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Unravelling the spatiotemporal evolution of the Cenozoic Andean (Altiplano-Puna) plateau has been one of the most intriguing problems of South American geology. Despite a number of investigations, the early deformation and uplift history of this area remained largely enigmatic. This paper analyses the Paleogene tectono-sedimentary history of the Casa Grande Basin, in the present-day transition zone between the northern sector of the Puna Plateau and the northern part of the Argentine Eastern Cordillera. Our detailed mapping of synsedimentary structures records the onset of regional contractional deformation during the middle Eocene, revealing reactivation of Cretaceous extensional structures and the development of doubly vergent thrusts. This is in agreement with records from other southern parts of the Puna Plateau and the Eastern Cordillera. These observations indicate the existence of an Eocene broken foreland setting within the region, characterized by low-lying compressional basins and ranges with spatially disparate sectors of deformation, which was subsequently subjected to regional uplift resulting in the attainment of present-day elevations during the Neogene.
Previous thermomechanical modeling studies indicated that variations in the temperature and strength of the crystalline crust might be responsible for the juxtaposition of domains with thin-skinned and thick-skinned crustal deformation along strike the foreland of the central Andes. However, there is no evidence supporting this hypothesis from data-integrative models. We aim to derive the density structure of the lithosphere by means of integrated 3-D density modeling, in order to provide a new basis for discussions of compositional variations within the crust and for future thermal and rheological modeling studies. Therefore, we utilize available geological and geophysical data to obtain a structural and density model of the uppermost 200km of the Earth. The derived model is consistent with the observed Bouguer gravity field. Our results indicate that the crystalline crust in northern Argentina can be represented by a lighter upper crust (2,800kg/m(3)) and a denser lower crust (3,100kg/m(3)). We find new evidence for high bulk crustal densities >3,000kg/m(3) in the northern Pampia terrane. These could originate from subducted Puncoviscana wackes or pelites that ponded to the base of the crystalline crust in the late Proterozoic or indicate increasing bulk content of mafic material. The precise composition of the northern foreland crust, whether mafic or felsic, has significant implications for further thermomechanical models and the rheological behavior of the lithosphere. A detailed sensitivity analysis of the input parameters indicates that the model results are robust with respect to the given uncertainties of the input data.
Intraplate earthquakes pose a significant seismic hazard in densely populated rift systems like the Lower Rhine Graben in Central Europe. While the locations of most faults in this region are well known, constraints on their seismogenic potential and earthquake recurrence are limited. In particular, the Holocene deformation history of active faults remains enigmatic. In an exposure excavated across the Schafberg fault in the southwestern Lower Rhine Graben, south of Untermaubach, in the epicentral region of the 1756 Duren earthquake (M (L) 6.2), we mapped a complex deformation zone in Holocene fluvial sediments. We document evidence for at least one paleoearthquake that resulted in vertical surface displacement of 1.2 +/- 0.2 m. The most recent earthquake is constrained to have occurred after 815 AD, and we have modeled three possible earthquake scenarios constraining the timing of the latest event. Coseismic deformation is characterized by vertical offset of sedimentary contacts distributed over a 10-m-wide central damage zone. Faults were identified where they fracture and offset pebbles in the vertically displaced gravel layers and fracture orientation is consistent with the orientation of the Schafberg fault. This study provides the first constraint on the most recent surface-rupturing earthquake on the Schafberg fault. We cannot rule out that this fault acted as the source of the 1756 Duren earthquake. Our study emphasizes the importance of, and the need for, paleoseismic studies in this and other intracontinental regions, in particular on faults with subtle geomorphic expression that would not typically be recognized as being potentially seismically active. Our study documents textural features in unconsolidated sediment that formed in response to coseismic rupturing of the underlying bedrock fault. We suggest that these features, e.g., abundant oriented transgranular fractures in their context, should be added to the list of criteria used to identify a fault as potentially active. Such information would result in an increase of the number of potentially active faults that contribute to seismic hazards of intracontinental regions.
