Ernst Willingshofer

The significance of Gosau-type basins for the Late Cretaceous tectonic history of the Alpine-Carpathian Belt.

Abstract A key feature of Late Cretaceous tectonics throughout the Alpine-Carpathian-Pannonian (ALCAPA) region is the synchronous formation of sedimentary basins (Gosau basins) and exhumation of metamorphic domes. Initial subsidence, spatially varying in time (Cenomanian-Santonian), within Gosau-type basins is associated with the development of a fluvial-lacustrine to shallow marine environment and the deposition of conglomerates, sandstones, coal-bearing marls and rudist limestones were deposited. The progressive deepening of the marine basins is documented by a second subsidence pulse during the Campanian-Early Maastrichtian leading to the establishment of an open marine environment. Gosau basins on top of the Northern Calcareous Alps and equivalent nappes of the Western Carpathians which were located at or close to the northern to northwestern active margin of the Austroalpine realm (external basins) locally subsided below the CCD. In contrast a distinctly shallower water environment prevailed in Gosau basins in central areas of the actively evolving Alpine-Carpathian mountain chain (internal basins). Deposition of flysch-type deposits is common for the deep-water facies. Subsidence analysis of internal Gosau basins were performed and their mutual relationship to coevally exhuming metamorphic domes, documented by a number of geochronologic data, is emphasised. A compilation of these data revealed a diachronic evolution of the ALCAPA region. Major vertical movements post-dating nappe imbrication and metamorphism started first in the Tisza-Dacia related orogenic compartments (East-, South Carpathians and Apuseni Mts.) as early as Late Aptian, whereas exhumation and subsequent cooling of metamorphic crust in the East Alpine-West Carpathian domain occurs from the Cenomanian onward. This characteristic basement-basin relationship suggests a strong coupling between lithospheric and surface processes, largely controlled by the rheology of the orogenic system. Formation of internal Gosau basins is seen in context with collapse of thickened and gravitationally unstable continental crust.

Decoupling along plate boundaries: Key variable controlling the mode of deformation and the geometry of collisional mountain belts

The consequences of decoupling between weak orogenic wedges and strong adjacent foreland plates are investigated by means of lithospheric-scale analogue modeling. Decoupling is implemented in the three-layer models by lubrication of the inclined boundary between a strong foreland and a weak orogenic wedge. Plate boundaries are orthogonal to the convergence direction. Experimental results show that strong decoupling between the foreland and the orogenic wedge leads to underthrusting of the former underneath the orogenic wedge and deformation of the orogenic wedge by folding, shearing, and minor backthrusting. Shortening is mainly taken up along the main overthrust, the decoupled boundary, and within the orogenic wedge, leaving the indenter devoid of deformation. In contrast, strong coupling between the foreland and the orogenic wedge favors buckling, involving both the weak zone and the strong plates. The results of these end-member models have implications for collision zones, for example, the Eastern Alps in Europe, such that the switch from localized deformation within the orogenic wedge during the Oligocene–middle Miocene to orogen-scale uplift and deformation during the late Miocene–Pliocene involving the foreland and indenter plates, respectively, is interpreted as reflecting a change from a decoupled to a coupled system.

Lithospheric-scale analogue modelling of collision zones with a pre-existing weak zone

