Paleo-fl uid fl ow conditions are reconstructed for an exhumed faulted and fractured sandstone aquifer, the Jurassic Aztec Sandstone at Valley of Fire, Nevada. This reconstruction is based on detailed mapping of multicolored alteration patterns that resulted from syndepositional reddening of the eolian sandstone and repeated episodes of dissolution, mobilization, and reprecipitation of iron oxide and hydroxide. A fi rst stage of bleaching and local redeposition of hematite is attributed to upward migration of reducing basinal fl uid during and subsequent to Late Cretaceous Sevier thrusting and foreland deposition of clastic sediments. A second stage of bleaching and iron remobilization, precipitating predominantly goethite and minor iron sulfates, occurred during Miocene strike-slip faulting associated with Basin and Range tectonics. This second stage is explained by mixing of reducing sulfi derich basinal fl uid with meteoric water entering the aquifer. The distribution of alteration patterns indicates that regional-scale fl uid migration pathways were controlled by stratigraphic contacts and by thrust faults, whereas the outcrop-scale focusing of fl ow was controlled by structural heterogeneities such as joints, joint-based faults, and deformation bands as well as the sedimentary architecture. The complex interaction of structural heterogeneities with alteration is consistent with their measured hydraulic properties, demonstrating the signifi cance of structural heterogeneities for focused fl uid fl ow in a porous sandstone aquifer.

S U M M A R YThe structure and seismicity of the subduction zone of central Costa Rica have been investigated with local earthquake tomography down to ca. 50 km depth. Seismic traveltime data sets of three on-and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations, 3-D structure of P-wave velocity and V p /V s ratio using about 2000 highquality events. The seismicity and slab geometry as well as V p and V p /V s show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest, a seamount province in the centre and the subducting Cocos Ridge in the southeast of the investigation area. Three prominent features can be identified in the V p and V p /V s tomograms: a high-velocity zone with a perturbation of 4-10 per cent representing the subducting slab, a low-velocity zone (10-20 per cent) in the forearc crust probably caused by deformation, fluid release and hydration and a low-velocity zone below the volcanic arc related to upwelling fluids and magma. Unlike previously suggested, the dip of the subducting slab does not decrease to the south. Instead, an average steepening of the plate interface from 30 • to 45 • is observed from north to south and a transition from a plane to a step-shaped plate interface. This is connected with a change in the deformation style of the overriding plate where roughly planar, partly conjugated, clusters of seismicity of regionally varying dip are observed. It can be shown that the central Costa Rica Deformation Belt represents a deep crustal transition zone extending from the surface down to 40 km depth. This transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well.

Abstract. The distribution of flow within conductive joint sets is influenced by the geometric arrangement of joints and the hydraulic properties of both joints and matrix. We use finite element simulations with an equivalent porous media joint representation to understand the distribution of flow through joints and porous matrix. Isolated joints in a porous media create characteristic flow perturbations in the matrix with reduced fluid potentials near the upstream joint tip, elevated potentials near the downstream tip, and flow shadows adjacent to the joint. In more complex joint systems, flow in any given joint is influenced by its proximity to other joints, resulting in characteristic enhancement or reduction of flow velocities. The permeability ratio (equivalent joint permeability divided by matrix permeability) plays a major role in determining the distribution of flow within complex joint systems. When the permeability ratio is <3.0 orders of magnitude, all joints are indirectly connected to the flow system through the matrix. As joint conductivity increases, flow becomes increasingly localized into directly connected joints. When the permeability ratio exceeds 6.5 orders of magnitude, significant flow occurs only in the directly connected joints. We compare these numerical results with field observations from an ancient reactive flow system now exposed at the Earth's surface. In the field, 32% of joints are associated with chemically altered halos. By explicitly representing mapped joint distributions in numerical simulations, we estimate that 32% of the joints would conduct significant volumes of fluid if joint permeability is 5 orders of magnitude greater than the matrix permeability. This corresponds to an insitu joint aperture of 2.3 mm, closely resembling the 1.8-mm average joint aperture measured on the present-day outcrop.

Abstract. At the Valley of Fire State Park, southeastern Nevada, there is a unique relationship between deformation bands and the distribution of diagenetic mineralization in the Aztec Sandstone, a reservoir and aquifer analog exposed at the surface. Distinct diagenetic alteration fronts are refracted where they cross deformation bands. Modeling this refraction as resulting from the advective transport of a nonreactive solute, we are able to back out that deformation band permeability is reduced by an average of 1.3 orders of magnitude relative to the surrounding rock with a range of 0.7 to 2.1 orders of magnitude. The geometric relationship utilized in our approach formed in the subsurface during diagenesis and provides an in situ measurement of permeability not affected by uplift and erosion or by damage associated with collecting a sample from a well bore. Our estimation is about 1 order of magnitude lower than values reported in the literature for similar rock types. IntroductionCharacterization of fluid flow pathways for managing and exploiting petroleum reservoirs and groundwater aquifers requires knowledge of the distribution of permeability within the reservoir. During deformation, physical discontinuities may develop in a rock mass, contributing to the hydrologic heterogeneity and altering fluid flow pathways. These discontinuities have their own hydraulic properties and can occur in complex geometric distributions. Our research analyzes the spatial distribution of diagenetic mineralization exposed in outcrop to infer the in situ permeability of deformation bands, a type of discontinuity that is present in many siliciclastic reservoirs. Studying the distribution of diagenetic minerals provides insight into paleofluid flow systems and is one of the few techniques available to aid in characterizing in situ permeability of faulted and fractured rock. rock outcrops, on samples taken from rock outcrops or on cores extracted from wells. Stress conditions on these samples at the time of measurement are markedly different than subsurface conditions in aquifers and reservoirs, and samples collected at the outcrop have been subject to weathering and erosion since the rock was exposed at the surface.We contend that extended exposure at the surface, or the process of extracting the sample and reloading it in the laboratory, may have a significant influence on deformation band permeability. The new method presented in this paper uses an indirect approach to estimate deformation band permeability rather than a direct measurement on a sample. The indirect approach hinges on a geometric relationship formed in the subsurface during diagenesis that remains invariant during unloading and erosion and so offers the opportunity to accurately estimate permeability at subsurface conditions from attributes observable at Earth's surface. The key relationship is that the angle of refraction of a distinct alteration front is related to the permeability contrast between the undeformed rock matrix and a deformation band. Field Obse...

S U M M A R YThe continental margin of Nicaragua and Costa Rica is characterized by significant lateral changes from north to south such as a decreasing dip of the slab, a decreasing magma production and a shift in the volcanic front. To investigate this transition, a joint on-and offshore local earthquake tomography was performed. Low P-wave velocities and high V p /V s ratios, indicative for hydration, were found in the upper-mantle and lowermost crust beneath the Sandino Basin. The mantle wedge hydration can be estimated to 2.5 wt. per cent beneath south Nicaragua. In contrast, the mantle wedge beneath north Costa Rica is weakly or not hydrated. The hydration leads to a local gap in the seismicity in Nicaragua. The lateral transition between the hydrated and non-hydrated areas occurs within a distance of about 10 km. This transition coincides with a change in the crustal thickness in the order of 5-10 km, thickening to the south, and in the tectonic regimes. The change in the tectonic regimes towards a stronger extension along the margin of Nicaragua could be the key for understanding the observations: the extension may support the opening of pathways for a wide zone of fluid migration and hydration through the overriding plate which are identified with areas of low V p , high V p /V s and low seismicity.