E. H. Rutter

On the internal structure and mechanics of large strike-slip fault zones: Field observations of the Carboneras fault in southeastern Spain

Abstract Deciphering the internal structure of large fault zones is fundamental if a proper understanding is to be gained of their mechanical, hydrological and seismological properties. To this end, new detailed mapping and microstructural observations of the excellently exposed Carboneras fault zone in southeastern Spain have been used to elucidate both the internal arrangement of fault products and their likely mechanical properties. The fault is a 40 km offset strike-slip fault, which constitutes part of the Africa–Iberia plate boundary. The zone of faulting is ∼1 km in width not including the associated damage zone surrounding the fault. It is composed of continuous strands of phyllosilicate-rich fault gouge that bound lenses of variably broken-up protolith. This arrangement provides a number of fluid flow and fluid sealing possibilities within the fault zone. The gouge strands exhibit distributed deformation and are inferred to have strain hardening and/or velocity hardening characteristics. Also included in the fault zone are blocks of dolomite that contain thin (

Comparisons of water and argon permeability in natural clay-bearing fault gouge under high pressure at 20°C

Quantification of fluid transport through fault zones is critical for the understanding of fault mechanics and prediction of subsurface fluid flow. The permeability of clay-bearing fault gouge has been determined using first argon then water as pore fluids under total confining pressures ranging up to 200 MPa and pore pressures of 40 MPa at room temperature. Use of the two pore fluids allows interactions between the gouge and pore fluids to be examined. Natural clay-bearing fault gouge recovered from surface exposures of the Carboneras fault zone in southeastern Spain was used and was collected in such a way that the in situ microstructure was preserved. Cores were collected in directions relative to the well-developed planar fabrics seen in these types of fault rock. The mineralogy of the gouges included muscovite/illite, chlorite, and quartz, with minor amounts of gypsum, albite, and graphite. Glycolation of the gouge showed no discernible amounts of swelling phases. Grain size analyses revealed a bimodal grain size distribution, with the 50 wt %) This fraction contained predominantly clay phases. Permeabilities in the range of 10 -17 m -22 to 10 m 2 were measured. Experimental results show that the previous highest in situ effective pressure to which the fault gouge had been subjected (overconsolidation pressure) could not be determined from changes in permeability. Differences between water and argon permeabilities determined on the same sample amounted to ∼1 order of magnitude, even if the sample had been pressure cycled (reduction to zero and reapplication of both confining and pore pressure) using argon as pore fluid until asymptotic values for permeability had been attained. Volumetric strain measurements showed no enhanced compaction due to the introduction of water as the pore fluid, leading to the conclusion that the reduction in permeability must be due to physicochemical interactions of the water with the fault gouge. The low permeabilities measured support models invoking high fluid pressure weakening of large faults with minimal fluid loss. The stability of structured water films with varying temperature, water pressure and water chemistry may produce a heterogeneous permeability profile with depth in fault zones.

Preferred crystallographic orientation development during the plastic and superplastic flow of calcite rocks

Abstract Grain-size sensitive flow of fine-grained rocks, such as are often found in mylonitic shear zones, is commonly inferred to be accomplished by grain-boundary sliding, so that no preferred crystallographic orientation is expected to develop, pre-existing fabrics are supposed to weaken and grain shapes to remain equant. However, many fine-grained mylonites commonly exhibit marked preferred crystallographic orientation, whilst displaying a stable but weak grain-shape fabric. We have examined preferred crystallographic orientation and shape fabric development during the flow of synthetic, ultrafine-grained ( 700°C). The strong crystal-plastic fabric develops more slowly but is not eliminated through the broad transition into true superplastic flow. This is interpreted to reflect an important role of intracrystalline plasticity in the accommodation of grain-boundary sliding. From experiments designed to investigate the extent of survival or modification of an initial twinning fabric through recrystallization and subsequent hot working, it was shown that although the microstructure could be totally annealed, the early formed fabric survived even subsequent high-temperature crystal-plastic or superplastic flow. This may explain the frequent occurrence of strong, low-temperature fabrics in calcite ultramylonites.

