Structure of the lithosphere below the southern margin of the East European Craton (Ukraine and Russia) from gravity and seismic data

The present study was undertaken with the objective of deriving constraints from available geological and geophysical data for understanding the tectonic setting and processes controlling the evolution of the southern margin of the East European Craton (EEC). The study area includes the inverted southernmost part of the intracratonic Dnieper-Donets Basin (DDB)–Donbas Foldbelt (DF), its southeastern prolongation along the margin of the EEC–the sedimentary succession of the Karpinsky Swell (KS), the southwestern part of the Peri-Caspian Basin (PCB), and the Scythian Plate (SP). These structures are adjacent to a zone, along which the crust was reworked and/or accreted to the EEC since the late Palaeozoic. In the Bouguer gravity field, the southern margin of the EEC is marked by an arc of gravity highs, correlating with uplifted Palaeozoic rocks covered by thin Mesozoic and younger sediments. A three-dimensional (3D) gravity analysis has been carried out to investigate further the crustal structure of this area. The sedimentary succession has been modelled as two heterogeneous layers—Mesozoic–Cenozoic and Palaeozoic—in the analysis. The base of the sedimentary succession (top of the crystalline Precambrian basement) lies at a depth up to 22 km in the PCB and DF–KS areas. The residual gravity field, obtained by subtracting the gravitational effect of the sedimentary succession from the observed gravity field, reveals a distinct elongate zone of positive anomalies along the axis of the DF–KS with amplitudes of 100–140 mGal and an anomaly of 180 mGal in the PCB. These anomalies are interpreted to reflect a heterogeneous lithosphere structure below the supracrustal, sedimentary layers: i.e., Moho topography and/or the existence of high-density material in the crystalline crust and uppermost mantle. Previously published data support the existence of a high-density body in the crystalline crust along the DDB axis, including the DF, caused by an intrusion of mafic and ultramafic rocks during Late Palaeozoic rifting. A reinterpretation of existing Deep Seismic Sounding (DSS) data on a profile crossing the central KS suggests that the nature of a high-velocity/density layer in the lower crust (crust–mantle transition zone) is not the same as that of below the DF. Rather than being a prolongation of the DDB–DF intracratonic rift zone, the present analysis suggests that the KS comprises, at least in part, an accretionary zone between the EEC and the SP formed after the Palaeozoic.     

Tectonics of the Beja-Acebuches Ophiolite: a major suture in the Iberian Variscan Foldbelt

The Beja-Acebuches Ophiolitic Complex (BAOC) (south Portugal/Spain) corresponds to a high grade metamorphic belt along the boundary between Ossa-Morena and the South Portuguese Zones and comprises a lithostratigraphic sequence including (from top to bottom) metabasalts, (metamorphosed) multiple dyke intrusions in gabbro, flasergabbros and metaserpentinites. It is affected by three deformation phases. D₁ affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north-north-east; this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the Serpa antiform, affecting Precambrian basement and Cambrian cover rocks. The obduction polarity ist north-eastwards, similar to the subduction polarity that generates the Beja Gabbroic Complex (BGC), implying a flake geometry. The second deformation phase, D₂, is represented by an intense WNW-ESE sinistral shear event which is responsible for the shattered appearance of the suture; D₂ is reactivated later by a more brittle D₃ event involving thrusting to the south-west, again with a sinistral component. ⁴⁰Ar/³⁹Ar isotopic ages were obtained for (metamorphosed) multiple dyke intrusions in the BAOC's gabbro (342.6 ± 1.4 Ma), for metagabbroic cumulates (340.7 ± 1.9 Ma), and for the undeformed/unmetamorphosed BGC (341.1 ± 1.3 Ma) occurring to the north of the ophiolitic suture. These ages reflect a last regional cooling event in the area which post-dates the ophiolite emplacement and the intrusion of the BGC through this oceanic sequence. Correspondence to: P. Fonseca     

Foldbelts with early salt withdrawal and diapirism: Physical model and examples from the northern Gulf of Mexico and the Flinders Ranges, Australia

A physical experiment shows that shortening applied to existing diapirs and minibasins produces anomalous structural styles that are unlike those of more typical foldbelts. Strong minibasins remain largely undeformed while weak diapirs localize contractional strain. Short diapirs form the cores to folds and thrusted folds, whereas tall diapirs are squeezed and often welded, commonly leading to the extrusion of allochthonous material. Key features of the model are observed in real examples. In the northern Gulf of Mexico passive margin, minibasins were originally separated by a polygonal pattern of deep salt ridges, with diapirs located at ridge intersections. Gravity spreading resulted in squeezed diapirs (and associated allochthonous salt) connected by variably oriented contractional, extensional, and strike-slip structures. In the Flinders Ranges convergent-margin foldbelt of South Australia, preexisting diapirs were squeezed, welded, and thrusted, with anticlines plunging away in multiple directions, so that minibasins are surrounded by highly variable structures. A different geometry is observed in La Popa Basin, Mexico, where squeezing of a linear salt wall produced a vertical weld with diapirs at the terminations, rather than the culmination. In all areas, foldbelt geometries are strongly influenced by the preestablished salt-minibasin architecture.