Subvertical Faults Research Articles

Self-potential investigation of moraine dam seepage

Self-potential (SP) and electrical resistivity measurements are used to investigate seepage at a remote moraine dam in the Sierra Nevada of California. The site is a small terminal moraine impounding roughly 300,000 m3 of water at ~ 3400 m a.s.l. Suspicious fine sediment in a small lake at the dam's downstream toe prompted initial concerns that anomalous seepage may be eroding matrix material from the moraine. 235 individual SP measurements covering the surface of the dam were collected in order to investigate electrokinetic current sources resulting from seepage, while resistivity soundings probed moraine stratigraphy and suggest that the till contains interstitial ice. Contoured SP data reveal a non-uniform voltage distribution over the moraine dam and two distinct negative SP anomalies. The first, located in the central area of the moraine, shows a broad negative SP zone around the crest and increasingly positive SP moving downhill towards both the upstream and downstream toes. This anomaly can be explained by shallow gravitational groundwater flow in the near subsurface combined with upward groundwater flux through evapotranspiration; numerical simulation of the combined effect matches field data well. The second SP anomaly has a tightly localized distribution and can be explained by vertically descending flow into a bedrock fault conduit. Our conceptual seepage model suggests that flow travels from Dana Lake first at the boundary of ice-filled moraine and bedrock before converging on a concentrated channel in the subvertical fault zone. Positive SP near the dam abutments results from groundwater inflow from adjacent hillslopes. Combined analyses suggest that seepage erosion is not currently affecting the moraine dam, and that the sediment observed on the bed of the downstream toe lake is likely a remnant of past outflow events.

The development of cavities and clastic infills along fault-related fractures in Tertiary basalts on the NE Atlantic margin

Field-based geological observations have revealed the hitherto unrecognised development of post-magmatic, brittle deformation structures cutting Tertiary volcanic rocks in the Faroe Islands. These faults and fractures are characteristically associated with different styles of clastic sedimentary infill including: 1) 0.3–1.0 m thick clastic units infilling open fractures formed along pre-existing steeply-dipping to sub-vertical faults; 2) 0.1–0.6 m thick sub-horizontal clastic units displaying internal features consistent with deposition from flowing water passing through complex open subterranean cavity systems within fractured basalts; 3) Anastomosing mm-scale and planar dm-scale clastic intrusion features mobilised and emplaced during transient, fault-related overpressuring events along pre-existing fractures cutting the surrounding volcanic units. The infill features provide evidence for the existence of sustained open cavities in the sub-surface. The clastic materials are commonly internally affected by later fault-related deformation and lack mineralisation, unlike all preceding faulting episodes in the Faroes region, perhaps reflecting their near-surface development. We believe structures equivalent to these features may occur widely in other parts of the NE Atlantic margin, particularly along the outer arcs of gentle regional-scale fold hinges. The uncemented fracture-hosted clastic infills potentially represent important fluid migration pathways within the otherwise low permeability Cenozoic volcanic sequences of the NE Atlantic region.

Porosity prediction from seismic attributes of the Ordovician Trenton-Black River groups, Rochester field, southern Ontario

This article integrates three-dimensional (3-D) seismic attributes and log data to determine porosity distribution of the Ordovician Trenton-Black River groups within the Rochester field, southern Ontario. The rocks are composed of tight limestone, parts of which were dolomitized to form porous reservoir rock. Previous studies of the Trenton-Black River dolomite reservoirs have indicated a close relationship between faulting and reservoir development, but few published studies have attempted to examine these relationships using 3-D seismic data. This study explores the stratigraphy and structure of the Rochester fault-related dolomite reservoir using 3-D seismic data and neural networks to predict porosity. By predicting porosity using seismic attributes, vertical and lateral distributions of porosity that can be used to guide development and exploration drilling for optimal hydrocarbon recovery were obtained. The sites of highest porosity were found to be along and within the fault zones. Faults extending from the basement into overlying Paleozoic rocks are composed of several short-plane, vertical, and subvertical fault segments. However, some of these faults appear to have originated and died within the Paleozoic rocks; they cannot be traced to the basement because of little or no offset where they penetrate the basement. Although the five identified attributes are considered important in exploration for fault-related dolomite reservoirs, the single most important attribute to employ is the amplitude envelope because the other attributes are mathematically related to it. Furthermore, the sags that are seen in the Rochester field are due to the combined effects of low-velocity pushdown and faulting. Methods and results presented in this study can be used to explore and develop fault-related dolomite reservoirs elsewhere in similar geologic settings.