Experimental intracrystalline plastic flow in hot-pressed synthetic quartzite prepared from Brazilian quartz crystals

Samples of synthetic, ultrafine-grained quartzite were prepared by hot-pressing aggregates of crushed, clear Brazilian quartz of mean grain size 0.4 μm at 300 MPa and 1373 and 1473 K. The samples displayed rapid grain-growth to ca. 12-20 μm with <2% porosity at 1473 K, facilitated by the 0.6 wt% of water adsorbed onto the grain boundaries during sample preparation. This water could be driven off by preheating, thereby preventing grain-growth. Sufficient water was incorporated during growth into the coarsened samples to render them weak and ductile. These were deformed experimentally in the β-quartz field using an argon gas medium apparatus at 300 MPa confining pressure at temperatures mainly between T = 1273 and 1473 K. Ductile flow was described by the flow law: ∈ = 10 - 4 . 9 3 σ 2 . 9 7 f(H 2 O)exp(-242000/RT) with stress, a, in MPa, and grain size, d, in microns and strain rate ∈ in s - 1 . R is the gas constant and f(H 2 O) is water fugacity. Based on observation of grain flattening, optical strain features, shapes and densities of dislocation arrays, and flow law parameters, samples deformed dominantly by dislocation creep, with some contribution from grain-boundary sliding. Bearing in mind possible changes in the flow law parameters over the extrapolation interval and possible effects of the α-β phase transition, extrapolation to natural strain rates and temperatures predicts plastic flow at higher flow stresses at the same water fugacity at greenschist facies temperatures than previously published flow laws.

The gas permeability of clay-bearing fault gouge at 20°C

Abstract Permeability measurements of clay-bearing fault gouges have been made using the pulse-transient and pore pressure oscillation methods with argon as the pore fluid, and at effective pressures up to 160 MPa and at a constant pore pressure of 40 MPa. Samples were collected from the outcrop of a major transpressional fault in southeastern Spain, in three orthogonal directions relative to the planar fabrics developed within the fault zone. Measurements show the gouge to exhibit permeability anisotropies of up to 3 orders of magnitude. In addition, pressure cycling of the gouge reduced the permeability by 2 to 3 orders of magnitude after 5 pressure cycles, when minimum permeabilities slightly less than 10−21 m2 were measured.

Structural geometry, lower crustal magmatic underplating and lithospheric stretching in the Ivrea-Verbano zone, northern Italy

Abstract The Ivrea-Verbano zone is believed to represent an up-ended cross-section through the lower continental crust as it existed at the end of the Hercynian orogeny in the Southern Alps. Structurally-oriented geological mapping has been carried out in the central and northern parts of the Ivrea-Verbano zone, a region some 30 km along strike. The geology of the southern half of the zone is dominated by a large basic-ultrabasic complex (the Mafic Formation), which is in contact with a strongly banded metasedimentary + metabasic sequence to the north. Particular attention was paid to: (a) the structural relationship between the rocks of the Mafic Formation and their envelope of high-grade metasedimentary and metabasic rocks; (b) the geometrical configuration throughout the Ivrea-Verbano zone of high-temperature shear zones, which accommodated post-Hercynian crustal extension; and (c) the geometry of late, low-temperature faulting, the effects of which have been removed in order to produce a restoration of the structure as it existed during the post-Hercynian extensional phase. The intrusion of large volumes of basic magma (ca 50% of the outcrop area) to form the rocks of the Mafic Formation appears to be coeval with the onset of extension ( ca 280 Ma). The main basic body has a laccolithic form, which was originally more than 10 km thick. Overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot, immediately subsolidus or partially molten body, incorporating its envelope of hot metasediments into a large, originally recumbent fold. A geometrical association with a high-temperature, low-angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion. The Ivrea-Verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating, which may aid in the interpretation of present-day deep seismic profiles. It also demonstrates the geometry of a network of conjugate, high-temperature, low-angle shear zones in a well-layered lower crustal section.

Lithosphere rheology—a note of caution

Abstract It has become common practice to use laboratory-determined, low-temperature frictional sliding data, together with power-law equations for ‘steady-state’ creep of rocks at high temperatures, to construct inferred profiles of rock strength and mode of failure with depth in the lithosphere. In some cases this may involve unwarranted extrapolation of rock mechanics behaviour beyond the region of its validity. The form of transition from frictional behaviour to intracrystalline plastic flow in the Earth is much more complex than is suggested by the above model. Within the regime of plastic flow, substantial changes in flow-strength and mode of failure (whether flow becomes localized into shear zones) may accompany microstructural changes developed over large strains. When deformation is accompanied by metamorphic changes, existing flow laws for rocks are likely to be wholly inapplicable. Until a clearer understanding has emerged of the full range of expression of the rheological behaviour of rocks, existing models of lithosphere rheology should be treated with caution.

