Juan Carlos Balanyá

Miocene extensional detachments in the outcropping basement of the northern Alboran Basin (Betics) and their tectonic implications

Whether or not there are extensional detachment faults in the Alboran basement can be tested directly because a part of the Alboran Basin is now emerged. These detachments, related to crustal thinning beneath the Alboran Basin, occurred from the Aquitanian to Tortonian. The resulting extensional geometries can be described in general terms. During the Serravalian a considerable southwest extension of the basin took place, accompanied by south-southeast extension in the northern Gibraltar Arc. Other detachments affected by Serravalian extension can be found. The spreading of the Alboran was nearly coeval with roughly westward migration of the Gibraltar mountain front.

Autopsy on a dead spreading center: The Phoenix Ridge, Drake Passage, Antarctica

New bathymetric and magnetic anomaly data from the Phoenix Ridge, Antarctica, show that extinction of all three remaining segments occurred at the time of magnetic chron C2A (3.3 ± 0.2 Ma), synchronous with a ridge-trench collision south of the Hero Fracture Zone. This implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision. Spreading rates slowed abruptly at the time of chron C4 (7.8 ± 0.3 Ma), probably as a result of extinction of the West Scotia Ridge, which would have led to an increase in slip rate and transpressional stress across the Shackleton Fracture Zone. Spectacular, high-relief ridges flanking the extinct spreading center, mapped for the first time using multibeam swath bathymetry, are interpreted as a consequence of a reduction in spreading rate, involving a temporary magma oversupply immediately prior to extinction.

Tectonics of an extinct ridge-transform intersection, Drake Passage (Antarctica)

New swath bathymetric, multichannel seismic and magnetic data reveal the complexity of the intersection between the extinct West Scotia Ridge (WSR) and the Shackleton Fracture Zone (SFZ), a first-order NW-SE trending high-relief ridge cutting across the Drake Passage. The SFZ is composed of shallow, ridge segments and depressions, largely parallel to the fracture zone with an `en echelon pattern in plan view. These features are bounded by tectonic lineaments, interpreted as faults. The axial valley of the spreading center intersects the fracture zone in a complex area of deformation, where N120°xa0E lineaments and E–W faults anastomose on both sides of the intersection. The fracture zone developed within an extensional regime, which facilitated the formation of oceanic transverse ridges parallel to the fracture zone and depressions attributed to pull-apart basins, bounded by normal and strike-slip faults.On the multichannel seismic (MCS) profiles, the igneous crust is well stratified, with numerous discontinuous high-amplitude reflectors and many irregular diffractions at the top, and a thicker layer below. The latter has sparse and weak reflectors, although it locally contains strong, dipping reflections. A bright, slightly undulating reflector observed below the spreading center axial valley at about 0.75xa0s (twt) depth in the igneous crust is interpreted as an indication of the relict axial magma chamber. Deep, high-amplitude subhorizontal and slightly dipping reflections are observed between 1.8 and 3.2 s (twt) below sea floor, but are preferentially located at about 2.8–3.0xa0s (twt) depth. Where these reflections are more continuous they may represent the Mohorovicic seismic discontinuity. More locally, short (2–3xa0km long), very high-amplitude reflections observed at 3.6 and 4.3xa0s (twt) depth below sea floor are attributed to an interlayered upper mantle transition zone. The MCS profiles also show a pattern of regularly spaced, steep-inclined reflectors, which cut across layers 2 and 3 of the oceanic crust. These reflectors are attributed to deformation under a transpressional regime that developed along the SFZ, shortly after spreading ceased at the WSR. Magnetic anomalies 5 to 5xa0E may be confidently identified on the flanks of the WSR. Our spreading model assumes slow rates (ca. 10–20xa0mm/yr), with slight asymmetries favoring the southeastern flank between 5C and 5, and the northwestern flank between 5 and extinction. The spreading rate asymmetry means that accretion was slower during formation of the steeper, shallower, southeastern flank than of the northwestern flank.

Elongated domes in extended orogens: A mode of mountain uplift in the Betics (southeast Spain)

The Sierra Nevada elongated dome in the Betic hinterland (westernmost Mediterranean region) formed by polymetamorphic, non-melted rocks involving crustal thickening and subsequent exhumation via extensional denudation including both normal faulting and vertical ductile thinning. Core rocks record a clockwise P-T-t path with segments of quasi-isothermal decompression that do not cross the melting solidi. Doming was caused by the interference of two orthogonal sets of Miocene-Pliocene, large-scale open folds (trending roughly E-W and N-S) that warp both WSWdirected extensional detachments and the footwall regional foliation. N-S folds were generated by a rolling hinge mechanism while E-W folds formed due to shortening perpendicular to the direction of extension. Strike-slip faults striking subparallel to the direction of extension laterally bound the domes, adjoining highly extended domains to less extended blocks. Using a three-dimensional model of the crustal structure of the Sierra Nevada elongated dome constrained by surface geological data, the relationships with present-day topography, and the deep crustal structure, this paper explores the role of crustal flow in the origin and evolution of the dome. Collectively, the crustal structure, the rheological considerations, and other geophysical data suggest the occurrence of flow channels at two levels: mid-crustal depths and the deep crust. Flow in the upper channel is closely related to the mode of footwall denudation by detachment unroofing. The flowing channel in the deep crust is probably induced by the NW-SE crustal thinning pattern inferred for the region, with a relatively thick crust at the NW, and is likely to be oblique to the direction of extension in the upper crust. A geometric model assuming footwall deformation by subvertical simple shear examines the possible exhumation paths of the lower-plate rocks and the evolution of the dome core in the upper crust during extension. In this model, the dome width measured parallel to the direction of extension can be used to estimate the amount of horizontal extension, once the dip of the non-readjusted segment of the detachment is well constrained. Finally, we also discuss two interesting associated problems

Contractional and extensional tectonics in Flysch and Penibetic units (Gibraltar Arc, SW Spain) : new constraints on emplacement mechanisms

Abstract The Aljibe Unit (Flysch Trough Complex), which can be described as a thin-skinned contractional belt, represents a major element of the Gibraltar Arc accretionary prism. It is shown that this unit underwent WNW–ESE shortening during the Miocene, as did the underlying Penibetic units. The organization of the Aljibe unit suggests a push-from-behind emplacement mechanism during this first contractional event. By contrast, the current boundary between the Penibetic and Aljibe units is found to be a normal, low-to-moderate angle fault that was later folded. This extensional faulting occurred between two compressional pulses (Lower-to-Middle and Upper Miocene in age) with similar shortening directions.