Browsing by Author "Frei, Dirk"

Highly elevated and well-preserved peneplains are characteristic geomorphic features of the Tibetan plateau in the northern Lhasa Terrane, north-northwest of Nam Co. The peneplains were carved in granitoids and in their metasedimentary host formations. We use multi-method geochronology (zircon U-Pb and [U-Th]/He dating and apatite fission track and [U-Th]/He dating) to constrain the post-emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas. LA-ICP-MS U-Pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 Ma and 85 Ma, and a third group records Paleocene volcanic activity (63-58 Ma) in the Nam Co area. The low-temperature thermochronometers indicate common age groups for the entire Nam Co area: zircon (U-Th)/He ages cluster around 75 Ma, apatite fission track ages around 60 Ma and apatite (U-Th)/He ages around 50 Ma. Modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest Cretaceous to earliest Paleogene time. By Middle Eocene time the relief was already flat, documented by a thin alluvial sediment sequence covering a part of the planated area. The present-day horst and graben structure of the peneplains is a Late Cenozoic feature triggered by E-W extension of the Tibetan Plateau. The new thermochronological data precisely bracket the age of the planation to Early Eocene, i.e. between ca. 55 and 45 Ma. The erosional base level can be deduced from the presence of Early Cretaceous zircon grains in Eocene strata of Bengal Basin. The sediment generated during exhumation of the Nam Co area was transported by an Early Cenozoic river system into the ocean, suggesting that planation occurred at low elevation. (C) 2013 Elsevier Ltd. All rights reserved.

The geodynamic history of the Precambrian basement in central North Africa as well as the age and provenance of its sedimentary cover sequence are still poorly constrained. Here we present first detrital zircon ages (obtained by IA-SF-ICP-MS and SHRIMP) from Paleozoic and Mesozoic sandstones of the eastern Murzuq Basin, southern Libya, which unconformably overlie the Saharan Metacraton. Establishing the age and provenance of these sandstones has important implications for our understanding of the evolution of northern Gondwana during the Paleozoic, especially for reconstructions of paleo-source areas and transport paths. Detrital zircons from the sandstones show mainly early Paleozoic to Neoarchean ages with four main age populations, at 2750-2500 Ma (8%), 2200-1750 Ma (16%), 1060-920 Ma (18%), and 720-530 Ma (39%). About 13% of all concordant grains yield ages of 1600-1000 Ma. In addition, there are 9 zircon grains (0.7% of all concordant grains) with ages of 3600-2800 Ma. The presence of a high number of ca. 1 Ga zircons is enigmatic and their origin is controversial. Besides direct sourcing from ca. 1 Ga igneous rocks in eastern Chad and ca. 1 Ga igneous rocks along the southeastern margins of the Congo and Tanzania cratons, recycling of Neoproterozoic sediments containing ca. 1 Ga zircons is another alternative hypothesis to explain the presence of ca. 1 Ga zircons in the Paleozoic sedimentary sequence of central North Africa. The ubiquitous occurrence of ca. 1 Ga zircons in Paleozoic sediments of southern Libya provides insights into the correlation and paleotectonic arrangement of Gondwana-derived terranes, present, for example, in the eastern Mediterranean and in southwestern Europe. Current paleotectonic models of dextral terrane transport along the northern Gondwana margin during the early Paleozoic may need to be revised. (C) 2011 Elsevier B.V. All rights reserved.

Heavy mineral provenance data presented in this article reinforce evidence for significant late Palaeogene deformation and relief formation along the western margin of the Central Andean Plateau. Late EoceneOligocene onset of molasse-type sedimentation records initial range uplift. Strikingly different basement sources of sediments deposited to the east and west of the Late Palaeogene range indicate that initial relief development was governed by a bivergent thrust system. Significantly higher sediment accumulation rates to the east of the range compared to the west suggest that the generated relief acted already as an effective orographic barrier at that time. Higher precipitation and denudation along the eastern slope facilitated deeper erosion of the trust belt.

Timing, amount, and mechanisms of uplift in the Central Andes have been a matter of debate in the last decade. Our study is based on the Cenozoic Moquegua Group deposited in the forearc basin between the Western Cordillera and the Coastal Cordillera in southern Peru from similar to 50 to similar to 4 Ma. The Moquegua Group consists mainly of mud-flat to fluvial siliciclastic sediments with upsection increasing grain size and volcanic intercalations. Detrital zircon U-Pb dating and fission track thermochronology allow us to refine previous sediment provenance models and to constrain the timing of Late Eocene to Early Miocene Andean uplift. Uplift-related provenance and fades changes started around 35 Ma and thus predate major voluminous ignimbrite eruptions that started at similar to 25 by up to 10 Ma. Therefore magmatic addition to the crust cannot be an important driving factor for crustal thickening and uplift at Late Eocene to Early Oligocene time. Changes in subduction regime and the subducting plate geometry are suggested to control the formation of significant relief in the area of the future Western Cordillera which acts as an efficient large-scale drainage divide between Altiplano and forearc from at least 15.5 to 19 degrees S already at similar to 35 Ma. The model integrates the coincidence of (i) onset of provenance change no later than 35 Ma, (ii) drastic decrease in convergence rates at similar to 40, (iii) a flat-subduction period at around similar to 40 to similar to 30 Ma leading to strong interplate coupling, and (iv) strong decrease in volcanic activity between 45 and 30 Ma. (C) 2013 Elsevier Ltd. All rights reserved.

