Minor Normal Faults Research Articles

Three-Dimensional Integrated Geo-Static Modeling for Prospect Identification and Reserve Estimation in the Middle Miocene Multi-Reservoirs: A Case Study from Amal Field, Southern Gulf of Suez Province

Middle Miocene reservoirs in the southern part of the Gulf of Suez province are characterized by geometrical uncertainties due to their structural settings, lateral facies change, different lithologies, and diverse reservoir quality. Therefore, in this study, detailed 3D geo-static models were constructed by integrating multiple datasets, including 2D seismic sections and digital well-logs. The 3D models were constructed for the Belayim Formation (Hammam Faraun Member), Kareem Formation (Markha Member), and Rudies Formation (Upper Rudies Member) with detailed structuration, zonation, and layering for Amal Field in the southern Gulf of Suez province to assess the hydrocarbon potential, calculate accurate reserves, recommend development and exploration plans, and propose locations for future drilling. The resultant structural model exhibited a compartmentalized area of major and minor normal faults trending NW–SE, forming structurally high potential hydrocarbon trapping locations in the study area. The petrophysical models indicated the good potentiality of Hammam Faraun as a reservoir with porosity values of 15–23%, increasing towards the central part of the area, volume of shale (Vsh) of 21–31%, water saturation (Sw) of 34–49%, and sand thickness increasing toward the northeastern part of the area. The Markha Member was also interpreted as a good reservoir, with porosity values of 15–22%, increasing towards the southeastern part of the area, Vsh of 13–29%, Sw of 16–38%, and sandy facies accumulating in the central horst block. Upper Rudies exhibits good reservoir properties with porosity values of 16–23%, Vsh of 29–37%, Sw of 35–40%, and good sandy facies in the central horst block of the area. The study results showed hydrocarbon potential in the central horst block of the study area for the Middle Miocene multi-reservoirs.

Aftershock study of the 2019 Ambon earthquake using moment tensor inversion: identification of fault reactivation in northern Banda, Indonesia

On September 26, 2019, an Mw 6.5 earthquake occurred 23 km northeast of Ambon City, Indonesia, followed by numerous aftershock series related to a complex fault network reactivation in the Ambon and Seram region. Using moment tensor inversion, we identify the kinematics of fault reactivation based on the focal mechanism solution of 20 aftershocks with Mw > 3.2 and analyze the earthquake sequence from both focal mechanism solutions and spatiotemporal seismicity. The MTs solution of aftershocks revealed three different characteristics of fault reactivation: (i) a 35 km long N-S oriented main fault characterized by dextral strike-slip (ii) a NE-SW reverse fault segment with a ~ 55° northeastward dip located in southwest Seram, and (iii) two strike-slip segments (NNW-SSE and NNE-SSW trends) and an E-W normal fault in Ambon Island. Analysis of spatiotemporal seismicity with the MTs solution suggests that the Mw 6.5 Ambon aftershock sequences can be described as follows: (i) an Mw 6.5 mainshock rupture that was primarily made up of a major strike-slip component and an insignificant minor normal fault; (ii) first aftershock cluster propagate along the main N-S ruptures, followed by the strike-slip and normal cluster in Ambon Island (iii) The reverse fault events cluster appeared next in Southwest Seram. The presence of complex strike-slip segments in Ambon agrees with the regional structure trends in Halmahera, located in the north of the study area, while the E-W oriented normal fault might be related to the eastward velocity increase in Banda Arc, which causes extensional deformation. Given that the fault reactivation identified in Ambon and Seram is located close to the densely populated urban regions of Ambon City and Kairatu, the analysis of future seismic hazards related to this fault reactivation should consider the risks in a region with complex fault settings.Graphical

A Review of the Global Polygonal Faults: Are They Playing a Big Role in Fluid Migration?

