Walter Kurz

The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties

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. This book captures current research into understanding the complexities of fault-zone internal structure, and their control on mechanical and fluid-flow properties of the upper crust. A wide variety of approaches are presented, from geological field studies and laboratory analyses of fault-zone and fault-rock properties to numerical fluid-flow modelling, and from seismological data analyses to coupled hydraulic and rheological modelling. The publication aims to illustrate the importance of understanding fault-zone complexity by integrating such diverse approaches, and its impact on the rheological and fluid-flow behaviour of fault zones in different contexts.

The Exhumation of Eclogite-Facies Metamorphic Rocks—a Review of Models Confronted with Examples from the Alps

We summarize recently proposed models for the exhumation of (ultra-) high pressure ([U]HP) metamorphic units within collisional orogens. In terms of mechanics, exhumation is either caused by buoyancy or by externally applied stresses. In terms of kinematics, exhumation is either by erosion, extension, or extrusion. The tectonic scenarios that may lead to the exhumation of (U)HP units are: (1) flow within a low-angle or high-angle corner; (2) entrance of the continental margin into the subduction zone; (3) cessation of subduction; (4) slab breakoff; (5) extension in the backarc; and (6) serpentinization of the mantle wedge. The applicability of these models are discussed for some (U)HP terrains in the Alps, in particular the Koralm Complex of the Austroalpine Unit, and the Internal Penninic Nappes of the Eastern, Central, and Western Alps, (in particular, the Eclogite Zone, the Adula Nappe, and the Monte Rosa Nappe). Within the Austroalpine Koralm Complex of the Eastern Alps, the exhumation of HP rocks seems to be related to the development of a major detachment within the lower crust (Plattengneis shear zone), subsequent to the formation of an orogenic wedge. The central parts of the Koralm Complex were exhumed to a larger extent than the northern, and in particular the southern parts, as indicated by a continuous decrease of pressures toward north and south. Thus, we suggest that the Plattengneis shear zone was not related to the compressional phase during nappe stacking, but to the extensional exhumation of the Middle Austroalpine basement complexes that were previously affected by eclogite facies metamorphism. The tectonometamorphic evolution of (U)HP Internal Penninic Nappes, which are assumed to have originated from the European continental margin, may be characterized by(buoyancy-driven) extrusion of eclogite facies crustal wedges from the subduction channel with a low-angle corner geometry. During the early phase of exhumation, the pressure-temperature path is characterized by cooling in the Dora Maira Massif, the Gran Paradiso, the Monte Rosa Nappe, and the Eclogite Zone; this argues for exhumation within a still-active subduction zone. In the Adula Nappe, decompression was isothermal. At amphibolite- to greenschist facies conditions, the exhumation mechanism of these units changed to crustal extension, accompanied by minor heating during decompression (except for the Adula Nappe). This argues for cessation of subduction and removal of the cooling effect.

Exhumation of the Tauern window, Eastern Alps

Abstract The exhumation of metamorphic domes within orogenic belts is exemplified by the Tauern window in the Eastern Alps. There, the exhumation is related to partitioning of final orogenic shortening into deep-seated thrusts, near-surface antiformal bending forming brachyanticlines, and almost orogen-parallel strike-slip faults due to oblique continental plate collision. Crustal thickening by formation of an antiformal stack within upper to middle crustal portions of the lower lithosphere is a prerequisite of late-stage orogenic window formation. Low-angle normal faults at releasing steps of crustal-scale strike-slip faults accomodate tectonic unloading of synchronously thickened crust and extension along strike of the orogen, forming pull-apart metamorphic domes. Initiation of low-angle normal faults is largely controlled by rock rheology, especially at the brittle-ductile transitional level within the lithosphere. Several mechanisms may contribute to uplift and exhumation of previously buried crust within such a setting: (1) Shortening along deep-seated blind thrusts results in the formation of brachyanticlines and bending of metamorphic isograds; (2) oversteps of strike-slip faults within the wrench zone control the final geometry of the window; (3) unloading by tectonic unroofing and erosional denudation; and (4) vertical extrusion of crustal scale wedges. Rapid decompression of previously buried crust results in nearly isothermal exhumation paths, and enhanced fluid circulation along subvertical tensile fractures (hydrothermal ore and silicate veins) that formed due to overall coaxial stretching of lower plate crust.

Eclogite meso- and microfabrics: implications for the burial and exhumation history of eclogites in the Tauern Window (Eastern Alps) from P-T-d paths

Abstract The Eclogite Zone of the central-southern Tauern Window comprises eclogites and associated high-pressure metasediments that are intercalated between Penninic basement units in the foot-wall and an imbricate stack in the hanging-wall. This nappe stack consists of continental basement units, cover sequences of a distal continental slope, and the main part of the ophiolitic Glockner Nappe. Textures and microfabrics of eclogites from the central-southern Tauern Window allow the establishment of the eclogitic and post-eclogitic deformation histories from burial by subduction to subsequent exhumation. The metamorphic evolution of the eclogites is documented by: (1) initial greenschist to blueschist facies; (2) eclogite facies at ca. 550–630°C and ±20 kbar; (3) a second blueschist facies overprint (ca. 450°C, 10–15 kbar); and (4) exhumation to pressures of ca. 6–7 kbar at 500–550°C (upper greenschist to lower amphibolite facies). An eclogitic penetrative foliation (S1) and S-dipping stretching lineation (L1) formed during the final stages of subduction of the eclogite-bearing unit. The P T evolution of the eclogites during D1 documents the final increment of the prograde P T path. Later blueschist facies metamorphism is contemporaneous to top-to-the-N emplacement of the eclogite-bearing unit onto continental basement units within a subduction zone. We discuss two models of eclogite emplacement. Most probably, emplacement is achieved by a thrust along the base of the eclogite-bearing unit. Alternatively, the P T path of the eclogite facies rocks suggests an emplacement model similar to corner flow. Subsequent to eclogite and nappe emplacement, the nappe stack is entirely affected by a penetrative deformation (D2) that resulted in the development of a mylonitic foliation (S2) and an E-W-oriented stretching lineation (L2) within lower amphibolite to upper greenschist facies. This deformation is related to a crustal-scale detachment zone, that formed during the exhumation of the Penninic units.

Deformation partitioning during updoming of the Sonnblick area in the Tauern Window (Eastern Alps, Austria)

The Sonnblick Dome forms a NE-vergent dome structure cored by basement gneisses within the southeastern Tauern Window of the Eastern Alps (Austria). A succession of ductile and brittle deformation stages documents doming and exhumation subsequent to the thermal peak of metamorphism. Contrasting deformation geometries within internal parts and along the margins of the dome are explained in terms of deformation partitioning. Subhorizontal shortening is documented by subvertical en-echelon extensional gashes within central parts of the dome. Subhorizontal as well as subvertical flattening is also documented by fold structures. During dextral transpression strike-slip is accommodated along the NE margin (Sonnblick Lamella, Moll Valley Fault) and the southern margin (Moser Fault) of the Sonnblick Dome, while vertical thickening occurred within the interior of the dome. Crustal thickening triggered unroofing and extension parallel to the dome axis which is documented by ductile low-angle normal faults in the uppermost structural sections of the dome. This normal fault system contributed to footwall uplift and exhumation of the dome structure. The Sonnblick Lamella, associated with the dextral Moll Valley Fault, forms a stretching fault where deformation is concentrated along a potential zone of weakness. This fault is interpreted to represent the transition from vertical thickening within the dome to vertical thinning at the dome margins. During upbending, the dome structure passed the isotropic stress surface that is characterized by equality of σ1 and σ2. This is documented in a perturbation of the orientations of principal stress axes σ1 and σ2, while σ3 remains constant in orientation. Transpression contributed substantially to updoming and to rapid, nearly isothermal, exhumation subsequent to the thermal peak of metamorphism.

Progressive development of lattice preferred orientations (LPOs) of naturally deformed quartz within a transpressional collision zone (Panafrican Orogen in the Eastern Desert of Egypt)

Abstract Lattice preferred orientations (LPOs) of quartz were used to establish differences in deformation geometry, finite strain, and temperature within a transpressional collision zone within the Panafrican Orogen in the Eastern Desert of Egypt. Metamorphic and/or magmatic core complexes in the area are bordered in the NW and SW by ductile sinistral NW-trending strike-slip zones and low angle normal faults (LANFs). Simultaneous activity of both fault systems suggests bulk W–E shortening coeval with orogen-parallel extension. Displacement partitioned into orogen-parallel sinistral strike-slip faults and LANFs. This study compares both quartz-LPOs in shear-zones and normal faults. From south to north, quartz c-axis data show a continuous evolution along orogen-parallel strike-slip faults from maxima in Y , with a slight tendency to oblique single girdles at the margins of the Wadi Beitan and Hafafit complexes, to asymmetric crossed girdles and oblique single girdles along the margins of the Sibai and Meatiq complexes. The NW-directed LANFs to the NW of the Hafafit and the SE-directed LANFs to the SE of the Sibai show maxima in Y . The SE-directed LANF at the SE margin of the Meatiq complex shows symmetric crossed girdles, indicating coaxial deformation geometry. Oblique single girdles and maxima in Y occur in the southern part of the orogen, whereas crossed girdle distributions dominate in the northern part. The variation in quartz c-axis patterns is explained in terms of decreasing metamorphic grade during deformation from the S (medium to high grade) to the N (low grade), and decreasing finite strain. This is in accordance with the general progression of transpressional tectonics and exhumation of core complexes from S to N.

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.

Constriction during exhumation: Evidence from eclogite microstructures

The Eclogite zone (Tauern Window, Eastern Alps, Austria) represents one of only a few examples of high-pressure units providing both the prograde and the retrograde metamorphic evolution. The eclogites are associated with rocks originating from continental and transitional crust that also have been affected by high-pressure metamorphism. Eclogites that formed along the prograde path indicate metamorphic conditions of 17-20 kbar (1700-2000 MPa) at 550-580 °C; the peak assemblages formed at 21-25 kbar (2100-2500 MPa) at 600-620 °C. Omphacite microstructures, in particular shape fabrics and crystallographic preferred orientations, indicate that the final phases of the prograde evolution were characterized by flattening strain. The evolution at the pressure peak and along the exhumation path shows a constrictional strain geometry. This change is interpreted to have been controlled by the force balance between slab pull (related to an oceanic slab) and the buoyancy of subducted adjacent continental crustal material, including the eclogites. At a certain lithospheric level, where the subducted continental rocks were entirely surrounded by high-density lower-crustal and upper-mantle material, the buoyancy forces exceeded the slab-pull forces. This force change resulted in the buoyancy-driven extrusion of continental material between two lithospheric plates. The part of the subducted slab that remained in buoyant equilibrium was therefore affected by constriction. Moreover, this process may have resulted in breakoff of the subducted oceanic slab. If so, the cessation of slab pull resulted in accelerated buoyancy-driven extrusion of the continental material. Extrusion of these high-pressure sheets would have been accompanied by constriction subparallel to the subduction channel as well.

Tectonometamorphic Evolution of the Austroalpine Nappe Complex in the Central Eastern Alps—Consequences for the Eo-Alpine Evolution of the Eastern Alps

In the east-central part of the Eastern Alps, three major deformation events can be distinguished within the Koralm Complex and adjacent units (Plankogel Complex, Gleinalm Complex, Seckau Crystalline Complex, Paleozoic of Graz). A first deformation event D1 is characterized by the formation of a penetrative foliation and an E-W stretching lineation. Remnants of deformational micro-structures indicate a top-to-the-west sense of shear during this deformation event. Most of the D1-related fabrics were overprinted by subsequent metamorphism. This metamorphic event did affect the presumed tectonic boundary between the Koralm Complex and the Gleinalm Complex below. Particularly, D2 is related to the Plattengneis shear zone, which formed in the uppermost structural sections of the Koralm Complex, characterized by a N-S-oriented stretching lineation. Eclogites in the footwall have been affected by this deformation event, too. This deformation event is associated with pure shear in the central parts of the Koralm Complex, probably with top-to-the-south displacement in the southern parts, and top-to-the-north displacement in the northern parts. Deformation within the Plattengneis and the eclogites below occurred along the decompressional path, indicated by decreasing minimum pressures within the eclogites, and by northward and southward decreasing pressures and temperatures. The Plattengneis shear zone continuously passes into a low-angle normal fault in the northeastern part of the Koralm, forming the contact between the Koralm Complex and the Paleozoic of Graz. Thus, the Plattengneis shear zone primarily formed as an extensional structure and triggered exhumation of the eclogites. D3-related structures are restricted to distinct low-angle normal shear zones along the northern and southern margins of the Koralm Complex, with top-to-the-N/NE and top-to-the-S/SE displacement, respectively. These are related to the juxtaposition of exhumed high-pressure rocks of the Koralm Complex, and medium- to low-grade metamorphic units above. According to this evolution, the Cretaceous collisional process (Eo-Alpine cycle), which formed the present Austroalpine Nappe Complex, may be subdivided into two distinct phases: The first phase is the (ES)E-ward subduction and closure of the Hallstatt-Meliata Basin, resulting in the assembly of the Upper Austroalpine Nappe Complex. After closing of the Hallstatt-Meliata oceanic basin during the Late Jurassic, the Cretaceous orogeny in the Eastern Alps encompasses the collision between (south)easternmost parts of the Austroalpine continental crust and a continental fragment to the east. The second phase involves the southward underplating of the southern Apulian continental margin and resulted in the imbrication of the Middle Austroalpine basement complex. These units were additionally affected by pronounced metamorphim, increasing from greenschist-facies conditions in the northern parts to amphibolite- and eclogite-facies conditions in its southern-most parts. Continuous underplating was accompanied by extension in the internal parts of the orogen, resulting in the formation of an extensional detachment in the lower crust (Plattengneis shear zone), and exhumation of high-pressure metamorphic rocks during the Late Cretaceous. Coeval extension in the upper plate resulted in the formation of the Gosau sedimentary basins. Toward the north, the Plattengneis shear zone continuously climbed toward shallower crustal levels, and passed into a foreland-directed thrust. This thrust is conjectured to have affected the Upper Austroalpine Nappe Complex as well as the formation of distinct out-of-sequence thrusts.

Subduction initiation and ophiolite crust: new insights from IODP drilling

ABSTRACT International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.