2015 | Taylor, R.L., Rutter, E.H., Nippress, S.E.J., Brodie, K.H.

The Carboneras fault zone forms part of a major strike-slip fault system in SE Spain, striking NE–SW, and accommodating up to 40 km displacement. It affects basement metamorphic rocks and unconformably overlying upper Miocene sediments and volcanic rocks. High-resolution shallow seismic tomographic sections were made across the fault zone in two localities. From the same areas, fault rocks and their wallrocks were collected for laboratory seismic velocity measurements. The laboratory data were corrected for the substantial effects of near-surface crack damage. By combining these results with geological cross sections, forward velocity models for the fault zone were constructed to compare with field seismic measurements and hence to ‘ground-truth’ the inferences made from them. These velocity/depth relationships matched moderately well with those extracted from the in-situ tomography results. Aspects of the in-situ seismic sections matched features on the forward-modelled sections, but the comparisons showed that it is important to have some degree of foreknowledge of the geology to be able successfully to interpret seismic tomography sections as an exploration tool.​

Keywords: Carboneras Fault Zone; Brittle faults; Ultrasonic velocity; Seismic tomography; Velocity modelling; Fractures

2013 | Taylor, R.L. | PHD THESIS

The Carboneras fault zone (CFZ, Almería Province, SE Spain) is a major NE-SW trending tectonic lineament that marks part of the diffuse plate boundary between Iberia and Africa. Developed within a basement terrain dominated by mica schist, the fault system comprises two main strands within a complex zone up to 1 km wide. Between these two strands is a braided network of left-lateral strike-slip, phyllosilicate-rich fault gouge bands, ranging between 1 and 20 m in thickness, passively exhumed from up to 3 km depth. The excellent exposure in a semi-arid environment, the wide range of rock types and fault structures represented and the practicality of carrying out in-situ geophysical studies makes this fault zone particularly well suited to verifying and interpreting the results of in-situ seismic investigations. Integration of elements of field study, laboratory analysis and modelling has aided interpretation of the internal structure of the fault zone. Ultrasonic measurements were made using standard equipment over confining and pore pressure ranges appropriate to the upper 10 km of the continental crust. Seismic velocities have also been approximated from modal analysis and mineral phase elastic properties and adjusted for the effects of porosity. In-situ seismic investigations recorded P-wave velocities 40-60% lower than those measured in the laboratory under corresponding pressures and at ambient temperatures for hard rock samples. Fault gouge velocities measured in the laboratory, however, are comparable to those measured in the field because, unlike the host rocks, fault gouges are only pervasively micro-fractured and lack the populations of long cracks (larger than the sample size) that cause slowing of the velocities measured in the field. By modelling the effect of fractures on seismic velocity (by superimposing upon the laboratory seismic data the effects of crack damage) the gap between field- and laboratory-scale seismic investigations has been bridged. Densities of macroscopic cracks were assessed by measuring outcrop lengths on planar rock exposures. Assuming crack length follows a power law relation to frequency, this fixes a portion of the power spectrum, which is then extrapolated to cover the likely full range of crack sizes. The equations of Budiansky and O'Connell (1976), linking crack density to elastic moduli, were used to calculate modified acoustic velocities, and the effects of the wide range of crack sizes were incorporated by breaking the distribution down into small sub-populations of limited range of crack density. Finally, the effect of overburden pressure causing progressively smaller cracks to close was incorporated to predict velocity versus depth of burial (i.e. pressure). Determination of rock physical properties from laboratory analysis and sections constructed from geological mapping provides a representation of velocity from selected parts of the Carboneras fault zone. First break tomography images show particularly well the location of steeply-inclined fault cores, and these correlate generally well with geological mapping and laboratory velocity measurements corrected for the effect of cracks. The decoration of the fault zone with intrusive igneous material is well correlated with the results of geological observations. Comparisons made between the field (seismic) inversion model and laboratory forward velocity model in El Saltador valley show the laboratory and field velocity measurements made within the fault zone can be reconciled by accounting for the effects of crack damage in field data.