Groundwater flow analysis using different geothermal constraints: The case study of Acqui Terme area, northwestern Italy

We review some analytical techniques that use underground thermal data as tracers of groundwater flow. These techniques allow the evaluation of the Darcy velocity in shallow aquifers of mid-low permeability and the evaluation of heat gain/loss by conduction in deeper aquifers. Examples of application are then given for the Acqui Terme hydrothermal system, located in the Tertiary Piedmont Basin (northwestern Italy). The analysis of borehole temperatures allowed the inference of the hydraulic features of the sedimentary cover of the hydrothermal system. The results show the presence of a relatively weak flow, with upward and horizontal components, only in conglomerates occurring at the base of the marly impermeable cover. The analysis of the heat transported in the deep parts of the hydrothermal system was approached by splitting the water path into different sections, each with given shape, slope and hydraulic properties. The recharge area is situated in the upland, south of the discharge area. Meteoric water initially descends and then flows horizontally within the fractured metamorphic basement of the basin, heating by conduction. Finally, from a reservoir positioned at intermediate depths, hot water reaches rapidly the surface through a sub-vertical fault. This scheme of deep water flow is constrained by the regional surface heat flow and the local geothermal gradient, and it is consistent with data of rock–water equilibrium temperature.

Normal faults and thrusts reactivated by deep fluids: The 6 April 2009 Mw 6.3 L'Aquila earthquake, central Italy

On 6 April 2009, a Mw = 6.3 earthquake occurred in the central Apennines (Italy) damaging the city of L'Aquila and the surrounding country. We relocate the October 2008 to 6 April 2009 foreshocks and about 2000 aftershocks occurred between 6 and 30 April 2009 by applying a double‐difference technique and determine the stress field from focal mechanisms. The events concentrate in the upper 15 km of the crust. Three main NW‐SE to NNW‐SSE striking, 30°–45° and 80°–90° dipping faults were activated during the seismic sequence. Among these, a normal fault and a thrust were reactivated with dip‐slip movements in response to NE‐SW extension. The structural maturity of the seismogenic fault system is lower than that displayed by other systems in southern Apennines because of the lower strain rate of the central sector of the chain with respect to the southern one. VP/VS increases progressively from October 2008 to the 6 April 2009 main shock occurrence along a NW‐SE strike because of an increment in pore fluid pressure along the fault planes. Pore pressure diffusion controls the space‐time evolution of aftershocks. A hydraulic diffusivity of 80 m2 s−1 and a seismogenic permeability of about 10−12 m2 suggest the involvement of gas‐rich (CO2) fluids within a highly fractured medium. Suprahydrostatic, high fluid pressure (about 200 MPa at 10 km of depth) within overpressurized traps, bounded by preexisting structural and/or lithological discontinuities at the lower upper crust boundary, are required to activate the April 2009 sequence. Traps are the storage zone of CO2‐rich fluids uprising from the underlying, about 20 km deep, metasomatized mantle wedge. These traps easily occur in extensional regimes like in the axial sector of Apennines but are difficult to form in strike‐slip regimes, where subvertical faults may cross the entire crust. In the Apennines, fluids may activate faults responsible for earthquakes up to Mw = 5–6. Deep fluids more than tectonic stress may control the seismotectogenesis of accretionary wedges.

The tsunamigenic earthquake of April 1, 1946, in the Fox Islands (Aleutian island arc)