Abstract Lithospheric-scale analogue experiments have been conducted to investigate the influence of strength heterogeneities on the distribution and mode of crustal-scale deformation, on the resulting geometry of the deformed area, and on its topographic expression. Strength heterogeneities were incorporated by varying the strength of the crust and upper mantle analogue layers and by implementing a weak plate or part-of-a-plate between two stronger ones. Three (brittle crust/viscous crust/strong viscous upper mantle) and four (brittle crust/viscous crust/brittle upper mantle/strong viscous upper mantle) layer models were confined by a weak silicone layer on one side in order to contain but not oppose lateral extrusion. Experimental results show that relative strength contrasts between converging plates and intervening weak plates control the location and the shape of deformation sites taken as ‘collision orogens’. If the contrast is small, internal deformation of the strong plates through fore- and backthrusting occurs early in the deformation history. However, the bulk system is dominated by buckling that nucleates on the weak plate whose antiformal topography is highest; model Moho of the bordering stronger plates is deepest under these conditions. If the contrast is large, deformation remains localized within the weak plate for a larger amount of shortening and develops a root zone below a narrow deformation belt, which coincides with the locus of maximum topography. Implementing a buoyant, low-viscosity layer above the model Moho of the weak plate favours the development of asymmetric model orogens notwithstanding the initial symmetric setup. Once the asymmetry is established strain remains localized in thrust faults and ductile shear zones documenting foreland directed displacement of the model orogen. Such laterally and vertically irregular configurations have applications in continent-continent collision settings such as the Eastern Alps. First-order mechanical boundary conditions recognized from modelling to be favourable to the post early Oligocene tectonics of the Eastern Alps include: (1) subtle rather than high-strength contrasts between the Adriatic indentor and the strongly deformed region comprising Penninic and Austroalpine units to the north of it; (2) decoupling of Penninic continental upper crust from its substratum to allow for crustal-scale buckling of the Tauern Window; (3) weak mechanical behaviour of the European lower crust during collision to account for its constant thickness along the TRANSALP deep seismic transect; and (4) the direct continuation of the basal detachment underlying the fold and thrust belt in the hangingwall of the European plate with a wide ductile shear zone in the core of the orogen, which separates the European from the Adriatic plate.

Subduction and deformation of the continental lithosphere in response to plate and crust-mantle coupling

Physical analogue experiments are used to investigate the effect of plate and intra-lithospheric coupling on the efficiency of continental lithosphere subduction and the style of collision. Key parameters investigated in this study are: the degree of plate coupling, regulated by the viscosity ratio of the decoupling zone and the surrounding crust and/or mantle lithosphere; and the depth of decoupling. The experimental results show that subduction of the slab is deepest in cases with strong decoupling at the plate interface and at the level of the lower crust of the downgoing plate, with upper-plate deformation restricted to the area close to the plate contact. In these cases, the strongly asymmetric orogenic wedge is widest, consists of a series of imbricated upper-crustal slices derived from the lower plate, and lacks a retro-wedge. In contrast, a reduced strength contrast across the plate interface, at the depth of either the lithospheric mantle or the ductile crust, leads to a combination of subduction and thickening of the mantle lithosphere in both the upper and the lower plates. The degree of plate coupling determines the efficiency of subduction of continental lithosphere under conditions of collision of neutrally buoyant lithospheres, whereas the vertical position of decoupling horizons within the subducting plate controls the amount of subducted lower crust. Transfer of strain to the upper plate depends critically on (1) the degree of plate coupling, with stronger coupling leading to more deformation, and (2) the presence of decoupling horizons within the upper plate, which act as strain guides to propagate deformation into the upper plate. The experimental results explain the geometry and the sequence of deformation in subduction dominated orogens, such as the Carpathians or the Dinarides, and provide a mechanical basis for the transfer of strain to the upper plate.

Thermomechanical consequences of Cretaceous continent-continent collision in the eastern Alps (Austria): insights from two-dimensional modeling.

We use two-dimensional numerical modeling techniques to investigate the thermomechanical consequences of closure of the Meliata-Hallstatt ocean and consequent Cretaceous continent-continent collision in the eastern Alps (Austria). In the modeling a lower plate position of the Austro-Alpine (AA) continental block is adopted during collision with the Upper Juvavic-Silice block. The thermal structure of the lithosphere was calculated for major AA tectonic units (Upper, Middle, and Lower Austro-Alpine) by integration of the transient heat flow equation along an approximately NW–SE cross section east of the Tauern Window. Indications of the rheological evolution of the AA were determined by calculating strength profiles at key stages of the Cretaceous orogeny, making use of the thermal modeling predictions combined with rock mechanics data. Cooling in the upper plate and lower greenschist facies metamorphism within footwall parts of the lower Upper Austro-Alpine (UA) plate, related to SE directed underthrusting of the UA beneath the Upper Juvavic-Silice block at circa 100 Ma, were predicted by the numerical model. The observed pressure-temperature path for deeply buried Middle Austro-Alpine (MA) upper crustal units and their subsequent isothermal exhumation are best reproduced assuming a pressure peak at 95 Ma and exhumation rates ranging between 4 and 7.5 mm yr−1. From the modeling results, we deduce that the temperature evolution during eclogite exhumation is primarily dependent on rates of tectonic movements and largely independent of the mode of exhumation (thrusting versus erosion). Furthermore, very rapid postmetamorphic exhumation of southern Lower Austro-Alpine (LA) units is predicted in order to account for subsequent cooling. This is constrained by 40Ar/39Ar data. The cooling paths of MA and LA rocks appear to be primarily controlled by their near-surface positions at the end of the Cretaceous rather than by other processes such as concurrent underthrusting. Upward advection of heat by rapid exhumation induced thermal weakening of the thickened crust. The inferred weakness of the central parts of the orogenic system may play an important role during detachment-related tectonic unroofing, orogenic collapse, and concomitant basin formation (central Alpine Gosau basins).

