1.1. Structure and Paleogeography of Austroalpine Nappes
The Austroalpine nappes may be subdivided into the Lower Austroalpine (former NW passive margin of the Austroalpine-Apulian continent), the Northern Calcareous Alps, and the Central Austroalpine (= all nappes above the Lower Austroalpine, south of the NCA, and N of the Periadriatic Fault). We subdivide the Central Austroalpine into an upper part with only anchizonal to greenschist-facies Cretaceous metamorphism (UCA=Upper Central Austroalpine, including Northern Greywacke Zone, Gurktal nappes, Graz Paleozoic etc.), and a lower part (LCA=Lower Central Austroalpine) with stronger Cretaceous metamorphism, including eclogite-bearing nappes in Koralpe, Saualpe, Pohorje, etc. The Bajuvaric nappes of the Northern Calcareous Alps are connected by transgressive contacts with the LCA, and the Tirolic nappes with the UCA.
Within the eclogite-bearing units, maximum P-T conditions increase towards SE. During Cretaceous orogeny, the UCA represented the upper plate, and the LCA the lower plate of a southeast-dipping subduction zone. This subduction zone was intra-continental. It does not coincide with the suture of the Meliata Ocean, neither in space (the Cretaceous subduction boundary is below the UCA, whereas the Meliata suture was originally above the Tirolic nappes and therefore above the UCA) nor in time (high-P metamorphism took place in the Meliata rocks at c. 150 Ma, in the LCA at c. 95-90 Ma). Moreover, no relics of Meliata oceanic crust are found between LCA and UCA. Permian gabbros in the LCA, e.g. in Koralpe, are not related to Meliata (which opened in the Middle Triassic) but to the widespread Permian rifting event affecting all paleogeographic units of the Alps.
1.2. Cenozoic evolution of the Eastern Alps and adjacent orogens
The Cenozoic evolution of the Eastern Alps is mainly characterized by the continent-continent collision between the European lower plate in the north and the Adriatic upper plate in the south, subsequent to the consumption of Penninic oceanic domains. While the external parts of the European plate were deeply subducted and affected by the detachment of crustal slices and nappe stacking, most parts of the Adria- derived units, termed as Australpine and Southalpine, were almost completely exhumed and therefore close to the surface.
During Cenozoic times the Eastern Alps are therefore characterized by a contrasting evolution of the lower plate units, i.e. the subduction of Penninic and Subpenninic units and their subsequent exhumation and upper plate units mainly affected by brittle faulting during orogen-parallel escape tectonics (lateral extrusion).
Faults are primary focuses of both fluid migration and deformation in the upper crust. The recognition that faults are typically heterogeneous zones of deformed material, not simple discrete fractures, has fundamental implications for the way geoscientists predict fluid migration in fault zones, as well as leading to new concepts in understanding seismic/aseismic strain accommodation. Research foucusses on the understanding of the complexities of fault zone internal structure, control on mechanical and fluid flow properties of the upper crust, and their influence on crustal fluid flow and strain accommodation.
Faults control both fluid migration and deformation in the Earth’s crust. Faults are typically complex zones of deformed fault rock, yet are too often considered as discrete surfaces for modelling these processes.
Schematic model of shear zone evolution during layer-parallel shear deduced from field and sample observations. (a) Formation of distinct cross joints at high angles to the pre-existing bedding/foliation planes. (b) Formation of joint- bounded slices, rotation of slices, reactivation of joints as shears with antithetic displacement, and formation of secondary joints at the tips and internal parts of slices. Widening of the fault zone is inhibited by external compressive stresses at high angles to the SZB, stylolites are formed at low angles to the SZB, perpendicular to maximum principal stress orientation. (c) Kinking, fracturing and disintegration of slices by bookshelf rotation, developing into a cataclastic shear zone at advanced stages of displacement. (d) Cementation of disintegrated slices and subsequent formation of new high-angle joints. (e) Second cycle of brecciation. The newly formed fragments consist of both slice fragments and fragments of sparitic cement. σ┴: effective normal stress acting perpendicular to the externally imposed general shear direction. σII: effective normal stress acting parallel to the externally imposed general shear direction (from Hausegger et al., in press).
