• Boundary condition controls on the high-sand-flux regions of Mars

      Chojnacki, Matthew; Banks, Maria E.; Fenton, Lori K.; Urso, Anna C.; Univ Arizona, Lunar & Planetary Lab (GEOLOGICAL SOC AMER, INC, 2019-05)
      Wind has been an enduring geologic agent throughout the history of Mars, but it is often unclear where and why sediment is mobile in the current epoch. We investigated whether eolian bed-form (dune and ripple) transport rates are depressed or enhanced in some areas by local or regional boundary conditions (e.g., topography, sand supply/availability). Bed-form heights, migration rates, and sand fluxes all span two to three orders of magnitude across Mars, but we found that areas with the highest sand fluxes are concentrated in three regions: Syrtis Major, Hellespontus Montes, and the north polar erg. All regions are located near prominent transition zones of topography (e.g., basins, polar caps) and thermophysical properties (e.g., albedo variations); these are not known to be critical terrestrial boundary conditions. The two regions adjacent to major impact basins (Hellas and Isidis Planitia) showed radially outward upslope winds driving sand movement, although seasonally reversing wind regimes were also observed. The northern polar dunes yielded the highest known fluxes on the planet, driven by summer katabatic winds modulated by the seasonal CO2 cap retreat-processes not known to affect terrestrial dunes. In contrast, southern dune fields (<45 degrees S) were less mobile, likely as a result of seasonal frost and ground ice suppressing sand availability. Results suggest that, unlike on Earth, large-scale topographic and thermophysical variabilities play a leading role in driving sand fluxes on Mars.
    • Cretaceous shortening and exhumation history of the South Pamir terrane

      Chapman, James B.; Robinson, Alexander C.; Carrapa, Barbara; Villarreal, Dustin; Worthington, James; DeCelles, Peter G.; Kapp, Paul; Gadoev, Mustafo; Oimahmadov, Ilhomjon; Gehrels, George; et al. (GEOLOGICAL SOC AMER, INC, 2018-08)
      Despite Miocene extension and exhumation of middle to lower crust in a series of gneiss domes and interpreted Cenozoic delamination of the lower crust, the crust in the modern Pamir Mountains is among the thickest in the world. Cenozoic shortening, crustal thickening, and prograde metamorphism in the Pamir have been associated with India-Asia collision. However, new mapping in the South Pamir terrane indicates relatively minor, distributed shortening since the Jurassic, which occurs in a thrust belt overprinted by late Cenozoic transpression. The thrust belt connects with the Rushan-Pshart suture zone, a Mesozoic terrane boundary. New detrital zircon U-Pb and detrital zircon fission track ages of synorogemc clastic rocks exposed in the footwall of thrust faults in the South Pamir thrust belt provide maximum deposition ages (76-112 Ma), which are interpreted to document Cretaceous shortening prior to India-Asia collision. Furthermore, zircon (U-Th)/He and apatite (U-Th)/He data from the South Pamir terrane generally record cooling ages of ca. 102-44 Ma, suggesting limited Cenozoic exhumation. These results (1) are consistent with widespread Cretaceous deformation throughout the Pamir-Tibet orogen with limited Cenozoic upper crustal shortening in the South Pamir terrane, (2) together with previous studies, allow for the possibility that the upper crust of the Pamir orogen was characterized by net extension during the Cenozoic rather than net shortening, and (3) are consistent with models that relate Cenozoic crustal thickening to the insertion of Indian lower crust beneath the Pamir. Lower crustal thickening of the South Pamir terrane is difficult to reconcile with the prograde metamorphic history of gneiss domes in the South Pamir terrane and may require a relatively shallow (<15-20 km) shear zone separating lower crustal contraction from upper crustal extension.
    • East African weathering dynamics controlled by vegetation-climate feedbacks

