J. I. Soto

Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean

High-grade metamorphic rocks drilled at Ocean Drilling Program Site 976 in the Alboran Sea show a PT path characterized by decompression from about 1050 MPa (40 km depth) to 350 MPa (13 km depth) accompanied by an increase in temperature from about 550°±50°C to 675°±25°C. The Ar/Ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20.5–18 Ma, which coincides with the initiation of sedimentation in the Alboran Sea basin. The Alboran Sea formed by Miocene extension on the site of a Late Cretaceous? to Paleogene contractional orogen, and extension coincided with thrusting in the peripheral parts of the Betic-Rif arc, which surrounds the basin on three sides. Thermal modeling of the PT path was carried out with the aim of constraining geodynamic models for the formation of the basin. Variables considered in the modeling included (1) the thickness and thermal gradient of the postorogenic lithosphere; (2) the radiogenic heat production in the thickened crust; (3) the time gap (pause) between the end of contractional tectonics and the start of extension; (4) removal of lithospheric mantle below 125, 75, or 62.5 km; and (5) the rate of extension. The only combinations of variables that produce modeled PT paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause (to produce high temperatures in rocks initially at 40 km depth), removal of lithosphere below 62.5 km (to produce further heating during decompression), extension by a factor of 3 in 6 m.y. (to delay the attainment of the maximum temperature until the rocks reached shallow depths), and final exhumation and cooling in about 3.3 m.y. (to satisfy radiometric and petrological constraints). This gives a maximum of about 9 m.y. for exhumation from 40 km depth to the surface. Lithospheric stretching in response to plate-boundary forces such as trench rollback, without removal of lithosphere, cannot explain the late onset of heating and the high temperatures reached by these rocks. Removal of lithosphere at depths significantly greater than 62.5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature. Only a combination of significant postcollisional radiogenic heating, then wholesale removal of lithospheric mantle below the orogenic crust, followed by rapid stretching can explain the observed PT path. These results appear to support some form of lithospheric delamination as the primary cause for the formation of the Alboran Sea basin.

Lithospheric Structure Beneath the Alboran Basin: Results from 3D Gravity Modeling and Tectonic Relevance

A three-dimensional gravity modeling combined with integrated heat flow and elevation modeling is conducted to map out the crustal and lithospheric mantle thickness in the Alboran Basin, in the westernmost Mediterranean. A “sediment”-corrected Bouguer anomaly has been derived using a depth-to-the-basement map and densities determined from well logs and seismic data. The gravity effect of the base of the lithosphere has been removed from the sediment-corrected Bouguer anomaly to obtain a “crustal” Bouguer anomaly, which has been inverted for crustal thickness. The resulting lithospheric structure is further constrained by elevation data under the assumption of local isostasy. The low residual elevation anomalies obtained (±100 m in average) suggest that the area is in local isostasy, particularly the medium- and long-wavelength topography features. Variations in crustal thickness range from 36 km underneath the Betic and Rif Chains to <12 km beneath the easternmost part of the Alboran Sea Basin, in the transition to the South Balearic Basin. In western Alboran the Moho lies at a rather constant depth of ∼18 km, deepening sharply toward the Gibraltar Strait down to 30–32 km. The base of the lithosphere shallows from 140 km depth in the Gibraltar Strait to <45 km depth in the easternmost Alboran Sea. Lithospheric thinning penetrates to the southeastern side of the Iberian margin crosscutting topographic highs of the central and eastern Betic Chain. Our results favor mantle delamination produced by detachment and subsequent peeling away of the lithospheric mantle rather than convective removal of the lithospheric mantle either by orogenic collapse or detachment and sinking of a lithospheric slab.

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

Mud volcanoes in the Alboran Sea: evidence from micropaleontological and geophysical data.

During the BASACALB-TTR9 cruise of the R/V Professor Logachev (1999), two mud volcanoes (called Marrakech and Granada) were discovered in the southern sector of the mud diapir province in the West Alboran Basin (WAB). This paper presents micropaleontological and geophysical data on these mud volcanoes from gravity core samples, sidescan sonar (OKEAN) images and high-resolution seismic lines. Mud breccia recovered from the Granada mud volcano is matrix-supported with well-consolidated clasts of limestone, marlstone, claystone, siltstone, sandstone and mudstone, whereas mud breccia from the Marrakech mud volcano contains unconsolidated clasts. The mud breccia matrix contains abundant Miocene calcareous nannofossils (CN), together with Pliocene^Pleistocene species and reworked late Cretaceous and Paleocene^Eocene species. CN dating indicates that clasts in the mud breccia derive from late Cretaceous, Paleocene, Eocene, and probable Miocene sediments. These data suggest that the mud diapirs and mud volcanoes in the WAB can be derived from the olistostromes of Unit VI, the basal stratigraphic sequence in the Alboran Sea basin. Unit VIconsists of lower Miocene sediments that incorporated late Cretaceous and Paleocene^Eocene materials and basement-derived rock fragments. The mud volcanic deposits are covered by a thin drape of pelagic marls, suggesting that these two volcanoes are currently inactive. Structures determined on highresolution seismic profiles across mud volcanoes and surrounding diapirs correspond to the late-stage, Pliocene-toQuaternary diapir development. This stage is thought to have developed during a compressional tectonic setting that produced folding and wrench tectonics throughout the basin. Mud ascent at that time resulted in active diapirism and mud volcanoes on the seafloor. < 2002 Elsevier Science B.V. All rights reserved.

Decompression and high-temperature–low-pressure metamorphism in the exhumed floor of an extensional basin, Alboran Sea, western Mediterranean

Leg 161 of the Ocean Drilling Program (ODP) has made a major contribution to our understanding of the origin of the Alboran Basin by demonstrating that it is underlain by rocks of continental origin that have undergone high-temperature metamorphism and melting at exceptionally low pressure after exhumation and decompression. Basement rocks recovered from Site 976 consist of high-grade schist and gneiss derived from aluminous sediments, and minor amounts of marble, granitic dikes, and migmatitic segregations of granitic material. Mineral assemblages and textural relations show that an early assemblage including biotite, garnet, staurolite, plagioclase, and rutile is overprinted by a second assemblage of biotite, sillimanite, plagioclase, potassium feldspar, and ilmenite. Both assemblages are overprinted by andalusite, potassium feldspar, and minor garnet. Migmatitic gneiss contains relict andalusite, overprinted by sillimanite and cordierite coexisting with granitic leucosome. Preliminary pressure-temperature estimates suggest that the metamorphic evolution followed an approximately isothermal decompression path from 7 to 3 kbar at temperatures in the range 580 to 630 °C. After decompression, granitic melts formed at 670 °C, after andalusite breakdown and within the sillimanite stability field. The cored rocks closely resemble high-grade metamorphic rocks in the adjacent Betic Cordillera of southern Spain, which yield early Miocene radiometric dates. At ODP Site 976 they are overlain by middle Miocene marine sediments. The combination of exhumation in an extensional tectonic environment and the evidence for high and increasing temperature during exhumation provide support for and new constraints on current models for the basin that involve the removal of lithospheric mantle below a zone of continental collision, accompanied or followed by extension.

Thermal structure of the crust in the Gibraltar Arc: Influence on active tectonics in the western Mediterranean

We have modeled thermal structure of the crust in the western Mediterranean on the basis of inversion of heat flow and elevation in the context of Airys isostatic equilibrium. Modeling results reveal dramatic variations in crustal temperatures within the Gibraltar Arc region. The steep gradients in crustal thickness, together with the regional heat flow pattern lie at the origin of temperature anomalies. Temperatures at the base of the crust range between 700°C in the eastern Betics and the connection between the Rif and Tell belts in North Africa, where a high heat flow anomaly occurs. High-temperature zones define a hotter region (>650°C) running SW–NE across the central part of the Alboran Sea. These results agree with other geophysical evidence (e.g., low deep-crust Vp and Pn values) suggesting the occurrence of high temperatures in the deepest crust. According to the estimated silicic composition of the deep crust in the area, temperatures in some regions are appropriate for partial melting under muscovite dehydration conditions (>700°C). We have estimated that average partial melting ranges from ∼12% (maximum XL) in the western Tell region to ∼6% in the eastern Betics. This process modifies physical and mechanical properties of the deep crust enhancing crust-mantle decoupling and deep crustal flow with concomitant surface uplift. These mechanisms explain why high topography and active E–W extension occur transecting the overall orogenic trend of the Gibraltar Arc.

Pliocene to Recent mud diapirism and related mud volcanoes in the Alboran Sea (Western Mediterranean)

Abstract High resolution and multichannel seismic profiles depict the Pliocene to Recent evolution of the mud diapirism in the West Alboran Basin (WAB) and its relationship with the Miocene diapir province that occupies the WAB depocentre. During the early to middle Miocene period of basin extension (16 to 9 Ma), normal faulting triggered the diapirism from mobile overpressured shale containing olistostromes. Plio-Quaternary diapirism evolved as a second main stage of diapiric activity and developed throughout the subsequent contractive tectonic evolution of the basin (9 Ma to Holocene). Mud volcanoes, discovered to the south of the WAB, developed on the flank of Recent diapirs as a consequence of the rise of fluidized sediments through diapiric bodies and/or through fractures connecting with deeper diapirs. During the Pliocene to Recent, some diapirs stopped ascending, leading to the production of collapse structures on their tops due to lateral subsurface mud migration and/or fluid escape. Other cylindrical shaped diapirs continued rising and produced mud volcanoes on the sea floor. All the studied volcanoes seem to be currently inactive. Two major pulses of diapiric rise have been distinguished during the Pliocene to Recent contractive evolution of the basin.

PTMAFIC: software package for thermometry, barometry, and activity calculations in mafic rocks using an IBM-compatible computer

Abstract PTMAFIC (v. 2.0) is a program for IBM-compatible personal computers with minimum hardware requirements, that can be used in thermometric and barometric calculations by petrologists concerned with the determination of P-T paths in mafic and ultramafic rocks. The program creates ASCII files of normalized chemical analyses of amphiboles, garnet, epidote, ilmenite, muscovite, pyroxenes (ortho- and clinopyroxene), olivine, plagioclase, and spinel. The program carries out a wide range of stoichiometric and thermodynamic calculations on these minerals, such as cation site-allocations at the different structural positions, the molar fractions and mole percent of the end-member terms, and the application of different thermodynamic models for calculation of activity coefficients and activities. The program contains calibrations published for 13 geothermometers and 12 geobarometers applicable to mafic and ultramafic rocks that cover metamorphic conditions, ranging from granulite and eclogite facies to green-schist facies.

A general deformation matrix for three-dimensions

A deformation that is obtained by any simultaneous combination of two steady-state progressive deformations: simple shearing and a coaxial progressive deformation, involving or not a volume change, can be expressed by a single transformation, or deformation matrix. In the general situation of simple shearing in a direction non-orthogonal with the principal strains of the coaxial progressive deformation, this deformation matrix is a function of the strain components and the orientation of shearing. In this example, two coordinate systems are defined: one for the coaxial progressive deformation (xi system), where the principal and intermediate strains are two horizontal coordinate axes, and another for the simple shear (xit’ system), with any orientation in space. For steady-state progressive deformations, from the direction cosines matrix that defines the orientation of shear strains in the xi coordinate system, an asymmetric finite-deformation matrix is derived. From this deformation matrix, the orientation and ellipticity of the strain ellipse, or the strain ellipsoid for three-dimensional deformations, can be determined. This deformation matrix also can be described as a combination of a rigid-body rotation and a stretching represented by a general coaxial progressive deformation. The kinematic vorticity number (Wk is derived for the general deformation matrix to characterize the non-coaxiality of the three-dimensional deformation. An application of the deformation matrix concept is given as an example, analyzing the changes in orientation and stretching that variously-oriented passive linear markers undergo after a general two-dimensional deformation. The influence of the kinematic vorticity number, the simple and pure shear strains, and the obliquity between the two deformation components, on the linear marker distribution after deformation is discussed.

Normal faulting driven by denudational isostatic rebound

Fluvial piracy in the Guadix-Baza basin (southeast Spain) promoted erosion of a high volume of sediments during the late Quaternary, after the former internal drainage of this basin changed to external due to headward erosion by the Guadalquivir River. As a response to load release, this basin underwent uplift, which, in turn, enhanced fluvial incision and erosional processes. Differential erosion within the Guadix-Baza basin resulted in dissimilar isostatic rebound. We numerically model the lithospheric response to sediment unloading and show a total rising of 15 m in the Guadix subbasin compared to 2 m in the Baza subbasin. This differential uplift is likely to have been accommodated along the Baza normal fault, which has been active throughout the Quaternary. Modeling results suggest that ∼13% of the total Quaternary fault throw directly arises from the differential isostatic readjustment between subbasins. This example is one of the first estimates of fault slip partitioning between tectonic and isostatic effects due to unloading driven by river incision.