An interpretation of the type, size, and location of the source of the Aleutian earthquake on April 1, 1946, which was characterized by the highest intensity (I = 4), is proposed. The earthquake source is a subvertical reverse fault striking along the island arc and dipping at an angle of 85° toward the deep-sea trench. The reverse fault is located in the lower part of the island slope, within the eastern termination of the Aleutian terrace. The western end of the reverse fault is located in the area of the Krenitsyn Islands (λ ∼ 165°W), where the pattern of isobaths changes, and an abrupt widening of the shelf part of the Fox Islands takes place. Large (M S ∼ 7) shocks, preceding the 1946 earthquake, occurred here in 1940, 1942, and 1944. Structural inhomogeneities in the island slope in the area of the Sanak Islands (λ ∼ 162°W) determine the eastern edge of the source-reverse fault, whose length within the specified boundaries is about 200 km. The mean magnitude of the earthquake corresponding to such a source is ∼8.3. According to the regular relation between the rupture length and the mean movement, the vertical displacement of the ocean floor in the source region could attain 5–6 m. A significant vertical displacement of the ocean floor over its large length (∼200 km) was responsible for the high tsunamigenic ability of this earthquake. A favorable combination in the source area of the topographic and other conditions necessary for the tsunami formation could additionally contribute to an increase in the intensity of the tsunami. The earthquake of April 1, 1946, in the Fox Islands, as well as the tsunamigenic earthquakes of March 9, 1957, in the Andreanof Islands and February 4, 1965, in the Rat Islands, does not belong to the class of “slow” earthquakes.

Structural model of the Trieste Gulf: A proposal

Abstract The onshore geology of the Trieste Gulf is well known in Italian reliefs, as well as in Slovenian and Croatian ones (Istria peninsula). In the northeastern and southern part carbonate platforms outcrop mainly in shelf facies of the Cretaceous to the Paleogene ages and are covered by Eocene flysch deposits. In the northern part the Gulf is closed by the coastal line of the Friuli plain composed mainly of glacio-fluvial deposits of Quaternary age. From a structural point of view, overthrusts set in a typical Dinaric NW–SE direction dominate and where one can identifiy many tectonic units with a SW vergence, tending to overlap the Mesozoic–Paleogene carbonate units which lie over the Eocene terrigenous ones. In this general picture, some sub-vertical faults with an anti-Dinaric NE–SW orientation and a strike-slip activity, cut the previous overthrusts. On the other hand the offshore geological knowledge of the Gulf is scarce due also to the absence, in the inner part of the Gulf, of exploration wells. Only recent studies on some tectonic structures of the Trieste Gulf revealed by geophysical explorations made a few years ago, have suggested making a hypothesis on its formation that has never been studied before. The model proposed here starts from the hypothesis that all tectonic lines that come from the Italian, Slovene and Croatian coastline, continue offshore. This is validated by highlighting all the geometries and morphologic anomalies obtained by analyzing various documents that regard the buried subsoil and obtained through geophysical prospections. With this idea, the maps showing the isobaths of the top of the limestones, the depth and structures of the top of the flysch, the isobaths of the base of the Quaternary deposits and the morphology of the actual sea bed were compared. All these documents present many morphological elements, found repeatedly on maps with different finalities but which appear coherent with the general picture of the structural outcrop and with the starting hypothesis. Hence, the Gulf of Trieste, is situated on a graben defined by master faults in an anti-Dinaric NE–SE direction. These extend into the Gulf from two vertical faults documented in the onshore with a transtensional effect, and cut the NW–SE Dinaric thrusts that carry on length-wise into the offshore from the Istria coast. On the basis of the geometry of the top of the buried limestones and other pieces of evidence, we hypothesize also a new line, NE–SW oriented (Aquileia line) that could actually separate two buried sectors of limestone platform each with a different evolution.

Potential of Electrical Resistivity Tomography to Detect Fault Zones in Limestone and Argillaceous Formations in the Experimental Platform of Tournemire, France

The Experimental platform of Tournemire (Aveyron, France) developed by IRSN (French Institute for Radiological Protection and Nuclear Safety) is located in a tunnel excavated in a clay-rock formation interbedded between two limestone formations. A well-identified regional fault crosscuts this subhorizontal sedimentary succession, and a subvertical secondary fault zone is intercepted in the clay-rock by drifts and boreholes in the tunnel at a depth of about 250 m. A 2D electrical resistivity survey was carried out along a 2.5 km baseline, and a takeout of 40 m was used to assess the potential of this method to detect faults from the ground surface. In the 300 m-thick zone investigated by the survey, electrical resistivity images reveal several subvertical low-resistivity discontinuities. One of these discontinuities corresponds to the position of the Cernon fault, a major regional fault. One of the subvertical conductive discontinuities crossing the upper limestone formation is consistent with the prolongation towards the ground surface of the secondary fault zone identified in the clay-rock formation from the tunnel. Moreover, this secondary fault zone corresponds to the upward prolongation of a subvertical fault identified in the lower limestone using a 3D high-resolution seismic reflection survey. This type of large-scale electrical resistivity survey is therefore a useful tool for identifying faults in superficial layers from the ground surface and is complementary to 3D seismic reflection surveys.