Polygonal faults (PFs) have been widely found in over 100 sedimentary basins worldwide, mainly in marine setting on continental margins and intracratonic sedimentary basins. PFs are characterized by layer-bound minor normal faults arranged in polygonal patterns and multi-direction strikes, most of which were developed in fine-grained sediments rich in claystone. The genesis mechanisms of PFs have been recognized as non-tectonic, mainly including shear failure, syneresis, density inversion, low coefficients of friction and gravity sliding theory. Previous studies often regarded PFs as conduits playing a role in favouring fluid migration, which were also regarded as fluid source expelled during PF generation. However, some researchers proposed that the PF-bearing layer is generally impermeable and may act as a seal. To better understand the role of PFs in fluid migration and their contrition to the shallower hydrocarbon accumulation, a geophysical review of global PFs case studies is conducted in this paper. A comprehensive analysis is carried out based on abundant case studies, with key parameters analyzed including the PFs characteristics (e.g., buried depth, lithology, size, etc.), formation mechanisms of PFs, tectonic-sedimentary setting, and the spatial relationship of PFs, nearby major fluid migration pathways and high amplitude anomalies. The compilation of global PF case studies of shows that hydrocarbon fluids from the deep is more likely to migrate upward through normal big faults, gas chimneys or unconformities surface around PFs. In most cases, PFs play limited roles in fluid migration, such as in South China Sea where hydrocarbon accumulations have been observed under the PFs and the PF-bearing layer may act as a seal trapping the free gas underneath. Generally, the role of PFs in fluid migration should be determined through a comprehensive analysis of the characteristics of PFs, surrounding amplitude anomalies, hydrocarbon distribution, and even the sealing ability of PFs which needs further study. It is more likely that the PFs play a limited role in vertical fluid migration and related shallower hydrocarbon accumulation and distribution.

Interplay between base‐salt relief, progradational sediment loading and salt tectonics in the Nordkapp Basin, Barents Sea – Part II

AbstractReprocessed, regional, 2D seismic reflection profiles, 3D seismic volumes and well data (exploration and shallow boreholes), combined with 2D structural restorations and 1D backstripping were used to study the post‐salt evolution of the Nordkapp Basin in Barents Sea. The post‐salt evolution took place above a pre‐salt basement and basin configuration affected by multiple rift events that influenced the depositional facies and thickness of Pennsylvanian‐lower Permian‐layered evaporite sequence. Initially, regional mid‐late Permian extension reactivated pre‐salt Carboniferous faults, caused minor normal faulting in the Permian strata and triggered slight salt mobilization towards structural highs. The main phase of salt mobilization occurred during earliest Triassic when thick and rapidly prograding sediments entered from the east into the Nordkapp Basin. In the early‐mid‐Triassic, the change in the direction of progradation and sediment entry‐points shifted to the NW led to rotation of the earlier‐formed mini‐basins and shift of dominant salt evacuation direction to the south. The prograding sediment influx direction, sediment transport velocity and thickness influenced the dynamics of the early to late passive diapirism, salt expulsion and depletion along the strike of the basin. The basin topography resulting from salt highs and mini‐basin lows strongly affected the Triassic progradational fairways and determined the distinct sediment routing patterns. Minor rejuvenation of the salt structures and rotation of the mini‐basins took place at the Triassic–Jurassic transition, due to far‐field stresses caused by the evolving Novaya Zemlya fold‐and‐thrust belt to the east. This rejuvenation influenced the sediment dispersal routings and caused formation of dwarf secondary mini‐basins. The second and main rejuvenation phase took place during likely early‐mid‐Eocene when propagated far‐field stresses from the transpressional Eurekan/Spitsbergen orogeny to the NW inverted pre‐salt normal faults, reactivated the structural highs and rejuvenated the salt structures. The studied processes and study outcomes can be applicable to other evaporite‐dominated basins worldwide.

