Kurt Decker

Balancing lateral orogenic float of the Eastern Alps

Abstract Oligocene to Miocene post-collisional shortening between the Adriatic and European plates was compensated by frontal thrusting onto the Molasse foreland basin and by contemporaneous lateral wedging of the Austroalpine upper plate. Balancing of the upper plate shortening by horizontal retrodeformation of lateral escaping and extruding wedges of the Austroalpine lid enables an evaluation of the total post-collisional deformation of the hangingwall plate. Quantification of the north–south shortening and east–west extension of the upper plate is derived from displacement data of major faults that dissect the Austroalpine wedges. Indentation of the South Alpine unit corresponds to 64 km north–south shortening and a minimum of 120 km of east–west extension. Lateral wedging affected the Eastern Alps east of the Giudicarie fault. West of the Giudicarie fault, north–south shortening was compensated by 50 to 80 km of backthrusting in the Lombardian thrust system of the Southern Alps. The main structures that bound the escaping wedges to the north are the Inntal fault system (ca. 50 km sinistral offset), the Konigsee–Lammertal–Traunsee (KLT) fault (10 km) and the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault system (60 km). These faults, as well as a number of minor faults with displacements less than 10 km, root in the basal detachment of the Alps. The thin-skinned nature of lateral escape-related structures north of the SEMP line is documented by industry reflection seismic lines crossing the Northern Calcareous Alps (NCA) and the frontal thrust of the Eastern Alps. Complex triangle zones with passive roof backthrusts of Middle Miocene Molasse sediments formed in front of the laterally escaping wedges of the northern Eastern Alps. The aim of this paper is a semiquantitative reconstruction of the upper plate of the Eastern Alps. Most of the data is published elsewhere.

THE TERTIARY DYNAMICS OF THE NORTHERN EASTERN ALPS (AUSTRIA) : CHANGING PALAEOSTRESSES IN A COLLISIONAL PLATE BOUNDARY

Abstract Six Tertiary deviatoric palaeostress tensor groups from 165 stations in the Calcareous Alps describe the upper crustal dynamics of the leading edge of the Adriatic plate during protracted continental collision with the European lower plate. Palaeostress changes are correlated to the kinematic evolution of the plate boundary and to independently derived plate kinematic data of the Alpine-Carpathian-Pannonian area. Each palaeostress direction is defined by 32 to 89 individual tensors which constrain: (1) Late Eocene to ?Oligocene NW-directed compression during thrusting and dextral shearing on WNW-striking faults; NW-directed compression along the northern margin of the Adriatic plate is correlated to its N-directed translation combined with a counterclockwise rotation. (2a) ?Late Eocene to Early Miocene N-directed compression during N-directed thrusting of Penninic and Helvetic units and the Molasse; (2b) Early to Middle Miocene N-directed strike-slip compression during the onset of eastward lateral extrusion; the change from NW- to N-directed compression is interpreted to result from strain partitioning at the Periadriatic fault which has formed the new Adriatic plate boundary since the Oligocene. Dextral shearing on this fault accommodated continued anticlockwise rotation and decoupled the northern Eastern Alps from the rotational component. N-directed compression in the Calcareous Alps paralleled the N-directed translation of the Adriatic plate. (3) Middle Miocene NE-directed compression of thrust- and strike-slip type during lateral extrusion; NE-directed compression in the Calcareous Alps resulted from the drag of the eastward extruding central Eastern Alps. Sinistral shear stress was transmitted across the ENE-trending Salzach-Ennstal fault. NE-oriented σ1-axes include 45° with the E-W-striking zone of distributed sinistral shear in the Calcareous Alps. (4) Middle Miocene E-directed extension associated with orogen-parallel normal faulting of the central Eastern Alps; E-directed extension paralleled the direction of mass transfer towards the Pannonian Basin during lateral extrusion. Normal faulting was induced by reduced lateral confinement due to removal of material east of the Alps. On the large scale, eastward motion was enabled by the E-directed retreat of the Eastern Carpathian subduction zone. (5) Late Miocene E-directed compression; orogen parallel compression obstructed lateral extrusion. The stress change from extension to E-directed compression was caused by the end of subduction retreat in the Eastern Carpathians due to dynamic changes in the subduction zone. (6) N-directed extension; Late- to post-Miocene extension paralleled recent topographic slopes in the foothill of the Eastern Alps. Palaeostresses changes correspond to distinct phases in the kinematic evolution of the northern Eastern Alps and to the systematic formation and reactivation of major fault zones. WNW-striking faults changed deformation styles from dextral shear to dextral transpression, reverse faulting and finally to sinistral shear. The Early to Middle Miocene Salzach-Ennstal fault which formed the northern boundary of the extruded central Eastern Alps changed from sinistral transpression to sinistral shear, sinistral transtension and finally to dextral shear.

