Origin of the Betic-Rif mountain belt
TL;DR: In this paper, a model for the Miocene evolution of the Betic-Rif mountain belts is proposed, which is compatible with the evolution of rest of the western Mediterranean.
Abstract: In recent years, the origin of the Betic-Rif orocline has been the subject of considerable debate. Much of this debate has focused on mechanisms required to generate rapid late-orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic-Rif mountain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present-day Calabrian Arc. The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic-Rif orocline during the early to middle Miocene.
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TL;DR: In this article, a model for the Cenozoic development of the region of SE Asia and the SW Pacific is presented and its implications are discussed, accompanied by computer animations in a variety of formats.
2,272 citations
TL;DR: A number of tectonic events occurred contemporaneously in the Mediterranean region and the Middle East 30-25 Myr ago as discussed by the authors, which are contemporaneous to or immediately followed a strong reduction of the northward absolute motion of Africa.
Abstract: A number of tectonic events occurred contemporaneously in the Mediterranean region and the Middle East 30–25 Myr ago. These events are contemporaneous to or immediately followed a strong reduction of the northward absolute motion of Africa. Geological observations in the Neogene extensional basins of the Mediterranean region reveal that extension started synchronously from west to east 30–25 Myr ago. In the western Mediterranean it started in the Gulf of Lion, Valencia trough, and Alboran Sea as well as between the Maures massif and Corsica between 33 and 27 Ma ago. It then propagated eastward and southward to form to Liguro-Provencal basin and the Tyrrhenian Sea. In the eastern Mediterranean, extension started in the Aegean Sea before the deposition of marine sediments onto the collapsed Hellenides in the Aquitanian and before the cooling of high-temperature metamorphic core complexes between 20 and 25 Ma. Foundering of the inner zones of the Carpathians and extension in the Panonnian basin also started in the late Oligocene-early Miocene. The body of the Afro-Arabian plate first collided with Eurasia in the eastern Mediterranean region progressively from the Eocene to the Oligocene. Extensional tectonics was first recorded in the Gulf of Aden, Afar triple junction, and Red Sea region also in the Oligocene. A general magmatic surge occurred above all African hot spots, especially the Afar one. We explore the possibility that these drastic changes in the stress regime of the Mediterranean region and Middle East and the contemporaneous volcanic event were triggerred by the Africa/Arabia-Eurasia collision, which slowed down the motion of Africa. The present-day Mediterranean Sea was then locked between two collision zones, and the velocity of retreat of the African slab increased and became larger than the velocity of convergence leading to backarc extension. East of the Caucasus and northern Zagros collision zone the Afro-Arabian plate was still pulled by the slab pull force in the Zagros subduction zone, which created extensional stresses in the northeast corner of the Afro-Arabian plate. The Arabian plate was formed by propagation of a crack from the Carlsberg ridge westward toward the weak part of the African lithosphere above the Afar plume.
925 citations
TL;DR: In this article, the authors describe the evolution of the western Mediterranean subduction zone (WMSZ) during the last 35 Myr by combining new and previous geological data, new tomographic images of the Western Mediterranean mantle, and plate kinematics.
Abstract: [1] The western Mediterranean subduction zone (WMSZ) extends from the northern Apennine to southern Spain and turns around forming the narrow and tight Calabrian and Gibraltar Arcs. The evolution of the WMSZ is characterized by a first phase of orogenic wedging followed, from 30 Ma on, by trench retreat and back-arc extension. Combining new and previous geological data, new tomographic images of the western Mediterranean mantle, and plate kinematics, we describe the evolution of the WMSZ during the last 35 Myr. Our reconstruction shows that the two arcs form by fragmentation of the 1500 km long WMSZ in small, narrow slabs. Once formed, these two narrow slabs retreat outward, producing back-arc extension and large scale rotation of the flanks, shaping the arcs. The Gibraltar Arc first formed during the middle Miocene, while the Calabrian Arc formed later, during the late Miocene-Pliocene. Despite the different paleogeographic settings, the mechanism of rupture and backward migration of the narrow slabs presents similarities on both sides of the western Mediterranean, suggesting that the slab deformation is also driven by lateral mantle flow that is particularly efficient in a restricted (upper mantle) style of mantle convection.
884 citations
Cites background from "Origin of the Betic-Rif mountain be..."
...[18] At the Oranie-Melilla region (Algeria-Morocco border) (Figure 3) it is possible to identify the other major discontinuity in the WMSZ [Frizon de Lamotte et al., 1991; Lonergan and White, 1997]....
