Bruno Goffé

Midcrustal shear zones in postorogenic extension: Example from the northern Tyrrhenian Sea

Metamorphic core complexes of the Aegean region have revealed midcrustal, shallow-dipping extensional shear zones. These shear zones display constant kinematic indicators over large regions (100–200 km). We analyze the example of the northern Tyrrhenian Sea and then compare it to the Aegean region. We first summarize our observations on ductile extension and metamorphic evolution in the northern Tyrrhenian Sea from Alpine Corsica to Tuscany. (1) Extension migrated from west to east from the early Miocene in Corsica to the Recent in the Apennines; (2) Extension is accommodated by shallow east dipping extensional shear zones at the depth of the brittle-ductile transition, from the early Miocene to the Pliocene. (3) West dipping normal faults accommodate extension on the eastern side of the volcanic arc. (4) Extension is preceded along the convergence front by the formation of a thrust wedge, where high-pressure and low-temperature conditions are recorded; maximum PT conditions decrease toward the east, and PT paths are systematically very cold, suggesting that a large part of the exhumation occurred during synorogenic extension. We discuss the possible mechanisms that account for constant shear sense over large domains. The model involves retreat of the slab and migration of the volcanic arc. Partially molten lower crust acts as a low strength zone where extensional strain is localized. Eastward motion of the upper mantle as a consequence of the migration of the slab induced a component of shear toward the volcanic arc at the base of the stronger upper crust. In the weak upper mantle and lower crust, to the west of the volcanic arc, extensional stresses are not transmitted; this produces a top-to-the-east sense of shear at the base of the upper crust that migrates eastward, following arc migration.

On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy

The applicability of Raman spectroscopy to characterize disordered and heterogeneous carbonaceous materials (CM) is discussed, by considering both natural and synthetic CM. First, different analytical mismatches during the measurement are discussed and technical indications are provided in order to eliminate them. Second, the accuracy and relevance of the different parameters obtained by the decomposition of spectra by conventional fitting procedure, is reviewed. Lastly, a new Raman technique (Raman area mode microspectroscopy) giving an homogeneous repartition of power within a large laser beam is presented, this technique being powerful to study strongly heterogeneous CM and/or photosensitive samples.

Miocene detachment in Crete and exhumation P-T-t paths of high-pressure metamorphic rocks

A major problem posed by the geology of Crete is the horizontal contact of an upper unit without Miocene metamorphism onto a metamorphosed lower one with Early Miocene high pressure/low temperature (HP/LT) parageneses. This very sharp contact is roughly parallel to the major Oligo-Miocene thrust planes which were reactivated as a large-scale detachment which allowed exhumation of high-pressure units. We describe the extensional deformation and the metamorphic evolution of the lower plate. Most first-order deformation features relate to the retrogression from high-pressure to low-pressure conditions. A N-S pervasive stretching is observed everywhere, often associated with a top-to-the-north sense of shear. The extreme variation of thickness of the Phyllite-Quartzite nappe (upper part of the lower plate) is probably the result of large-scale boudinage similar to the one seen in large outcrops. The most important observation is the systematic occurrence of fresh carpholite immediately below the base of the Tripolitza nappe except in northwestern Crete where a late extensional shear zone is present. Deeper in the nappe pile carpholite is systematically retrograded. This observation reveals a drastically different PT history for the upper part of the Phyllite-Quartzite nappe. It also suggests that the late extensional shear zone found along the northern side of Crete cuts inside the metamorphic structure and brings the nonmetamorphosed Tripolitza nappe directly in contact with the deeper parts of the Phyllite-Quartzite nappe. PT-t paths suggest a fast temperature decrease in the top of the Phyllite-Quartzite during retrogression and, hence, during the top-to-the-north shear. The deeper part of the Phyllite-Quartzite nappe shows a low-temperature regime throughout, but its PT path includes an isothermal decompression in the first stage. We produce a tentative map of domains having experienced similar PT trajectories during decompression. The overall cool regime is related to the continuous underthrusting of cola continental units during exhumation. Isothermal decompression observed in the core of the Phyllite-Quartzite Nappe implies fast exhumation during extension and the faster cooling of the upper part is related to a continuous displacement toward the north of a cooler unit during exhumation. Single grain 39Ar-40Ar ages obtained on phengites (15–25 Ma) in various structural sites are in good agreement with these conclusions and with the geological context suggesting that underthrusting of cold units at the front the accretionnary complex occurred contemporaneously with unroofing below a north dipping detachment near the top of the wedge. The age of this detachment is bracketed between the end of the high-pressure event (20 Ma) and the deposition of the breccia (Early to Middle Miocene) in the Neogene basins.

KINEMATIC, THERMAL AND PETROLOGICAL MODEL OF THE CENTRAL ALPS : LEPONTINE METAMORPHISM IN THE UPPER CRUST AND ECLOGITISATION OF THE LOWER CRUST

Abstract Seismic and seismological studies as well as gravimetric models indicate that a slab of European lithospheric mantle and lower crust is currently underthrust below the Apulian crust. We assume a simple kinematic model in which the lower and upper subducted European crusts are decoupled along a decollement. The lower crust goes into subduction without deformation. The upper crust deforms by pure shear with a horizontal compressional axis. The total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry, different distributions of erosion being used. The computed temperature field is steady-state if the kinematic model applies during a minimum time of 40 Myr for a convergence rate of 8 mm/yr. Equilibrium mineral assemblages and densities are determined from the computed P, T conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust. Assuming local isostasy, the density model fits the average topographic profile across the Central Alps. The P-T-t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure Oligocene metamorphism in the Lepontine dome. The peak calculated temperature for the deepest non subducted crustal rocks is 600°C for a pressure of 0.8 GPa, near the lower limit of high-pressure amphibolites. We thus propose that the Lepontine metamorphism corresponds to the steady-state thermal regime. However, either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt. The computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km. We conclude that the Moho limiting the deepest part of the root may correspond to the eclogitisation phase change. Lower crust eclogites have a density comparable to or higher than that of the mantle, depending on their chemical composition (3.37 for andesitic eclogites, 3.56 for gabbroic eclogites). Thus, andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink.

Thermal structure and exhumation history of the Lesser Himalaya in central Nepal

The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last ~20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°–550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°–50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e., an increase from about 3–4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid-Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle-ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m^2/yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units.

How does the Alpine belt end between Spain and Morocco

The Betic-Rif arcuate mountain belt (southern Spain, northern Morocco) has been interpreted as a symmetrical collisional orogen, partly collapsed through convective removal of its lithospheric mantle root, or else as resulting of the African plate subduction beneath Iberia, with further extension due either to slab break-off or to slab retreat. In both cases, the Betic-Rif orogen would show little continuity with the western Alps. However, it can be recognized in this belt a composite orocline which includes a deformed, exotic terrane, i.e. the Alboran Terrane, thrust through oceanic/transitional crust-floored units onto two distinct plates, i.e. the Iberian and African plates. During the Jurassic-Early Cretaceous, the yet undeformed Alboran Terrane was part of a larger, Alkapeca microcontinent bounded by two arms of the Tethyan-African oceanic domain, alike the Sesia-Margna Austroalpine block further to the northeast. Blueschist- and eclogite-facies metamorphism affected the Alkapeka northern margin and adjacent oceanic crust during the Late Cretaceous-Eocene interval. This testifies the occurrence of a SE-dipping subduction zone which is regarded as the SW projection of the western Alps subduction zone. During the late Eocene-Oligocene, the Alkapeca-Iberia collision triggered back-thrust tectonics, then NW-dipping subduction of the African margin beneath the Alboran Terrane. This Maghrebian-Apenninic subduction resulted in the Mediterranean basin opening, and drifting of the deformed Alkapeca fragments through slab roll back process and back-arc extension, as reported in several publications. In the Gibraltar area, the western tip of the Apenninic-Maghrebian subduction merges with that of the Alpine-Betic subduction zone, and their Neogene roll back resulted in the Alboran Terrane collage astride the Azores-Gibraltar transpressive plate boundary. Therefore, the Betic-Rif belt appears as an asymmetrical, subduction/collision orogen formed through a protracted evolution straightfully related to the Alpine-Apenninic mountain building.

Kinematic, thermal and petrological model of the Himalayas: constraints related to metamorphism within the underthrust indian crust and topographic elevation

Abstract The maximum average elevation of the Alpine-Himalayan belt does not exceed 2.5 km from the Alps to the Zagros but abruptly increases to 5 km in the High Himalayas. The limitation of the elevation to 2.5 km can be explained if eclogitization occurs below 55 km from the Alps to the Zagros. The 5-km elevation of the High Himalayas is due to the presence of crustal-like densities to 75 km and this presence requires a mechanism to prevent the eclogitization to occur above 75 km there. Thermal models are constructed assuming a geometry based on the INDEPTH seismic profile, which shows underthrusting of the whole Indian crust below the crustal wedge of the High Himalayas, and possibly as far as the Indus-Tsangpo suture. Assuming a rate of underthrusting of about 15 mm/yr on a 10° dip fault plane, undeformed Indian crust had already reached the Indus-Tsangpo suture at the end of the main phase of thrusting along the Main Central Thrust (MCT) 20 myr ago. Consequently, it is possible to estimate the current isotherms in the Himalayan belt from a single thrust model. The high temperatures and partial melting observed in the Higher Himalayan Crystallines (HHC) imply that similar conditions should occur within the upper crustal wedge. Our models show that these conditions at the base of the upper crustal wedge can be obtained by the conjunction of a thick upper crustal pile (with radioactive heat production 2 × 10 −6 W/m 3 ) and a high erosion rate (1 mm/yr). The absence of eclogitization above 75 km within the underthrust Indian lower crust can be explained either if the temperature is higher than 800°C (in the granulite field) or lower than 500–550°C, within the blueschist field. Because the fast rate of underthrusting of the lower crust prevents its heating, the temperature there cannot reach values in excess of 800°C. On the other hand, with reasonable thermal parameters, temperatures of less than 500–550°C may prevail. In this case, the 5-km elevation can be reproduced if the transformation in blueschist is inhibited in the lower crust, presumably because of low water availability. We consequently propose that the combination of a relatively cold underthrust Precambrian crust and of a fast rate of underthrusting prevents eclogitization above 75 km and enables the mountain belt to reach altitudes of 5 km. Thus the main difference between the Zagros and the Himalayas would be the critically slower rate of underthrusting in the Zagros compared to the Himalayas.

Ductile extension in alpine Corsica

Ductile deformation in high-pressure ( P )-low temperature ( T ) conditions due to the westward thrusting of oceanic material onto a continental basement in alpine Corsica is overprinted by a late deformation event with a reverse shear sense (eastward) that took place in less severe P-T conditions. We show that the late deformation can be linked to extension during rifting and spreading of the Liguro Provencal basin from late Oligocene to late-middle Miocene time. Major compressive thrust contacts were reactivated as ductile normal faults and, in some units, only a penetrative eastward shear can be observed. This extension following the thickening of the crust brought tectonic units which underwent very different P- T conditions during the earlier stage into close contact. The Balagne nappe, which shows neither significant ductile deformation nor metamorphism, directly overlies the high- P units. The extensional deformation is distributed through the entire thickness of the nappe stack but is more important along the major thrust contacts, which localize the strain. The geometry of the crustal extension is controlled by that of the early compressive thrusts. The latest structures are east-dipping brittle normal faults which bound the early to middle Miocene Saint Florent half graben.

Exhumation of deep crustal metamorphic rocks and crustal extension in arc and back-arc regions

We adress the problem of exhumation of metamorphic rocks in arc and back-arc positions based on observations of the Aegean and Tyrrhenian back-arc regions. In both regions back-arc extension has been active for 25 Ma at least, while convergence and crustal thickening was going on at the thrust front. Compressional structures are reworked in the back-arc domain as extensional flat-lying detachments which allow unroofing of the metamorphic pile. Metamorphic rocks suffer first a dynamic burial during nappe stacking and then fast exhumation when they reach the extensional region. We describe a continuous process of burial and exhumation that is contemporaneous with the migration of the thrust front toward external zones. The front of extension follows behind at approximately one hundred kilometers. At the convergence front, continuous subduction of a cold oceanic slab or continuous accretion of continental units keeps a cool P-T gradient and HP-LT metamorphic rocks are formed at depth. The overall cool environment leads to a deep brittle-ductile transition and thus to a resistant crust which can sustain its large thickness. While the thrust front propagates forward, parts of the accretionary wedge which were formed earlier enter the back-arc domain which can be already under extension and which is warmer because of a greater distance to the cold slab and the presence of the volcanic arc. The brittle-ductile transition rises in the thick crust which then becomes unstable and subsequently collapses. The exhumation history of metamorphic rocks below detachments is two-fold. (1) A constantely thickening crust is unroofed by flat-lying detachments which rework early thrusts. This first period is characterized by a good preservation of HP-LT parageneses. (2) The second period is when rocks are in the zone of crustal-scale extension of the back-arc region. Deep crustal metamorphics are exhumed below flat-lying detachements which affect the whole crust.

Uplift of Tibet: from eclogites to granulites — implications for the Andean Plateau and the Variscan belt

Abstract Using a simplified petrogenetic grid and a one-dimensional thermal model to compute the evolution of the density of the crust with time, the contribution to the uplift of Tibet of metamorphic transitions in the lower crust is shown to be large. The computations are made for a lower crust of either mafic or intermediate composition. A mafic lower crust would have a mantle-like density, incompatible with seismological results; this conclusion implies that any pre-thickening gabbroic lower crust has been removed during the orogenic process. A lower crust of intermediate composition would be affected by about 3 km of post-thickening uplift, over a few tens of millions of years. This uplift is essentially the result of the eclogite to granulite transformation. The resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere. Such a process would also explain why the Neogene uplift of Tibet to an average elevation of 5 km appears to have occurred without crustal shortening. The same model may explain the uplift of the Altiplano-Puna Plateau, which also occurred without surface shortening since about 10 Ma. A comparison is made with the now collapsed Variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years.