Manel Fernandez

Catastrophic flood of the Mediterranean after the Messinian salinity crisis

The Mediterranean Sea became disconnected from the world’s oceans and mostly desiccated by evaporation about 5.6 million years ago during the Messinian salinity crisis. The Atlantic waters found a way through the present Gibraltar Strait and rapidly refilled the Mediterranean 5.33 million years ago in an event known as the Zanclean flood. The nature, abruptness and evolution of this flood remain poorly constrained. Borehole and seismic data show incisions over 250 m deep on both sides of the Gibraltar Strait that have previously been attributed to fluvial erosion during the desiccation. Here we show the continuity of this 200-km-long channel across the strait and explain its morphology as the result of erosion by the flooding waters, adopting an incision model validated in mountain rivers. This model in turn allows us to estimate the duration of the flood. Although the available data are limited, our findings suggest that the feedback between water flow and incision in the early stages of flooding imply discharges of about 108 m3 s-1 (three orders of magnitude larger than the present Amazon River) and incision rates above 0.4 m per day. Although the flood started at low water discharges that may have lasted for up to several thousand years, our results suggest that 90 per cent of the water was transferred in a short period ranging from a few months to two years. This extremely abrupt flood may have involved peak rates of sea level rise in the Mediterranean of more than ten metres per day.

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.

Integrated lithospheric modeling combining thermal, gravity, and local isostasy analysis: Application to the NE Spanish Geotransect

A two-dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal, gravity, and local isostasy analyses is presented. Gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure, while absolute elevation is used to determine the lithospheric mantle thickness. The calculation is performed using a finite element technique that links the different physical equations. The program optionally calculates the temperature at any material boundary and, with given rheological parameters, the strength distribution and the total lithospheric strength in selected columns. We apply the algorithm to the Northeastern Spanish Geotransect which extends from the Pyrenees to the Balearic Promontory and along which a strong variation in crustal and lithospheric thickness is evident. We assess the use of two different inferred density models for the lithospheric mantle: The first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference; the second model assumes a constant density for the whole lithospheric mantle. Although conceptually the two hypotheses differ substantially, the results obtained do not show significant differences. Lithospheric thicknesses of 120–130 km below the Pyrenees, 60–65 km in the Valencia Trough, and 65–75 km below the Balearic Promontory are deduced. In all cases the mean lithospheric mantle density has to be 40–60 kg m−3 higher than the asthenospheric density. The algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature-density-elevation relationship.

The transition from linear to diffuse plate boundary in the Azores–Gibraltar region: results from a thin-sheet model

Abstract We use the thin-sheet plane-stress approach to study the present-day dynamic behavior of the plate boundary between Eurasia and Africa along the Azores–Gibraltar region. This plate boundary, which extends from the Azores triple junction to the Gibraltar strait, shows a tectonic regime that changes from transtension in the west to transpression in the east, with a strike–slip motion in its central segment. Seismological data reveal that the western and central segments are currently marked by a linear series of earthquakes indicating that the plate boundary is located in a narrow zone. In contrast, the eastern segment is not so well defined and deformation spreads over a much broader area. To apply the thin-sheet approach, we combined heat flow, elevation and crustal thickness data to calculate the steady-state geotherm and the total strength of the lithosphere. Several models with different fault friction coefficients and geometries at the eastern segment of the plate boundary were tested. Results are compared with the maximum compressive stress directions from the World Stress Map, and the calculated seismic strain rates and slip vectors from earthquake data. The best fitting models are consistent with the rotation pole of Argus et al. [D.F. Argus et al., J. Geophys. Res. 94 (1989) 5585–5602], and show that the rheological behavior of the plate boundary must necessarily change from the western and central segments to the eastern segment. The diffuse character of the plate boundary east of the Gorringe Bank is dominated by the transition from oceanic to continental lithosphere, the weakness of the Alboran domain, and the convergence between the African and the Eurasian plates. The displacement of the Alboran domain relative to the African plate may play a major role in stress propagation through the Iberian Peninsula and its Atlantic margin.

Heat flow in the Alboran Sea, western Mediterranean

Abstract The results of the first regional heat flow survey carried out in the Alboran Basin are presented. The survey consists of 98 heat flow measurements obtained using a violin type probe, 697 nautic miles of gravity profiles, 1446 nautic miles of bathymetric survey, and 22 gravity cores. A remarkable difference in heat flow patterns exists between the western (WAB) and eastern (EAB) parts of the Alboran Basin. The average heat flow in the WAB is 69 ± 6 mW m −2 with a generally increasing trend towards the centre and to the east. In contrast, the heat flow pattern in the EAB shows an average value of 124 ± 8 mW m −2 and it is maintained rather constant for the overall area. Superimposed on this general pattern there are some local thermal anomalies, associated with hydrothermal activity, which have been detected in the central WAB (up to 123 mW m −2 ), in the South Alboran Basin (SAB) (up to 153 mW m −2 ) and in the Djibouti Bank (DB) (up to 254 mW m −2 ). After corrections for thermal refraction, sedimentation and cooling of volcanic bodies, the resulting heat flow distribution in the WAB is smoother, but still shows the increasing trend towards the centre and to the east. In the EAB, the application of these corrections did not lead to any noticeable changes. A 1-D approach that combines heat flow data, crustal structure and elevation shows a dramatic decrease in lithospheric thickness from the WAB (50–90 km) to the EAB (38–40 km). Likewise, the resulting crustal thickness is around 14–16 km in the central part of the WAB, increasing towards the borders of the basin, whereas in the EAB the crustal thickness varies between 12.5 and 14.5 km in its western part, and between 10 and 11.5 km in its eastern part.

The role of rheology in extensional basin formation modelling

The rheology of the lithosphere determines its deformation under given initial and boundary conditions. This paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling. Since strength envelopes are often used in models, we review the uncertainties (in temperature and rheological parameters) and assumptions (in type of rheology and mode of deformation) involved in their construction. Models of extensional basins are classified into three groups: kinematic, kinematic with rheological constraints, and dynamic. Rheology enters kinematic models only implicitly, in the assumption of an isostatic compensation mechanism. We show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere, which requires a flexural response. Kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension. Dynamic models are the only ones to explicitly introduce rheological constitutive equations (usually in plane strain or plane stress). They usually, however, require the presence of an initial perturbation (thickness variations, pre-existing faults, thermal inhomogeneities, rheological inhomogeneities). The mechanical boundary conditions (kinematic and dynamic) and the thermal boundary conditions (constant temperature or constant heat flux at the lower boundary of the lithosphere) may result in negative/positive feedbacks leading to cessation/acceleration of extension. We conclude that, while kinematic models (with rheological constraints if possible) are very successful in accounting for the observed characteristics of sedimentary basins, dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution.

Crustal-scale cross-sections across the NW Zagros belt: implications for the Arabian margin reconstruction

Quantified balanced and restored crustal cross-sections across the NW Zagros Mountains are presented in this work integrating geological and geophysical local and global datasets. The balanced crustal cross-section reproduces the surficial folding and thrusting of the thick cover succession, including the near top of the Sarvak Formation (~90 Ma) that forms the top of the restored crustal cross-section. The base of the Arabian crust in the balanced cross-section is constrained by recently published seismic receiver function results showing a deepening of the Moho from 42 ± 2 km in the undeformed foreland basin to 56 ± 2 km beneath the High Zagros. The internal parts of the deformed crustal cross-section are constrained by new seismic tomographic sections imaging a ~50° NE-dipping sharp contact between the Arabian and Iranian crusts. These surfaces bound an area of 10800 km 2 that should be kept constant during the Zagros orogeny. The Arabian crustal cross-section is restored using six different tectonosedimentary domains according to their sedimentary facies and palaeobathymetries, and assuming Airy isostasy and area conservation. While the two southwestern domains were directly determined from well-constrained surface data, the reconstruction of the distal domains to the NE was made using the recent margin model of Wrobel-Daveau et al . (2010) and fitting the total area calculated in the balanced cross-section. The Arabian continental–oceanic boundary, at the time corresponding to the near top of the Sarvak Formation, is located 169 km to the NE of the trace of the Main Recent Fault. Shortening is estimated at ~180 km for the cover rocks and ~149 km for the Arabian basement, including all compressional events from Late Cretaceous to Recent time, with an average shortening rate of ~2 mm yr −1 for the last 90 Ma.

Mantle unrooting in collisional settings

Abstract We present a two-dimensional numerical model to study the thermo-mechanical evolution of the lithosphere under a convergence regime in order to define the conditions that lead to lithospheric mantle break-up and consequent unrooting. A Newtonian rheology with a temperature-dependent viscosity is considered. The system is not closed and horizontal flow through lateral boundaries is permitted. A horizontal velocity is imposed at the top of the model to simulate compression, whereas velocity vanishes at the bottom of the model. The initial conditions correspond to a homogeneous lithosphere with a constant heat production in the crust. The analysis of variations of maximum shear stress, strain rate, and total kinetic energy allowed us to define four major stages during the mantle unrooting process: orogenic growth, initiation of gravitational instability until lithospheric failure, sinking of the detached lithosphere, and relaxation of the system. Numerical results also show that the conditions for lithospheric unrooting strongly depend on the convergence velocity, the wideness of the deformation zone, and the imposed rheology.

Lateral diapiric emplacement of Triassic evaporites at the southern margin of the Guadalquivir Basin, Spain

Abstract The Guadalquivir Basin is the Neogene foreland basin of the central and western Betic thrust belt in southern Spain. At the boundary between the basin and the outcrops of thrust nappes of Mesozoic limestones of the Prebetic and Subbetic is a broad belt of outcrops of Triassic evaporitic sediments with scattered younger rocks: the so-called ‘Olistostrome’ unit. This is highly deformed, in places chaotic, and its mode of emplacement has been attributed by various authors to olistostromal debris flow, diapirism, or tectonic melange. Studies of outcrop data in conjunction with seismic and well data, integrated using restorable cross-sections lead us to propose the following sequence of emplacement mechanisms. (a) Loading above a Triassic evaporite formation, probably in the Intermediate Units depositional zone, by north vergent thrusting of thick nappes of Mesozoic sediments, causes northward expulsion of evaporitic sediments between a basal thrust and the base of the limestones. (b) Continued thrust loading drives the diapiric body forwards ahead of the thrust belt, into the floor of the deepening Miocene foreland basin. The body includes blocks of Triassic rocks in normal stratigraphic sequence, as well as blocks of younger rocks broken off the leading hanging-wall cutoffs of the nappes. (c) When the diapiric body reaches the sea-floor of the basin, its top becomes subject to modification by sedimentary processes such as dissolution of evaporites leaving a cap rock and debris flow, both submarine and subaerial but rarely, if ever, forming true olistostromes. (d) At the leading edge of the diapir, northward compression of Miocene basin sediments results in thin-skinned thrusting within these sediments, and formation of duplex structures with a north-dipping monoclinal deformation front. Results from analogue and numerical modelling match the main geological features observed in the study area, thus supporting the plausibility of the proposed lateral diapiric emplacement of the chaotic unit.

Density structure and buoyancy of the oceanic lithosphere revisited

[1] The density structure of the lithospheric and sublithospheric oceanic mantle is assessed with an integrating methodology that incorporates mineral physics, geochemical, petrological, and geophysical data. Compressibility, partial melting, and compositional layering are considered in addition to the standard thermal modelling. The results indicate that due to differences in the degree of melt depletion and crust segregation, the depth-averaged density of old oceanic plates with thermal thicknesses of ∼ 105 ± 5 km is always lower than the density of the underlying sublithospheric mantle. Moreover, representative depth-averaged density contrasts between the plate and the adiabatic mantle, Ap, do not exceed values of ∼40 kg m -3 , in contrast to what is assumed (∇p > 70 kg m -3 ) in many geodynamic models. Thus, the role of ∇p in triggering/assisting processes such as subduction initiation may be less critical than previously thought.