Terra Nova
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Deciphering tectonic‐ and erosion‐driven exhumation of the Nevado–Filábride Complex (Betic Cordillera, Southern Spain) by low temperature thermochronologyTerra Nova, 23, 257–263, 2011 Abstract New apatite (U‐Th)/He and fission‐track data from the Nevado–Filábride complex in the Sierra Nevada of southern Spain are used to constrain the Neogene exhumation history. Apatite (U‐Th)/He ages are close to fission‐track ages in western Sierra Nevada indicating that rapid cooling occurred at 8–6 Ma, consistent with exhumation due to extension along the Mecina Fault. The western Sierra de los Filabres cooled rapidly at c . 12 Ma, while the central Sierra de los Filabres experienced less rapid cooling at 8 Ma. The age distribution in the Sierra de los Filabres can be explained by exhumation due to flexural uplift to the south‐west of the footwall of the Mecina Fault. The lower cooling rates in the central Sierra de los Filabres suggest that folding and erosion were the main exhumation processes during the Late Tortonian. Terra Nova, 00, 1–7, 2011
Terra Nova - Tập 23 Số 4 - Trang 257-263 - 2011
A 400‐km‐long piggyback basin (Upper Aptian–Lower Cenomanian) in the Eastern AlpsThe Late Aptian to Early Cenomanian Tannheim–Losenstein basin constitutes an early, deep‐marine piggyback trough which formed on the Cretaceous orogenic wedge of the Eastern Alps. The narrow basin extended over more than 400 km from the western part of the Northern Calcareous Alps into the Western Carpathians (Slovakia), as suggested by similarities in stratigraphy – e.g. the common coarsening upward succession of marls, sandstones, and conglomerates – and by similarities in timing of deformation and the uniform composition, e.g. similar heavy mineral assemblages. The coarsening‐upward succession resulted from the progradation of a coarse‐grained slope apron into a hemipelagic basin. The composition of detrital material constitutes evidence for a uniform source area to the north, along the entire length of the basin, comprising continental basement, Mesozoic sediments and remnants of ophiolites. The basin formation marked the onset of compression along the northern Austroalpine plate boundary.
Terra Nova - Tập 13 Số 6 - Trang 401-406 - 2001
Archaean crustal growth through successive partial melting events in an oceanic plateau‐like setting in the Tanzania CratonAbstract The detrital zircon population in quartzitic conglomerates from the northern Tanzania Craton yield ages between 2640 Ma and 2790 Ma which includes most of the igneous history from this part of the craton. The igneous evolution is characterised by mafic volcanism with an oceanic plateau‐like geochemical signature at ~2800 Ma followed by diorite and tonalite–trondhjemite–granodiorite dominated magmatism between 2790 and 2700 Ma, which transitioned into more evolved high‐K magmatism between 2700 and 2620 Ma. The εHf values of the detrital zircons range from +2.4 to −1.4 and change with time from radiogenic Hf pre‐2700 Ma (98% positive εHf ) to unradiogenic Hf post‐2700 Ma (41% positive εHf ). The petrological progression from mafic to felsic crust is reflected in the detrital age distribution and Hf isotopes and is consistent with juvenile mafic crust slowly maturing into more evolved felsic crust through a series of successive partial melting events in an oceanic‐plateau‐like environment.
Terra Nova - Tập 30 Số 3 - Trang 169-178 - 2018
Metamorphic decarbonation, silicate weathering and the long‐term carbon cycleABSTRACT The seawater 87 Sr/86 Sr curve implies a 50–100 Myr episodicity in weathering rate which requires a corresponding variation in CO2 degassing from the solid earth to the atmosphere. It is proposed that this is caused by orogenesis, which both produces CO2 as a result of metamorphic decarbonation reactions, and consumes extra CO2 as a consequence of erosion‐enhanced weathering. Global climate on the geological time‐scale is therefore contTolled by the difference between the relatively large and variable orogenic‐moderated degassing and weathering CO2 fluxes.
Terra Nova - Tập 8 Số 3 - Trang 270-276 - 1996
The nature and geological history of the deep crust under the Eifel, GermanyABSTRACT The crust ≈ 10–20 km under the Eifel is composed of amphibolite‐facies metasediments and meta‐igneous rocks of tonalitic to granodioritic composition; mafic granulites occupy the base of the crust down to a Moho depth between about 29 and 34 km. The meta‐granodiorites and meta‐tonalites have I‐type chemical characteristics and appear to have formed approximately 400 Myr ago by partial melting of a lower crustal source. Amphibolite‐facies metamorphism probably followed within the same orogeny. During the Quaternary, many amphibolite‐facies rocks were subjected to contact heating in crustal magma chambers and/or during transport to the earth's surface. Contact heating is also recorded in radiogenic isotope compositions of minerals from one xenolith. A genetic link between meta‐igneous amphibolites and the deeper crustal mafic granulites can neither be proven nor discounted by the isotope data. If there is a genetic relationship, it requires fractionation of a mafic magma in the lower crust and assimilation of metasediments and separation of a highly evolved melt.
Terra Nova - Tập 4 Số 1 - Trang 53-62 - 1992
Determining cooling rates from mica 40Ar/39Ar thermochronology data: Effect of cooling path shapeAbstract Tectonic models are commonly underpinned by metamorphic cooling rates derived from diffusive‐loss thermochronology data. Such cooling ages are usually linked to temperature via Dodson's closure temperature (T C ) formulation, which specifies a 1/time‐shaped cooling path (Contributions to Mineralogy and Petrology, 1973, 40, 259). Geologists, however, commonly discuss cooling rates as a linear temperature/time shape. We present the results of a series of simple finite‐difference diffusion models for Ar diffusion in muscovite and biotite that show that the difference in recorded age between 1/t and linear cooling paths increases significantly with hotter starting temperatures, slower cooling rates and smaller grain sizes. Our results show that it is essential to constrain the cooling path shape in order to make meaningful interpretations of the measured data.
Terra Nova - Tập 31 Số 3 - Trang 234-246 - 2019
Subduction and eduction of continental crust: major mechanisms during continent‐continent collision and orogenic extensional collapse, a model based on the south Norwegian CaledonidesABSTRACT During continental collision in the middle Silurian, the thickness of the lithosphere under the Caledonides of S. Norway was doubled by subduction of the western margin of Baltica, including the Western Gneiss Region, under Laurentia. Crustal rocks of the Baltic plate reached sub‐Moho depths of near 100 km or more as inferred from the presence of coesite in eclogites. Isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage. The subducted lithosphere experienced vertical constrictional strains as a result of slab‐pull by its heavy and cold root. Eduction of the deeply buried crustal material was initiated by decoupling of the Thermal Boundary Layer in the subducted lithosphere. Isostatic rebound resulted in very rapid uplift (1–2 mm yr‐1 ), and the deep crust was exhumed, mainly by tectonic extensional stripping over a period of 30–40 Myr. The eduction was probably related to a rolling hinge, footwall uplift mechanism, and the early high‐pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels. The model explains the present‐day normal crustal thickness under the exhumed deep rocks without necessarily invoking large‐scale lateral flow of material in the lower crust or igneous underplating.
Terra Nova - Tập 3 Số 3 - Trang 303-310 - 1991
The origin and significance of metamorphosed tectonic blocks in mélanges: evidence from Sulawesi, IndonesiaABSTRACT Metamorphosed tectonic blocks (or ‘knockers’) are widespread but volumetrically minor constituents of many circum‐Pacific mélange belts, Due to the common lack of an exposed in situ provenance and to the seemingly chaotic field disposition of most block‐bearing mélanges, their origin and uplift history are problematic and controversial. On the Indonesian island of Sulawesi a block‐bearing mélange is overlain by an ophiolite nappe, the base of which is characterized by a metamorphic sole sequence. Petrological, geochemical and geochronological data indicate a direct genetic relationship between high‐grade tectonic blocks in the mélange and amphibolites in the metamorphic sole. Amphibolite precursors to lower temperature blueschist assemblages are virtually ubiquitous in the tectonic blocks and subdivisions based on the nature of the overprinting relationships can be systematically correlated with block distribution patterns orientated subparallel to the strike of the mélange belt. It is suggested here that the high‐grade tectonic blocks originated in a thin, thermally zoned metamorphic sheet welded to the oceanic hangingwall plate at the inception of subduction. Break‐up of this sequence at depth, by tectonic erosion, led to dispersal of fragments into a newly developed serpentinite mélange wedge. Blocks experienced abrupt changes in P‐T‐X conditions due to a combination of hydration in the new fluid‐rich environment, gradual cooling of the hangingwall over time and continuing underflow dragging sheared blocks deeper into the subduction zone, prior to upflow. Blocks plucked from the hangingwall at different depths and at different times evidently experienced uplift in different flow channels, resulting in block concentrations, with P‐T‐t paths characteristic of their source and flow trajectory, at systematically greater distances from the subduction zone hangingwall. The elucidation of the origin and significance of tectonic blocks in Sulawesi has important implications not only for the tectonometamorphic evolution of similar inclusions in other mélange belts, but also for models of the inception and early stages of subduction.
Terra Nova - Tập 8 Số 4 - Trang 312-323 - 1996
The origin of magnesite deposits from the Polish Foresudetic Block ophiolites: preliminary δ13C and δ18O investigationsABSTRACT Twenty‐three magnesite samples from vein deposits, found in ultramafic rocks related to ophiolites in the north‐east Bohemian Massif, have been examined for carbon and oxygen isotope ratios. In these rocks two stages of antigorite serpentinization took place prior to magnesite precipitation. δ18 OSMOW in magnesites ranges from 24.5 to 27.9%o (corrected for acid fractionation factor) which leads us to conclude a meteoric origin of the ore‐forming solution. This inference is further supported by δ13 C values (−18.7 to −4.4%o) which point to a biogenic origin of carbon in the magnesites.
Terra Nova - Tập 2 Số 2 - Trang 154-159 - 1990
13C/12C ratio double cyclicity in a Miocene browncoal: isotopic signals and orbital forcingA detailed stable carbon isotopic profile of a late Miocene browncoal seam from the Lower Rhine Embayment, Germany, reveals two clear separate cycles of different frequency: this is the first instance of this type of isotopic signature being recognized within coals. The ratio of the two frequencies suggests this isotopic signal possibly resulted from climatic or vegetational responses to orbital forcing. The cyclicity has been enhanced by the application of conventional filtering methods on the data set. An analysis of the seam’s palynology indicates a correlation between heavier isotopic compositions and the presence of Sequoiapollenites polyformosus
Terra Nova - Tập 9 Số 1 - Trang 19-23 - 1997
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