Stratigraphic significance and resolution of spectral reflectance logs in Lower Devonian carbonates of the Barrandian area, Czech Republic; a correlation with magnetic susceptibility and gamma-ray logs

Spectral reflectance (SR) is a relatively novel technique which is used as a proxy for organic carbon and siliciclastic input in carbonate sediments, in particular in core research. This study summarizes results from high-resolution outcrop SR logging of a Lower Devonian distal ramp carbonate section of the Barrandian area, Czech Republic. The SR data were correlated with parallel petrophysical and geochemical data with an aim to improving their cyclostratigraphic and event stratigraphic interpretation potential and the gaps SR data can fill. Transgressive–regressive trends, indicated by facies stacking patterns, CaCO 3 data and spectral gamma-ray (GRS) logs, are well correlated in the regional context and point to third-order eustatic sea-level fluctuations. Peak regressions of the TR cycles are sensitively marked by CIE L* a* b* parameters and percent reflectance in colour bands. The SR and magnetic susceptibility (MS) data are markedly cyclic on a more detailed scale. Their inferred periods vary between 49.2 and 205 kyr indicating that they may have been forced by orbital cycles in the Milankovitch band. The SR and MS cycles are either in phase or out of phase. The latter cycles point to a strong diagenetic forcing of the colour cyclicity. Hematite concentration cycles, inferred from SR signal in the Pragian red hemipelagic carbonates, are interpreted as reflecting changing bottom oxygenation and subsurface redox gradients during early diagenesis. The CIE L* parameter was found to be related to the presence of diagenetic hematite, which indicates that cyclostratigraphic analysis of simple greyscale data can be affected by considerable error due to chromacity. Although the CIE L* parameter is partly correlated with CaCO 3 concentrations, the MS data are not, which suggests that the MS signal can be significantly influenced by diagenetic minerals and its cyclostratigraphic interpretation can be potentially misleading. The SR data, backed up by GRS, MS and CaCO 3 data indicate that the red colour in the Pragian hemipelagic limestones originated from early diagenetic hematite precipitation under the conditions of an oxygenated ocean floor. This suggests that deep-sea bottom oxygentation may have been in operation during the early Devonian, which is traditionally perceived as a typical greenhouse period. The SR data can be obscured in several m thick zones of elevated CIE L* and other colour changes (“leaching zones”) related to postdepositional alteration along subvertical faults, however, this fault-related telogenic alteration has little influence on the MS signal. The combined use of SR and MS has a synergic effect and can significantly improve the interpretation of their respective cyclic signals, particularly in relation to the diagenetic overprint and possible carriers of the MS.

Impact of gently dipping discontinuities on basement aquifer recharge: An example from Ploemeur (Brittany, France)

Classically, geological surveys of water resources in hard-rock aquifers are made from aerial photographs or geophysical techniques that basically permit to detect vertical features. On the other hand, aquifers only linked to vertical structures are very limited in space and offer in most cases very poor water resources. In the present case, we do show that an outstanding aquifer in Brittany is linked to a geological context associated with low-angle faults possibly directly connected with vertical feeders. In particular, we show through a high-resolution gravimetric survey that the highly fractured contact between granite and micaschists, which constitutes the main path for groundwater flow, is a gently dipping structure. Combined gravimetric, magnetic and geological data allowed us to establish the overall aquifer geometry by running a three-dimensional forward model. In addition, details about the shape of the contact have been obtained using an iterative scheme based on the method of Oldenburg (1974). The gravimetric model confirms the presence of sub-vertical faults that may constitute important drains for the aquifer recharge. Thus, the geological context associated with exceptional water resources for this crystalline aquifer is characterized by a sub-horizontal to gently dipping contact between granite and micaschists. Only such a geological context can allow sufficient recharge to provide the main water supply for a town of 18,000 inhabitants at an average rate of about a billion of cubic meter per year since 1991. Thus, instead of looking for possible vertical structures like in classical hard-rock hydrogeology, it appears much more efficient to detect sub-horizontal permeable fractures and faults for providing consistent water resources.