On the structure and mechanical properties of large strike-slip faults

Abstract Elucidation of the internal structure of fault zones is paramount for understanding their mechanical, seismological and hydraulic properties. In order to observe representative brittle fault zone structures, it is preferable that the fault be passively exhumed from seismogenic depths and the exposure must be in arid or semi-arid environments where the fragile rocks are not subject to extensive weathering. Field observations of two such faults are used to constrain their likely mechanical properties. One fault is the Carboneras fault in southeastern Spain, where the predominant country rocks are phyllosilicate-rich lithologies, and the other is part of the Atacama fault system in northern Chile, where faults pass through crystalline rocks of acidic to intermediate composition. The Carboneras fault is a left lateral fault with several tens of kilometres offset exhumed from approximately 4 km depth, and displays multiple strands of clay-bearing fault gouge, each several metres wide, that contain variably fractured lenses of protolithic mica schists. The strain is evenly distributed across the gouge layers, in accordance with the measured laboratory mechanical behaviour which shows predominantly strain hardening characteristics. The overall width of the fault zone is several hundred metres. Additionally, there are blocks of dolomitic material that are contained within the fault zones that show extremely localized deformation in the form of faults several centimetres wide. These are typically arranged at an angle of c. 20° to the overall fault plane. These differing types of fault rock products allow for the possibility of ‘mixed mode’ seismicity, with fault creep occurring along the strands of velocity strengthening clay-rich gouge, punctuated by small seismic events that nucleate on the velocity weakening localized faults within the dolomite blocks. The Caleta Coloso fault in northern Chile has a left-lateral offset of at least 5 km and was exhumed from 5–10 km depth. The fault core is represented by a 200–300 m wide zone of hydrothermally altered protocataclasite and ultracataclasite. This is surrounded by a zone of micro and macro-fractures on the order of 150 m thick. The fault core shows a heterogeneous distribution of strain, with alternate layers of ultracataclasite and lower strain material. The strain-weakening behaviour of crystalline rocks might be expected to produce highly localized zones of deformation, and thus the wide core zone must be a result of additional process such as precipitation strengthening or geometric irregularities along the fault plane.

Submarine salt flows in the central Red Sea

The central Red Sea, an oceanic basin floored by Miocene evaporites reaching kilome ters in thickness in places, is at an early stage of development, where seafl oor spreading has geologically only recently replaced continental rifting. Surveys using a high-resolution multibeam echo sounder around Thetis Deep, a new spreading center, have revealed a remarkable series of structures resembling viscous gravity fl ows, which are here interpreted as originating from fl owage of the evaporites laterally unloaded by axial rifting and other processes developing the relief of the deep. The fl ow margins are marked by stream-wise lineaments and some apparently rotated markers . Their fronts in the fl oor of the deep are rounded in plan view and profi le. Their surfaces contain small, closely spaced features resembling extensional faults. In one area below declining gradients, the surface contains along-slope ridges and valleys typical of compression folds (ogives). Flowparallel lineaments and extensional faults lie, respectively, parallel and orthogonal to the direction of maximum seabed gradient. Movement is apparently heterogeneous, at least in part because of varied blocking by relief in underlying basement observed protruding between fl ows. Flowage is currently transporting materials into the fl oor of the deep where they have the potential to become incorporated into the young oceanic crust by repeated eruption of axial lavas over them. In the light of these new data, we reexamine the possibility and implications of fl owage in the South Atlantic marginal evaporites, in particular, whether fl owage contaminated early oceanic crust in such areas.

High‐pressure‐high‐temperature seismic velocity structure of the midcrustal and lower crustal rocks of the Ivrea‐Verbano zone and Serie dei Laghi, NW Italy

The Ivrea-Verbano zone and the adjacent Serie dei Laghi, in the inner arc of the western Alps (NW Italy), display an upended cross section through much of the thickness of the continental crust as it existed in that region in Triassic/Jurassic time. We collected a suite of oriented rock samples that represent most of the volume of the rocks of the region. P and S wave velocity measurements were made in three orthogonal directions, related to the mesoscopic fabric of the rocks, at room temperature and up to 550 MPa confining pressure. Combined high-temperature and high-pressure measurements were made up to 700°C on a subset of the samples. The lithologic units were divided into 23 different principal rock types, and we present velocity data averaged in terms of these groups. Vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section. These show that Vp and Vs both increase and anisotropy decreases systematically with depth of burial, reflecting variations in rock type and metamorphic grade with depth. Vp and Vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding, with the slowest direction tending to be normal to the foliation or banding. The Vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust. These correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution.