Mario Sánchez-Gómez

New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190

Moore, G. F., Taira, A., Klaus, A., Becker, L., Boeckel, B., Cragg, B. A., Dean, A., Fergusson, C. L., Henry, P., Hirano, S., Hisamitsu, T. et al. (2001). New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems, 2, Article No: 2001GC000166.

Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betics, Gibraltar Arc)

In the western Alpine system, Neogene extensional tectonics triggered the development of marine basins on the concave side of tight orogenic arcs, as happened within the Alboran Crustal Domain, the hinterland of the Gibraltar Arc. A detailed analysis of the structural and metamorphic records of one of the main Alboran Domain complexes, however, plainly reveals a complex tectonic evolution prior to the development of the Miocene arc/back arc system, which includes a major intraorogenic extensional event. This large-scale subvertical shortening, that can be assessed from the PT paths of representative tectonic units, was subsequent to the continental crust subduction inferred from high pressure-low temperature mineral asssemblages. The crustal section was thinned in nearly isothermal conditions, its thickness being reducted to at least 1/3 of the initial value. Yet still before the Miocene, a second contractional event led to the overthrusting of high-grade metamorphic rocks over other low-grade rocks, accompanied by subordinate overturning of the metamorphic zones. Since migration of the Gibraltar Arc is roughly balanced by crustal spreading in the back arc, available data concerning Miocene extension suggest that the Alboran Domain can be restored to its appropriate position several hundred kilometers to the east. Thus a collision belt that underwent significant intraorogenic extension must have existed in what is now the western South-Balearic basin.

Geochronology of clasts in allochthonous Miocene sedimentary sequences on Mykonos and Paros Islands: implications for back-arc extension in the Aegean Sea

In the Cyclades, low-angle normal faults juxtaposed Miocene sedimentary rock units lying over Alpine blueschist- and greenschist-facies metamorphic rocks and Miocene granites. The sedimentary units in the hanging wall were deposited in fault-bounded basins while their footwalls progressively emerged through the ductile and brittle crust. The sedimentation in the basins evolved from marine turbidites in the Early Miocene to shallow/continental conglomerates during the Late Miocene. The transition from marine to fan delta sedimentation was coeval with widespread magmatism and possibly reflects true crustal uplift. It is inferred that the sequence is no younger than 8 Ma. K–Ar and 40Ar/39Ar geochronology, petrology and petrography of the clasts deposited on the hanging wall provide proof of progressive exhumation of the footwall and reveal the nature of the overburden that covered the Cycladic blueschist belt during Alpine orogeny in the Tertiary. Abundant metamorphic clasts yielding mica cooling ages between 80 and 100 Ma occur throughout the sedimentary section and probably pertain to a vast Pelagonian-type rock mass that covered the internal Hellenides from the Olympos to the central Cyclades. In addition, volcanic clasts dated at 10 Ma reveal the existence of a hitherto unknown volcanic province of that age in the central Aegean. Miocene crustal extension and exhumation of granitic plutons is recorded in the detrital sequence in a concentration of 10 Ma granitic clasts restricted to the top of the conglomerate sequence. A group of metamorphic clasts that yielded ages of 13–16 Ma possibly represents exhumation of mid crustal levels. Clasts similar to the currently-exposed Cycladic Blueschist Unit, such as 40 Ma old blueschists, eclogites and marble, were not found, thus indicating the very late exposure of these footwall rocks.

Coaxial flattening at deep levels of orogenic belts: evidence from blueschists and eclogites on Syros and Sifnos (Cyclades, Greece)

This work presents new Structural data from a high-pressure/low-temperature (HP/LT) metamorphic terrane exposed on the islands of Syros and Sifnos (Cyclades, Greece). The structure and the metamorphism of a relatively coherent HP/LT rock section were studied in order to elucidate how strain was accommodated at deep crustal levels during the formation and exhumation of HP/LT rocks. At least three deformation phases associated with eclogite- and blueschist-facies conditions (P = 8-15 kbar; T = 400-550 degreesC) were recognised. The earliest deformation fabric (S1), preserved as inclusion trails within garnet porphyroblasts, is aligned to define a sub-vertical schistosity (at present orientation), which is frequently orthogonal to the flat matrix schistosity (S2), and may indicate that deep crustal thickening involved upright folding. The currently dominant fabric in the HP rock section, S2, is Usually moderately dipping and locally contains NW-trending glaucophane lineations, symmetric pressure-shadows and eclogitic boudins. The symmetric structures associated with this fabric seem to indicate coaxial vertical thinning, although the existence of non-coaxial structures out of the study area cannot be excluded. Glaucophane-bearing shear bands (S3), with top-to-NW sense of shearing, locally crosscut the earlier structures. The latest recognised fabric (D4) is scarce and often absent within the HP rocks. It is associated with top-to-NE kinematic criteria that formed at greenschist-facies conditions (P = 4-7 kbar; T = 400-450 degreesC). Based on these observations, it is suggested that partitioning of strain occurred at different crustal levels and at different times. Deep crustal deformation was governed by thickening via upright folding followed by coaxial vertical thinning, whereas non-coaxial shearing occurred when the rocks were already exhumed to relatively shallow crustal levels. The earliest fabrics (D1 to D3) pertain to Alpine orogenesis and possibly to syn-orogenic extension, whereas the latest correspond to whole-crust back-are extension