Structural evolution and the partitioning of deformation during basin growth and inversion: A case study from the Mizen Basin Celtic Sea, offshore Ireland

AbstractThe Celtic Sea basins lie on the continental shelf between Ireland and northwest France and consist of a series of ENE–WSW trending elongate basins that extend from St George’s Channel Basin in the east to the Fastnet Basin in the west. The basins, which contain Triassic to Neogene stratigraphic sequences, evolved through a complex geological history that includes multiple Mesozoic rift stages and later Cenozoic inversion. The Mizen Basin represents the NW termination of the Celtic Sea basins and consists of two NE–SW‐trending half‐grabens developed as a result of the reactivation of Palaeozoic (Caledonian, Lower Carboniferous and Variscan) faults. The faults bounding the Mizen Basin were active as normal faults from Early Triassic to Late Cretaceous times. Most of the fault displacement took place during Berriasian to Hauterivian (Early Cretaceous) times, with a NW–SE direction of extension. A later phase of Aptian to Cenomanian (Early to Late Cretaceous) N–S‐oriented extension gave rise to E–W‐striking minor normal faults and reactivation of the pre‐existing basin bounding faults that propagated upwards as left‐stepping arrays of segmented normal faults. In common with most of the Celtic Sea basins, the Mizen Basin experienced a period of major erosion, attributed to tectonic uplift, during the Paleocene. Approximately N–S Alpine regional compression‐causing basin inversion is dated as Middle Eocene to Miocene by a well‐preserved syn‐inversion stratigraphy. Reverse reactivation of the basin bounding faults was broadly synchronous with the formation of a set of near‐orthogonal NW–SE dextral strike‐slip faults so that compression was partitioned onto two fault sets, the geometrical configuration of which is partly inherited from Palaeozoic basement structure. The segmented character of the fault forming the southern boundary of the Mizen Basin was preserved during Alpine inversion so that Cenozoic reverse displacement distribution on syn‐inversion horizons mirrors the earlier extensional displacements. Segmentation of normal faults therefore controls the geometry and location of inversion structures, including inversion anticlines and the back rotation of earlier relay ramps.

Seismic Structural Analysis of the Alam-El Bueib Formation, Qasr Oil Field, North Western desert, Egypt

The Qasr oil and gas Field is located in the north western desert of Egypt. It belongs to the southeastern part of the Lower Jurassic-Cretaceous Shushan Basin. The Lower Cretaceous Alam-El Bueib formation composed of clastic rocks with noticeable carbonate proportions, and forms multiple oil-bearing sandstone reservoirs in Qasr field. The study aims to define and analyze the Surface and subsurface structural features which are a key issue in assessing reservoir quality. Through this integrated approach, one may be able to identify lithologies and fluids in this region and provide possibly new hydrocarbon fairways for exploration. For this purpose, seismic and well data were interpreted and mapped in order to visualize the subsurface structure of the Cretaceous section. Results show the effect of NE-SW, NW-SE, and E-W trending normal faulting on the Lower Cretaceous Alam-El Bueib formation and is extended to the Upper Cretaceous Abu Roash Formation. The effect of folding is minimal but can be detected. These normal faults are related to the extensional tectonics which affected the north western desert of Egypt during the Mesozoic. One reverse fault is detected in the eastern part and is related mostly to the inversion tectonics in the Late Mesozoic. The depth structure contour maps of the Alam-El Bueib horizons (AEB-1, AEB-3A, and AEB-3D) show several major normal faults trending NE-SW and minor normal faults trending NW-SE. One larger branching normal fault trending E-W and lies to the south of the study area. These step-normal faults divide the area into a number of tilted structural blocks which are shallower in the south and deepen to the north. The area of study was most probably affected by E-W trending normal faults during the opening of the Atlantic Ocean in the Jurassic. Later right-lateral compression resulted from the movement of Laurasia against North Africa, changed their trend into NE-SW faults with minor NW-SE trending folds. These compressive stresses are also responsible for the reverse faulting resulted by inversion in the Late Mesozoic.