Far-field effects of Late Miocene subduction in the Eastern Carpathians: E-W compression and inversion of structures in the Alpine-Carpathian-Pannonian region

Kinematic and paleostress data constrain a Late Miocene E-W compressional event that affected the entire Alpine-Carpathian-Pannonian system after 9 Ma and prior to 5.3 Ma. The maximum horizontal compression directions obtained from 110 stations show a mean σ1 orientation of 083°. Deformation is mainly strike-slip. E-W directed compression followed Early to Middle Miocene upper plate extension in the Pannonian Basin which was caused by the retreating subduction boundary in the outer Carpathians. This compressional event terminated Early to Middle Miocene eastward lateral extrusion of the Eastern Alps and reverted strike-slip faults which bounded earlier extruded wedges such as the Salzachtal-Ennstal fault, the Periadriatic fault, the Mur-Murz-Vienna Basin fault system, and strike-slip faults in the Western Carpathians. E-W compression caused the reorientation of the extension direction of the Alpine crustal stack. East-directed tectonic unroofing of the metamorphic domes in the central Eastern Alps terminated between 9 and 6 Ma and Early to Middle Miocene orogen-parallel E-W extension switched to Late Miocene N-S extension which parallels modern topographic slopes. In the Pannonian Basin the change from synrift extension to Late Miocene compression during the postrift phase caused the pronounced postrift subsidence by stress induced downward flexure of the loaded lithosphere. We relate Late Miocene E-W compression to coeval late-stage west- directed subduction of the European plate below the Eastern Carpathians. Slab pull of the subducted plate caused subduction roll back and upper plate extension of the Pannonian area up to the Middle Miocene. Subduction slowed down and ceased when up to 60-km-thick buoyant continental crust entered the subduction zone. During the short period of continued convergence E-W -directed compressive stress was transmitted across the subduction boundary into the upper plate. Compressive stress was transferred through the uppermost brittle crust of the earlier extended Pannonian region into the Eastern Alps, up to 1400 km behind the subduction zone. E-W compression terminated during the Pliocene when the Pliocene to recent NNW-SSE compressive stress field was established in Central Europe.

The Pieniny Klippen Belt in the Western Carpathians of northeastern Slovakia: Structural evidence for transpression

Abstract The Pieniny Klippen Belt represents a 600-km-long but only a few kilometers wide suture zone in the Carpathian orogenic belt. Based on a quantitative analysis along a part of its NW-trending segment in northeastern Slovakia, we present structural data supporting transpression, the continuous interaction of strike-slip shearing, horizontal shortening, and vertical lengthening, as a major deformation style in its polyphase deformation history. Dextral transpression is expressed in the map scale and outcrop fault pattern, the oblique orientation of fold axes to the faults bounding the Klippen Belt, and extension parallel to the fold axes. The transpression-related strain field is described and quantified by the analysis of: (1) orientation of rotated fold axes (displaying an acute angle to the margins of the Klippen Belt); (2) orientation and geometry of paleostress derived from mesoscale fault-striae analysis (E-trend of σ3-trajectories and flattening geometry); and (3) the deformation history indicated by extension veins (non-coaxial regime). Different techniques using fault-striae data quantify paleostress and subdivide heterogeneous data sets mathematically into homogeneous subsets. The observed deformation history is modelled as a homogeneous transpression deformation. The best-fitting model requires a NW-trending (present-day orientation) external contraction direction (e.g., plate-slip vector), and predicts 16% fold axes parallel extension and 23% axial plane normal shortening.