The role of the Kupferschiefer in the formation of hydrothermal base metal mineralization in the Spessart ore district, Germany: insight from detailed sulfur isotope studies

The Spessart district (SW Germany), located at the southwestern margin of the Permian Kupferschiefer basin in Central Europe, hosts abundant stratabound and structurally controlled base metal mineralization. The mineralization styles identified are (1) stratabound Cu-Pb-Zn-(Ag) ores in Zechstein sedimentary rocks, (2) structurally controlled Cu-As-(Ag) ores in Zechstein sedimentary rocks, (3) crosscutting Co-Ni-(Bi)-As and Cu-Fe-As veins, (4) stratabound metasomatic Fe-Mn carbonate ores in Zechstein dolomite, (5) barren barite veins, and (6) Fe-Mn-As veins in Permian rhyolites. Building on previous work that involved mineralogical, textural, and chemical characterization of the major mineralization types, we have performed a comprehensive sulfur isotope study that applied both conventional and novel laser-ablation multi-collector inductively coupled plasma mass spectrometry techniques. The δ34S values of sulfide minerals from the different ore types are consistently negative and highly variable, in the range between −44.5‰ and −3.9‰, whereas the δ34S values of barite are all positive in the range between 4.7‰ and 18.9‰. Remarkably, stratabound and structurally controlled mineralization in Zechstein sedimentary rocks has the least negative δ34S values, whereas vein-type deposits have consistently more negative δ34S values. The observed pattern of sulfide δ34S values can be best interpreted in terms of fluid mixing at the basement-cover interface. Hydrothermal fluids originating from the crystalline basement migrated upward along subvertical fault zones and were periodically injected into groundwaters that were flowing in the post-Variscan sedimentary cover. These groundwaters had interacted with the Zechstein sedimentary rocks, resulting in fluids characterized by elevated concentrations of reduced sulfur (with negative δ34S values) and alkaline pH. Repeated mixing between both chemically contrasting fluids caused rapid and efficient precipitation of sulfide ore minerals in hydrothermal veins with highly variable but distinctly negative δ34S values.

Mode of emplacement of the Slaufrudalur Pluton, Southeast Iceland inferred from three-dimensional GPS mapping and model building

The Slaufrudalur Pluton is a granitic pluton in the Tertiary lava pile of Southeast Iceland. Excellent exposures of its roof and walls made it possible to map the shape of the pluton with high-resolution GPS. Based on the GPS mapping and field observations, we reconstructed the three-dimensional shape of the Slaufrudalur Pluton with the aim to test which implications on the mechanisms of emplacement can be derived from this approach. The reconstructed pluton shape is characterised by steep walls and a flat roof at map-scale. This shape and the internal compositional layering indicate that the pluton was probably emplaced by cauldron subsidence along subvertical faults that are parallel with the strike of the regional fissure swarms. At the roof contact, the pluton exploited the original layering of the flood basalts. At outcrop-scale, however, the roof was modified by magmatic stoping, which resulted in a step-like pattern on a scale that cannot be resolved in the three-dimensional model. Hence, the reconstruction of the three-dimensional shape of the Slaufrudalur Pluton, combined with structural field studies, provided valuable information about the mechanism of its emplacement on pluton-scale. For a comprehensive understanding of all mechanisms involved in the emplacement of the Slaufrudalur Pluton, detailed structural field studies remain essential.

Variscan structure in the eastern part of the Pallaresa massif, Axial Zone of the Pyrenees (NW Andorra). Tectonic implications

