectonics Program [EAR-071069]6 month embargo; published online: 26 December 2018This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Recognizing Allogenic Controls on the Stratigraphic Architecture of Ancient Alluvial Fans in the Western US

Alluvial fans are a significant part of the sediment routing system, forming distinctive steep, fan-shaped deposits of coarse-grained detritus where rivers lose flow velocity after exiting confined mountain drainages. Processes on the fan are influenced by both internal (autogenic) feedback cycles like channel avulsion and by external (allogenic) conditions such as climate and tectonics. These conditions in turn influence the stratigraphic architecture (i.e., the pattern of channel stacking and sizes) within the fan. Studying stratigraphic architecture of alluvial fans can, therefore, provide insight into controls on fan deposition. We employ UAV-based photogrammetric models to analyze the stratigraphic architecture of two well-exposed ancient alluvial fans in the western US - the Eocene Richards Mountain Conglomerate and the Cretaceous Echo Canyon Conglomerate. Both fans were deposited under relatively warm, wet climates and compressional tectonic regimes. We use a seven-fold hierarchy of bounding surfaces and associated lithosomes to describe alluvial fan architecture. First- through fourth-order surfaces and lithosomes represent bedform to channel-scale features influenced primarily by autogenic processes on the fan. Controls on fifth-order surfaces/lithosomes have historically been poorly understood, but probably represent fanhead trench migration and lobe construction. Sixth-order surfaces bound individual alluvial fans and seventh-order surfaces correspond to formation boundaries. These are controlled primarily by tectonics. The fifth-order architectural style of the deposits in our two study areas is significantly different and we use this difference to try to isolate a primary control on fifth-order alluvial architecture. Average width:height ratios of fifth-order lithosomes are nearly twice as high for Echo Canyon (112:1) than for Richards Mountain (64:1). This indicates that active channels on the Echo Canyon fan were more mobile than those on the Richards Mountain fan. We attribute this to a more seasonal climate and less vegetation during the deposition of the Echo Canyon Conglomerate (relative to Richards Mountain). This would have increased lateral migration by destabilizing channels through increased sediment flux and flood events. Our results imply that fifth-order stratigraphic architecture of ancient alluvial fans may provide insight into allogenic processes related to paleoclimate. They also indicate risk of increased geologic hazards on alluvial fans where anthropogenic climate change increases future climate variability.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Rapid Geodetic Shortening Across the Eastern Cordillera of NW Argentina Observed by the Puna-Andes GPS Array

We present crustal velocities for 29 continuously recording GPS stations from the southern central Andes across the Puna, Eastern Cordillera, and Santa Barbara system for the period between the 27 February 2010 Maule and 1 April 2014 Iquique earthquakes in a South American frame. The velocity field exhibits a systematic decrease in magnitude from ~35 mm/yr near the trench to <1 mm/yr within the craton. We forward model loading on the Nazca-South America (NZ-SA) subduction interface using back slip on elastic dislocations to approximate a fully locked interface from 10 to 50 km depth. We generate an ensemble of models by iterating over the percentage of NZ-SA convergence accommodated at the subduction interface. Velocity residuals calculated for each model demonstrate that locking on the NZ-SA interface is insufficient to reproduce the observed velocities. We model deformation associated with a back-arc décollement using an edge dislocation, estimating model parameters from the velocity residuals for each forward model of the subduction interface ensemble using a Bayesian approach. We realize our best fit to the thrust-perpendicular velocity field with 70 ± 5% of NZ-SA convergence accommodated at the subduction interface and a slip rate of 9.1 ± 0.9 mm/yr on the fold-thrust belt décollement. We also estimate a locking depth of 14 ± 9 km, which places the downdip extent of the locked zone 135 ± 20 km from the thrust front. The thrust-parallel component of velocity is fit by a constant shear strain rate of −19 × 10−9 yr−1, equivalent to clockwise rigid block rotation of the back arc at a rate of 1.1°/Myr.Fil: Mcfarland, Phillip K.. University of Arizona; Estados UnidosFil: Bennett, Richard A.. University of Arizona; Estados UnidosFil: Alvarado, Patricia Monica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Decelles, Peter G.. University of Arizona; Estados Unido

Lithospheric evolution of the Andean fold-thrust belt

Abstract We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 188 and 208 S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by~70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold-thrust belt at 40 Ma through the eastward propagation of a 200-400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera-Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24-23 Ma, and further volcanism recorded by 12-7 Ma crustal ignimbrites. 4) After~20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold-thrust belt indicate that the average shortening rate has remained fairly constant (~8-10 mm/year) through time with possible slowing (~5-7 mm/year) in the last 15-20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera-Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination.

The Liuqu Conglomerate, southern Tibet: Early Miocene basin development related to deformation within the Great Counter Thrust system

The rapid pace of climate and environmental changes requires some degree of adaptation, to forestall or avoid severe impacts. Adaptive capacity and water security are concepts used to guide the ways in which resource managers plan for and manage change. Yet the assessment of adaptive capacity and water security remains elusive, due to flaws in guiding concepts, paucity or inadequacy of data, and multiple difficulties in measuring the effectiveness of management prescriptions at scales relevant to decision-making. We draw on conceptual framings and empirical findings of the thirteen articles in this special issue and seek to respond to key questions with respect to metrics for the measurement, governance, information accessibility, and robustness of the knowledge produced in conjunction with ideas related to adaptive capacity and water security. Three overarching conclusions from this body of work are (a) systematic cross comparisons of metrics, using the same models and indicators, are needed to validate the reliability of evaluation instruments for adaptive capacity and water security, (b) the robustness of metrics to applications across multiple scales of analysis can be enhanced by a 'metrics plus' approach that combines well-designed quantitative metrics with in-depth qualitative methods that provide rich context and local knowledge, and (c) changes in the governance of science policy can address deficits in public participation, foster knowledge exchange, and encourage the co-development of adaptive processes and approaches (e.g., risk-based framing) that move beyond development and use of static indicators and metrics.12 month embargo; First Published on July 28, 2016This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Extensional Collapse along the Sevier Desert Reflection, Northern Sevier Desert Basin, Western United States: Comment and Reply

Comment and reply on the Sevier Desert reflection debate. Comment abstract: Coogan and DeCelles (1996) provided a welcome addition to the debate on the Sevier Desert reflection. The evidence and arguments presented on the nature of this subsurface feature merit particular scrutiny, as they bear directly on a first order issue in tectonics: the mechanical paradox of low-angle normal faults. Field geologists have argued that in some cases such faults must have moved at dips of 20° or less; tectonophysicists maintain that such interpretations are inconsistent with our present knowledge of rock mechanics, and seismologists have yet to record a single earthquake that can be related unequivocally to slip on a low-angle normal fault. If, as Coogan and DeCelles (1996) and others have argued, the seismically imaged Sevier Desert reflection of west-central Utah is a rooted detachment fault with as much as 39 km of top-to-the-west slip, the seismic-reflection geometry effectively requires normal-sense slip on a surface dipping 11°. We believe, however, that geometry can also support alternative interpretations. Reply abstract: In their comparison of two seismic reflection profiles across the western margin of the Sevier Desert basin, Anders et al. fall victims to a classic pitfall of seismic interpretation—the misinterpretation of multiples as primary stratal reflections. Their error lies in ignoring the acquisition parameters of the two data sets and overlooking fundamental characteristics of long-path multiples. The industry profiles in their are published in Mitchell and McDonald (1987) for detailed inspection. We focus on two attributes of long-path multiples to document the error in Anders et al.’s analysis: the effects of fold on multiple identification and attenuation, and the periodicity of multiples