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.
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We present a global inventory of transform continental margins, based on both a worldwide comparison of continent-ocean boundary identifications with oceanic fracture zones traces, and a compilation of published regional studies. This inventory increases the number of identified transform margins from 29 to 78. These margins represent 16% of continental margins in cumulative length and 31% of non-convergent margins. We include morphological data, published upper crustal sections, continent to ocean transition locations and published Moho shape data in the new database. This review confirms that continent to ocean transitions are sharper at transform margins than at divergent margins. It also emphasizes the structural diversity of transform margins. Associated with one third of transform margins, we define marginal plateaus as a new type of relief that corresponds to a flat but deep surface inside the continental slope, and that may be inherited from crustal thinning prior to transform faulting.Transform margin initiation appears to be favoured along propagating oceans and within cold and thick lithospheres.
The eastern Demerara Plateau offshore French Guiana was surveyed in 2003 during the GUYAPLAC cruise (multibeam bathymetry and acoustic imagery, 6-channel seismic reflection and 3.5 kHz echosounding). The data show the "post-transform" Cenozoic that the series located on the outer part of the plateau (below c. 2000 m) contain at least twelve stacked mass transport deposits (MTDs) that have recorded a history of large-scale slope failure, as well as two main normal fault sets that provide possible pathways for upward fluid migration through the series, reaching at high as the uppermost MTDs. Seabed data show that the area above the failures is characterized by circular-to-elongate (slope-parallel) depressions interpreted as fluid seeps (pockmarks), some of them have been modified by along slope currents. We suggest that the development of the MTDs to results from the combinaiton of the presence of fluid overpressure at depth the geometry of the margin's deep structure, in particular the existence of a 'free borderlateral border' on the outermost plateau. Our results also emphasise the role of stratigraphic décollements within the Cenozoic series. Highlights ► Cenozoic succession of the Demerara Plateau contains a complex of at least 12 MTDs. ► Sedimentary undulations and circular to elongated depressions occur on the seafloor. ► Faults provide fluid migration pathways through the MTDs (biogenic or thermogenic). ► Detachments occur on stratigraphic horizons that outcrop along the continental slope. ► Fluid overpressures and the distal transform free border seem to control MTD dynamics.
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