Brittle fault zones typically show a complex internal structure with a continuous change in structural characteristics along strike and depth. Concerning cross sections through brittle fault zones, the internal structure usually can be sub-divided into a host rock domain, a damage zone, a transition zone and a fault. The width and elongation of these distinct domains along strike, however, is highly variable and may depend on the lithology of the protolith, displacement along the fault zone, strain rate, the orientation of principal stresses relative to the fault zone, and the magnitude of differential (effective) stress as well as shear and normal stresses and fluid pressure.
(a, b) Example of the internal structure of a fault zone along the SEMP fault system from Hausegger and Kurz (2013). Fault direction strikes NW-SE (218/72). Fault core thickness varies from 20 to 50cm. The damage zone shows fractures with close to intermediate spacing. Secondary calcite cementation is dominant. (c) Detail of the internal fault core structure. Slickensides, sub-parallel to the main fault direction, border internal shear bands and divide the fault core in various domains of grain size distribution and fragment/matrix ratio. (d) Fault core cataclasite sample with fine- and coarser grained domains, bordered by internal slickensides, striking sub-parallel to the main fault direction. Fault core cataclasites are composed of comminuted dolomite fragments (or particles) embedded in a fine grained matrix of secondary calcite and pulverized dolomite.
(a-c) Orientation data, strike of classified faults and related structures along the SEMP fault system from Hausegger and Kurz (2013). (d-f) Paleostress analysis carried out by Improved Right Dihedral method. Fault types are illustrated in black squares (Fault type I), grey circles (Fault type II) and white triangles (Fault type III). (d) Fault type I at site Haindlkar is sub-divided into two domains. Black squares indicate steep fault planes with dip angles over 60°. White rhombuses mark fault planes with dip angels less than 60°. (e, h) Conjugated type II faults at site Brandwald are indicated by black/white circles. (g-i) Equivalent Mohr diagrams illustrate the stress ratio R (by convention σ1 = 100 and σ3 = 0) and indicate the orientation and relative magnitude of shear stress (τ) of classified faults. Type I and Type II faults develop in orientations of high to maximum shear stress. Type III faults exhibit variable orientations and therefore variable magnitudes of shear stress (τ).
By applying distinct thermochronological methods with closure temperatures ranging from ~450° to ~40°C (40Ar/39Ar dating on white mica, zircon and apatite fission track (ZFT, AFT), and apatite U/Th-He) the thermochronological evolution of distinct tectonic units of the Eastern Alps can be revealed. This provides the reconstruction of their tectonic evolution during allows the timing of distinct phases of fault activity.
Fission track (zircon, apatite) and (U-Th)/He ages in Ma together with track length distributions of confined horizontal tracks in apatite along the southeastern margin of the Tauern Window (after Wölfler et al. 2008).
Kinematic evolution along the southeastern margin of the Tauern Window. Phase 1: Katschberg- and Polinik normal faults merge together in the Mölltal fault; exhumation of Hochalm Dome and Polinik Block started around 19 Ma. Phase 2: from 10.6 Ma on the Sonnblick Dome is the extracted body between the Mölltal- and the Moser fault. H.D. Hochalm Dome; S.D. Sonnblick Dome; P.B. Polinik Block; K.B. Kreuzeck Block (after Wölfler et al 2008).
Isotopic dating of metamorphic minerals places fundamental constraints on the rates and mechanisms of burial and exhumation in collisional orogens. The Eclogite Zone in the Tauern Window has been the focus of many studies on subduction-related high-pressure metamorphism.