Mesozoic halokinesis and basement inheritance in the eastern Provence fold-thrust belt, SE France

The Provence Chain incorporated preexisting halokinetic and basement features which have played a key role in the structural evolution of the thrust systems. Field structural data, previously published geological maps and exploration well data have been used to interpret a new ~80 km-long balanced and restored cross section across the eastern Provence fold-thrust belt. The geological data and cross section construction suggest evidence for Mesozoic salt domes and minibasins embedded in the thin-skinned thrusts, and upper Paleozoic extensional structures transported on new thick-skinned thrusts. According to field data, pre-contractional palinspastic restoration suggests that salt domes and minibasins grew mainly during Rhaetien to late Jurassic times and intermittently until the early Santonian. Minibasins are mainly filled by carbonate systems associated with gravitational instabilities in minor normal faulting. Minibasin sinking into middle-upper Triassic evaporitic-carbonate succession has been mainly controlled by sediment loading. However, the initiation of salt movements might have been controlled by the initial geometry of the Triassic strata and probably extensional basement faulting. Preexisting salt structures have controlled thrust emplacements and their kinematics. Synorogenic deposits indicate a classical hinterland to foreland sequence of thrust inversion of the margin during the upper Cretaceous-Eocene Pyrenean-Provence compression. Diapiric structures were mainly eroded during and after the Pyrenean-Provence compression then reactivated during Oligocene extension and Miocene to present-day Alpine compression. Cross section balancing shows a total horizontal shortening of 28.5 km related to the Pyrenean-Provence and Alpine compressions. The eastern Provence fold-thrust belt integrates preexisting halokinetic folding which can be misinterpreted as resulting to compression. This suggests that a significant amount of folding in fold-thrust belts can results from an early halokinetic fold system developed during the pre-contractional passive margin evolution.

The hydrothermal uranium and some other metal deposits of the extensional faulting during the advanced opening of the Red Sea, Central Eastern Desert, Egypt

The hydrothermal uranium and some other metal deposits occur within the zone of the major NW-SE trending normal fault and its associated minor normal faults that cut the Miocene clastic-carbonate sediments at Ochre-Um Greifat and Wadi Wizr locations. Samples from these locations were subjected to mineralogical and geochemical studies in order to determine their concentrations of strategic and economic elements (e.g., U, Th, Mo, Zn, Pb, As and V) as well as their petrogenesis. The mineralogical studies were confirmed by X-ray diffraction (XRD) and the environmental scanning electron microscopy coupled with energy dispersive spectroscopy ESEM/EDX. The geochemical data was determined by ICP-MS and ICP-OES (Acme Lab. of Canada). The results showed that the Ochre-Um Greifat deposits are enriched in U and Zn elements, with significant concentrations of Cu and Th while Wadi Wizr deposits are enriched in Mo, Zn, Pb and As elements. The discrimination diagrams based on major and trace elements indicated that the Ochre-Um Greifat and Wadi Wizr deposits are of hydrothermal origin. Additionally, uranium deposit was subjected to the supergene processes. Moreover, radionuclides showed that the state of disequilibrium as a result of physicochemical conditions of the hydrothermal solutions and the nature of ambient rocks. The study concluded that the locations need further geological and geophysical studies to determine if nuclear materials (uranium and molybdenum) and other associated elements are economically valuable.

3D Seismic Structural Study of the Lower Cretaceous Sequence in Kifl oil field_ central Iraq

This research focused up on the Seismic reflection study of a 268.7 km2 area located in the central of Iraq within the Karbala province (Kifl area). The seismic data were reprocessed by Anadarko Petroleum Corporation and its partners Dome International and Vitol with OEC in 2005, the result of processing works has improved the quality of seismic sections in most of the study area. The study area was interpreted by using 3-D seismic data from the Oil Exploration Company. By studying the seismic sections and applying seismic attributes (instantaneous phase) Faults were picked across 3D seismic volume of the studied reflectors. The study area affected by a major fault and minor normal faults, Two fault system has been observed in the study area; the major normal fault of (NW-SE) trending and minor normal faults of (NE-SW) trending, with a small displacement are influencing the studied reflectors (NahrUmr, Shuaiba, Zubair and Ratawi reflectors). Time, velocity and depth maps are prepared depending on the structural interpretation of the picked reflectors, The structural interpretation of these reflectors shows a structural anticline (Asymmetrical anticline) extending in NW-SE trend and plunges to the southeast with a dip angle about 5 degrees. and the general dip towards the east.

The nature and origin of bed-parallel slip in Kardia Mine, Ptolemais Basin, Greece

Normal faulting in Kardia Lignite Mine within the Pliocene-Quaternary Ptolemais Basin, Northern Greece, was associated with broadly contemporaneous top to the north bed-parallel slip arising from hangingwall rollover on a km-scale displacement normal fault outside the mine. The direction of bed-parallel slip is perpendicular to the strike of minor normal faults (<50 m displacement), which therefore provide markers for measuring the magnitude of bed-parallel slip. Associated slip data measured during 6 years of mine extraction provide constraints on the nature and kinematics of slip distributions over bed-parallel slip-surfaces, a generic issue which has previously not been possible to analyse for other extensional fault systems.Bed-parallel slip gradients lie within the range of fault displacement gradients observed in global datasets, but are lower than the gradients developed on normal faults in Kardia mine. Bed-parallel slip-surfaces are segmented both parallel and normal to the slip direction and form both soft- and hard-linked relay zones on a wide range of scales. Zones of layer-bound minor normal faults, forming extensional duplexes or arrays of “bookshelf” faults between bounding slip-surfaces, accommodate the transfer of slip across associated releasing relay zones.

Complexity of hydrogeologic regime around an ancient low‐angle thrust fault revealed by multidisciplinary field study

AbstractCo‐located and integrated observation of the surface and subsurface is necessary to characterize fault zone hydrogeology. The spectacular cliff‐face exposure of the Champlain Thrust fault at Lone Rock Point, Vermont, and a nearby well‐field site provides the opportunity for co‐located structural and hydrogeologic field observations. We mapped the prominent structural features of the Champlain Thrust fault and discrete groundwater seeps in outcrop, and also drilled through the fault near the outcrop and determined aquifer parameters from aquifer pumping tests. In outcrop, the fault core thickness varies on the meter scale, splays out into multiple strands, and is offset by a minor normal fault. Groundwater seeps are prevalent in the heavily fractured footwall, but limited in the fault core and hanging wall, suggesting that at the cliff face the water table is generally near the fault core and groundwater flow in the hanging wall is limited. Enrichment of more soluble minerals in cemented fault rock associated with older strands of the fault system may play an important role in localizing karst features in the hanging wall. At the well‐field site, the Champlain Thrust fault is offset significantly by a high‐angle normal fault, the water table is near the surface, and aquifer pumping tests reveal a complex hydrogeologic system, with karst and steep fractures as strong hydraulic conduits in the hanging wall and fault core. The most salient features of the fault zone hydrogeology in the surface and subsurface data are different, but can be integrated into a preliminary conceptual model. Together, the surface and subsurface methods underscore and emphasize the complexity and heterogeneity of the hydrogeology of this low‐angle sedimentary fault.

Magmatic cycles pace tectonic and morphological expression of rifting (Afar depression, Ethiopia)

The existence of narrow axial volcanic zones of mid-oceanic ridges testifies of the underlying concentration of both melt distribution and tectonic strain. As a result of repeated diking and faulting, axial volcanic zones therefore represent a spectacular topographic expression of plate divergence. However, the submarine location of oceanic ridges makes it difficult to constrain the interplay between tectonic and magmatic processes in time and space. In this study, we use the Dabbahu–Manda Hararo (DMH) magmatic rift segment (Afar, Ethiopia) to provide quantitative constraints on the response of tectonic processes to variations in magma supply at divergent plate boundaries. The DMH magmatic rift segment is considered an analogue of an oceanic ridge, exhibiting a fault pattern, extension rate and topographic relief comparable to intermediate- to slow-spreading ridges. Here, we focus on the northern and central parts of DMH rift, where we present quantitative slip rates for the past 40 kyr for major and minor normal fault scarps in the vicinity of a recent (September 2005) dike intrusion. The data obtained show that the axial valley topography has been created by enhanced slip rates that occurred during periods of limited volcanism, suggestive of reduced magmatic activity, probably in association with changes in strain distribution in the crust. Our results indicate that the development of the axial valley topography has been regulated by the lifetimes of the magma reservoirs and their spatial distribution along the segment, and thus to the magmatic cycles of replenishment/differentiation (<100 kyr). Our findings are also consistent with magma-induced deformation in magma-rich rift segments. The record of two tectonic events of metric vertical amplitude on the fault that accommodated the most part of surface displacement during the 2005 dike intrusion suggests that the latter type of intrusion occurs roughly every 10 kyr in the northern part of the DMH segment.

PALEOSTRESS ANALYSIS OF KAROO SUPERGROUP OF THE TSHIPISE-PAFURI BASIN, SOUTH AFRICA

The Tshipise-Pafuri basin is located in the Limpopo Province, north-eastern South Africa. This paper presents the first paleostress-analysis results from fault-slip data on the sedimentary rocks of Karoo Supergroup of late Palaeozoic to early Mesozoic in Tshipise-Pafuri basin. Stress inversion of 251 fault-slip data was performed using an improved Right-Dihedral method, followed by rotational optimization (WINTENSOR Program, Delvaux, 2012). Fault-slip data including fault planes, striations and sense of movements, were obtained from rock exposures distributed in ten stations in the basin. The orientation of the principal stress axes (σ1, σ2 and σ3) and the ratio of the principal stress differences (R) show a paleostress field marking two main stress regimes. The main strain field in the Tshipise-Pafuri basin is characterised by west to east to east-northeast to west-southwest extension and the second strain field is a minor north to south to west-northwest to east-southeast reactivated extension. It includes two paleostress regimes, one with dominantly dip-slip normal faulting (extensional regime) and a minor normal faulting (extensional regime). The north-eastern margin of the Kaapvaal craton was down-faulted into a graben structure, in which the pre-Karoo Soutpansberg Group was deposited. This faulting, which controlled graben formation, continued during the deposition of the Karoo sediments and was reactivated in post-Karoo times, resulting in a very complex structural setting. The east-west trends of the stress fields in the basin follows the east-west elongated low lying Limpopo Mobile Belt.

The tectonic evolution of the southeastern Terceira Rift/São Miguel region (Azores)

The eastern Azores Archipelago with São Miguel being the dominant subaerial structure is located at the intersection of an oceanic rift (Terceira Rift) with a major transform fault (Gloria Fault) representing the westernmost part of the Nubian–Eurasian plate boundary. The evolution of islands, bathymetric highs and basin margins involves strong volcanism, but the controlling geodynamic and tectonic processes are currently under debate. In order to study this evolution, multibeam bathymetry and marine seismic reflection data were collected to image faults and stratigraphy. The basins of the southeastern Terceira Rift are rift valleys whose southwestern and northeastern margins are defined by few major normal faults and several minor normal faults, respectively. Since São Miguel in between the rift valleys shows an unusual W–E orientation, it is supposed to be located on a leaky transform. South of the island and separated by a N120° trending graben system, the Monacco Bank represents a N160° oriented flat topped volcanic ridge dominated by tilted fault blocks. Up to six seismic units are interpreted for each basin. Although volcanic ridges hamper a direct linking of depositional strata between the rift and adjacent basins, the individual seismic stratigraphic units have distinct characteristics. Using these units to provide a consistent relative chrono-stratigraphic scheme for the entire study area, we suggest that the evolution of the southeastern Terceira Rift occurred in two stages. Considering age constrains from previous studies, we conclude that N140° structures developed orthogonal to the SW–NE direction of plate-tectonic extension before ~10Ma. The N160° trending volcanic ridges and faults developed later as the plate tectonic spreading direction changed to WSW–ENE. Hence, the evolution of the southeastern Terceira Rift domain is predominantly controlled by plate kinematics and lithospheric stress forming a kind of a re-organized rift system.

