Cornelia Spiegel

Morphotectonic evolution of the central Kenya rift flanks: Implications for late Cenozoic environmental change in East Africa

The Kenya rift valley is the classic example of an active continental rift zone. We report the rift flank cooling history based on a combination of previous apatite fission track (AFT) and new (U-Th)/He (AHe) data. Our results corroborate the Late Cretaceous rapid cooling episode of continent-wide significance revealed previously by AFT dating. Post-Cretaceous cooling of the eastern rift flank was slow with net cooling of 3400 m in central Kenya. This timing also largely coincides with the uplift of the Western Rift flanks in Uganda and Congo and with the change toward drier conditions and grassland-dominated vegetation in East Africa. We propose that the regional morphotectonic evolution of the Kenyan rift flanks contributed to late Cenozoic environmental change in East Africa, thus superimposing a pronounced local effect on global climate change at that time.

Thermal history of the central Gotthard and Aar massifs, European Alps: Evidence for steady state, long‐term exhumation

[1] Quantifying long‐term exhumation rates is a prerequisite for understanding the geodynamic evolution of orogens and their exogenic and endogenic driving forces. Here we reconstruct the exhumation history of the central Aar and Gotthard external crystalline massifs in the European Alps using apatite and zircon fission track and apatite (U‐Th)/He data. Age‐elevation relationships and time‐temperature paths derived from thermal history modeling are interpreted to reflect nearly constant exhumation of ∼0.5 km/Ma since ∼14 Ma. A slightly accelerated rate (∼0.7 km/Ma) occurred from 16 to 14 Ma and again from 10 to 7 Ma. Faster exhumation between 16 and 14 Ma is most likely linked to indentation of the Adriatic wedge and related thrusting along the Alpine sole thrust, which, in turn, caused uplift and exhumation in the external crystalline massifs. The data suggest nearly steady, moderate exhumation rates since ∼14 Ma, regardless of major exogenic and endogenic forces such as a change to wetter climate conditions around 5 Ma or orogen‐perpendicular extension initiated in Pliocene times. Recent uplift and denudation rates, interpreted to be the result of climate fluctuations and associated increase in erosional efficiency, are nearly twice this ∼0.5 km/Ma paleoexhumation rate.

Toward a comprehensive provenance analysis: A multi-method approach and its implications for the evolution of the Central Alps

In this study, we discuss potential problems connected with using geochronological data from foreland basins to unravel exhumation histories of the hinterland. In particular, we compare the results of a provenance analysis solely based on zircon fission-track ages from the foreland basin with a multi-method approach based on (i) the aforementioned zircon fission-track data, (ii) Nd isotope ratios of detrital epidote, and (iii) sediment accumulation rates in the foreland basins. For the example of the Central European Alps, we demonstrate that the multi-method approach can lead to highly different interpretations in terms of hinterland exhumation and geodynamic evolution. This is due to the fact that fission-track dating on detrital zircons alone only monitors the exhumation and erosion of zircon-containing lithologies and therefore only of restricted areas of the hinterland while the combination with Nd isotope ratios on detrital epidote also includes the erosion of zircon-free or -poor units such as basic magmatic rocks. A comparison of zircon fission-track and epidote Nd data with the sediment accumulation curve shows whether hinterland exhumation was predominantly caused by tectonic or by erosional denudation. Furthermore, we discuss some problems that may arise from using geochronological data from foreland basins to assess the maturity of a mountain belt in the hinterland. Applied to the Central Alps, our combined approach shows that the metamorphic core became exposed simultaneously over large areas by one sudden pulse of exhumation between 21 and 20 Ma. The main trigger for that exhumation event was tectonic denudation which is consistent with a geodynamic setting of large-scale extension. The Central Alps did not achieve exhumational steady-state conditions before 14 Ma.