The uplift history of Tibet is crucial for understanding the geodynamic and paleoclimatologic evolution of Asia; however, it remains controversial whether Tibet attained its high elevation before or after India collided with Asia similar to 50 m.y. ago. Here we use thermochronologic and cosmogenic nuclide data from a large bedrock peneplain in southern Tibet to shed light on the timing of the uplift. The studied peneplain, which was carved into Cretaceous granitoids and Jurassic metasediments, is located in the northern Lhasa block at an altitude of similar to 5300 m. Thermal modeling based on (U-Th)/He ages of apatite and zircon, and apatite fission track data, indicate cooling and exhumation of the granitoids between ca. 70 and ca. 55 Ma, followed by a rapid decline in exhumation rate from similar to 300 m/m.y. to similar to 10 m/m.y. between ca. 55 and ca. 48 Ma. Since then, the peneplain has been a rather stable geomorphic feature, as indicated by low local and catchment-wide erosion rates of 6-11 m/m.y. and 11-16 m/m.y., respectively, which were derived from cosmogenic Be-10 concentrations in bedrock, grus, and stream sediment. The prolonged phase of erosion and planation that ended ca. 50 Ma removed 3-6 km of rock from the peneplain region, likely accomplished by laterally migrating rivers. The lack of equivalent sediments in the northern Lhasa block and the presence of a regional unconformity in the southern Lhasa block indicate that the rivers delivered this material to the ocean. This implies that erosion and peneplanation proceeded at low elevation until India's collision with Asia induced crustal thickening, surface uplift, and long-term preservation of the peneplain.

Sandwiched between the Adriatic Carbonate Platform and the Dinaride Ophiolite Zone, the Bosnian Flysch forms a c. 3000 m thick, intensely folded stack of Upper Jurassic to Cretaceous mixed carbonate and siliciclastic sediments in the Dinarides. New petrographic, heavy mineral, zircon U/Pb and fission-track data as well as biostratigraphic evidence allow us to reconstruct the palaeogeology of the source areas of the Bosnian Flysch basin in late Mesozoic times. Middle Jurassic intraoceanic subduction of the Neotethys was shortly followed by exhumation of the overriding oceanic plate. Trench sedimentation was controlled by a dual sediment supply from the sub-ophiolitic high-grade metamorphic soles and from the distal continental margin of the Adriatic plate. Following obduction onto Adria, from the Jurassic–Cretaceous transition onwards a vast clastic wedge (Vranduk Formation) was developed in front of the leading edge, fed by continental basement units of Adria that experienced Early Cretaceous synsedimentary cooling, by the overlying ophiolitic thrust sheets and by redeposited elements of coeval Urgonian facies reefs grown on the thrust wedge complex. Following mid-Cretaceous deformation and thermal overprint of the Vranduk Formation, the depozone migrated further towards SW and received increasing amounts of redeposited carbonate detritus released from the Adriatic Carbonate Platform margin (Ugar Formation). Subordinate siliciclastic source components indicate changing source rocks on the upper plate, with ophiolites becoming subordinate. The zone of the continental basement previously affected by the Late Jurassic–Early Cretaceous thermal imprint has been removed; instead, the basement mostly supplied detritus with a wide range of pre-Jurassic cooling ages. However, a c. 80 Ma, largely synsedimentary cooling event is also recorded by the Ugar Formation, that contrasts the predominantly Early Cretaceous cooling of the Adriatic basement and suggests, at least locally, a fast exhumation.

We introduce and propose zircon M257 as a future reference material for the determination of zircon U-Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS-determined, mean isotopic ratios (2s uncertainties) of 0.09100 +/- 0.00003 for (206)pb/(238)U and 0.7392 +/- 0.0003 for (207)pb/(235)U. Its (206)pb/(238)U age is 561.3 +/- 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U-Pb system is concordant within uncertainty of decay constants. Zircon M257 contains similar to 840 mu g g(-1) U (Th/U similar to 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit-cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the "Sri Lankan trends", with actinide concentrations, U-Pb age, and the calculated alpha fluence of 1.66 x 10(18) g(-1). This, and a (U+Th)/He age of 419 +/- 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc-silicate skarn.