We present element distribution maps and lutetium-hafnium (Lu-Hf) garnet ages of three samples from the Eclogite Zone. All samples display almost unaltered eclogite-facies assemblages and garnets preserve growth zoning. Lu-Hf ages are thus considered as formation ages recording metamorphism towards peak-pressure conditions. In the sample with the smallest grain size, garnet shows regular bell-shaped element distributions with respect to manganese and the iron-magnesium ratio. A six-point isochron of this sample yields 32.76 ± 0.5 Ma (MSWD=1.06), interpreted as the age of Alpine eclogite-facies metamorphism. In one of the other two, coarser-grained samples garnet chemistry is identical. The third sample, however, shows complex zoning in large garnet crystals. Cores with a very low iron-magnesium ratios are surrounded by a second garnet generation, which is very similar to the Alpine generation in the other two samples. The two coarser-grained samples yield scattered ages between 26.9 ± 9.8 Ma and 62.7 ± 1.8 Ma for individual garnet-whole-rock pairs as the analysed garnet fractions display very different 176Hf/177Hf vs. 176Lu/177Hf ratios. This scatter reflects varying degrees of mixing between Alpine and pre-Alpine garnet fractions that is recorded in the cores of the third sample. The results confirm the Rb-Sr-whole rock ages of Glodny et al. (2005). Despite the problems this result causes for conventional tectonic reconstructions, the eclogites from the Eclogite Zone in the Tauern Window have to be considered as Lower Oligocene in age and are thus the youngest eclogites of the Alps identified so far.
Lu-Hf isochron plots for samples A) FRT5, B) FRT2 and C) FRT8. If not shown, 2σ uncertainties used in regressions are smaller than symbol sizes (from Nagel et al., 2013.
The Costa Rica Seismogenesis Project (CRISP) is designed to elucidate the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. The CRISP study area is located offshore the Osa Peninsula where the incoming Cocos ridge has lifted the seismogenic zone to within reach of scientific drilling. This area is characterized by low sediment supply, a fast convergence rate, abundant plate interface seismicity, and a change in subducting plate relief along strike. In addition to elucidating processes at erosional convergent margins, this project is complementary to other deep fault drilling projects (e.g., The Nankai Trough Seismogenic Zone Experiment and J-Fast).
Expedition 344 is the second expedition of CRISP Program A (Integrated Ocean Drilling Program Proposal 537A-Full5), a first step toward the deep riser drilling through the seismogenic zone. The focus of CRISP Program A is on the shallow lithologic, hydrologic, stress and thermal conditions that lead to unstable slip in the seismogenic zone. Together with Expedition 334, the first expedition of CRISP Program A, these data provide exciting insights into the nature of seismogenesis and erosive plate boundaries.
Site map of IODP CRISP (from Expedition 344 Scientists, 2013).
The IBM system is the type locality for studying oceanic crustal accretion immediately following subduction initiation. It is sufficiently old that it carries a full record of the evolution of crustal accretion from the start of subduction to the start of normal arc volcanism and sufficiently young that the key features have not been excessively disturbed by subsequent erosion or deformation. Intra-oceanic arcs are built on oceanic crust and are sites of formation of juvenile continental crust. Most active intra-oceanic arcs are located in the western Pacific. Among these, the IBM system stands out as a natural scientific target. This predominantly submarine convergent plate boundary is the result of ~52 m.y. of subduction of the Pacific plate beneath the eastern margin of the Philippine Sea plate. Stretching for 2800 km from the Izu Peninsula, Japan, to Guam, USA, the IBM system has been extensively surveyed and is a very suitable natural laboratory for IODP expeditions aimed at understanding subduction initiation, arc evolution, and continental crust formation. A scientific advantage of studying the IBM system is its broad background of scientific investigation resulting from its designation as a focus site by the U.S. National Science Foundation MARGINS-Subduction Factory experiment and similar efforts in Japan. We know when subduction and arc construction began, even if the precise paleogeography is controversial, and there is a good time-space record of crustal development.
The objectives for Expedition 352 were to drill through the entire volcanic sequence of the Bonin forearc to (1) obtain a high-fidelity record of magmatic evolution during subduction initiation and early arc development, (2) test the hypothesis that forearc basalt lies beneath boninite and to understand chemical gradients within these units and across the transition, (3) use drilling results to understand how mantle melting processes evolve during and after subduction initiation, and (4) test the hypothesis that the forearc lithosphere created during subduction initiation is the birthplace of supra-subduction zone ophiolites.
Site map of IODP Expedition 352 (Izu Bonin Mariana Forearc) (provided by Gail Christeson).