2012 | Taylor, R.L., Rutter, E.H., Nippress, S.E.J., Brodie, K. | London Petrophysical Society Invited  Lecture | INVITED TALK

*RECIPIENT OF IAIN HILLIER UNIVERSITY GRANT FROM LONDON PETROPHYSICAL SOCIETY*

2011 | Taylor, R.L., Rutter, E.H., Nippress, S.E.J., Brodie, K. | Geological Society London, Rock Deformation from Field, Experiments and Theory Meeting in Honour of Ernie Rutter  | POSTER

The Carboneras fault zone of SE Spain provides an excellent opportunity to study a major tectonic lineament by integrating elements of field study, laboratory analysis and modelling.  The excellent exposure in a semi-arid environment provides the opportunity to complete detailed geological mapping and directly sample rocks for testing in the laboratory.  Coupled with field seismic investigations, a more complete picture of fault zone structure emerges.  By integrating both laboratory experiments and geological and geophysical fieldwork enables 'ground-truthing' of results. 

   In-situ seismic investigations recorded P-wave velocities 40-60 % lower than those measured in the laboratory under corresponding pressures and at ambient temperatures for 'hard rock' samples.  Fault gouge velocities measured in the laboratory however, are comparable to those observed in the field.  We attribute this to the fact that unlike the hostrocks, fault gouges are only pervasively micro-fractured and lack the populations of long cracks (larger than the sample size for laboratory measurements) that cause slowing of velocities measured in the field. 

   In order to bridge the gap between field- and laboratory-scale we modelled the effect of populations of large (greater than a cored specimen size) cracks on P-wave velocity.  This required the effects of crack damage to be superimposed upon the laboratory seismic data.  Densities of macroscopic cracks were assessed by measuring outcrop lengths on planar exposures  both directly in the field and from photographs.  Assuming crack length follows a power law relation to frequency, this fixes a portion of the power spectrum, which is then extrapolated to cover the likely full range of crack sizes.  The equations of Budiansky and O'Connell, linking crack density to elastic modulii, were used to calculate modified acoustic velocities, and the effects of the wide range of crack sizes were incorporated by breaking the distribution down into small populations of limited range of crack density.  Finally, the effect of overburden pressure causing progressively smaller cracks to close was incorporated to predict velocity versus depth of burial. In this way it has proved possible to reconcile laboratory and field velocity measurements made within the fault zone. 

2011 | Taylor, R.L., Rutter, E.H., Nippress, S.E.J., Brodie, K. | Geological Society London, Rock Deformation from Field, Experiments and Theory Meeting in Honour of Ernie Rutter | TALK

The Carboneras fault zone (CFZ) is a major NE-SW trending tectonic lineament that marks part of the diffuse plate boundary between Iberia and Africa.  Developed within a basement terrain dominated by mica schist, the fault system comprises two main strands within a complex zone up to 1 km wide.  Between these two strands is a braided network of left-lateral strike-slip, phyllosilicate-rich fault gouge bands, ranging between 1 and 20 m in thickness,  passively exhumed from up to 3 km depth.  The excellent exposure and wide range of rock types and fault structures represented and the practicality of carrying out in-situ geophysical studies makes this fault zone particularly well suited to verifying and interpreting the results of in-situ seismic investigations.

Interpretation of the internal structure of the fault zone is based on (a) detailed geological mapping, (b) field seismic experiments (high resolution refraction and guided wave studies), and (c) laboratory analysis, comprising high pressure velocity measurements, optical and SEM microstructural studies and modal proportions calculated from XRD measurements.  This three-pronged approached has combined an array of data to produce a detailed understanding of the structure of the fault zone by ‘ground-truth’ comparisons with field seismic experiments and exposed geology.  Further, modelling the effect of fractures on seismic velocity can bridge the gap between field- and laboratory-scale seismic investigations. 

Ultrasonic measurements were made using standard equipment over confining and pore pressure ranges appropriate to the upper 10 km of the continental crust.  Seismic velocities have also been approximated from modal analysis and mineral phase elastic properties, and adjusted for the effects of porosity.  Pore volumometry is essential for velocity determination on porous rocks so that any permanent damage due to application of effective pressure can be accounted for.         

Determination of rock physical properties from laboratory analysis and sections constructed from geological mapping provide a representation of density and velocity from selected parts of the Carboneras fault zone.  First break tomography images show particularly well the location of steeply-inclined fault cores, and these correlate well with geological mapping and laboratory velocity measurements.  The decoration of the fault zone with intrusive igneous material is well correlated with the results of geological observations.  However, for these rocks and others (e.g. mica schist, marl) laboratory seismic results do not compare well with P-wave velocities in the field, that are typically 40-60 % lower than those measured at atmospheric and relatively low pressures in the laboratory. This demanded modelling of the effect of cracks that are larger than laboratory core samples on seismic velocities in order to reconcile the two sets of results.