A potential human footprint on Western Central African rainforests before the Common Era has become the focus of an ongoing controversy. Between 3,000 y ago and 2,000 y ago, regional pollen sequences indicate a replacement of mature rainforests by a forest-savannah mosaic including pioneer trees. Although some studies suggested an anthropogenic influence on this forest fragmentation, current interpretations based on pollen data attribute the "rainforest crisis" to climate change toward a drier, more seasonal climate. A rigorous test of this hypothesis, however, requires climate proxies independent of vegetation changes. Here we resolve this controversy through a continuous 10,500-y record of both vegetation and hydrological changes from Lake Barombi in Southwest Cameroon based on changes in carbon and hydrogen isotope compositions of plant waxes. delta C-13-inferred vegetation changes confirm a prominent and abrupt appearance of C-4 plants in the Lake Barombi catchment, at 2,600 calendar years before AD 1950 (cal y BP), followed by an equally sudden return to rainforest vegetation at 2,020 cal y BP. delta D values from the same plant wax compounds, however, show no simultaneous hydrological change. Based on the combination of these data with a comprehensive regional archaeological database we provide evidence that humans triggered the rainforest fragmentation 2,600 y ago. Our findings suggest that technological developments, including agricultural practices and iron metallurgy, possibly related to the large-scale Bantu expansion, significantly impacted the ecosystems before the Common Era.
The Central Pontides of N Turkey represents a mobile orogenic belt of the southern Eurasian margin that experienced several phases of exhumation associated with the consumption of different branches of the Neo-Tethys Ocean and the amalgamation of continental domains. Our new low-temperature thermochronology data help to constrain the timing of these episodes, providing new insights into associated geodynamic processes. In particular, our data suggest that exhumation occurred at (1) similar to 110 to 90Ma, most likely during tectonic accretion and exhumation of metamorphic rocks from the subduction zone; (2) from similar to 60 to 40Ma, during the collision of the Kirehir and Anatolide-Tauride microcontinental domains with the Eurasian margin; (3) from similar to 0 to 25Ma, either during the early stages of the Arabia-Eurasia collision (soft collision) when the Arabian passive margin reached the trench, implying 70 to 530km of subduction of the Arabian passive margin, or during a phase of trench advance predating hard collision at similar to 20Ma; and (4) similar to 11Ma to the present, during transpression associated with the westward motion of Anatolia. Our findings document the punctuated nature of fault-related exhumation, with episodes of fast cooling followed by periods of slow cooling or subsidence, the role of inverted normal faults in controlling the Paleogene exhumation pattern, and of the North Anatolian Fault in dictating the most recent pattern of exhumation.
Despite remarkable tectonostratigraphic similarities along the Himalayan arc, pronounced topographic and exhumational variability exists in different morphotectonic segments. The processes responsible for this segmentation are debated. Of particular interest is a 30- to 40-km-wide orogen-parallel belt of rapid exhumation that extends from central Nepal to the western Himalaya and its possible linkage to a midcrustal ramp in the basal decollement, and the related growth of Lesser Himalayan duplex structures. Here we present 26 new apatite fission track cooling ages from the Beas-Lahul region, at the transition from the Central to the Western Himalaya (77 degrees-78 degrees E) to investigate segmentation in the Himalayan arc from a thermochronologic perspective. Together with previously published data from this part of the orogen, we document significant lateral changes in exhumation between the Dhauladar Range to the west, the Beas-Lahul region, and the Sutlej area to the east of the study area. In contrast to the Himalayan front farther east, exhumation in the far western sectors is focused at the frontal parts of the mountain range and associated with the hanging wall of the Main Boundary Thrust fault ramp. Our results allow us to spatially correlate the termination of the rapid exhumation belt with a midcrustal ramp to the west. We suggest that a plunging anticline at the northwestern edge of the Larji-Kullu-Rampur window represents the termination of the Central Himalayan segment, which is related to the evolution of the Lesser Himalayan duplex. Key Points
Terrestrial cosmogenic nuclide (TCN) concentrations in fluvial sediment, from which denudation rates are commonly inferred, can be affected by hillslope processes. TCN concentrations in gravel and sand may differ if localized, deep-excavation processes (e.g. landslides, debris flows) affect the contributing catchment, whereas the TCN concentrations of sand and gravel tend to be more similar when diffusional processes like soil creep and sheetwash are dominant. To date, however, no study has systematically compared TCN concentrations in different detrital grain-size fractions with a detailed inventory of hillslope processes from the entire catchment. Here we compare concentrations of the TCN Be-10 in 20 detrital sand samples from the Quebrada del Toro (southern Central Andes, Argentina) to a hillslope-process inventory from each contributing catchment. Our comparison reveals a shift from low-slope gullying and scree production in slowly denuding, low-slope areas to steep-slope gullying and landsliding in fast-denuding, steep areas. To investigate whether the nature of hillslope processes (locally excavating or more uniformly denuding) may be reflected in a comparison of the Be-10 concentrations of sand and gravel, we define the normalized sand-gravel index (NSGI) as the Be-10-concentration difference between sand and gravel divided by their summed concentrations. We find a positive, linear relationship between the NSGI and median slope, such that our NSGI values broadly reflect the shift in hillslope processes from low-slope gullying and scree production to steep-slope gullying and landsliding. Higher NSGI values characterize regions affected by steep-slope gullying or landsliding. We relate the large scatter in the relationship, which is exhibited particularly in low-slope areas, to reduced hillslope-channel connectivity and associated transient sediment storage within those catchments. While high NSGI values in well-connected catchments are a reliable signal of deep-excavation processes, hillslope excavation processes may not be reliably recorded by NSGI values where sediment experiences transient storage. (c) 2018 John Wiley & Sons, Ltd.
We analysed the interplay between coastal uplift, sea level change in the Black Sea, and incision of the Kizilirmak River in northern Turkey. These processes have created multiple co-genetic fluvial and marine terrace sequences that serve as excellent strain markers to assess the ongoing evolution of the Pontide orogenic wedge and the growth of the northern margin of the Central Anatolian Plateau. We used high-resolution topographic data, OSL ages, and published information on past sea levels to analyse the spatiotemporal evolution of these terraces; we derived a regional uplift model for the northward advancing orogenic wedge that supports the notion of laterally variable uplift rates along the flanks of the Pontides. The best-fit uplift model defines a constant long-term uplift rate of 0.28 +/- 0.07 m/ka for the last 545 ka. This model explains the evolution of the terrace sequence in light of active tectonic processes and superposed cycles of climate-controlled sea-level change. Our new data reveal regional uplift characteristics that are comparable to the inner sectors of the Central Pontides; accordingly, the rate of uplift diminishes with increasing distance from the main strand of the restraining bend of the North Anatolian Fault Zone (NAFZ). This spatial relationship between the regional impact of the restraining bend of the NAFZ and uplift of the Pontide wedge thus suggests a strong link between the activity of the NAFZ, deformation and uplift in the Pontide orogenic wedge, and the sustained lateral growth of the Central Anatolian Plateau flank. (C) 2018 Elsevier Ltd. All rights reserved.
The aim of this study is to investigate the shal-
low thermal field differences for two differently aged pas-
sive continental margins by analyzing regional variations in
geothermal gradient and exploring the controlling factors for
these variations. Hence, we analyzed two previously pub-
lished 3-D conductive and lithospheric-scale thermal models
of the Southwest African and the Norwegian passive mar-
gins. These 3-D models differentiate various sedimentary,
crustal, and mantle units and integrate different geophysi-
cal data such as seismic observations and the gravity field.
We extracted the temperature–depth distributions in 1 km
intervals down to 6 km below the upper thermal boundary
condition. The geothermal gradient was then calculated for
these intervals between the upper thermal boundary condi-
tion and the respective depth levels (1, 2, 3, 4, 5, and 6 km
below the upper thermal boundary condition). According to
our results, the geothermal gradient decreases with increas-
ing depth and shows varying lateral trends and values for
these two different margins. We compare the 3-D geologi-
cal structural models and the geothermal gradient variations
for both thermal models and show how radiogenic heat pro-
duction, sediment insulating effect, and thermal lithosphere–
asthenosphere boundary (LAB) depth influence the shallow
thermal field pattern. The results indicate an ongoing process
of oceanic mantle cooling at the young Norwegian margin
compared with the old SW African passive margin that seems
to be thermally equilibrated in the present day.
Abstract. The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D models differentiate various sedimentary, crustal, and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature–depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition). According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3-D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal lithosphere–asthenosphere boundary (LAB) depth influence the shallow thermal field pattern. The results indicate an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated in the present day.
Abstract. The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D models differentiate various sedimentary, crustal, and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature–depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition). According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3-D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal lithosphere–asthenosphere boundary (LAB) depth influence the shallow thermal field pattern. The results indicate an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated in the present day.