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...The hypothesis of slab retreat has been also extended to the case of the Gibraltar Arc [Royden, 1993; Lonergan and White, 1997]....
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01 Dec 1999
619 citations
TL;DR: In this paper, the evolution of the Central Mediterranean subduction zone is reconstructed using geophysical and geophysical constraints, and the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross-sections is derived.
Abstract: SUMMARY Geological and geophysical constraints to reconstruct the evolution of the Central Mediterranean subduction zone are presented. Geological observations such as upper plate stratigraphy, HP–LT metamorphic assemblages, foredeep/trench stratigraphy, arc volcanism and the back-arc extension process are used to define the infant stage of the subduction zone and its latest, back-arc phase. Based on this data set, the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross-sections from the northern Apennines and from Calabria to the Gulf of Lyon can be derived. Further, the reconstruction is used to unravel the main evolutionary trends of the subduction process. Results of this analysis indicate that (1) subduction in the Central Mediterranean is as old as 80 Myr, (2) the slab descended slowly into the mantle during the first 20–30 Myr (subduction speeds were probably less than 1 cm year x1 ), (3) subduction accelerated afterwards, producing arc volcanism and back-arc extension and (4) the slab reached the 660 km transition zone after 60–70 Myr. This time-dependent scenario, where a slow initiation is followed by a roughly exponential increase in the subduction speed, can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull. Finally, the third stage is controlled by the interaction between the slab and the 660 km transition zone. In the southern region, this results in an important re-shaping of the slab and intermittent pulses of back-arc extension. In the northern region, the decrease in the trench retreat can be explained by the entrance of light continental material at the trench.
618 citations
Cites background from "Origin of the Betic-Rif mountain be..."
...Shown are the distribution of HP alpine metamorphism, the average ages and the distribution of volcanism (from Lonergan & White 1997)....
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References
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TL;DR: In this paper, an arc migration model was proposed to explain the dynamic relationship between extension in the Tyrrhenian basin and compression in the Apennines, and the estimated contemporaneous (post-middle Miocene) amounts of extension and shortening in the apennines appear to be very similar.
Abstract: Previously proposed models for the evolution of the Tyrrhenian basin-Apenninic arc system do not seem to satisfactorily explain the dynamic relationship between extension in the Tyrrhenian and compression in the Apennines. The most important regional plate kinematic constraints that any model has to satisfy in this case are: (1) the timing of extension in the Tyrrhenian and compression in the Apennines, (2) the amount of shortening in the Apennines, (3) the amount of extension in the Tyrrhenian, and (4) Africa-Europe relative motion. The estimated contemporaneous (post-middle Miocene) amounts of extension in the Tyrrhenian and of shortening in the Apennines appear to be very similar. The extension in the Tyrrhenian Sea is mostly accomplished in an E-W direction, and cannot be straightforwardly related to the calculated N-S Africa-Europe convergence. A model of outward arc migration fits all these constraints. In a subducting system, the subduction zone is expected to migrate outward due to the sinking of the underthrusting plate into the mantle. The formation of a back-arc or internal basin, i.e. of a basin internal to the surrounding belt of compression, (in this case the Tyrrhenian Sea) is then expected to take place if the motion of the overriding plate does not compensate for the retreat of the subduction zone. The sediment cover will be stripped from the underthrusting plate by the outward migrating arc of the overriding plate, and will accumulate to form an accretionary wedge. This accretionary body will grow outward in time, and will eventually become an orogenic belt, (in this case the present Apennines) when the migrating arc collides with the stable continental foreland on the subducting plate. An arc migration model satisfactorily accounts for the basic features of the Tyrrhenian-Apennine system and for its evolution from 17 Ma to the present, and appears to be analogous to the tectonic evolution of other back-arc settings both inside and outside the Mediterranean region. An interesting implication of the proposed accretionary origin of the Apennines is that the problematic “Argille Scagliose” (scaly clays) melange units might have been emplaced as overpressured mud diapirs, as observed in other accretionary prisms, and not by gravity slides from the internal zones.
1,745 citations
TL;DR: In this article, a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens is presented, which integrates the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database.
Abstract: Summary The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.
1,545 citations
TL;DR: The extensional collapse of orogens offers a partial explanation for why oceans cyclically close and reopen in roughly the same places, preservation of very high pressure metamorphic rocks, for the return of orogenic large crustal thicknesses to normal without very much erosional denudation.
