Jaume Vergés

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.

Eastern Pyrenees and related foreland basins: pre-, syn- and post-collisional crustal-scale cross-sections

Abstract A new crustal-scale cross-section through the Eastern Pyrenees shows a minimum of 125 km of total shortening across the belt. Convergence rates of 6 mm/yr (during early and middle Eocene time) between the northern domain of the Iberian plate and Europe can be evaluated from calculated shortening rates in both sides of the orogen. Two stages of orogenic growth can be determined in the Eastern Pyrenean transect. A first stage (from Early Cretaceous to middle Lutetian time) is characterized by a low topography, submarine emplacement of the thrust front, fast rates of south-directed shortening up to 5 mm/yr and widespread marine foreland deposition. This stage is also characterized by equivalent amounts of mountain erosion and detrital foreland accumulation. A second stage (middle Lutetian to late Oligocene) is marked by an increase in structural relief, subaerial emplacement, a decrease in shortening rates and widespread continental sedimentation. This leads towards a non-equilibrium condition in which mountain erosion is almost three times the foreland basin accumulation, leading to a large by-pass of sediments towards the Atlantic before the final endorrheic stage of the basin. Erosion rates based on area conservation between middle Lutetian and present day sections in a two-dimensional calculation indicate an average of 0.15 mm/yr. This rise is lower than middle Lutetian to early Miocene rock uplift rates in the Eastern Pyrenees, which account for 0.2–0.35 mm/yr, suggesting that erosion has been discontinuous through time. Inferred maximum river incision rates since the middle Miocene opening of the Ebro Basin towards the Mediterranean Sea account for less than 0.1 mm/yr.

Interplay between tectonics, climate, and fluvial transport during the Cenozoic evolution of the Ebro Basin (NE Iberia)

[1] Three-dimensional modeling that integrates fluvial sediment transport, crustal-scale tectonic deformation, and lithospheric flexural subsidence is carried out to simulate the landscape and drainage evolution of the Ebro sedimentary basin (NE Iberia). The Ebro Basin underwent a long period of closed intramountain drainage as a result of tectonic topography generation at the Pyrenees, the Iberian Range, and the Catalan Coastal Range. In the late Oligocene, the Catalan Coastal Range underwent extension leading to the formation of the Valencia Trough (NW Mediterranean), but the Ebro Basin remained closed for nearly 15 Myr more before the Ebro River cut through the remnants of the topographic barrier. This drainage opening caused widespread basin incision that shaped spectacular outcrops of the syntectonic and posttectonic infill. Here we investigate the processes controlling these major drainage changes. The modeling results, constrained by a large data set on the tectonic and transport evolution of the area, predict a closed phase characterized by a large lake in the central eastern Ebro Basin. Dry climatic conditions probably lowered the lake level and contributed, together with rift flank uplift, to prolong this endorheic basin stage. The age and amount of reworked sediment after the opening are consistent with an onset of basin incision between 13 and 8.5 Ma as a result of lake capture by escarpment erosion and lake level rise associated with sediment accumulation and wetter climatic conditions. Sea level changes in the Mediterranean had no major impact in the large-scale drainage evolution of the Ebro Basin. INDEX TERMS: 1625 Global Change: Geomorphology and weathering (1824, 1886); 1815 Hydrology: Erosion and sedimentation; 3210 Mathematical Geophysics: Modeling; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 8102 Tectonophysics: Continental contractional orogenic belts; KEYWORDS: Pyrenees, drainage evolution, lake, sediment budget, erosion, flexure

Thrusting and foreland basin evolution in the Southern Pyrenees

The geometry and the infill of the south Pyrenean foreland basin mainly depend on the tectonic subsidence history due to the flexure of the crust, which in turn can be related to the structural evolution of the mountain chain at a crustal scale. Characteristics of the infill of the basin as well as the relationships between structures and synorogenic deposits allow distinction of four stages in the evolution of the south Pyrenean foreland basin. These stages can be related to the structural evolution of the crust as deduced from partial restored cross-section construction through the central Pyrenees. Stage I (Upper Cretaceous) is characterized by strong subsident turbiditic troughs deposited over a thinned crust. Uniform distribution of continental facies and a crust restored to its initial thickness corresponds to the stage II (Palaeocene). During Stage III, turbiditic troughs developed synchronously with the initial subduction of the lower crust (Lower and Middle Eocene). Stage IV (Uppermost Eocene-Oligocene) is characterized by continental deposition coeval with the increase of the crustal cross-sectional area, produced by both piggy-back and break-back thrust sequences.

Bed-by-bed fold growth by kink-band migration: Sant llorenç de Morunys, eastern Pyrenees

Ahstract<rowth strata deposited over and against the flank of the Sant Llorenc de Morunys fold during its final stages of deformation have been mapped at high resolution as the basis for unraveling the kinematics of fold growth. We use restoration techniques based on normal balancing assumptions to decipher the detailed kinematic history of folding. The progressive restorations, as well as balanced forward modeling, show that the last few hundred meters of fold growth were dominated by kink-band migration of a sort that is typical of much faultrelated folding. The kink-band migration has produced complex anticlinal hinge-zone geometry, including segmented fold hinges linked by disconformities and unconformities, which has direct and detailed explanation in terms of fluctuations in deposition rate relative to curved-hinge kink-band migration rate. Large fluctuations in the convolution of non-steady sedimentation and deformation are demonstrated, although the absolute fluctuations in deformation and sedimentation are unknown. At a length scale of 100 m, kink-band migration with little or no deposition is interspersed with sedimentation with little or no deformation. At the length scale of 500 m, deposition ranges from 200% to 50% of uplift. 0 1997 Elsevier Science Ltd. All rights reserved.

Progressive evolution of a fault-related fold pair from growth strata geometries, Sant Llorenç de Morunys, SE Pyrenees

New structural-stratigraphical mapping constrains the three-dimensional kinematics and mechanisms of Eocene-Oligocene growth folding at Sant Llorenc de Morunys (NE Ebro basin, Spain). A 1 km wide sub-vertical panel of syntectonic alluvial gravels passes southwards via a highly asymmetrical growth fold-pair to shallowlydipping strata. The axial surface of the anticline comprises either continuous or en echelon segments while that of the syncline is concave and usually continuous. While converging upwards, the axial surfaces do not define growth triangles. Principal and subsidiary growth unconformities and thickness changes occur across both axial surfaces and the common limb. Dips within the common limb decrease up-stratigraphy and up-dip. Mesostructures indicate that internal deformation was ongoing during folding at all stratigraphical levels, and concentration of cleavage in the syncline indicates that this hinge was essentially fixed. Sequential restoration of three profiles shows that folds amplified principally by limb rotation but incorporated minor passive hinge migration. Particle movement vectors, generated by section restoration, are arcuate about a hinterland pinpoint. A new trishear model of fault propagation folding involving non-rigid limb rotation reproduces the rounded hinge forms, thickening geometries and limb dip variations observed. Simple kink band migration models (fixed axis and constant thickness theories) do not replicate these features.

Thrust sequences in the eastern Spanish Pyrenees

Detailed mapping of the South Pyrenean zone in the eastern Spanish Pyrenees shows that a great number of Alpine thrusts affect the basement and the cover. The geometry of the thrust structures and their propagation sequence can be interpreted from balanced sections. The folding dies out downwards, from the higher thrust sheets to the lower ones and demonstrates a piggy-back thrusting sequence. These thrusts form a duplex. The sole thrust of this duplex is the Vallfogona thrust. The roof thrust is rooted somewhere within the Axial Zone. The thrusts climb southwards from the Axial Zone basement up into Tertiary cover sediments. These thrusts have been folded so that they now dip to the south and form the southern side of the Axial Zone antiformal stack. Locally, smaller antiformal stacks developed and produced strong folds in the higher thrust sheets. The South Pyrenean zone studied is equivalent to the Gavarnie nappe and the connection between them, below the detached South Pyrenean Central unit, may be the Nogueres zone.

Mapping active faults offshore Portugal (36°N–38°N): Implications for seismic hazard assessment along the southwest Iberian margin

Swath-bathymetry and acoustic-backscatter data from the southwest Iberian margin, which hosts the present-day boundary between the European and African plates, reveal the surficial expression of several fault structures <100 km offshore of Portugal. High-resolution and multichannel seismic reflection profiles collected perpendicular to these structures show folding and reverse faulting of the Quaternary units, suggesting present-day tectonic activity. Successive submarine-landslide deposits at the base of the scarps provide evidence of cyclic fault activity. The location and dimension of these newly identified structures agree with the modeled source suggested for the A.D. 1755 Lisbon earthquake and tsunami, possibly the most destructive event in western Europe during historical time. These fault escarpments and deformed seafloor sediments associated with a cluster of shallow seismicity suggest that these thrusts are active and may pose a significant earthquake and tsunami hazard to the coasts of Portugal, Spain, and Morocco.

Late Cretaceous–Paleocene formation of the proto–Zagros foreland basin, Lurestan Province, SW Iran

Late Cretaceous emplacement of ophiolitic-radiolaritic thrust sheets over the Arabian passive margin was the first manifestation of the protracted closure of the Neotethys Ocean, which ended with the continental collision between Arabia and central Iran and the formation of the present Zagros fold belt. This tectonic stacking produced a flexural basin (the Amiran Basin: 400 × 200 km in size) in the northwest Zagros that was filled with a 1225-m-thick shallowing-upward detrital succession made up of the Amiran, Taleh Zang, and Kashkan Formations. This succession sits unconformably above the Late Cretaceous Gurpi Formation and is overlain by the Oligocene-Miocene Shahbazan-Asmari carbonate succession. Dating of the Amiran-Kashkan succession is based on detailed biostratigraphy using large foraminifera and calcareous nannoplankton. The Cretaceous-Tertiary (K-T) boundary is located within the uppermost 25-45 m of the Gurpi Formation. The overlying Amiran and Taleh Zang Formations have been dated as Paleocene in age. However, the base of the Paleocene within the Gurpi Formation lacks NP1 and NP2 zones, implying a hiatus of ∼2 m.y. at ca. 65.5 Ma, which is inferred to correspond to an early folding phase near the Cretaceous-Paleocene boundary. The upper part of the Kashkan Formation is dated to the earliest Eocene by palynostratigraphy. A large hiatus (or very slow deposition) lasting about 15 m.y. occurs between the Kashkan and Shahbazan Formations in the studied region. The base of the prograding Shahbazan platform deposits is dated by 87Sr/86Sr stratigraphy at ca. 33.9 Ma. The upper part of the Asmari Formation is dated as early-middle Miocene using foraminifera associations. Reconstruction of the Amiran-Taleh Zang-Kashkan succession of the Amiran Basin indicates a thickening of the basin fill from the southern pinch-out along the SE flank of the Kabir Kuh anticline to SW of the Khorramabad anticline, where the flexure is at least 900 m. In contrast, the NE part of the basin underwent coeval contraction and uplift of ∼1300 m. Superimposed smaller undulations onto the large-scale flexure are interpreted as Late Cretaceous-Paleocene folds. Regional comparisons (SE Zagros, Oman, and Turkey) indicate that Late Cretaceous-Early Tertiary deformation affected the entire NE margin of Arabia but that compression was not synchronous, being younger in Lurestan than in the NW Persian Gulf where inversion tectonics occurred from Turonian to mid-Campanian times. The long sedimentary hiatus spanning most of the middle and late Eocene must have been related to deep lithospheric processes linked to the initial events of the protracted closure of the Neotethys Ocean between Arabia and central Iran. The tectono-sedimentary history recorded in the Zagros Basin may help to understand early foreland basin growth in other orogens in which subsequent continental collision has obliterated these early events.

Reconstruction of topography and related depositional systems during active thrusting

Reliable reconstruction of former topography in deformed regions is commonly difficult, due to degradation of former erosional and depositional surfaces. In contrast to most modem landscapes, however, ancient localities can sometimes provide clearer insights on subsurface geometries of deposition, deformation, and erosion and on their variations through time. In some exceptional circumstances, ancient depositional sequences are preserved in direct juxtaposition with the structures that controlled their geometrical and sedimentological character. We describe here the evolving topography and depositional responses caused by the late Eocene growth of a detachment fold and related thrusts in the southern Pyrenees. Topography within these deforming systems can be reconstructed on the basis of (1) relief associated with paleovalleys, (2) geometric relationships of syntectonic strata with adjacent structures, and (3) relief of hanging walls above depositional or erosional surfaces of the same age. Onlapping, offlapping, and overlapping stratigraphic relationships are interpreted in the context of the relative rate of sediment accumulation versus the rate of uplift of the crest of the fold. In the study area, two contrasting fluvial systems provided sediment to the deforming area: a large longitudinal system, flowing parallel to the fold axes and carrying detritus from the distant hinterland, and a smaller transverse system that carried locally derived clasts. During fold growth, syntectonic sedimentary beds (growth strata) were progressively rotated in the forelimb of the fold. Proximal unconformities developed in the forelimb growth strata, when accumulation rates were low. Topographic relief on the backlimb of the growing fold caused transverse paleovalleys (>150 m deep) to be incised at high angles to the fold axis. A switch from incision to infilling of the paleovalleys appears to be controlled by relative rates of subsidence, sediment supply and accumulation, and uplift. During an interval of rapid accumulation and low rates of subsidence and uplift, the effects of rising local base levels propagated up the transverse valleys, where they initiated backfilling of the paleovalleys. As deformation began on an adjacent, more hinterlandward thrust, waning growth of the detachment fold permitted depositional overlap of its crest, as sedimentation shifted toward the hinterland. Subsequently, as the new footwall was folded, longitudinal rivers filled the space formerly occupied by transverse rivers, and a new detachment fold grew in the very shallow (<25 m) subsurface. Although similar examples are scarce in the geological record, the synthesis from this Pyrenean locale illustrates how stratal geometries, reconstructed river patterns, precise stratigraphic ages, and preserved erosional surfaces can be combined to reconstruct evolving topography during active folding and faulting in terrestrial environments.