Nd and Sr isotopic ratios and trace element geochemistry of epidote from the Swiss Molasse Basin as provenance indicators: implications for the reconstruction of the exhumation history of the Central Alps

Abstract Epidote is a common heavy mineral in sandstones. Its use as a provenance indicator, however, is limited because it appears in a wide range of lithologies and geological settings. In order to discriminate between mantle- and crust-derived source rocks, we measured isotopic and trace element compositions of detrital epidote from sandstones of the Swiss Molasse Basin. The compositions of the detrital epidotes were compared to epidotes from Penninic metabasites and Austroalpine metagranites from the present-day exposures and to epidote from ophiolitic and metagranitic pebbles from the molasse basin. We found that Nd isotopic ratios of epidote allow a good discrimination between different types of source rocks with eNd values ranging between 6.1 and 8.6 for metabasic lithologies and between −9.7 and −2.8 for metagranitic lithologies. Sr isotope ratios reveal similar trends but do not allow such a clear discrimination. Ba, U, Rb, Nb and Th contents of epidote correlate well with the eNd values, yielding the same provenance signal. According to isotopic and trace element compositions, three types of epidote-bearing sandstones can be distinguished. One received epidote from Lower Austroalpine metagranites, another from Penninic ophiolites and a third type of sandstone received epidote from both sources. The provenance of the detrital epidote has important bearing on the exhumation history of the Central Alps, elucidating the transition between the erosion of Lower Austroalpine felsic lithologies and the onset of erosion of the Penninic lower plate. According to our data, the Penninic lower plate became exposed simultaneously in the hinterland of all molasse fans at 21–20 Ma. At the same time, the relief collapsed, as reflected by a drastic decrease of sediment accumulation rates in the foreland basin. The combination of both events fits well to the geodynamic scenario of large-scale lateral extension processes affecting the Central Alps in post-collisional times.

Paleogeography and catchment evolution in a mobile orogenic belt: the Central Alps in Oligo-Miocene times

Abstract In this study, we reconstruct the surface evolution of the Oligo–Miocene Central Alps using geochronological, geochemical and petrographical methods on the foreland basin sediments of both flanks of the mountain range. Our model is illustrated in four sketch maps of different time slices between mid-Oligocene to Middle Miocene times. For each time slice, we try to (1) give a palinspastic reconstruction of the Central Alps, based on the post-collisional lateral extrusion model, (2) show which tectonic units had become exposed to the surface due to exhumation processes in the Central Alps, (3) describe the thermochronologic evolution of lithological units formerly exposed but completely eroded today, (4) differentiate the catchment areas of the paleo-river systems which delivered debris to the foreland basins, and (5) describe the position of the main drainage divide relative to the exposed tectonic units.

Introduction: Detrital thermochronology

Bernet, M., and Spiegel, C., 2004, Introduction: Detrital thermochronology, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 1–00. For permission to copy, contact editing@geosociety.org.

What perturbs isotherms? An assessment using fission-track thermochronology and thermal modelling along the Gotthard transect, Central Alps

Abstract Interpretation of low-temperature thermochronological data usually relies on assumptions on the shape of isotherms. Recently, a number of thermal modelling approaches investigate and predict the theoretical influence of topography on isotherms. The application and proof of these predictions is not well confirmed by measured data. Here we present apatite fission-track (AFT) data from samples collected along the Gotthard road tunnel and its corresponding surface line to test these predictions. AFT ages broadly cluster around 6 Ma along the tunnel. No correlation of tunnel ages with superimposed topography is seen, which means that topography-induced perturbation of isotherms under given boundary conditions (topographic wavelength 12 km; relief 1.5 km; exhumation rate 0.45 km Ma−1) can be neglected for the interpretation of AFT ages. Thus, in areas characterized by similar topographies and exhumation rates, apparent exhumation rates deduced from the age–elevation relationship (AER) of AFT data need no correction for topography-induced perturbation of isotherms. Three-dimensional (3D) numerical thermal modelling was carried out incorporating thermally relevant parameters and mechanisms, such as topography, geology, thermal conductivities and heat production. Modelling reveals a strong influence on the shape of isotherms caused by spatially variable thermal parameters, especially heat production rates. Therefore, not only topography has to be considered for interpreting low-temperature thermochronological data, but also other parameters like heat production rates. Supplementary material: 1. Electron microprobe analyses, 2. Topography and model extend, 3. Model parameters are all available online at http://www.geolsoc.org.uk/SUP18380.