2011 | Taylor, R.L., Rutter, E.H., Brodie, K., Faulkner, D.R., Nippress, S.E.J., Rietbrock, A. and Haberland, C. | European Geosciences Union General Assembly 2011, Vienna, Austria | POSTER and PUBLISHED ABSTRACT

The Carboneras fault zone (CFZ) is a major NE-SW trending tectonic lineament in SE Spain. Of Miocene through Recent age, it separates the volcanic Cabo de Gata terrain from the tract of uplifted metamorphic basement blocks and post-orogenic basins that comprise the Betic Cordilleras lying to the NW. The CFZ consists of two main strands, about 100m apart, each containing several metres thickness of low metamorphic grade, clay-bearing fault gouge, formed in the uppermost 3 to 5 km of the crust. Outside the fault cores, there is widespread cataclastic damage done to the country rocks, plus some subsidiary fault strands. The excellent exposure of the fault rocks and their protoliths makes them particularly well suited to verifying the results of in-situ seismic investigations. Seismic methods are widely used to investigate fault rock structures, but commonly in regions that are less well exposed that the CFZ. Two high resolution seismic reflection/refraction transects were carried out in river valleys cutting the fault zone, and tomographic sections have been constructed. Samples of fault rocks and their protoliths were also collected for laboratory measurements of acoustic wave velocities. Inevitably, the shallow seismic investigations are strongly affected by the cracking on a range of scales that has been done to the country rocks during fault motions, whereas the small samples used for laboratory acoustic measurements are relatively pristine. Despite this, of course, there is still marked sensitivity of velocity to pressure at low pressures as small cracks are progressively closed. To reconcile the results obtained using the two approaches requires the effects of crack damage to be superimposed upon the laboratory seismic data. To assess the density of macroscopic cracks, outcrop crack lengths were measured in the field and from photographs. Assuming crack length follows a power law relation to frequency, this fixes a small portion of the power spectrum, which is then extrapolated to cover the likely full range of crack sizes. The equations of Budiansky and O’Connell linking crack density to elastic moduli were used to calculate modified acoustic velocities, and the effects of the wide range of crack sizes were incorporated by breaking the distribution down into bins of limited range of crack density. In this way it has proved possible to reconcile the laboratory and field velocity measurements. The seismic tomography results show particularly well the location of steeply-dipping fault cores and the decoration of fault zones with intrusive igneous material, and these correlate well with the results of geological observations. 

2010 | Taylor, R.L., Rutter, E.H., Faulkner, D.R., Nippress, S.E.J. | Tectonic Studies Group AGM , Durham | POSTER

*WINNER OF BEST STUDENT POSTER PRIZE*

​The Carboneras fault, SE Spain, is an exceptionally well-exposed, large offset strike-slip fault. The rock products of faulting presently seen were formed at 3-5 km depth, on the basis of deformation mechanisms and clay mineralogy developed from the breakdown of feldspars and micas of the metamorphic basement rocks. Structural characteristics observed and the mechanical characteristics inferred from laboratory work can be compared to observations of the San Andreas fault at the SAFOD drilling site around Parkfield, CA. Similarities suggest that the structures seen in the Carboneras fault may be analogous to the San Andreas fault at corresponding depths. 

It is widely held that upper crustal fault zones consist of a number of fundamental components: fault core, damage zone and protolith. Conceptual fault zone structure models describe only the end-member fault structures, with an array of fault zone structures existing between them. The structure of the Carboneras fault is best described as dominated by anastomising strands of clay-bearing fault gouge, chiefly derived from graphite mica schist and phyllites, enclosing lenticular slivers of damaged rock. Such distributed deformation is usually produced by strain hardening and/or velocity hardening deformation characteristics.

We have measured acoustic wave velocities of a representative suite of rocks of the Carboneras fault zone, to aid interpretation of in-situ high-resolution seismic refraction and reflections studies. Velocities were measured on intact samples over a range of pressures. At low pressures velocities are strongly pressure sensitive, until sufficient pressure is applied to close cracks and/or collapse pores. Pore volumometry was used to track changes in porosity during pressurization cycles and volume corrections applied to high porosity dry rocks. At any given pressure, the range of rock types present exhibits dry compressional wave velocities ranging from 1.154 to 6.681 km/s.  The slowest rocks are the clay-bearing fault gouges, that display unusual behaviour that suggests a large fraction of collapsible pore space recovers elastically upon depressurization. The behaviour is comparable to graphite.

