Ana Crespo-Blanc

Extension versus compression during the Miocene tectonic evolution of the Betic chain. Late folding of normal fault systems

The westernmost part of the Mediterranean Alpine Belt is represented by the Betic-Rif orogenic belt, around the Gibraltar Arc, which in turn surrounds the Alboran Basin. In the Betic Chain, early and middle Miocene crustal thinning of the Alboran basement is well established, as extensional low-angle normal faults and detachment faults, developed in both ductile and brittle conditions, thinned a previously thickened crust. In the Alboran Domain of the central Betics, two main extensional episodes are evidenced: a Langhian one, with a north-northwestward transport direction, followed by a west-southwestward extension, Serravallian in age. Therefore all the units heretofore considered to be thrust nappes are, in reality, extensional units bounded by low-angle normal faults. The cortical segment studied formed the basement of the Miocene Alboran Basin, in which progressively deeper basement units were covered by younger marine sediments as a result of extensional denudation processes. The age of these sediments clearly dates the faulting. The extensional evolution during the Miocene is much more complex than the past models suggest. During the upper Miocene, these extensional systems were folded as the result of a compressive regime, which allowed them to be well exposed. Compression in the Gibraltar Arc is nearly contemporaneous with extension, and the westward migration of the compression through its footwall is related with the extensional spreading.

Structure and kinematics of the South Iberian paleomargin and its relationship with the Flysch Trough units: extensional tectonics within the Gibraltar Arc fold-and-thrust belt (western Betics)

Abstract The superposition of structures observed in a selected area of the external zones of the Betic orogen, situated in the westernmost segment of the peri-Mediterranean mountain belt, reveals an alternation of compressional and extensional events during the overall convergence of the African and Eurasian plates that bound the system. An Early Miocene compressional event produced the formation of the so-called Gibraltar Arc fold-and-thrust belt, formed by the units derived from the South Iberian paleomargin and the Flysch Trough. This wedge constitutes the footwall of the Gibraltar Thrust, whose hanging wall corresponds to the Alboran Domain. The outward migration of the compressional front (W-to-NWward in the South Iberian segment) was followed by the outward migration of the extensional locus, related to the Middle Miocene rifting that led to the Alboran Basin subsidence. The tectonic inversion of the Gibraltar Thrust was accompanied by normal faulting of the innermost part of the paleomargin and the Flysch Trough units. Finally, the Alboran region underwent N–S-to-NW–SE compression from Late Miocene onwards, which caused the uplift of the Gibraltar Arc area.

The Southern Iberian Shear Zone: a major boundary in the Hercynian folded belt

A long ductile shear zone extending across the northern parts of Sevilla and Huelva provinces, southwest Spain, and very probably stretching further west into Portugal, is characterized by a conspicuously developed L-S fabric and the existence of various mylonitic textures. The shear zone, closely associated with amphibolites of tholeiitic affinity, represents an important oceanic suture of the Hercynian belt in Europe. The sense of displacement inferred from microstructures is consistent with southwest-directed upthrusting evolving to a left-lateral shearing movement.

Role of décollement material with different rheological properties in the structure of the Aljibe thrust imbricate (Flysch Trough, Gibraltar Arc): an analogue modelling approach

Abstract The Miocene Aljibe thrust imbricate (Flysch Trough), in the external zone of the Gibraltar Arc, shows large changes in structural style towards the foreland. Rheological variation of the decollement rocks—Triassic evaporitic rocks and Mesozoic limestones—appears to be the main factor controlling the contrasts in structural style that characterise adjacent wedge domains. In order to test this hypothesis, we carried out analogue experiments involving three-dimensional variations of decollement material. A plate of viscous material was placed in the central part of the models, at the base of the sand multilayers. Different geometries of the viscous plate were used. Experimental results show a strong link between the rheological properties of the decollement material and the structure of the overlying thrust wedge. These results compare well with the internal geometry and vergence distribution in the Aljibe thrust imbricate. Indeed, the tectonic style of the wedge can be explained as a consequence of variations of rock type along the basal decollement.

Continental collision, crustal thinning and nappe forming during the pre-Miocene evolution of the Alpujarride Complex (Alboran Domain, Betics)

The tectono-metamorphic evolution of the Alpujarride Complex within the internal zones of the Gibraltar Arc indicates that the Alboran Crustal Domain underwent several contractional and extensional events. The superposition of structures and the metamorphic evolution observed in the Alpujarride Complex, and in particular in the Adra unit, suggest that these events are as follows: (a) the first thickening event (D1) results from continental subduction, and is evidenced by the presence of pre-S2 high-pressure mineral relicts; (b) the thinning and associated vertical shortening event (D2) is indicated by an almost isothermal decompression during which the S2 main foliation developed; (c) kilometric-scale N-vergent recumbent folds of the condensed metamorphic sequences, and thrust and nappe-forming in the Alpujarride Complex, suggest that a second thickening event (D3) took place; (d) the second thinning event (D4), which affects the whole Alboran Domain, is evidenced by extensional fault systems resulting in the opening of the Alboran Sea. The oldest synrift deposits are of the Late Oligocene and Early Miocene, suggesting that D1 to D3 events are pre-Miocene; and (e) from the Late Tortonian to the Pliocene, the extensional systems were folded due to a continuous N-S to NW-SE compression.

Exhumation during a continental collision inferred from the tectonometamorphic evolution of the Alpujarride Complex in the central Betics (Alboran Domain, SE Spain)

The tectonometamorphic evolution of the Alpujarride Complex within the internal zones of the Betics reveals a complex structural and metamorphic succession of continental collision, synmetamorphic exhumation, nappe forming in the final stage of exhumation, and rifting. A complex tectonic evolution is deduced from the superposition of structures observed in several Alpujarride units of the central Betics together with their metamorphic record. The following sequence of events is suggested: (1)A first stacking event (D1) is deduced from the presence of pre-S2 high-pressure-low-temperature metamorphic assemblages, (2) The synmetamorphic exhumation is inferred from the almost isothermal decompression pressure-temperature paths during which the S2 main foliation developed; the condensation of the mineral zones perpendicularly to the S2 foliation suggests large-scale vertical shortening during event D2, (3) A new stacking event (D3) is inferred from post-metamorphic thrusts and nappes, which are probably associated with kilometer-scale recumbent folds affecting the S2 main foliation, (4) A thinning event (D4), revealed by extensional fault systems, is associated with crustal spreading on the concave side of the Gibraltar Arc, which resulted in the opening of the Alboran Basin from the early Miocene (Burdigalian), (5) From late Tortonian to Pliocene a continuous N-S to NW-SE compression took place (D5). Stratigraphic, paleontological, and geochronological data suggest that event D3 could be lower Miocene in age (Aquitanian); consequently, event D1 and D2 would be pre-Miocene.

Interference pattern of extensional fault systems: a case study of the Miocene rifting of the Alboran basement (North of Sierra Nevada, Betic Chain)

Abstract In the Betic-Rif orogenic belt, in the westernmost part of the Mediterranean, early and middle Miocene crustal thinning of the upper part of the Alboran basement is well established by previous studies. In the Alboran domain of the central Betics, the present distribution of the Alpujarride units results from the extensional dismembering of a pre-Miocene nappe stack under brittle conditions. The interference of two subperpendicular and successive extensional fault systems can explain the current geometric pattern of the Alpujarride units: upper-Burdigalian-Langhian north-south extension was followed by west- to southwestward extension of Serravallian age. Northeast of Sierra Nevada, these two extensional systems have resulted in a spectacular chocolate tablet megastructure and the cropping out, at any one vertical sequence, of a varying number of extensional units belonging to the Alpujarride complex. This pattern can be considered representative of the middle Miocene tectonics of the entire Alboran domain in the Betics, and illustrates the development of rifting processes in the upper crust.

Superimposed folding and oblique structures in the palaeomargin-derived units of the Central Betics (SW Spain)

The Subbetic units consist of a deformed wedge of sedimentary rocks deriving from the South Iberian palaeomargin, in the westernmost segment of the Alpine–Mediterranean orogenic belt. In the central Betics, shortening produced folding, thrusting and nappe-stacking. The structural trend is NE–SW to ENE–WSW, and the internal structure of the thrust sheets, which shows no preferred vergence, is strongly controlled by the rheology of the décollement level, a viscous layer of Triassic evaporites, and of the overlying carbonate rock sequence. The relationships of piggyback basin Miocene sediments with the main structures show that the latter developed during the Aquitanian–Burdigalian transition. Superimposed folding is described for the first time in the study area. The origin of the interference cannot be determined at present, but may be related to initially oblique structures refolded during progressive deformation. A revision of previously published palaeomagnetic data in the light of the data presented in this paper, in conjunction with a model of the structural evolution of the Subbetic units of the western Betics, permits us to establish milestones of the tectonic evolution of the external zones in the northern branch of the Gibraltar Arc.

Arc-parallel vs back-arc extension in the Western Gibraltar arc: Is the Gibraltar forearc still active?

Extremely tight arcs, framed within the Eurasia-Africa convergence region, developed during the Neogene on both sides of the western Mediterranean. A complex interplate deformation zone has been invoked to explain their structural trend-line patterns, the shortening directions and the development of back-arc basins. Updated structural and kinematic maps, combined with earthquake data covering the complete hinge zone of the western Gibraltar arc help us to explore the mode of strain partitioning from 25My ago to present. During the Miocene, the strain partitioning pattern showed arc-perpendicular shortening in the active orogenic wedge –assessed from the radial pattern of tectonic transport directions– accompained by subhorizontal stretching. Structures accommodating stretching fall into two categories on the basis of their space distribution and their relationships with the structural trend-line pattern: i) arc-parallel stretching structures in the external wedge (mainly normal faults and conjugate strike-slip faults); and ii) extensional faults developed in the hinterland zone in which transport directions are centripetal towards the Alboran back-arc basin. Pliocene to Recent deformational structures together with focal solutions from crustal earthquakes (n=167; 1.5<Mw<6.3) support that this strain partitioning pattern still occurs. By contrast, the eastern end zones of the western Gibraltar arc, especially during the last 5My, underwent intense transpression tectonics with a NW to NNW main shortening axis. These results agree with a still active Gibraltar forearc, governed by westward migrating subduction retreat or subcontinental mantle delamination.

Structure and kinematics of a sinistral transpressive suture between the Ossa–Morena and the South Portuguese Zones, South Iberian Massif

The Ossa–Morena Zone and the South Portuguese Zone represent the southernmost Hercynian domains of the Iberian Massif. Lithostratigraphic, petrographic and structural differences exist between both zones. The boundary between the Ossa–Morena and the South Portuguese Zones is marked by a narrow belt of amphibolites of tholeiitic affinity and related oceanic sequences, the Beja–Acebuches ophiolites. The southernmost area of the Ossa–Morena Zone, which bounds the Acebuches amphibolites, is composed of medium to high grade metamorphic rocks. The Ossa–Morena Zone-South Portuguese Zone suture preserves a complex story of middle to upper Devonian (?) to upper Stephanian sinistral transpression. The resulting structures were generated in a ductile to brittle regime. Transpressive shear zones affect the ophiolitic remnants and the high grade rocks of the southernmost area of the Ossa–Morena Zone. The late Hercynian faulting combines sinistral strike-slip movements and some thrusting component, which cut the suture to produce fish-like megaboudins. The transpressive regime can be explained by the oblique collision of the two plates now represented by the Ossa–Morena Zone and the South Portuguese Zone during the continental accretion. On the scale of the Hercynian Belt, this sinistral transpression belongs to a network of comparable structures explained by the impingement of the Ibero-Armorican Terranes.