      Ivory, Sarah J.; McGlue, Michael M.; Ellis, Geoffrey S.; Boehlke, Adam; Lézine, Anne-Marie; Vincens, Annie; Cohen, Andrew S.; Univ Arizona, Dept Geosci (GEOLOGICAL SOC AMER, INC, 2017-09)
      Tropical weathering has important linkages to global biogeochemistry and landscape evolution in the East African rift. We disentangle the influences of climate and terrestrial vegetation on chemical weathering intensity and erosion at Lake Malawi using a long sediment record. Fossil pollen, microcharcoal, particle size, and mineralogy data affirm that the detrital clays accumulating in deep water within the lake are controlled by feedbacks between climate and hinterland forest composition. Particle-size patterns are also best explained by vegetation, through feedbacks with lake levels, wildfires, and erosion. We develop a new source-to-sink framework that links lacustrine sedimentation to hinterland vegetation in tropical rifts. Our analysis suggests that climate-vegetation interactions and their coupling to weathering/erosion could threaten future food security and has implications for accurately predicting petroleum play elements in continental rift basins.
    • Effects of Miocene–Pliocene global climate changes on continental sedimentation: A case study from the southern Central Andes

      Goddard, Andrea Stevens; Carrapa, Barbara; Univ Arizona, Dept Geosci (GEOLOGICAL SOC AMER, INC, 2018-07)
      Sedimentation rates are valuable proxies for changes in tectonics, climate, and sediment routing systems. We use sedimentation rates from the Bermejo foreland basin of the southern Central Andes to evaluate the role of global Miocene-Pliocene climate changes on continental erosion and sedimentation in non-glaciated landscapes. Our compilation identifies a tripling of sedimentation rates between ca. 10 and 8.5 Ma coinciding with a period of short-lived global warming and increased seasonality, and a decrease by half in sedimentation rates between ca. 6 and 5 Ma coinciding with increased global cooling and aridity. Both the increase and decrease in sedimentation rates occured during periods of heightened tectonic activity. Our results suggest that periods of aridity can reduce erosion and mask contemporaneous tectonic signals, and that more humid, variable climate conditions amplify the signal of tectonic forcing in the sedimentary record. This work shows that changes in sedimentation rates can accurately filter climatic variabilities out of the overprinting tectonic signal.
    • Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet

      Laskowski, Andrew K.; Kapp, Paul; Cai, Fulong; Univ Arizona, Dept Geosci (GEOLOGICAL SOC AMER, INC, 2018)
      The mechanisms for crustal thickening and exhumation along the Yarlung (IndiaAsia) suture in southern Tibet are under debate, because the magnitudes, relative timing, and interaction between the two dominant structures-the Great Counter thrust and Gangdese thrust-are largely unconstrained. In this study, we present new geologic mapping results from the Yarlung suture zone in the Lazi region, located similar to 350 km west of the city of Lhasa, along with new igneous (5 samples) and detrital (5 samples, 474 ages) U-Pb geochronology data to constrain the crystallization ages of Jurassic- Paleocene Gangdese arc rocks, the provenance of Tethyan Himalayan and Oligocene-Miocene Kailas Formation strata, and the minimum age (ca. 10 Ma) of the Great Counter thrust system. We supplement these data with a compilation of 124 previously published thermochronologic ages from Gangdese batholith, Kailas Formation, and Liuqu Formation rocks, revealing a dominance of 23-15 Ma cooling contemporaneous with slip across the Great Counter thrust system and other potentially linked structures. These data are systematically younger than 98 additional compiled thermochronologic ages from the northern Lhasa terrane, recording mainly Eocene cooling. Structural and thermochronologic data were combined with regional geological constraints, including International Deep Profiling of Tibet and the Himalaya (INDEPTH) seismic reflection data, to develop a new structural model for the Oligocene-Miocene evolution of the Tethyan Himalaya, Yarlung suture zone, and southern Lhasa terrane. We propose that a hinterland-dipping duplex beneath the Gangdese mountains, of which the Gangdese thrust is a component, is kinematically linked with a foreland-dipping passive roof duplex along the Yarlung suture zone, the Great Counter thrust system. The spatial and temporal convergence between the proposed duplex structures along the Yarlung suture zone and the South Tibetan detachment system indicate that they may be kinematically linked, though this relationship is not directly addressed in this study. Our interpretation, referred to as the Gangdese culmination model, explains why the Gangdese thrust system is only locally exposed (at relatively deeper structural levels) and provides a structural explanation for early Miocene crustal thickening along the Yarlung suture zone, relief generation along the modern Gangdese Mountains, early Miocene Yarlung River establishment, and creation of the modern internal drainage boundary along the southern Tibetan Plateau. The progression of deformation along the suture zone is consistent with tectonic models that implicate subduction dynamics as the dominant control on crustal deformation.
    • High-pressure Tethyan Himalaya rocks along the India-Asia suture zone in southern Tibet

      Laskowski, Andrew K.; Kapp, Paul; Vervoort, Jeff D.; Ding, Lin; Department of Geosciences, University of Arizona (GEOLOGICAL SOC AMER, INC, 2016-10)
      This study documents an early Cenozoic continental high-pressure (HP) metamorphic complex along the Yarlung (India-Asia) suture zone in southern Tibet. The complex is exposed in the Lopu Range, located similar to 600 km west of the city of Lhasa. HP rocks in the core of the complex have Indian passive-margin (Tethyan Himalaya Sequence) protoliths and are exposed in the footwall of a top-to-the-north, normal-sense shear zone. Phengite geobarometry, Zr-in-rutile geothermometry, and pseudosection modeling indicate that these rocks reached pressures >= 1.4 GPa at temperatures <= 600 degrees C. A meta-Tethyan graywacke yielded a garnet Lu-Hf date of 40.4 +/- 1.4 Ma, which is interpreted as the age of prograde metamorphism. Five Ar-Ar phengite ages between 39 and 34 Ma are interpreted to record the timing of exhumation to midcrustal depths (similar to 25 km) and concomitant retrogression. The structural geometry and pressure-temperature-time (P-T-t) history of Lopu Range rocks are similar to the Tso Morari and Kaghan Valley complexes, located >700 km to the northwest along the Indus suture zone. However, peak metamorphism and exhumation occurred similar to 6 m.y. later in the Lopu Range, and no ultrahigh-pressure assemblages have been identified. We propose a tectonic model that involves steep subduction of the Tethyan Himalaya continental margin at ca. 40 Ma, initial exhumation of HP metasedimentary rocks at ca. 39 Ma, and subsequent northward underthrusting of Greater Indian lithosphere shutting off Gangdese arc magmatism at ca. 38 Ma.
    • Intracontinental subduction beneath the Pamir Mountains: Constraints from thermokinematic modeling of shortening in the Tajik fold-and-thrust belt

      Chapman, James B.; Carrapa, Barbara; Ballato, Paolo; DeCelles, Peter G.; Worthington, James; Oimahmadov, Ilhomjon; Gadoev, Mustafo; Ketcham, Richard; Univ Arizona, Dept Geosci (GEOLOGICAL SOC AMER, INC, 2017)
      A regional, balanced cross section is presented for the thin-skinned Tajik fold-and-thrust belt, constrained by new structural and stratigraphic data, industrial well-log data, flexural modeling, and existing geologic and geophysical mapping. A sequential restoration of the section was calibrated with 15 new apatite (U-Th)/He ages and 7 new apatite fission-track ages from samples of the major thrust sheets within the Tajik fold-and-thrust belt. Thermokinematic modeling indicates that deformation in the Tajik fold-and-thrust belt began during the Miocene (prior to or ca. 17 Ma) and continues to near present, with long-term shortening rates of similar to 4-6 mm/yr and Pliocene to present rates of similar to 6-8 mm/yr. The Tajik fold-and-thrust belt can be characterized as two distinct, oppositely verging thrust belts. Deformation initiated at opposite margins of the Tajik foreland basin, adjacent the southwest Tian Shan and northwest Pamir Mountains, and propagated toward the center of the basin, eventually incorporating the foreland basin entirely into a composite fold-and-thrust belt. The western Tajik fold-and-thrust belt records at least 35-40 km of total shortening and is part of the greater Tian Shan orogenic system. The eastern Tajik fold-and-thrust belt records similar to 30 km of shortening linked to the Pamir Mountains. The amount of shortening in the Tajik fold-and-thrust belt is significantly less than predicted by models of intracontinental subduction, which call for subduction of an similar to 300-km-long slab of continental Tajik-Tarim lithosphere beneath the Pamir. Field observations and structural relationships suggest that the Mesozoic and younger sedimentary rocks of the Tajik Basin were deposited on and across the Northern Pamir terrane and then subsequently uplifted and eroded during orogenic growth, rather than undergoing subduction beneath the Pamir. The Paleozoic-Proterozoic(?) metasedimentary and igneous rocks exposed in the Northern Pamir terrane are equivalent to the middle-lower crust of the Tajik Basin, which has become incorporated into the Pamir orogen. We propose that the southdipping zone of deep seismicity beneath the Pamir, which is the basis for the intra-continental subduction model, is related to gravitational foundering (by delamination or large-scale dripping) of Pamir lower crust and mantle lithosphere. This contrasts with previous models that related the Pamir seismic zone to subduction with or without roll-back of Asian lithosphere. Delamination may explain the initiation of extension in the Pamir gneiss domes and does not require a change in plate boundary forces to switch between compressional and extensional regimes. Because the Pamir is the archetype for active subduction of continental lithosphere in the interior of continental plates (intracontinental subduction), the viability of this particular tectonic process may need to be reassessed.
    • The Liuqu Conglomerate, southern Tibet: Early Miocene basin development related to deformation within the Great Counter Thrust system

      Leary, Ryan J.; DeCelles, Peter G.; Quade, Jay; Gehrels, George E.; Waanders, Gerald; Department of Geosciences, University of Arizona (GEOLOGICAL SOC AMER, INC, 2016-10)
      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.
    • Paleo-Asian oceanic slab under the North China craton revealed by carbonatites derived from subducted limestones

      Chen, Chunfei; Liu, Yongsheng; Foley, Stephen F.; Ducea, Mihai N.; He, Detao; Hu, Zhaochu; Chen, Wei; Zong, Keqing; Department of Geosciences, University of Arizona (GEOLOGICAL SOC AMER, INC, 2016-12)
      It is widely accepted that the lithospheric mantle under the North China craton (NCC) has undergone comprehensive refertilization due to input from surrounding subducted slabs. However, the possible contribution from the Paleo-Asian oceanic slab to the north is poorly constrained, largely because of the lack of convincing evidence for the existence of this slab under the NCC. We report here carbonatite intruding Neogene alkali basalts in the Hannuoba region, close to the northern margin of the NCC. Trace element patterns with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti) and Ce anomalies, high Sr-87/Sr-86 ratios (0.70522-0.70796), and high delta O-18(SMOW) (standard mean ocean water) values (22.2%-23%) indicate that this carbonatite had a limestone precursor. However, the presence of coarse-grained mantle-derived clinopyroxene, orthopyroxene, and olivine, and chemical features of the carbonates suggest that the carbonate melts were derived from the mantle. The carbonates have high Nd-143/Nd-144 ratios (0.51282-0.51298) and show negative correlation between CaO and Ni contents, resulting from reaction between carbonate melt and peridotite. Considering the regional tectonic setting, the carbonatite probably formed by melting of subducted sedimentary carbonate rocks that formed part of the Paleo-Asian oceanic slab, and thus could provide the first direct evidence for the presence of the Paleo-Asian oceanic slab beneath the NCC.
    • Rapid Pliocene-Pleistocene erosion of the central Colorado Plateau documented by apatite thermochronology from the Henry Mountains

      Murray, K. E.; Reiners, P. W.; Thomson, S. N.; Univ Arizona, Dept Geosci (GEOLOGICAL SOC AMER, INC, 2016-06)
      Apatite thermochronology is, in principle, uniquely suited to document the Cenozoic erosion of the Colorado Plateau (southwestern United States) and settle generations of debate regarding the region's history of uplift, erosion, and fluvial incision. The protracted near-surface history of the Colorado Plateau bedrock, however, complicates the temperature sensitivity of apatite thermochronometers. This has confounded efforts to see clear evidence of late Cenozoic erosion, especially in the central Colorado Plateau, where this problem is compounded by the diverse detrital apatite grains in the region's sedimentary bedrock. We overcome this problem in the thermal aureole of the Oligocene Henry Mountains intrusive complex, where these sandstones have apatite (U-Th-Sm)/He ages younger than 26 Ma with positive-slope age-effective U trends (3-25 Ma, 5-180 ppm eU) that resolve a distinctive late Cenozoic history. Thermal modeling results strongly suggest that the central Colorado Plateau was a stable Miocene landscape that was rapidly exhumed similar to 1.5-2 km during the past 5 m.y., likely in the past 3-2 m.y., at time-averaged rates of similar to 250-700 m/m.y. This demonstrates that substantial late Cenozoic erosion of the north-central plateau interior postdates the ca. 5.6 Ma integration of the Colorado River that lowered regional base level.
    • Subduction termination through progressive slab deformation across Eastern Mediterranean subduction zones from updated P-wave tomography beneath Anatolia

      Portner, Daniel Evan; Delph, Jonathan R.; Biryol, C. Berk; Beck, Susan L.; Zandt, George; Özacar, A. Arda; Sandvol, Eric; Türkelli, Niyazi; Univ Arizona, Dept Geosci (GEOLOGICAL SOC AMER, INC, 2018-06)
      Using finite-frequency teleseismic P-wave tomography, we developed a new three-dimensional (3-D) velocity model of the mantle beneath Anatolia down to 900 km depth that reveals the structure and behavior of the sub-ducting African lithosphere beneath three convergent domains of Anatolia: the Aegean, Cyprean, and Bitlis-Zagros domains. The Aegean slab has a relatively simple structure and extends into the lower mantle; the Cyprean slab has a more complex structure, with a western section that extends to the lower mantle with a consistent dip and an eastern section that is broken up into several pieces; and the Bitlis slab appears severely deformed, with only fragments visible in the mantle transition zone and uppermost lower mantle. In addition to the subducting slabs, high-amplitude slow velocity anomalies are imaged in the shallow mantle beneath recently active volcanic centers, and a prominent fast velocity anomaly dominates the shallow mantle beneath northern Anatolia and the southern Black Sea. As a whole, our model confirms the presence of well-established slow and fast velocity anomalies in the upper mantle beneath Anatolia and motivates two major findings about Eastern Mediterranean subduction: (1) Each of the slabs penetrates into the lower mantle, making the Eastern Mediterranean unique within the Mediterranean system, and (2) the distinct character of each slab segment represents different stages of subduction termination through progressive slab deformation. Our findings on the destructive processes of subduction termination and slab detachment have significant implications for understanding of the post-detachment-behavior of subducted lithosphere.
    • Ultrafast magmatic buildup and diversification to produce continental crust during subduction

      Ducea, Mihai N.; Bergantz, George W.; Crowley, James L.; Otamendi, Juan; Department of Geosciences, University of Arizona (GEOLOGICAL SOC AMER, INC, 2017-03)
      The processes and fluxes that produce the distinct compositional structure of Earth's continental crust by subduction remain controversial. The rates of oceanic crust production, in contrast, are well quantified and are generally believed to be faster than those responsible for building magmatic systems in subduction settings. Here we show that a recently recognized crustal section, the 30-km-thick Ordovician Sierra Valle Fertil-Sierra Famatina complex in Argentina, was built magmatically within only similar to 4 m.y. More than half of the crustal section represents additions from the mantle, and is preserved as mafic igneous rocks and maficultramafic cumulates; the remainder is tonalite to granodiorite with evidence for widespread assimilation from highly melted metasedimentary units. U-Pb zircon geochronology reveals that the construction of the arc was not a simple bottom-up construction process. This continuous exposure of the arc crust allows the quantification of field constrained magmatic addition rates of 300-400 km(3) km(-1) m.y.(-1). These rates are similar to those determined for modern slow-spreading mid-ocean ridges and are of the same magnitude as magmatic addition rates required to build certain large segments of the continental masses such as the Arabian-Nubian shield, among others. The implication is that significant convective removal of arc roots is required over time in order to build the modern continental crust via subduction-related magmatism.