AbstractThe Variscan structures of the Axial Zone of the Pyrenees are traditionally separated into the “infrastructure” and the “suprastructure” domains. Three deformation events have been recognized within Cambro-Ordovician rocks in the eastern part of the Pallaresa massif. D1 consists of a slaty cleavage (S1), and no folds have been found associated with it. The D2 event is characterized by E-W trending and north-verging recumbent folds with an associated foliation (S2) that can be observed at all scales. The S2 foliation has a horizontal orientation, and is the main foliation in the northern and central parts of the study area. The D3 deformation event is distinguished by development of E-W trending, upright folds, of centimetric to metric scale. These folds are associated with a steep and rough crenulation cleavage (S3) in the southern part of the study area. Subvertical faults trending E-W are associated with D3 folds. Most of the structures recognized in the eastern part of the Pallaresa massif have developed under low metamorphic grade conditions, although close to the Aston and Hospitalet domes, in the vicinity of some igneous intrusions, the metamorphism is coeval with S2 foliation and may reach high grade conditions. A gradual transition can be observed from zones characterized by structures of type D2 to zones exhibiting a better development of D3 structures. This transition can be explained by a décollement level located in the Cambro-Ordovician succession. This level can be related to faults and D3 folds observed in the eastern part of the Pallaresa massif. Most of the structures can be explained in a contractional geological setting. In addition the relationships between the different Variscan structures of the eastern part of the Pallaresa massif are better described. The different models recently presented by some authors about the Variscan structure of the Pyrenees are discussed in the light of these new results.

Marine palaeoseismology from very high resolution seismic imaging: the Gondola Fault Zone (Adriatic foreland)

AbstractWe present a marine palaeoseismology analysis of a dense network of very high resolution seismic profiles along the Gondola Fault Zone (GFZ), a right‐lateral, E–W‐striking, active fault system in the Adriatic foreland. This case‐study aims to show how time and space variations in the activity of a dominantly right‐lateral fault system can be assessed using the vertical component of slip. The GFZ has been investigated for a length of 50 km. It includes two parallel subvertical fault sets and two main anticlines. The late Middle Pleistocene to Holocene vertical component of displacement along the fault is bell‐shaped, suggesting that in the long‐term the fault zone acts as a single, kinematically coherent structure. Slip rates are 0–0.18 mm a−1 and vary temporally on individual segments. This variability is consistent with a model in which individual fault segments rupture independently during earthquakes with magnitudes up to 6.4 and 1.3–1.8 ka recurrence intervals.

Interpretation of ambiguous reflection data by imaging processes: An example profile crossing steep submarine valley at the sea bottom

The determination of the geometry of a vertical structure is one of the most important problems in the conventional stacking process used as an imaging process. The horizontal layers in depths of the earth are easily displayed in the seismic time section, but the dipping and folded layers are not correctly displayed in the reflection data. The present study addresses the same problem for the data collected from a profile of a steep marine valley and that of an offshore extension of the Ganos fault at the westernmost part of the Tekirdağ basin, Marmara Sea, NW Turkey. In the previous studies, the fault was interpreted as a low-angle reverse fault juxtaposing between the Plio-Quaternary and the Miocene strata in the time migration section. However, the results of the extensive analyses in this study show that the close position of a newly interpreted Miocene anticline to the synformal structure of the sea bottom makes the data ambiguous, whereas the subvertical fault and the other events coming away from the fault plane causes complexity. To eliminate misinterpretation due to ambiguity and complexity as in the previous analysis, the data are reinterpreted by pre-stack Kirchhoff depth migration in addition to the conventional stacking process. The new interpretation is checked through the synthetics data created from the model having appropriate parameters of the data used. The results have shown the existence of an anticline that is cored by the Miocene strata and overlain by the Plio-Quaternary strata 200 m below the sea bottom. The Ganos fault is imaged as disturbing all the layers down the way and as having a listric geometry and steeper dip angle (75° N) at shallow depths but decreasing downward. This study examples the importance of considering the problems of the complex geometry and choosing appropriate imaging technique for analyzing seismic reflection data collected over an area with known vertical fault and related tectonic complexity.

Extensional stresses in the Colombian Eastern Cordillera fold-and-thrust belt (northern Andes): insights from 2D finite element modeling

Deformation and stress characteristics in the upper crust of the fold-and-thrust belt in the Colombian Eastern Cordillera were investigated by numerical analysis. The structural trend of Colombian Eastern Cordillera has long been considered a possible example of a true contraction orogen. The issue of the convergent displacement along an elastic structural body, which controls present-day deformation in this Cordillera is examined here. Modeling results are presented in terms of three parameters: 1) distributions, orientations, and magnitudes of principal stresses; 2) maximum shear stress (� max) contour; and 3) proximity to failure of elements within faults. Mohr-Coulomb failure criteria with bulk rock properties are applied to analyze the faults. The model shows extensional stresses in the crust at shallow crustal levels (from surface to about 6 km) despite overall contraction, and contraction at depth is confirmed. Measurement results indicate that, for homogeneous crustal thickening, extensional stresses are concentrated in the Servita half-gaben and Bucaramanga fault systems, where the vertical thrust faults and thickening processes are located. Our two-dimensional (2D) modeling results emphasize that extensional stresses are still active along the vertical to sub-vertical fault system in the fold-and-thrust belt of the Colombian Eastern Cordillera.