Fission track data from the Mesohellenic Trough and the Pelagonian zone in NW Greece: Cenozoic tectonics and exhumation of source areas

The Mesohellenic Trough (MHT) trends parallel to the structural fabric of the Hellenides and covers the suture between the Apulian microplate and the Pelagonian block. It comprises an up to ∼4 km thick middle Eocene to upper Miocene sedimentary sequence. We have studied the thermal history of the Pelagonian basement and the provenance of the detrital material in the clastic sediments of the MHT by fission track dating. Apatite and zircon fission track (AFT, ZFT) analysis is applied to samples from the Pelagonian microcontinent along the eastern border of the MHT, and AFT analysis to the sedimentary rocks in the southern MHT. Eocene AFT age populations in the sedimentary strata indicate a proximal position of the Pelagonian microcontinent, which shows the same or even younger AFT ages, as the source area of the detrital material in the MHT. Late Cretaceous to Palaeocene age populations point to a more distant or structurally higher (now eroded) source area. The Eocene orogenic event caused only weak thermal overprinting in rocks of the Pelagonian microcontinent. In its eastern part, the AFT ages show only partial resetting, if any, whereas in its western part the ages were clearly reset during the Eocene event. AFT age‐elevation relations, correlation of zircon and apatite FT ages from the same samples, and thermal modelling based on AFT ages and track length distributions were all used to reconstruct the low‐temperature cooling history of the Pelagonian basement adjacent to the MHT. The results document fast cooling and exhumation in the Eocene that were possibly related to thrusting and associated erosion, followed by slow cooling and exhumation during Oligocene and Miocene time. This scenario is confirmed by the AFT data from the detrital material in the MHT sedimentary strata. The slow cooling period coincides with a stagnation period or crustal extension and possible reheating, which is probably also responsible for the partial rejuvenation of the ages of the detrital apatites in the oldest (Eocene) formation in the sediment sequence of the MHT.

Origin of bentonites and detrital zircons of the Paleocene Basilika Formation, Svalbard

The Paleocene was a time of transition for the Arctic, with magmatic activity of the High Arctic Large Igneous Province giving way to magmatism of the North Atlantic Large Igneous Province in connection to plate tectonic changes in the Arctic and North Atlantic. In this study we investigate the Paleocene magmatic record and sediment pathways of the Basilika Formation exposed in the Central Tertiary Basin of Svalbard. By means of geochemistry, Sm–Nd isotopic signatures and zircon U–Pb geochronology we investigate the characteristics of several bentonite layers contained in the Basilika Formation, as well as the provenance of the intercalated clastic sediments. Our data show that the volcanic ash layers of the Basilika Formation, which were diagenetically altered to bentonites, originate from alkaline continental-rift magmatism such as the last, explosive stages of the High Arctic Large Igneous Province in North Greenland and the Canadian Arctic. The volcanic ash layers were deposited on Svalbard in a flat shelf environment with dominant sediment supply from the east. Dating of detrital zircons suggests that the detritus was derived from Siberian sources, primarily from the Verkhoyansk Fold-and-Thrust Belt, which would require transport over ~3000 km across the Arctic.

Exhumation history along the eastern Amundsen Sea coast, West Antarctica, revealed by low‐temperature thermochronology

West Antarctica experienced a complex tectonic history, which is still poorly documented, in part due to extensive ice cover. Here, we reconstruct the Cretaceous to present thermo-tectonic history of Pine Island Bay area and its adjacent coasts, based on a combination of apatite and zircon fission track and apatite (U-Th-Sm)/He thermochronology. In addition, we report petrographic information for the catchments of Pine Island, Thurston Island and Thwaites glaciers. Our data suggest that the underlying bedrock of the Pine Island and Thwaites Glacier catchments are very different and vary from granitoids to (Cenozoic?) volcanogenic sequences and low-grade metamorphics. Our thermochronology data show that the upper crustal rocks of Pine Island Bay experienced very rapid cooling during the late Cretaceous. We attribute this rapid cooling of basement rocks and associated reduction in mean elevation to tectonic denudation driven by gravitational collapse of the Cretaceous orogen along the proto-Pacific Gondwana margin. Rapid Cretaceous crustal cooling was followed by very slow cooling during the Cenozoic, with no erosional response – within the limits of thermochronological methods – to the onset of glaciation and subsequent climatic changes. Cenozoic rifting within the West Antarctic Rift appears to have had little effect on erosion processes around Pine Island Bay; instead, our data suggest Cenozoic crustal tilting towards Pine Island Trough, a major geomorphic feature previously suggested to be a branch of the rift system.