Neotectonic extrusion of the Eastern Alps: Constraints from U/Th dating of tectonically damaged speleothems

We suggest that the Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) fault, a major strike-slip system in the European Alps, is active. It has accommodated lateral extrusion of the central part of the Eastern Alps toward the Pannonian Basin. The main tectonic activity of this fault dates back to Oligocene and Miocene time, but until now it was largely unknown whether the fault was still active. We present here the first field evidence of neotectonic activity from a cave in the Hochschwab karst massif (Styria, Austria) that intersects a segment of the SEMP fault zone. Damaged speleothems in this cave include scratched flowstone (a hitherto undescribed feature), massive flowstone disrupted by a fault, and ruptured flowstone. The superposition of younger flowstone layers allows constraining the time frame of the tectonic events using U/Th dating. The youngest flowstone of the pre-damage generation is ca. 118 ka (end of the Last Interglacial) and the oldest post-event layer is ca. 9 ka (early Holocene). The tectonic event bracketed by these layers coincided with a growth hiatus during the last glacial period, consistent with the high alpine setting of the cave. Geologic evidence precludes deformation mechanisms other than tectonic. These new data are consistent with vectors of continuous global positioning system measurements as well as instrumental seismicity data, and collectively suggest that the SEMP is an active fault and that lateral extrusion of the Eastern Alps is ongoing.

Quantifying Early Miocene in‐sequence and out‐of‐sequence thrusting at the Alpine‐Carpathian junction

A detailed reconstruction of late Oligocene and Early Miocene thrusting at the leading edge of the East Alpine fold-thrust belt is achieved from well data, seismic, and interpretative cross sections. Data are used for constraining the paleogeographic positions of the Alpine thrusts, quantifying in-sequence/out-of-sequence thrust distances, assessing the timing of thrust propagation from structurally higher units into more external ones, and estimating thrust velocities. Results are depicted in five palinspastic maps for time slices between ~26 Ma and ~16 Ma. The termination of foreland-propagating thrusting at the Alpine front is apparently controlled by the subcrop topography of the European basement, which includes a major recess in the east leading to a diachronic along-strike termination of foreland-propagating thrusting with younger thrust ages and higher in-sequence thrust distances in the east. Early locking of foreland-propagating thrusting in the west causes prominent out-of-sequence thrusts which add to the in-sequence thrust distances there. Continuing consecutive detachment of foreland units in the east occurs at rather fast propagation velocities with time intervals between foreland-thrust-propagations ranging between 0.1 and 0.7 Ma. The resulting increase of in-sequence thrust distances from west to east is balanced by out-of-sequence thrusts in the west. The total amount of late Oligocene to Early Miocene thrusting is quantified with a minimum of 51 km. Average thrust velocities range between 4.6 and 5.2 mm/yr. This rate refers to the movement of the basal thrust at the leading edge of the fold-thrust belt, which occurs contemporaneous with the eastward lateral extrusion of the Eastern Alps in the hinterland.

Combined Geophysical, Geomorphological and Geological Studies at the active Lassee Segment of the Vienna Basin Fault System.

The Subandean Basins of South America extending from Trinidad to Tierra del Fuego have been the object of intensive exploratory activities (Fig. 1). The largest amount of hydrocarbons discovered during the last 30 years in these basins was found in complex structural terrains. A total of 59 Billion Barrels of Oil Equivalent (BBOE) have been discovered in areas affected by compressional tectonics. Of these basins, the largest discoveries are in the Furrial Trend of Venezuela (24 BBOE), followed by the Chaco area in Bolivia and Argentina (13 BBOE), the Llanos Foothills of Colombia (4.4 BBOE), and the Madre de Dios Basin of Peru (4.2 BBOE).