Fluid generation and distribution in the highest sediment input accretionary margin, the Makran

Fluids in subduction zones can influence seismogenic behaviour and prism morphology. The Eastern Makran subduction zone, offshore Pakistan, has a very thick incoming sediment section of up to 7.5 km, providing a large potential fluid source to the accretionary prism. A hydrate-related bottom simulating reflector (BSR), zones of high amplitude reflectivity, seafloor seep sites and reflective thrust faults are present across the accretionary prism, indicating the presence of fluids and suggesting active fluid migration. High amplitude free gas zones and seep sites are primarily associated with anticlinal hinge traps, and fluids here appear to be sourced from shallow biogenic sources and migrate to the seafloor along minor normal faults. There are no observed seep sites associated with the surface expression of the wedge thrust faults, potentially due to burial of the surface trace by failure of the steep thrust ridge slopes. Thrust fault reflectivity is restricted to the upper 3 km of sediment and the deeper décollement is non-reflective. We interpret that fluids and overpressure are not common in the deeper stratigraphic section. Thermal modelling of sediments at the deformation front suggests that the deeper sediment section is relatively dewatered and not currently contributing to fluid expulsion in the Makran accretionary prism.

Identification of the subsurface sulfide bodies responsible for acidity in Río Tinto source water, Spain

The acidic waters of the Río Tinto rise from several acidic springs that emerge in the area surrounding Peña de Hierro (Fernández-Remolar et al., 2005). These springs are located above minor normal faults that act as natural conduits for the water from the underlying deep aquifer. Although it has been suggested that the acidity of the river originates from the biooxidation of massive and stockwork sulfides (Fernández-Remolar et al., 2008a), the location of the source for these acidic solutions has not previously been established. This lack of evidence has been used to suggest that the acidity of the Río Tinto may be the product of the most conspicuous of the possible source, the extensive mining of the area over approximately the last 5000 years (Davis et al., 2000). In this paper, we report resistivity and time-domain electromagnetic sounding data from the Río Tinto aquifer to a depth of ∼600 m, revealing the locations for the acidic sources. Both types of data support the presence of two distinct geological units that we interpret as thrust sheets emplaced onto each other during the Variscan orogeny of the Carboniferous. These units, both of which contain massive and stockwork sulfides, act as the aquifer for the acidic waters of the Río Tinto. Under this scenario, which is in agreement with the geological record of the Río Tinto fluvial system for the past 6 Ma (Moreno et al., 2003), our results imply that mining activity had little influence on the generation of the acidic river waters.

Shallow gas and focused fluid flow systems in the Pearl River Mouth Basin, northern South China Sea

Analysis of three-dimensional seismic data and multibeam data from the Pearl River Mouth Basin in the northern South China Sea, reveals numerous focused fluid flow features and associated widespread shallow gas from 500m to 2000m water depth. Shallow gas is usually indicated by acoustic anomalies, such as enhanced reflections, acoustic turbidity, and acoustic blanking. Two types of tectonically and stratigraphically controlled fluid flow related systems are identified. The first type can be observed in the deep strata, including gas chimneys, mud diapirs, mud volcanoes, pipes, and large normal faults, which are usually located over basement highs and are related to overpressure mainly caused by gas generation. The focused fluid flow structures serve as pathways for upward migration of thermogenic fluids from source rocks or gas reservoirs. The second type is a shallow fluid flow system consisting of minor normal faults, migrating canyons, mass transport deposits, contourite, and pockmarks. The minor faults and migrating canyons provide pathways for fluids of thermogenic origin transported by the deep fluid flow system and of biogenic origin generated in the shallow succession. The mass transport deposits and contourite mainly serve as caprock. Small-scale fluid seepage is observed at the modern seabed expressed as pockmarks located above shallow gas accumulations. The distribution of shallow gas is controlled by the combination of these two systems. The focused fluid flow features and shallow gas are poorly studied in the study area and we propose a 3D model of focused fluid flow and shallow gas distribution that can be used more widely in other passive and active continental margins. Our results are important to the understanding of resources (hydrocarbon and gas hydrate) exploration in such a petroliferous basin and they must be taken into account when assessing seabed stability.

