Postglacial slip distribution along the Teton normal fault, northeastern Basin-and-Range Province (Wyoming, USA) derived from tectonically offset geomorphological features
1Institut fuer Geologie, Leibniz Universitaet Hannover; 2Institut fuer Geologie und Palaeontologie, Westfaelische Wilhelms-Universitaet Muenster
Along the eastern front of the Teton Range, northeastern Basin-and-Range Province, well-preserved fault scarps that formed across moraines, river terraces and other geomorphological features indicate that multiple earthquakes ruptured the range-bounding Teton normal fault after the Last Glacial Maximum (LGM). Here we use high-resolution digital elevation models derived from Lidar data to determine the vertical slip distribution along-strike of the Teton fault from 54 topographic profiles across tectonically offset geomorphological features along the entire Teton Range front. (Hampel et al., Geosphere, in press, https://doi.org/10.1130/GES02370.1). We find that offset LGM moraines and glacially striated surfaces show higher vertical displacements than younger fluvial terraces, which formed at valley exits upstream of LGM terminal moraines. Our results reveal that the tectonic offsets preserved in the faults scarps are post-LGM in age and that the postglacial slip distribution along-strike of the Teton fault is asymmetric with respect to the Teton Range center, with the maximum vertical displacements (27-23 m) being located north of Jenny Lake and along the southwestern shore of Jackson Lake. As indicated by earlier 3D numerical models, this asymmetric slip distribution results from postglacial unloading of the Teton fault, which experienced loading by the Yellowstone ice cap and valley glaciers in the Teton Range during the last glaciation.
Slip rate of the Danghe Nan Shan thrust fault from 10Be exposure dating of folded river terraces: Implications for the strain distribution in northern Tibet
1State Key Laboratory of Oil and Gas Reservoir and Geology Exploration and Division of Key Laboratory of Carbonate Reservoirs of CNPC, Southwest Petroleum University (Chengdu, China); 2Institut fuer Geologie und Palaeontologie, Westfaelische Wilhelms-Universitaet Muenster; 3Institut fuer Geologie, Leibniz Universitaet Hannover
The northeastward motion of the Tibetan Plateau along the Altyn Tagh strike-slip fault causes thrust faulting in three parallel mountain ranges (Qilian Shan, Daxue Shan, Danghe Nan Shan) in the plateau interior, and leads to NNE-directed crustal shortening and plateau growth. While slip rates at the plateau margin (i.e. along the Qilian Shan and the Altyn Tagh fault) are well constrained, rates of thrust faulting and the strain distribution in the plateau interior remain poorly resolved. Here, we use field investigations, a high-resolution DEM, and 10Be exposure dating to quantify the shortening rate across the Danghe Nan Shan thrust fault from fluvial terraces, which are deformed by a growing NNE-vergent anticline (Xu et al., Tectonics, 2021, https://doi.org/10.1029/2020TC006584). 10Be ages from two terrace levels range from 70±5 to 92±7 ka. When combined with uplift values of 37–68 m along the fold hinge, the 10Be ages yield a mean uplift rate of 0.6±0.2 mm/year. Using the cross-sectional area of the fold and the subsurface geometry of the listric thrust fault, we obtain a shortening rate of 0.8±0.2 mm/year, which is consistent with the rate of elastic strain accumulation recorded by GPS data. Together with published fault slip rates and GPS data, our results indicate that Tibet experiences NNE-directed shortening at a rate of ∼5 mm/year between Qaidam Basin and Hexi Corridor. In the plateau interior, this shortening is accommodated by several range-bounding thrust faults and closely coupled with the eastward decrease in the slip rate of the Altyn Tagh fault.
Force-balance changes at the subduction-to-collision transition and implications for mountain building
Institut für Geologie, Leibniz Universität Hannover, Germany
The elevation of mountain belts increases at the subduction-to-collision transition in response to crustal thickening and processes like slab breakoff, but the main parameters controlling how much mountain height increases remain poorly understood. Based on analytical and finite-element force-balance models, we show that the increase in mountain height depends mainly on the magnitude of the shear force along the plate boundary fault (megathrust) and the reduction of submarine margin relief. During oceanic subduction, the megathrust shear force is balanced by the gravitational effect of the margin relief and the deviatoric stresses in the upper plate are low. When the submarine margin relief is reduced during the closure of the ocean basin, the effect of the gravitational force decreases and the upper plate experiences enhanced deviatoric compression, which allows the mountain height to increase until the deviatoric stresses beneath the high mountains are low again. If the increase in mountain height cannot keep pace with the submarine relief reduction, the compression of the upper plate increases by a few tens of MPa, which promotes tectonic shortening and mountain building. Our findings indicate that mountain height can increase by hundreds of meters to a few kilometers during continental collision, depending primarily on the trench depth during the subduction stage and possible syncollisional changes of the megathrust shear force.
Megathrust shear force limits mountain height at convergent plate boundaries
1Institut für Geologie, Leibniz Universität Hannover, Germany; 2Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, Germany; 3GFZ Deutsches GeoForschungsZentrum Potsdam, Germany
The shear force along convergent plate boundary faults (megathrusts) determines the height of mountain ranges that can be mechanically sustained. Whether the true height of mountain ranges corresponds to this tectonically supported elevation is, however, debated. In particular, climate-dependent erosional processes are often assumed to exert a major control on mountain height, although this assumption has remained difficult to validate. To address this issue, we first constrained the shear force along active megathrusts from their rheological properties and then determined the tectonically supported elevation using a force balance model. This analysis revealed that the height of mountain ranges around the globe matches the tectonically supported elevation, irrespective of climatic conditions and the rate of erosion. This finding indicates that the height of mountain ranges is effectively limited by the megathrust shear force and implies that global differences in mountain height are at first-order tectonically controlled. Temporal variations in mountain height should therefore reflect long-term changes in the overall force balance rather than changes in climate and erosion.
Refining workflow for obtaining subseismic-scale fracture density along scan lines (P10) in reservoir analogs
Subseismic-scale geological information from reservoir analogs, when integrated with reservoir seismic data, substantially improves reservoir modelling. Wüstefeld et al (2018) developed a new workflow for 1) automated detection of subseismic-scale fracture surfaces exposed in reservoir analogs using terrestrial light detection and ranging (t-LIDAR), and 2) integration of the analog-fracture data in the standard industrial reservoir modelling routines (e.g., in Petrel software). In this workflow, the fracture surfaces detected along horizontal scan lines are used to derive one dimensional fracture density (P10) that is further used as an input for discrete fracture network modelling for the reservoir. Apparent P10 values along scanlines need to be corrected to get actual fracture densities (Terzaghi 1965).
We developed a script in MATLAB that uses the fracture surfaces data (detected through standard workflows in 3D point cloud data) to obtain Terzaghi-corrected P10 values for each fracture orientation. Based on the user-defined condition for subparallelness (e.g., angle between fractures < A°), the script uses normal vectors of the detected fracture surfaces to classify them into clusters of subparallel fractures. It then obtains the mean orientations of different subparallel-fractures-clusters. Finally, the normal vector corresponding to the mean orientation of each cluster and spatial positions of the detected fracture surfaces are used to calculate perpendicular distances between the subparallel fractures (i.e., Terzaghi-corrected P10 values). The corrected P10 values may then be used for further reservoir modelling approaches or distances between neighboring subparallel fractures can be used to assess clustering based on the normalized correlation count approach.
Climatic Fluctuations in the Early and Middle Copper Age - First Isotope Investigations at the Water Supply of Los Millares in SE Spain
1Goethe-Universität Frankfurt, Germany; 2Ruprecht-Karls-Universität Heidelberg, Germany; 3Ofitec 2011, O.T., Almería, Spain
The Copper Age fortification of Los Millares is located 15 km north of Almería in Andalusia. At least between 3,541 BC (+/- 92 years) and 2,591 BC (+/- 22 years) there existed a water conduit, the so-called "aqueduct of Los Millares", with which water was conveyed to the settlement from a spring located about 1.25 km outside. U and Sr isotope analyses of groundwater samples in the catchment area as well as of the calcareous sinter deposits of the conduit provided evidence that a mixture of thermal water and near-surface groundwater had flowed in.
An interpretation of the climate on the basis of the 13C/12C and 18O/16O ratios of the sinters is therefore not straightforward. However, climatic information can be obtained. The calcareous sinter from the area of the spring, which was formed at the end of the Copper Age or at the beginning of the Bronze Age, during the so-called 4.2 ka cal BP event, showed a percentage increase of the thermal water content of the spring water, which can be explained by a less groundwater recharge of the near-surface water during the long-lasting drought.
By means of Sr isotope investigations of all aqueduct sinters, precisely dated paleoclimate data of the early and middle Copper Age can be obtained. So far, these have only been carried out on four sinter samples. For further investigations of climatic fluctuations based on Sr isotope ratios, 56 sinter samples from the aqueduct and the spring are available. A corresponding study is in preparation.
Determination of phases of warm climate during MIS 3 in Central Europe based on precisely dated speleothems from Bleßberg Cave, Germany
1Institute for Geosciences, Johannes Gutenberg University, Mainz, 55122, Germany; 2Department of Geography and Environmental Sciences, Northumbria University, Newcastle upon Tyne, UK; 3German Research Centre for Geosciences, Potsdam, 14473, Germany; 4Max Planck Institute for Chemistry, Mainz, 55128, Germany
Speleothems provide a great opportunity for paleoclimate reconstruction because they occur almost worldwide and can be dated very precisely using the U-series disequilibrium method. The most commonly used climate proxies are stable isotope values (δ18O and δ13C) and trace elements. However, these are influenced by a variety of surface and in-cave processes, which results in a non-trivial interpretation of the speleothem proxy signals.
The last glacial period and in particular the Marine Isotope Stage (MIS) 3 is, compared to the recent warm phase, the Holocene, characterised by larger climate oscillations. These are detectable in δ18O records from Greenland ice cores and also climate archives in Europe, such as pollen or tree ring records. Unfortunately, little direct proxy evidence is available from central Europe, and the climatic and environmental conditions during MIS 3 remain largely enigmatic. Speleothem records from central Europe during MIS 3 are limited due to cold climate conditions and mainly restricted to the warmer southern or alpine regions.
Here we present the first results of two flowstones from Bleßberg Cave in Germany. Preliminary 230Th/U-ages make these flowstones the most northern continental growth of speleothems during MIS 3 in central Europe. Thus, these samples provide the unique opportunity to reconstruct climate variability during parts of the last glacial period. With the combination of several different proxies, such as stable isotopes, trace elements and the results from cave monitoring, we will be able to obtain detailed insights into environmental conditions in central Europe during MIS 3 and the Late Glacial.
Should we correct speleothem carbon isotope records for degassing and prior calcite precipitation?
1ETH Zurich, Switzerland; 2University Bern, Switzerland; 3Oxford University, UK; 4Xian University, China
The carbon isotopic signature acquired from soil/epikarst processes may be a primary environmental signal of interest to interpret from speleothem d13C. However, this signal can be modified by prior calcite precipitation effects. To date, despite laboratory demonstration of PCP effects and increasingly sophisticated models fo the governing processes, there has been limited effort to deconvolve the dual PCP and soil/epikarst effects on measured speleothem carbon isotope time series.
In this contribution we evaluate the feasibility, advantages, and disadvantages of using trace element ratios and the d44Ca calcium isotopic ratio to estimate PCP and isolate its contribution to the measured speleothem d13C. We assess the challenges in the PCP indicators, such as incongruent dissolution, detrital influence on trace elements, and temperature or saturation effects on partitioning. We use the CaveCalc model of multi-step degassing with equilibrium fractionation to estimate the minimum contribution of PCP on the measured d13C. We compare the resulting estimated initial (without PCP) d13C calculated for the speleothem with cave monitoring data of d13C of undegassed DIC in the modern system. We contrast the potential for estimates of absolute initial d13C with the potential to estimate the temporal trends in initial d13C.
A multi-proxy SST and surface seawater carbonate chemistry reconstruction of the post-Industrial Revolution Southwest Pacific
1Leibniz Centre for Tropical Marine Research (ZMT), Bremen, Germany; 2Faculty of Geosciences, University of Bremen, Bremen, Germany; 3Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA; 4MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany; 5Alfred Wegener Institute – Helmholz Centre for Polar and Marine Research (AWI), Bremerhaven, Germany; 6IRD-Sorbonne Universités, UPMC, Univ Paris 06-CNRS-MNHN, LOCEAN, Paris, France
Oceanic uptake of anthropogenic CO2 emissions induced global seawater pH decrease by 0.1 since the Industrial Revolution by altering ocean chemistry with the reduction of carbonate ion concentrations and the saturation states of aragonite. Massive tropical corals are ideal palaeoceanographic archives providing high-resolution records of the most recent few hundred years and offer a valuable extension to instrumental measurements. The South Pacific Convergence Zone (SPCZ), the largest persistent precipitation band in the Southern Hemisphere with an associated salinity front modulated by large-scale ocean-atmospheric interactions (El Niño/Southern Oscillation, Interdecadal Pacific Oscillation) may influence regional seawater CO2 absorption and pH variability. Here we present coral-based paleoclimatic reconstructions from two Porites sp. corals of the Southwest Pacific region from Tonga and Rotuma. The corals are analyzed using a multi-proxy approach (δ18O, Sr/Ca, Li/Mg, U/Ca, Sr-U) to assess optimal sea surface temperature reconstruction. Preliminary δ18O results from both corals suggest similar freshening and/or warming of the surface water for the last 30 years of the 20th century (Tonga: -0.0038‰ δ18O per year; Rotuma: -0.0033‰ δ18O per year). Coral B/Ca and δ11B results for the reconstruction of carbonate chemistry changes and to establish the longer-term variability of seawater pH were completed. Tonga Porites sp. δ11B signature indicate a significant decreasing trend since 1779, with a pronounced depletion in δ11B since the 1950s of -0.0626 per year. Ultimately, this study will explore the regional-scale oceanic response to increasing pCO2 and temperature, as well as the influence of interannual and decadal-interdecadal climatic fluctuations.