The small samples used in laboratory tests cannot capture the overall density of long cracks that are present in the damage zones in and around the fault zone. Using previously published theoretical analyses, we have attempted to estimate corrections to be applied to the laboratory-measured velocities to take into account crack damage. In this way it has proved possible to reconcile laboratory and in-situ seismic estimations of velocities, the first step towards modeling the seismic structure of the fault zone by combining the results of field geological mapping with the laboratory velocity measurements.

2010 | Taylor, R.L., Rutter, E.H., Faulkner, D.R. | Tectonic Studies Group AGM , Birmingham | POSTER

​The Carboneras Fault Zone (CFZ) Almeria Province, SE Spain is an excellent example of a well preserved major strike-slip fault zone trending NE-SW, with several tens of kilometers displacement.  Metamorphic basement and post-orogenic (Burdigalian-Messinian) sediments are exposed along a 15 km tract in the northeast of the zone; to the southwest the basement is buried by Plio-Quaternary sedimentary cover.  The exceptional preservation in a semi-arid environment provides the opportunity to map the fault zone in detail.  Fault rocks, formed at 3-6km depth throughout the Miocene period and exposed following uplift and erosion have been sampled for laboratory study. 

The CFZ has developed in a basement terrain dominated by mica schist, giving rise to a braided network of foliated fault gouge.  The width of the fault zone is several hundred meters, comprising individual strands of fault gouge a few meters thick.  Bounding the fault zone, two main strands of clay-bearing fault rocks are enclosed by a tract of low-grade metamorphic andalusite-bearing schists.  The geometry of the fault zone is constrained by the intrusion of andesitic rocks to the south and the period of orogenic activity (ending c. 25 Ma) marks the onset of fault movement.  The lithological relations observed permit very detailed determination of the internal structure of the fault complex and the time sequence development.

The interpretation of the internal structure of the fault zone is based on (a) detailed geological mapping and (b) laboratory analysis comprising high pressure acoustic velocity measurements, optical and SEM microstructural study.  Ultrasonic acoustic velocity measurements are made using standard equipment over confining and pore pressure ranges appropriate to the upper 10 km of the continental crust.  We attempt to account for frequency-dependent effects using static elastic property determinations.

Seismic velocities can be approximated from modal analysis and mineral phase elastic properties, adjusted for the effects of porosity.  Pore volumometry is essential for velocity determination on porous rocks so that any permanent damage due to application of effective pressure can be accounted for.

Density and velocity determinations from laboratory analysis and sections constructed from geological mapping will provide a ‘3D map’, or representation of density and velocity throughout the CFZ from which it will be possible to compute seismic behaviour, i.e. forward (deterministic) modeling of the fault zone.  In collaboration with the Universities of Liverpool and Granada, field seismic experiments (high resolution reflection and trapped wave studies) will be combined to produce a detailed understanding of the relations between structure and remotely sensed properties of the fault zone leading to the development of a three-dimensional synthetic velocity-density-stiffness model.  Forward seismic modeling of the CFZ will allow for ‘ground truth’ comparisons with field seismic experiments. 

2009 | Taylor, R.L., Rutter, E.H., Faulkner, D.R. | Deformation, Rheology and Tectonics, Liverpool | POSTER

​The Carboneras Fault Zone (CFZ) Almeria Province, SE Spain is an excellent example of a well preserved major strike-slip fault zone trending NE-SW, with several tens of kilometers displacement.  Metamorphic basement and post-orogenic (Burdigalian-Messinian) sediments are exposed along a 15 km tract in the northeast of the zone; to the southwest the basement is buried by Plio-Quaternary sedimentary cover.  The exceptional preservation in a semi-arid environment provides the opportunity to map the fault zone in detail.  Fault rocks, formed at 3-6km depth throughout the Miocene period and exposed following uplift and erosion have been sampled for laboratory study. 

The CFZ has developed in a basement terrain dominated by mica schist, giving rise to a braided network of foliated fault gouge.  The width of the fault zone is several hundred meters, comprising individual strands of fault gouge a few meters thick.  Bounding the fault zone, two main strands of clay-bearing fault rocks are enclosed by a tract of low-grade metamorphic andalusite-bearing schists.  The geometry of the fault zone is constrained by the intrusion of andesitic rocks to the south and the period of orogenic activity (ending c. 25 Ma) marks the onset of fault movement.  The lithological relations observed permit very detailed determination of the internal structure of the fault complex and the time sequence development.

The interpretation of the internal structure of the fault zone is based on (a) detailed geological mapping and (b) laboratory analysis comprising high pressure acoustic velocity measurements, optical and SEM microstructural study.  Ultrasonic acoustic velocity measurements are made using standard equipment over confining and pore pressure ranges appropriate to the upper 10 km of the continental crust.  We attempt to account for frequency-dependent effects using static elastic property determinations.

Seismic velocities can be approximated from modal analysis and mineral phase elastic properties, adjusted for the effects of porosity.  Pore volumometry is essential for velocity determination on porous rocks so that any permanent damage due to application of effective pressure can be accounted for.

Density and velocity determinations from laboratory analysis and sections constructed from geological mapping will provide a ‘3D map’, or representation of density and velocity throughout the CFZ from which it will be possible to compute seismic behaviour, i.e. forward (deterministic) modeling of the fault zone.  In collaboration with the Universities of Liverpool and Granada, field seismic experiments (high resolution reflection and trapped wave studies) will be combined to produce a detailed understanding of the relations between structure and remotely sensed properties of the fault zone leading to the development of a three-dimensional synthetic velocity-density-stiffness model.  Forward seismic modeling of the CFZ will allow for ‘ground truth’ comparisons with field seismic experiments. 

2008 | Taylor, R.L., Rutter, E.H., Faulkner, D.R. | Geological Society London Meeting on Fault Zones | INVITED POSTER

​Understanding and characterizing fault zone structure at depth is a key part of predicting the slip behaviour of faults in the brittle crust. Field studies are often hampered by poor and/or incomplete exposure, and may only show fault rocks formed at one depth if movements are purely strike-slip. Some faults now exposed at the surface have suffered significant modification during exhumation, such as overprinting of seismogenic structures by continued slip. To image active faults at depth requires remote sensing techniques. Thus seismology can potentially give information on the spatial distribution of seismicity, the size and extent of the fault zone, the fluid pressure and the level of fracture damage within the fault zone, but presently seismic studies of fault zones are not ‘calibrated’ against a forward model of expected properties based on field studies of real fault rocks. If an improved understanding of the controls on the seismic properties of well characterized fault rocks (P/S wave velocity, attenuation, shear wave splitting) were available, this would result in a better understanding of the physical properties of fault zones at depth.

This study forms part of a collaborative investigation involving the universities of Manchester and Liverpool that uses the Carboneras fault zone (CFZ) in S.E. Spain as an analog for certain types of major fault zone, and involves specifically:

• Characterization of the surface structure of the CFZ by detailed geological mapping

• Laboratory measurement over a range of physical conditions (pressure/pore fluid pressure) of the seismic properties of representative samples collected from the CFZ

• Construction of a synthetic seismic fault zone model based on surface mapping and laboratory measurements, for comparison with results from active experiments, which it is hoped eventually will be carried out in this region.

This work follows the same approach that was adopted for producing a synthetic deep seismic reflection profile of lower crustal rocks of the Ivrea-Verbano zone, NW Italy, based on geological mapping, section construction, laboratory seismic measurements and seismic modeling.  Several recent field studies have used ‘seismic fault zone waves’ to help interpret fault zone contiguity and dimensions (such as width) and the contrasting physical properties with the country rock. These waves consist of head waves and waves trapped in the slower rocks of the fault core. To be able to model fault zone wave propagation from forward modelling should represent a substantial aid to their interpretation when detected from field seismic studies.

Work currently focuses on the extension of earlier geological mapping in the CFZ so that the whole of the exposed tract, some 30 km long, is characterized in a way suited to the seismic modelling. Ultrasonic acoustic velocity measurements are being made on fault gouge sampled from the CFZ using standard equipment over confining and pore pressure ranges appropriate to the upper 10km of the continental crust. This study will expand on data collected previously and combine structural information with in-situ seismic surveys and laboratory tests. We attempt to account for frequency-dependent effects using static elastic property determinations. Pore volumometry is used because fault rocks can be quite porous and pore collapse under effective pressure can have a significant modifying effect on seismic properties.