Peter Zweigel

Kinematic evolution of the Romanian Carpathians

Abstract The regional pattern of contraction and extension directions and the evolution of the strain field from Paleogene to Neogene times enabled a reconstruction of the migration path of the Carpathian collision front. The Carpathian nappes were thrust around the Moesian Plate during Paleogene and Early Neogene times and protruded into a small oceanic embayment between the Moesian and European plates. The arc structure of the Carpathian fold-thrust belt was formed in Late Neogene times as a result of the eastward-escaping Tisza–Dacia block, due to N-directed convergence of the Adriatic plate and the retreating subduction of an oceanic slab. Brittle deformation structures in the Romanian Carpathians suggest three tectonic events related to major plate motions: (1) Paleogene to Middle Miocene NE to ENE contraction caused right-lateral curved strike-slip faults; (2) Middle Miocene to Pliocene fan-shaped orientations of contraction directions were caused by right-lateral oblique convergence in the Southern Carpathians, frontal convergence in the southern Eastern Carpathians and left-lateral convergence in the northern Eastern Carpathians; (3) Pleistocene to Holocene general E–W extension and N–S contraction in the Carpathian arc and local ESE–WNW contraction in the Vrancea area is related to the late roll back stage and break-off of the subducted slab in the bend area.

KINEMATICS OF AN ARCUATE FOLD-THRUST BELT : THE SOUTHERN EASTERN CARPATHIANS (ROMANIA)

Abstract Flysch and molasse nappes of the Eastern Carpathians strike mainly N–S and were folded and imbricated since the late-Early Cretaceous. Boundaries of stratigraphic and structural units and calculated fold-trend axes from the southern part of the Eastern Carpathians indicate a curvature of the orogen by 80°. Kinematic data from fault-slip analysis of all nappe systems exposed in the arc document an overall NW–SE contraction. Nappes of the external Moldavides nappe complex underwent deformation since the Late Cretaceous, peaking in the Miocene, and exhibit fanning subhorizontal contraction axes from E–W in the N-striking part of the Eastern Carpathians through WNW–ESE and NW–SE in the northern part of the arc to NNW–SSE in its southwestern part. Geometric relations between faults and folds indicate that this fan pattern reflects an extended interval and was contemporaneous to nappe stacking and subsequent continental collision. Statistical analysis reveals correlation between orientation of the fanning kinematic axes and the structural trend, where fanning of subhorizontal contraction axes is systematically smaller than change of strike of fold–thrust structures. Orogen-parallel extension in the Tarcau nappe of the Moldavides nappe complex is estimated to be ≤20%. This low value of orogen-parallel extension indicates a primary arc in contrast to bending of a previously straight fold–thrust belt. Low orogen-parallel extension and the observed pattern of structural and kinematic axes can be simulated in sand-wedge indentation models where the indenter moves oblique to its margins. In these models, thrust traces and fold axes are curved around the arc but are parallel to the margins of the indenter away from the corner area. The movement directions of the thrust faults in the models are deflected from normal to fault-strike towards the movement direction of the indenter, leading to fanning contraction directions similar to those observed in the Eastern Carpathian arc. Thus, the main controlling factors on arc formation were the plate margin geometry (i.e. the shape of the upper plate) and the movement direction of the upper plate relative to the orientation of its margins. The influence of the geometry of the continental margin in the lower plate was mainly by imposing constraints on the movement direction of the upper plate.

ARCUATE ACCRETIONARY WEDGE FORMATION AT CONVEX PLATE MARGIN CORNERS : RESULTS OF SANDBOX ANALOGUE EXPERIMENTS

Abstract Indentation of a rigid box into a sand layer leads to accretion of sand into a wedge which has an arcuate shape in map-view. Curvature of the wedge is most pronounced at the corners of the indentor, where deformation has been investigated in detail in this study. Several features of the arcuate wedge in the corner region depend on the indentation angle, i.e. the angle between the indentation direction and the normal to the frontal face of the indentor: (a) The ratio between the widths of the lateral and frontal parts of the wedge increases linearly with increasing indentation angle. (b) Orogen-parallel extension (OPE) in the corner area is initially high and leads thus to large finite OPE in the firstly formed nappes, but decreases and attains a stable level during progressive convergence. Finite OPE in the corner region is smaller at larger indentation angles. (c) Displacement vectors of accreted material exhibit fanning around the arc, but in oblique indentation the spread of their orientation is smaller than the change of structural strike (nappe traces and fold axes). Displacement vectors trend mainly in such positions normal to structural strike where displacement is parallel to the indentation direction. The results from these experiments can be applied to natural arcuate fold–thrust belts if natural conditions do not differ much from experimental boundary conditions. Such an application may give information about the former plate movement direction across an arcuate plate margin. In the example of the Eastern Carpathian arc, a Tertiary plate-movement direction of approx. 125–130° was determined. This direction is in accordance with current models of large-scale tectonics of the Carpathian region, which indicates the applicability of the experimental results to natural examples.

Reservoir geology of the Utsira Sand in the southern Viking Graben area - a site for potential CO2 storage.

Since October 1996 Statoil has started to inject CO2 coming kom the Sleipner Vest Fieldin the southem Viking Graben area into a saline aquifer at a depth of approximately 900 m. This is the first case of industrial scale CO2 storage in the world (1 million tons per year). Careful monitoring of the behavior of the storage facility is hence required. To this end different time-lapse seismic surveys have been planned (presented in a companion paper: Brevik et al., this volume). In this paper the interpretation of the base survey acquired before injection is presented. The most likely pathways for CO2 migration in the vicinity of the injection point have been indicated