Abstract: Lithospheric extension is sited, preferentially, along orogenic belts because they have a thicker continental crust, contain structural inhomogeneities, and suffer extensional orogenic collapse caused by body forces resulting from isostatically compensated elevation and sharp elevation gradients. Collapse occurs especially where rapid advective thinning of the shortened thermal boundary conduction layer occurs beneath an orogen and causes rapid uplift. Where boundary forces are compressional, extension is balanced by radial thrusting to form oroclinal loops around collapsed extensional basins. Where, as in the disruption of Pangea, boundary forces change rapidly from compressional to tensional, body force collapse is continued by general extension which may lead to continental splitting. Even where overall convergence is continuing, orogenic collapse may be enhanced by subduction rollback into small remnant oceans. The extensional collapse of orogens offers a partial explanation for why oceans cyclically close and reopen in roughly the same places, preservation of very high pressure metamorphic rocks, for the return of orogenic large crustal thicknesses to normal without very much erosional denudation with the widespread preservation of supracrustal sequences, high temperature metamorphic assemblages and the minimum-melting granite suite.
1,352 citations
TL;DR: In this article, the authors studied the instability of a boundary layer for a range of physical parameters (Rayleigh number, amounts of thickening, and boundary conditions) and derived expressions that related the growth of the instability and the time needed to remove the boundary layer as a function of the amount of horizontal shortening (f), the Rayleigh number (R), and the ratio (a/d) of the thicknesses of the rigid and fluid layers.
Abstract: When crust thickens during crustal shortening, the underlying mantle lithosphere must shorten and thicken also, causing the submersion of cold, dense material into the surrounding asthenosphere. For a range of physical parameters the thickened boundary layer that forms the transition from the strong lithosphere to the convecting asthenosphere may become unstable, detach, and sink into the asthenosphere, to be replaced by hotter asthenospheric material. We have studied the instability of a thickened boundary layer for a range of physical parameters (Rayleigh number), amounts of thickening, and boundary conditions. In all cases the fluid was overlain by a rigid, conducting layer. Extensive numerical experiments were made for fluids with stress-free boundary conditions, heated either from below or from within. From a simple physical description of the observed pattern of flow we derived expressions that related the growth of the instability and the time needed to remove the thickened boundary layer as a function of the amount of horizontal shortening (f), the Rayleigh number (R), and the ratio (a/d) of the thicknesses of the rigid and fluid layers. In our opinion, observations and theory agree well (within 10% for R > 105) and show that the speed with which the thickened boundary layer is removed increases with increasing f, R, and a/d. A limited series of runs with no-slip boundary conditions suggests approximately the same functional relationships but with the process 0–30% slower than with stress-free boundaries. For Rayleigh numbers comparable to those appropriate for upper mantle convection (105–107) the removal of the boundary layer occurs rapidly, in times less than the thermal time constant of the overlying rigid plate. Using typical values for the physical parameters in the earth, the boundary layer is removed in times less than the duration of deformation in some collision zones (30–50 m.y.). Thus we suspect that often the lower lithosphere is removed during the process of crustal shortening, causing the overlying crust and uppermost mantle to warm rapidly. This process is likely to contribute to the development of regional metamorphism and to the generation of latetectonic or posttectonic granites. We suspect, in fact, that in some cases the entire mantle lithosphere may detach from the lower crust during crustal shortening, exposing the crust to asthenospheric temperatures.
971 citations
TL;DR: Several features of the Alboran Sea suggest that it may have been a high collisional ridge in Paleogene time that subsequently underwent extensional-collapse, driving radial thrusting around the Gibraltar arc.
Abstract: Several features of the Alboran Sea suggest that it may have been a high collisional ridge in Paleogene time that subsequently underwent extensional-collapse, driving radial thrusting around the Gibraltar arc. (1) The basin is underlain by thin (13-20 km) continental crust, has an east-west-trending horst and graben morphology, was the locus of Neogene volcanism, and has subsided 2-4 km since the middle Miocene. (2) Extension and subsidence in the basin coincided in time with outwardly directed thrusting in the surrounding mountain chains. (3) Africa and Europe were converging slowly during this period, so extension must have been driven by internally generated forces. (4) Onshore, rocks metamorphosed at 40 km depth are exposed beneath major low-angle normal faults that separate them from low-grade rocks above. (5) Emplacement of solid bodies of Iherzolite at asthenospheric temperature into the base of the collisional edifice in late Oligocene time suggests detachment of the lithospheric root beneath the collision zone. This would have increased the surface elevation and the potential energy of the system and would have favored extensional collapse of the ridge.
822 citations