Seismostratigraphic and structural setting of the Malvinas Basin and its southern margin (Tierra del Fuego Atlantic offshore)

New multichannel seismic reflection profiles acquired off the Tierra del Fuego Atlantic margin, from the southern part of the Malvinas foreland basin to the inner sector of the Magallanes fold and thrust belt, combined with available commercial profiles and exploration wells, allowed to outline the sedimentary architecture of the foreland basin and the structure of its deformed southern margin. Five major unconformities were differentiated within the sedimentary fill of the southern Malvinas basin, which neighbours the offshore extension of the Magallanes basin in Tierra del Fuego. The unconformity-bounded units record the corresponding major evolutionary tectonostratigraphic phases of the southern part of the Malvinas basin, and the development of the Magallanes fold-and thrust belt during Mesozoic and Cenozoic times: Unit 1 - Pre-Jurassic basement; Unit 2 - Rift phase (Middle - Upper Jurassic); Unit 3 - Sag phase (Lower – Upper Cretaceous); Unit 4 – Foredeep transitional phase (Upper Cretaceous - Middle Eocene); Unit 5 - Foreland phase (Middle Eocene - Pleistocene). The southern edge of the Malvinas basin corresponds to the imbricate basement wedges of the Fuegian Cordillera, which shows a thick-skin structural style developed as a consequence of the Middle Tertiary Andean compressional tectonic phase. Large folds, with low angle NE-verging thrusts propagated the shortening basin-ward at shallow structural levels. These structures are superimposed by an array of left-lateral strike-slip lineaments pertaining to the EW trending Magallanes-Fagnano fault system. In the Tierra del Fuego region these structures represent the western segment of the South America – Scotia plate boundary. Several Neogene pull-apart basins were formed along the principal deformation zone in correspondence of step-overs and releasing bends. These basins show an evident asymmetry in the sedimentary architecture, and are bounded by sub-vertical faults that in some cases reach the sea-floor. Other transtensional features were also recognized in the inner sector of the fold-and-thrust belt together with the formation of restricted pull-apart basins.

Fault plane orientations of intermediate‐depth earthquakes in the Middle America Trench

Intermediate‐depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones. The orientation of presubduction faults is particularly well known offshore of Middle America, where seismic reflection profiles show outer rise faults dipping toward the trench and extending >20 km into the lithosphere. If water is transported along these faults and incorporated into hydrous minerals, the faults may be reactivated later when the minerals dehydrate. In this case, the fault plane orientations should be the same in the outer rise and at depth, after accounting for the angle of subduction. To test this hypothesis, we analyze the directivity of 54 large (MW ≥ 5.7) earthquakes between 35 and 220 km depth in the Middle America Trench. For 12 of these earthquakes, the directivity vector allows us to identify the fault plane of the focal mechanism. Between 35 and 85 km depth, we observe both subhorizontal and subvertical fault planes. The subvertical fault planes are consistent with the reactivation of outer rise faults, whereas the subhorizontal fault planes suggest the formation of new faults. Deeper than 85 km, we only observe subhorizontal faults, indicating that the outer rise faults are no longer being reactivated. The similarity with previous results from the colder Tonga‐Kermadec subduction zone suggests that the mechanism generating these earthquakes, and controlling fault plane orientations, depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate‐depth seismicity. Exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes.

