ALBERT

Description: Observations from deep boreholes at several locations worldwide, laboratory measurements of frictional strength on quartzo-feldspathic materials and earthquake focal mechanisms indicate that crustal faults are strong (apparent friction μ≥0.6). However, friction experiments on phyllosilicate-rich rocks and some geophysical data have demonstrated that some major faults are considerably weaker. This weakness is commonly considered to be characteristic of mature faults in which rocks are altered by prolonged deformation and fluid-rock interaction (i.e. San Andreas, Zuccale, and Nankai faults). In contrast, in this study we document fault weakening occurring along a marly shear zone in its infancy (〈30 m displacement). Geochemical mass balance calculation and microstructural data show that a massive calcite departure (up to 50 vol%) from the fault rocks facilitated the concentration and reorganization of weak phyllosilicate minerals along the shear surfaces. Friction experiments carried out on intact foliated samples of host marls and fault rocks demonstrated that this structural reorganization lead to a significant fault weakening and that the incipient structure has strength and slip behavior comparable to that of the major weak faults previously documented. These results indicate that some faults, especially those nucleating in lithologies rich of both clays and high-solubility minerals (such as calcite) might experience rapid mineralogical and structural alteration and become weak even in the early stages of their activity.

Description: Subduction of oceanic highs has been described in offshore accretionary wedges thanks to bathymetric and seismic data, and simulated in sand-box models. Where documented, it is limited to oceanic subduction of small culminations (seamounts). However, tracing this phenomenon in ancient and/or uplifted belts remains a challenge. Based on structural, geomorphological and thermal analysis, we describe a subducted horst in the NW Borneo Wedge. This is evidenced by the occurrence of several geomorphological anomalies: a strong bend of the structural trend of the Rajang–Crocker Belt and an area with ′hummocky′ texture representing dismantled packages of sediments. Some specific structural features (large back and out-of-sequence thrusts in the internal zones, complex folds rooted on shear structures in the accretionary wedge) also occur in this central part of the NW Borneo Wedge. The central area of the related deformation zone records several thermal anomalies. Terra Nova, 00, 1–9, 2011

Description: [1]  The Pyrenees, north of the North Pyrenean fault display a complex structure involving a succession of peridotite massifs, basement massifs and Mid- to Late-Cretaceous basins located in a narrow domain which was affected by a Mid-Cretaceous, pre-orogenic high-temperature low-pressure metamorphism. The Late Cretaceous basins were interpreted either as pull-apart basins formed during transcurrent motion of Iberia relative to Eurasia or as remnants of a larger extensional basin. Recent models support that peridotite massifs result from the exhumation of the mantle during this pre-orogenic event. The northern boundary of the Agly basement massif shows evidence of ductile deformation of the basal formations of the Agly sedimentary cover. Macroscopic and microscopic kinematics indicators consistent with asymmetry of crystallographic fabrics suggest normal sense of shear, and thus suggest detachment, at least partial, of the Mesozoic cover from its basement. Triassic to Early Cretaceous limestones are mylonitic and consistently shows a foliation, a NS- to NE-trending lineation, shear criteria suggesting top-to-the-North shearing and locally boudinage. At the microscopic scale, mylonites are characterized by a very fine grain-size, frequently 〈10 m μ . They contain larger, partially recrystallized calcite parent grains and undeformed quartz grains with calcite fringes crystallized in pressure shadows. In these mylonites, calcite systematically shows a weak but well-defined crystallographic preferred-orientations suggesting HT dislocation creep combined to diffusion creep and possibly grain boundary sliding in the finest fraction of the mylonites. Paleotemperatures determined using Raman spectrometry suggest synkinematic temperature conditions in the range 340–390°C, in good agreement with observed microstructures and calcite CPO. The mylonitic fabric in Mesozoic limestones is folded by EW-trending Pyrenean folds north of the Agly basement massif, attesting that this fabric formed before the Pyrenean orogeny. These data consistently support pre-orogenic extension under medium-temperature conditions of the northern Agly massif and likely of the massif itself. Since simultaneously (~100 Ma) a Mid-Cretaceous basin opened south of the basement massif, we suggest a model of pre-orogenic exhumation of the Agly massif in response to the regional extension associated to the rotation of Iberia. This model may explain the exhumation of the North Pyrenean massifs during a single pre-orogenic event that allowed the opening of extensional basins and the exhumation of the lithospheric mantle. All these structures being subsequently reworked during the Pyrenean orogeny.

Description: Current models for the Oligo-Miocene post-orogenic back-arc extension of the Aegean domain suggest that stretching is accommodated by two bivergent detachment systems of opposing shear sense. The co-existence in the Eocene of a top-to-the-S thrust at the base of the Cycladic Blueschists unit and top-to-the-N extensional shear zones at the roof raises the problem of differentiating synorogenic and post-orogenic deformations with similar directions and shear senses. Based on structural field data, this study shows that the post-orogenic deformation recorded in the Southern Cyclades is extremely asymmetric as the Cycladic Blueschists unit is pervasively affected by top-to-the-N shearing deformation distributed on four main shear zones. All activated in greenschist-facies conditions, some of these shear zones operated in the brittle regime during the final part of the exhumation. The Cycladic Blueschists/Cycladic Basement contact displays clear polyphased deformation with the preservation of top-to-the-S thrust kinematics. Thermal structure of the Cycladic Blueschists unit with regards to position of ductile shear zones was retrieved using the Raman Spectroscopy of Carbonaceous Material peak-metamorphic temperatures. This study shows a series of major metamorphic gaps accommodating an upward and stepwise decrease of more than 200 °C within the Cycladic Blueschists unit. Pressure-temperature estimates show that only lower parts of the Cycladic Blueschists unit recorded ca. 18–20 kbar for 530 °C peak-conditions. While flanking the West Cycladic Detachment System, which shows a top-to-the-S shear sense, the Southern Cyclades are dominated by a top-to-the-N non-coaxial shearing. Deformation is therefore genuinely asymmetric in the center of the Aegean domain.

Description: Abstract Constraining the way in which continental deformation is accommodated in time and space is essential to reconcile past plate movements with geological observations. Kinematic reconstructions of the Iberia‐Europe plate boundary are still debated. Here we focus on an inverted Mesozoic rift basin, the Cameros basin, which is part of the Iberian chain. We use a combination of detrital low‐temperature thermochronological techniques to define the time‐temperature evolution of the basin from Mesozoic rifting to Cenozoic collision. Zircon fission‐track analyses of Oligocene–Miocene sedimentary rocks yield two main age populations at ~170 ± 10 and ~100 ± 10 Ma, reflecting (i) an Early Jurassic thermal event related to the Atlantic‐Alpine Tethys opening and (ii) an Albo‐Cenomanian thermal event related to the Bay of Biscay opening. Thermal modeling of combined zircon fission‐track, apatite fission‐track, and apatite (U‐Th‐Sm)/He data reveals that collision‐related cooling of the Cameros basin started during the Paleocene (~60 Ma). A second cooling/exhumation phase of the basin is recorded from 35 to 25 Ma. Initial cooling occurred after a protracted postrift period characterized by persistence of high geothermal gradients, a feature also recognized in the Pyrenees. Our results show that the Iberian chain shared the same Early to Late Jurassic tectonothermal evolution with the Atlantic‐Alpine Tethyan rifted margins. From the Albian onward, the thermal evolution of the Cameros basin was very similar to that of the Pyrenees. This study shows that the preservation in mountain ranges of a succession of rifting events provide important clues for plate reconstructions.

Description: This paper presents a new balanced structural cross-section of the Jaca thrust-sheet-top basin of the southern Pyrenees combined with paleo-thermometry and apatite fission track (AFT) thermochronology data. The cross-section, based on field data and interpretation of industrial seismic reflection profiles, allows refinement of previous interpretations of the south-directed thrust system, involving the identification of new thrust faults, and of the kinematic relationships between basement and cover thrusts from the middle Eocene to the early Miocene. AFT analysis shows a southward decrease in the level of fission track resetting, from totally reset Paleozoic rocks and lower Eocene turbidites (indicative of heating to Tmax 〉 ~120 °C), to partially reset middle Eocene turbidites and no/very weak resetting in the upper Eocene-lower Oligocene molasse (Tmax 〈 ~60 °C). AFT results indicate a late Oligocene-early Miocene cooling event throughout the Axial Zone and Jaca Basin. Paleo-maximum temperatures determined by vitrinite reflectance measurements and Raman spectroscopy of carbonaceous material reach up to ~240 °C at the base of the turbidite succession. Inverse modelling of AFT and vitrinite reflectance data with the QTQt software for key samples show compatibility between vitrinite-derived Tmax and the AFT reset level for most of the samples. However, they also suggest that the highest temperatures determined in the lowermost turbidites correspond to a thermal anomaly rather than burial heating, possibly due to fluid circulation during thrust activity. From these results, we propose a new sequential restoration of the south Pyrenean thrust system propagation and related basin evolution.