Evolution, distribution, and characteristics of rifting in southern Ethiopia

Southern Ethiopia is a key region to understand the evolution of the East African rift system, since it is the area of interaction between the main Ethiopian rift (MER) and the Kenyan rift. However, geological data constraining rift evolution in this remote area are still relatively sparse. In this study the timing, distribution, and style of rifting in southern Ethiopia are constrained by new structural, geochronological, and geomorphological data. The border faults in the area are roughly parallel to preexisting basement fabrics and are progressively more oblique with respect to the regional Nubia–Somalia motion proceeding southward. Kinematic indicators along these faults are mainly dip slip, pointing to a progressive rotation of the computed direction of extension toward the south. Radiocarbon data indicate post 30 ka faulting at both western and eastern margins of the MER with limited axial deformation. Similarly, geomorphological data suggest recent fault activity along the western margins of the basins composing the Gofa Province and in the Chew Bahir basin. This supports that interaction between the MER and the Kenyan rift in southern Ethiopia occurs in a 200 km wide zone of ongoing deformation. Fault-related exhumation at ~10–12 Ma in the Gofa Province, as constrained by new apatite fission track data, occurred later than the ~20 Ma basement exhumation of the Chew Bahir basin, thus pointing to a northward propagation of the Kenyan rift-related extension in the area.

Structural evolution within an extruding wedge: model and application to the Alpine-Pannonian system

Continental escape or lateral extrusion often results from late-stage contraction within continental collision zones when convergence is partitioned into orthogonal contraction, crustal thickening, surface uplift, and sideward motion of fault-bounded blocks. Geometrical arguments suggest that each individual fault-bounded block suffers a specific sequence of deformation. The style of deformation also depends on the location within the block. This includes: (1) initial shortening at the continental couple (future zone of maximum shortening: ZMS); (2) formation of a conjugate shear fracture system and initiation of orogen-parallel displacement of the decoupled extruding block away from the ZMS; (3) because of the changing width of the escaping block away from the ZMS the style of internal deformation changes within the extruding block: (i) shortening (thrusting, folding), surface uplift at the ZMS; (ii) strike-slip faulting along confining wrench corridors and formation of pull-apart basins at oversteps of en echelon shear fractures; (iii) extension parallel and perpendicular to the displacement vector far away from the ZMS. (4) Finally, the extruding block is gradually overprinted by general, laterally expanding contraction that starts to develop from the ZMS. This inferred sequence of deformation is tested by the Oligocene to Recent development of the Alpine-Pannonian system where late stage formation and extrusion of an orogen-parallel block started during the Oligocene. Stages 2 and 3 developed during Early to Middle Miocene, and final general contraction occurred during Late Miocene to Recent.

STRAIN LOCALIZATION AT THE MARGINS OF STRONG LITHOSPHERIC DOMAINS: INSIGHTS FROM ANALOG MODELS

The lateral variation of the mechanical properties of continental lithosphere is an important factor controlling the localization of deformation and thus the deformation history and geometry of intraplate mountain belts. A series of three-layer lithospheric-scale analog models, with a strong domain (SD) embedded at various depths, are presented to investigate the development of topography and deformation patterns by having lateral heterogeneities within a weak continental lithosphere. The experiments, performed at a constant velocity and under normal gravity, indicate that the presence or absence of the SD controls whether deformation is localized or distributed at a lithospheric scale. Deformation and topography localize above the edges of the SD, while the SD region itself is characterized by minor amounts of surficial deformation and topography. The depth of the SD (within the ductile crust, ductile mantle lithosphere, or both) controls the pattern of deformation and thus the topography. The presence of a SD in the ductile crust or in the mantle results in limited surficial topographic effects but large variations in the Moho topography. Strong Moho deflection occurs when the SD is in the ductile crust, while the Moho remains almost flat when the SD is in the mantle. When the SD occupies the ductile lithosphere, the SD is tilted. These analog experiments provide insights into intraplate strain localization and could in particular explain the topography around the Tarim Basin, a lithospheric-scale heterogeneity north of the India-Asia collision zone.

The tectonic evolution of a critical segment of the Dinarides‐Alps connection: Kinematic and geochronological inferences from the Medvednica Mountains, NE Croatia

The transition zone between the Alps and Dinarides is a key area to investigate kinematic interactions of neighboring orogens with different subduction polarities. A study combining field kinematic and sedimentary data, microstructural observations, thermochronological data (Rb-Sr and fission track), and regional structures in the area of Medvednica Mountains has revealed a complex polyphase tectonic evolution. We document two novel stages of extensional exhumation. The first stage of extension took place along a Late Cretaceous detachment following the late Early Cretaceous nappe stacking, burial, and greenschist facies metamorphism. Two other shortening events that occurred during the latest Cretaceous-Oligocene were followed by a second event of extensional exhumation, characterized by asymmetric top-NE extension during the Miocene. Top-NW thrusting took place subsequently during the Pliocene inversion of the Pannonian Basin. The Cretaceous nappe burial, Late Cretaceous extension, and the Oligocene(-Earliest Miocene) contraction are events driven by the Alps evolution. In contrast, the latest Cretaceous-Eocene deformation reflects phases of Dinaridic contraction. Furthermore, the Miocene extension and subsequent inversion display kinematics similar with observations elsewhere in the Dinarides and Eastern Alps. All these processes demonstrate that the Medvednica Mountains were affected by Alpine phases of deformations to a much higher degree than previously thought. Similarly with what has been observed in other areas of contractional polarity changes, such as the Mediterranean, Black Sea, or New Guinea systems, the respective tectonic events are triggered by rheological weak zones which are critical for localizing the deformation created by both orogens.

P–T–t modelling of Proterozoic terranes in Lithuania: geodynamic implications for accretion of southwestern Fennoscandia

Abstract The East European Craton (EEC) consists of Archaean nucleii and accreted Proterozoic terranes. Pressure–Temperature–time (P–T–t) paths for the West Lithuanian Granulite Domain (WLG), the East Lithuanian Domain (ELD) and the Mid-Lithuanian Suture Zone (MLSZ) in the southwestern part of the EEC constrain the sequence of tectonic events and their deformational and metamorphic conditions. A kinematic and thermal model has been developed for the evolution of these terranes, given the accretion of the ELD to the Sarmatian and the WLG to the Fennoscandian continents, respectively, based on their structural and compositional differences. For initial surface heat flow values ranging from 80–90 mWm−2, the numerical model predicts a thermal relaxation phase of at least 30–50 m.y., following very slow accretion (0.54 cm/yr) of the WLG in a subduction-type environment, prior to granulite facies metamorphism at 1.80 Ga. Only the use of very high initial surface heat flow values (c. 120 mWm−2) allows the direct link of accretion to the observed granulite facies metamorphism. Exhumation of rocks to shallower crustal levels occurs still within granulite facies conditions, being accompanied by partial melting. An observed cooling-reheating event can be explained by underthrusting of the ELD beneath the WLG at c. 1.70–1.65 Ga in the MLSZ, which is a boundary between accretionary belts related to Fennoscandia and another set of such belts formed at the margin of Sarmatia. Subsequent amphibolite facies metamorphism within the MLSZ is followed by what is interpreted as a collapse related exhumation phase. The predicted temperature field indicates that the collapse might have led to a substantial elevation of isotherms, possibly triggering Rapakivi magmatism at 1.60–1.50 Ga.