Hydrologic models of modern and fossil geothermal systems in the Great Basin: Genetic implications for epithermal Au-Ag and Carlin-type gold deposits

The Great Basin region in the western United States contains active geothermal systems, large epithermal Au-Ag deposits, and world-class Carlin-type gold deposits. Temperature profiles, fluid inclusion studies, and isotopic evidence suggest that modern and fossil hydrothermal systems associated with gold mineralization share many common features, including the absence of a clear magmatic fluid source, discharge areas restricted to fault zones, and remarkably high temperatures (>200 °C) at shallow depths (200–1500 m). While the plumbing of these systems varies, geochemical and isotopic data collected at the Dixie Valley and Beowawe geothermal systems suggest that fluid circulation along fault zones was relatively deep (>5 km) and comprised of relatively unexchanged Pleistocene meteoric water with small (<2.5‰) shifts from the meteoric water line (MWL). Many fossil ore-forming systems were also dominated by meteoric water, but usually exhibit δ18O fluid-rock interactions with larger shifts of 5‰–20‰ from the MWL. Here we present a suite of two-dimensional regional (100 km) and local (40–50 km) scale hydrologic models that we have used to study the plumbing of modern and Tertiary hydrothermal systems of the Great Basin. Geologically and geophysically consistent cross sections were used to generate somewhat idealized hydrogeologic models for these systems that include the most important faults, aquifers, and confining units in their approximate configurations. Multiple constraints were used, including enthalpy, δ18O, silica compositions of fluids and/or rocks, groundwater residence times, fluid inclusion homogenization temperatures, and apatite fission track anomalies. Our results suggest that these hydrothermal systems were driven by natural thermal convection along anisotropic, subvertical faults connected in many cases at depth by permeable aquifers within favorable lithostratigraphic horizons. Those with minimal fluid δ18O shifts are restricted to high-permeability fault zones and relatively small-scale (~5 km), single-pass flow systems (e.g., Beowawe). Those with intermediate to large isotopic shifts (e.g., epithermal and Carlin-type Au) had larger-scale (~15 km) loop convection cells with a greater component of flow through marine sedimentary rocks at lower water/rock ratios and greater endowments of gold. Enthalpy calculations constrain the duration of Carlin-type gold systems to probably <200 k.y. Shallow heat flow gradients and fluid silica concentrations suggest that the duration of the modern Beowawe system is <5 k.y. However, fluid flow at Beowawe during the Quaternary must have been episodic with a net duration of ~200 k.y. to account for the amount of silica in the sinter deposits. In the Carlin trend, fluid circulation extended down into Paleozoic siliciclastic rocks, which afforded more mixing with isotopically enriched higher enthalpy fluids. Computed fission track ages along the Carlin trend included the convective effects, and ranged between 91.6 and 35.3 Ma. Older fission track ages occurred in zones of groundwater recharge, and the younger ages occurred in discharge areas. This is largely consistent with fission track ages reported in recent studies. We found that either an amagmatic system with more permeable faults (10−11 m2) or a magmatic system with less permeable faults (10−13 m2) could account for the published isotopic and thermal data along the Carlin trend systems. Localized high heat flow beneath the Muleshoe fault was needed to match fluid inclusion temperatures at Mule Canyon. However, both magmatic and amagmatic scenarios require the existence of deep, permeable faults to bring hot fluids to the near surface.

Earthquake mechanics and deformation in the Tonga‐Kermadec subduction zone from fault plane orientations of intermediate‐ and deep‐focus earthquakes

We make use of rupture directivity to analyze 82 deep earthquakes (≥100 km depth) in the Tonga‐Kermadec subduction zone. Identifying the fault planes for 25 of them, we are able to place new constraints on both the physical mechanism of intermediate‐ and deep‐focus earthquakes and deformation within the subducting slab. We find that half of deep earthquakes with MW ≥ 6 have detectable directivity. We compare the obtained fault orientations with those expected for the reactivation of outer‐rise normal faults and with those expected for the creation of new faults in response to the ambient stress field. Earthquakes &gt;300 km depth match the patterns expected for the creation of a new system of faults: we observe both subhorizontal and subvertical fault planes consistent with a downdip‐compressional stress field. Slip along these faults causes the slab to thicken. Rupture propagation shows no systematic directional pattern. In contrast, at intermediate depths (100–300 km), all ruptures propagate subhorizontally and all identified fault planes, whether in the upper or lower region of the double seismic zone, are subhorizontal. Rupture propagation tends to be directed away from the top surface of the slab. After accounting for the angle of subduction, the subhorizontal fault plane orientation is inconsistent with the orientation of outer‐rise normal faults, allowing us to rule out mechanisms that require the reactivation of these large surface faults. Subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field, suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation. If all deformation takes place on these subhorizontal faults, it would cause the slab to thin. Assuming the slab is incompressible, this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths.