Gilles Brocard

What does a mean mean? The temporal evolution of detrital cosmogenic denudation rates in a transient landscape

In equilibrium landscapes, 10 Be concentrations within detrital quartz grains are expected to quantitatively refl ect basin-wide denudation rates. In transient landscapes, though detrital quartz is derived from both the incising, adjusting lowland and the unadjusted, relict upland, the integrated 10 Be concentrations still provide a denudation rate averaged across the two domains. Because fi eld samples can provide only a snapshot of the current upstream-averaged erosion rate, we employ a numerical landscape evolution model to explore how 10 Be-derived denudation rates vary over time and space during transient adjustment. Model results suggest that the longitudinal pattern of mean denudation rates is generated by the river’s progressive dilution of low-volume, high-concentration detritus from relict uplands by the integration of high-volume, low-concentration detritus from adjusting lowlands. The proportion of these materials in any detrital sample depends on what fraction of the upstream area remains unadjusted. Because the boundary of the adjusting part of the landscape changes over time, the longitudinal trend in cosmogenic nuclide‐derived erosion rates changes over time. These insights are then used to guide our interpretation of geomorphic and longitudinal cosmogenic nuclide data from the South Fork Eel River (SFER) in the California Coast Range (United States). The northward-propagating crustal thickening and rock uplift associated with the passage of the Mendocino triple junction generates a mobile wave of uplift that progressively sweeps longitudinally down the SFER. The consequences of this forcing can be both replicated in the model environment and observed in the fi eld. The SFER contains transient landforms including knickpoints and river terraces along mainstem and tributary channels that defi ne a clear boundary between an incised, adjusting lowland and an unadjusted, relict upland. We report nine nested, basin-wide denudation rates in the mainstem of the SFER using terrestrial cosmogenic 10 Be in river-borne sediment. We fi nd that denudation rates increase in the downstream direction from ~0.2 mm/yr in the upper catchment to ~0.5 mm/yr at the outlet. Using comparisons to the modeled landscape, we show that this pattern of denudation rates, paired with the distribution of relict topography throughout the watershed, refl ect the immaturity of the landscape’s transient adjustment. Later in this modeled transient, the predicted erosion rates decrease downstream before they become uniform. This interpretation of our data has potentially far-reaching implications for quantifying the uplift history and response time of transient landscapes using cosmogenic nuclides.

RATE AND PROCESSES OF RIVER NETWORK REARRANGEMENT DURING INCIPIENT FAULTING: THE CASE OF THE CAHABON RIVER, GUATEMALA

Deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics. Although the reorganization of deeply incised drainage systems has been documented, the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood. To address this problem, we have explored the rapid drainage reorganization that affected the Cahabón River in Guatemala during the Quaternary. Sediment-provenance analysis, field mapping, and electrical resistivity tomography (ERT) imaging are used to reconstruct the geometry of the valley before the river was captured. Dating of the abandoned valley sediments by the 10Be-26Al burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes, such as the rate of tectonic deformation of the paleovalley, the rate of propagation of post-capture drainage reversal, and the rate at which canyons that formed at the capture sites have propagated along the paleovalley. Transtensional faulting started 1 to 3 million years ago, produced ground tilting and ground faulting along the Cahabón River, and thus generated differential uplift rate of 0.3 ± 0.1 up to 0.7 ±0.4 mm · y−1 along the rivers course. The river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence. Then, the river experienced two captures and one avulsion between 700 ky and 100 ky. The captures breached high-standing ridges that separate the Cahabón River from its captors. Captures occurred at specific points where ridges are made permeable by fault damage zones and/or soluble rocks. Groundwater flow from the Cahabón River down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges. Valley-fill formation and capture occurred in close temporal succession, suggesting a genetic link between the two. We suggest that the aquifers accumulated within the valley-fills, increased the head along the subterraneous system connecting the Cahabón River to its captors, and promoted their development. Upon capture, the breached valley experienced widespread drainage reversal toward the capture sites. We attribute the generalized reversal to combined effects of groundwater sapping in the valley-fill, axial drainage obstruction by lateral fans, and tectonic tilting. Drainage reversal increased the size of the captured areas by a factor of 4 to 6. At the capture sites, 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm · y−1 while deepening at a rate of 0.7 to 1.5 mm · y−1. At this rate, 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley. It is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field, which involved transient sedimentation along the rivers course. If the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium, then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced.

Relict landscape resistance to dissection by upstream migrating knickpoints

Expanses of subdued topographies are common at high elevation in mountain ranges. They are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds. Preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography. However, relict fragments could also possess intrinsic characteristics that make them resilient to dissection. We document here the propagation of a wave of dissection across an uplifted relict landscape in Puerto Rico. Using 10Be-26Al burial dating on cave sediments, we show that uplift started 4 Ma and that river knickpoints have since migrated very slowly across the landscape. Modern detrital 10Be erosion rates are consistent with these long-term rates of knickpoint retreat. Analysis of knickpoint distribution, combined with visual observations along the streambeds, indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat. The studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks. Earlier studies have shown that vegetation over the relict topography efficiently limits erosion, allowing for the formation of a thick saprolite underneath. Such slow erosion reduces streambed load fluxes delivered to the knickpoints, as well as bed load grain size. Both processes limit abrasion. Compounding the effect of slow abrasion, wide joint spacing in the bedrock makes plucking infrequent. Thus, the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream, reducing the celerity at which the relict, subdued landscape is dissected. We conclude that similar top-down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies.

New kinematic and geochronologic evidence for the Quaternary evolution of the Central Anatolian fault zone (CAFZ)

As the kinematics of active faults that bound the Anatolian plate are well studied, it is now essential to improve our understanding of the style and rates of intraplate deformation to constrain regional strain partitioning and improve seismic risk assessments. One of these internal structures, the Central Anatolian fault zone (CAFZ), was originally defined as a regionally significant left-lateral “tectonic escape” structure, stretching for 700 km in a NE direction across the Anatolian plate. We provide new structural, geomorphic, and geochronologic data for several key segments within the central part of the CAFZ that suggest that the sinistral motion has been overstated. The Ecemis fault, the southernmost part of the CAFZ, has a late-Quaternary minimum slip rate of 1.1 ± 0.4 mm a−1, slower than originally proposed. Farther north, the Erciyes fault has fed a linear array of monogenetic vents of the Erciyes stratovolcano and 40Ar/39Ar dating shows a syneruptive stress field of ESE-WNW extension from 580 ± 130 ka to 210 ± 180 ka. In the Erciyes basin, and central part of the CAFZ, we mapped and recharacterized the Erkilet and Gesi faults as predominantly extensional. These long-term geological rates support recent GPS observations that reveal ESE-WNW extension, which we propose as the driver of faulting since 2.73 ± 0.08 Ma. The slip rates and kinematics derived in this study are not typical of an “escape tectonic” structure. The CAFZ is a transtensional fault system that reactivates paleotectonic structures and accommodates E-W extension associated with the westward movement of Anatolia.

Low-temperature thermochronologic signature of range-divide migration and breaching in the North Cascades

Physical and numerical simulations of the development of mountain topography predict that asymmetric distributions of precipitation over a mountain range induce a migration of its drainage divide toward the driest flank in order to equilibrate erosion rates across the divide. Such migration is often inferred from existing asymmetries, but direct evidence for the migration is often lacking. New low-temperature apatite cooling ages from a transect across the northern North Cascades range (Washington, NW USA) and from two elevation profiles in the Skagit River valley record faster denudation on the western, wetter side of the range and lower denudation rates on the lee side of the range. This difference has already been documented further south along another transect across the range; however, in the south, the shift from young cooling ages to older ages occurs across the modern drainage divide. Here, further north, the shift occurs along a range-transverse valley within the Skagit Gorge. It has been proposed that the upper Skagit drainage was once a part of the leeward side of the range but started to drain toward the western side of the range across the Skagit Gorge in Quaternary time. Age-elevation profiles along the former drainage and in the Skagit Gorge restrict the onset of Skagit Gorge incision to the last 2 m.y., in agreement with 4 He/ 3 He data for the gorge floor. Breaching of the range drainage resulted in its displacement 40 km further east into the dry side of the range. In the 2000-m-deep, V-shaped Skagit Gorge, river stream power is still high, suggesting that incision of the gorge is still ongoing. Several other similar events have occurred along the range during the Pleistocene, supporting the proposed hypothesis that the repeated southward incursions of the Cordilleran ice sheet during this period triggered divide breaching and drainage reorganization by overflow of ice-dammed lakes at the front of the growing ice sheet. Since these events systematically rerouted streams toward the wet side of the range and resulted in leeward migration of the divide, we propose that in fact the Cordilleran ice sheet advance essentially catalyzed the adjustment of the mountain chain topography to the current orographic precipitation pattern.

A unified framework for modelling sediment fate from source to sink and its interactions with reef systems over geological times

Understanding the effects of climatic variability on sediment dynamics is hindered by limited ability of current models to simulate long-term evolution of sediment transfer from source to sink and associated morphological changes. We present a new approach based on a reduced-complexity model which computes over geological time: sediment transport from landmasses to coasts, reworking of marine sediments by longshore currents, and development of coral reef systems. Our framework links together the main sedimentary processes driving mixed siliciclastic-carbonate system dynamics. It offers a methodology for objective and quantitative sediment fate estimations over regional and millennial time-scales. A simulation of the Holocene evolution of the Great Barrier Reef shows: (1) how high sediment loads from catchments erosion prevented coral growth during the early transgression phase and favoured sediment gravity-flows in the deepest parts of the northern region basin floor (prior to 8 ka before present (BP)); (2) how the fine balance between climate, sea-level, and margin physiography enabled coral reefs to thrive under limited shelf sedimentation rates after ~6 ka BP; and, (3) how since 3 ka BP, with the decrease of accommodation space, reduced of vertical growth led to the lateral extension of reefs consistent with available observational data.

pyBadlands: A framework to simulate sediment transport, landscape dynamics and basin stratigraphic evolution through space and time

Understanding Earth surface responses in terms of sediment dynamics to climatic variability and tectonics forcing is hindered by limited ability of current models to simulate long-term evolution of sediment transfer and associated morphological changes. This paper presents pyBadlands, an open-source python-based framework which computes over geological time (1) sediment transport from landmasses to coasts, (2) reworking of marine sediments by longshore currents and (3) development of coral reef systems. pyBadlands is cross-platform, distributed under the GPLv3 license and available on GitHub (http://github.com/badlands-model). Here, we describe the underlying physical assumptions behind the simulated processes and the main options already available in the numerical framework. Along with the source code, a list of hands-on examples is provided that illustrates the model capabilities. In addition, pre and post-processing classes have been built and are accessible as a companion toolbox which comprises a series of workflows to efficiently build, quantify and explore simulation input and output files. While the framework has been primarily designed for research, its simplicity of use and portability makes it a great tool for teaching purposes.

Abrupt Change in Forest Height along a Tropical Elevation Gradient Detected Using Airborne Lidar

Most research on vegetation in mountain ranges focuses on elevation gradients as climate gradients, but elevation gradients are also the result of geological processes that build and deconstruct mountains. Recent findings from the Luquillo Mountains, Puerto Rico, have raised questions about whether erosion rates that vary due to past tectonic events and are spatially patterned in relation to elevation may drive vegetation patterns along elevation gradients. Here we use airborne light detection and ranging (LiDAR) technology to observe forest height over the Luquillo Mountain Range. We show that models with different functional forms for the two prominent bedrock types best describe the forest height-elevation patterns. On one bedrock type there are abrupt decreases in forest height with elevation approximated by a sigmoidal function, with the inflection point near the elevation of where other studies have shown there to be a sharp change in erosion rates triggered by a tectonic uplift event that began approximately 4.2 My ago. Our findings are consistent with broad geologically mediated vegetation patterns along the elevation gradient, consistent with a role for mountain building and deconstructing processes.

Ophiolite gabbro from source to sink: A record of tectonic and surface processes in Central Anatolia

Ophiolitic rocks derived from Tethyan seaways are abundant in Anatolia; many are in arrays that mark sutures between Eurasia, Gondwana, and continental ribbons and island arcs. Ophiolitic fragments also occur dispersed between sutures, indicating tectonic transport of possibly hundreds of kilometers. Scattered fragments of the Central Anatolian Ophiolite (CAO) have been interpreted as originating in oceans to the north, west, and/or south of their current locations, with implications for the magnitude and direction of transport and the relation of ophiolite obduction to regional metamorphism of the underlying continental-margin terrane (Central Anatolian Crystalline Complex [CACC]). Ophiolitic clasts (primarily gabbro) are widespread in sedimentary basins and alluvial terraces, particularly in the southern CACC. Petrologic and geochemical data from (meta)gabbro outcrops, gabbro clasts in conglomerates, and gabbro cobbles on alluvial terraces near the Niğde Massif, a metamorphic dome at the southern tip of the CACC, indicate paleosources and can be used to reconstruct the history of ophiolite emplacement, metamorphism, erosion, and dispersal. (Meta)gabbro at the northern margin of the Niğde Massif is geochemically similar to CAO gabbro: both have low Ti/V and depleted high field strength elements, typical of boninitic (forearc) magma, although Niğde mafic and associated ultramafic rocks were metamorphosed at middle to upper amphibolite facies, and the rest of the CAO were metamorphosed at (sub)greenschistfacies conditions. Amphibolite-facies mafic and ultramafic rocks near the contact with underlying CACC metasedimentary rocks have been ductilely deformed in mylonitic high-strain zones with top-to-south kinematics likely related to tectonic interleaving of ophiolitic and continental margin rocks at depth. The confinement of high-grade metaophiolite to the southern tip of the CACC may indicate oblique and diachronous obduction from south to north. Whole-rock trace-element data for gabbro clasts indicate that early to middle Miocene sediments were derived from mixed sources (CAO and Tauride ophiolites), whereas later Miocene sediments were sourced entirely from the CAO, even those on the opposite side (south) of the Niğde topographic high. These results may indicate that late Miocene uplift and arching of the Tauride Mountains at the southern margin of the Central Anatolian plateau drove reorganization of sediment dispersal and topographic disconnection of Miocene depocenters from their CAO sources. INTRODUCTION Ophiolite belts mark the locations of suture zones that represent the sites of oceans that have closed during subduction and collision. Ophiolites also occur as dispersed fragments between suture zones, indicating long-distance tectonic transport (obduction), in some cases possibly driving high-grade metamorphism of the underlying continental margin (e.g., Coleman, 1981; Yalınız et al., 1996; Searle and Cox, 1999; Floyd et al., 2000; Searle and Treloar, 2010). Anatolia is festooned with ophiolites derived from various strands of Tethyan seaways and now exposed in suture zones (Fig. 1). Other Anatolian ophiolites, such as the Central Anatolian Ophiolite (CAO), occur as isolated fragments dispersed between suture zones (Fig. 1). Based on paleotectonic reconstructions, obducted Anatolian ophiolite is inferred to have been “quite continuous over a vast region,” on the scale of the Semail ophiolite in Oman (Robertson et al., 2009). Late Cretaceous ophiolites occur throughout central and southern Anatolia and include the Tauride ophiolites emplaced on the Tauride carbonate platform in the south (now comprising the Tauride Mountains), ophiolites marking the Izmir-Ankara-Erzincan suture zone (IAESZ) to the north, and the Central Anatolian Ophiolite (CAO) within the interior of the Central Anatolian Crystalline Complex (CACC) (e.g., Dilek et al., 1999; Rolland et al., 2012; Parlak et al., 2013a). Middle Jurassic ages have been reported for ophiolitic mélange from the IAESZ where it forms the northern boundary of the CACC, although the obduction has been dated as Late Cretaceous (Çelik et al., 2011; Rolland et al., 2012). Under debate is the paleogeographic origin of the Late Cretaceous ophiolites exposed in Central Anatolia. Obduction of a large nappe composed of Jurassic and Cretaceous oceanic crust and derived from the Izmir-AnkaraErzincan strand of the Neotethys Ocean has been proposed for the origin of both the CAO and Tauride ophiolites (e.g., Hassig et al., 2016a, 2016b; van Hinsbergen et al., 2016). Others suggest that the Tauride ophiolites were emplaced from a subduction zone located between the CACC and Tauride terranes and now represented by the Inner Tauride suture zone (ITSZ) (e.g., Dilek et al., 1999; Parlak et al., 2013a). The Central Anatolian Ophiolite occurs as fragments of gabbro and related rocks dispersed among exposures of high-grade metasedimentary GEOSPHERE GEOSPHERE; v. 13, no. 5 doi:10.1130/GES01465.1 16 figures; 6 tables; 1 supplemental file CORRESPONDENCE: dwhitney@ umn .edu CITATION: Radwany, M., Whitney, D.L., Brocard, G., Umhoefer, P.J., and Teyssier, C., 2017, Ophiolite gabbro from source to sink: A record of tectonic and surface processes in Central Anatolia: Geosphere, v. 13, no. 5, p. 1329–1358, doi: 10 .1130 /GES01465.1. Received 22 November 2016 Revision received 4 April 2017 Accepted 26 June 2017 Published online 9 August 2017 For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

Flow Routing in the Karst of Puerto Rico

90% of Puerto Rico’s 2400 km2 of karst is located near the Atlantic Coast. The north coast limestone aquifer consists of Tertiary carbonates with an unconfined upper aquifer and a confined lower aquifer. The transmissivity of the lower aquifer is an order of magnitude smaller than the upper aquifer. About half of the recharge is rainfall on karst uplands that travels north to near-coast springs and wells. Approximately 148,000 m3/day is withdrawn from the lowland areas, some of which contain a variety of chemical contaminants (e.g., phthalates) introduced in the populated lowlands. Widespread use of septic tanks and of caves and dolines for trash disposal is also a threat of unknown magnitude to water quality. Although the general flow paths of the upper recharges are known, and scores of streams in caves (of hundreds) have been mapped in higher elevations, the specific routes to the final discharges are not well understood. Dye traces, and isotope comparison of different sectors of the lower discharge area, have not proved particularly useful to identify them, so some efforts have focused on defining the characteristics of accessible cave system flows to see whether they can provide useful analogs. Geologic strike and dip are major controls of cave passage orientations, followed by faulting or major joint sets.