A ‘core-complex-like structure’ formed by superimposed extension, folding and high-angle normal faulting. The Santi Petri dome (western Betics, Spain)

The Santi Petri dome (western Betics, southern Spain) shows a core-complex-like structure, where migmatitic gneisses and schists outcrop below low-grade slates and phyllites, all of which form the basement of the Neogene Malaga basin. The migmatites and schists suffered a coaxial-flattening event during isothermal decompression and were later exhumed by ductile ESE non-coaxial stretching. Further exhumation was achieved by W- to SW-transport brittle low-angle normal faulting. Subsequently these extensional structures were gently folded in the core of a NE/SW-oriented antiform during the Tortonian. Finally the Santi Petri domal geometry was accentuated by the interference of orthogonal high-angle faults with ENE–WSW and NNW–SSE orientation. This core-complex-like structure, formed by superposition of extensional and compressive tectonic events, does not represent a classical, purely extensional core complex, which shows that metamorphic structure and geometry are not decisive criteria to define a core-complex.Abstract The Santi Petri dome (western Betics, southern Spain) shows a core-complex-like structure, where migmatitic gneisses and schists outcrop below low-grade slates and phyllites, all of which form the basement of the Neogene Malaga basin. The migmatites and schists suffered a coaxial-flattening event during isothermal decompression and were later exhumed by ductile ESE non-coaxial stretching. Further exhumation was achieved by W- to SW-transport brittle low-angle normal faulting. Subsequently these extensional structures were gently folded in the core of a NE/SW-oriented antiform during the Tortonian. Finally the Santi Petri domal geometry was accentuated by the interference of orthogonal high-angle faults with ENE–WSW and NNW–SSE orientation. This core-complex-like structure, formed by superposition of extensional and compressive tectonic events, does not represent a classical, purely extensional core complex, which shows that metamorphic structure and geometry are not decisive criteria to define a core-complex.

An evaporite‐bearing accretionary complex in the northern front of the Betic‐Rif orogen

The Guadalquivir Accretionary Complex forms a largely oblique prism at the northern edge of the Betic-Rif orogen, where Miocene sediments plus allochthonous evaporite-bearing units were accreted during the displacement of the Alboran Domain towards the west. Traditional interpretations end the tectonic structuring of the Betic Cordillera at the present topographic front, beyond which gravitational and/or diapiric processes would predominate. However, this study shows pervasive tectonic deformation in the outer prism with coherent oblique shortening kinematics, which is achieved through an alternation of roughly N-S arcuate thrust systems connected by E-W transfer fault zones. These structures accord well with the geophysical models that propose westward rollback subduction. The main stage of tectonic activity occurred in the Early–Middle Miocene, but deformation lasted until the Quaternary with the same kinematics. Evaporite rocks played a leading role in the deformation as evidenced by the suite of ductile structures in gypsum distributed throughout the area. S- and L- gypsum tectonites, scaly clay fabrics, and brittle fabrics coexist and consistently indicate westward motion (top to 290°), with subordinate N-S contraction almost perpendicular to the transfer zones. This work reveals ductile tectonic fabrics in gypsum as a valuable tool to elucidate the structure and deformational history of complex tectonic melanges involving evaporites above the decollement level of accretionary wedges.

Natural Monument of the Volcano of Cancarix, Spain: A Case of Lamproite Phreatomagmatic Volcanism

The Natural Monument of the Volcano of Cancarix (south Spain) is of great scientific interest due to the particular lamproitic nature of the geological materials that constitute it. The Cancarix volcano and similar lamproite volcanoes in the southeastern parts of the Iberian Peninsula were emplaced during the Miocene in connection with transtensive structures at the north end of the Betic Cordillera. These structures reactivated older basement faults, which facilitated the ascending circulation of highly enriched Mg–K magma. Two episodes of material emission can be distinguished: (1) explosive volcanism due to the interaction between magma and groundwater from the karstic system of the host carbonate rocks, generating a phreatomagmatic complex as the final product (breccias and lavas); and (2) effusive volcanic activity with crystal-rich magma responsible for the lamproitic body. The tectonic structure, the main features of the phreatomagmatic ring, and the vertical columnar jointing at the edge of the volcanic constitute a wide spectrum of geological phenomena that justify its educational scientific value and its protection as a Natural Monument. Two geotrails present this natural space not only as an interpretive resource but also as an instructive reference for the conservation of a valuable part of Spain’s geological heritage. Further, the remarkable exhibition of materials along the two geotrails provides an excellent framework for promoting geology at different levels of knowledge.