The 2009 L’Aquila (Central Italy) Seismic Sequence.

On April 6 (01:32 UTC) 2009 a MW 6.1 normal faulting earthquake struck the axial area of the Abruzzo region in central Italy. The earthquake heavily damaged the city of L'Aquila and its surroundings, causing 308 casualties, 70,000 evacuees and incalculable losses to the cultural heritage. We present the geometry of the fault system composed of two main normal fault planes, reconstructed by means of seismicity distribution: almost 3000 events with ML≥1.9 occurred in the area during 2009. The events have been located with a 1D velocity model we computed for the area by using data of the seismic sequence. The mainshock, located at around a 9.3 km depth beneath the town of L'Aquila, activated a 50o (+/- 3) SW-dipping and ~135o NW-trending normal fault with a length of about 16 km. The aftershocks activated the whole 10 km of the upper crust up to the surface. The geometry of the fault is coherent with the mapped San Demetrio-Paganica and Mt. Stabiata normal faults. The whole normal fault system that reached about 40 km of length by the end of December in the NW-trending direction, was activated within the first few days of the sequence when most of the energetic events occurred. The main shock fault plane was activated by a foreshock sequence that culminated with a MW 4.0 on March 30 (13:38 UTC), showing extensional kinematics with a minor left lateral component. The second major structure, located to the north close to Campotosto village, is controlled by an MW 5.0 event, which occurred on the same day of the main shock (April 6 at 23:15 UTC), and by an MW 5.2 event (April 9 at 00:53 UTC). The fault plane shows a shallower dip angle with respect to the main fault plane, of about 35o with a tendency to flattening towards the deepest portion. Due to the lack of seismicity above a 5 km depth, the connection between this structure and the mapped Monti della Laga fault is not straightforward. This northern segment is recognisable for about 12-14 km of length, always NW-trending and forming a right lateral step with the main fault plane. The result is a en-echelon system overlapping for about 6 km. The seismicity pattern also highlights the activation of numerous minor normal fault segments within the whole fault system. The deepest is located at around a 13-15 km depth, south of the L'Aquila mainshock, and it seems to be antithetic to the main fault plane.

Quaternary tectonics in the central Interandean Valley, Ecuador: Fault-propagation folds, transfer faults and the Cotopaxi Volcano

We studied the Quaternary tectonics of the central Interandean Valley (IV, Ecuador), around the active Cotopaxi volcano, by field geological–structural survey, analysis of seismicity, precise levelling of river terraces and numerical modelling. North of the volcano, there are main Quaternary west-dipping reverse faults located along the western side of the valley. At the Cotopaxi foothills, we found NNE-SSW-striking, vertical, right-lateral oblique strike-slip faults and E-W-striking normal faults, which offset Pleistocene deposits. South of the volcano, there are several Quaternary folds that show minor normal faults at the hinge zone linked to local extension at the extrados. The core of the folds sometimes shows reverse faults. Most of the folds have flexure geometry whereas the Guambalo fold shows a double flexure resembling a huge box fold. By numerical modelling we computed the best solution that fits the observed geometry as fault-propagation folds. The fold system shows a double vergence linked to reverse faults dipping to the west along the western side of the IV and to the east along the eastern side. Computed stress tensors by striated fault inversion indicate a Quaternary E-W-trending horizontal greatest principal stress (σ1) and a vertical least principal stress (σ3) south of Cotopaxi, whereas at the volcano σ1 is ENE-WSW and σ3 is also horizontal. At the Yanayacu flexure we measured an increment of deformation along late Quaternary river deposits, suggesting a potentially very recent activity. These results are consistent with the distribution of crustal seismicity. We consider a larger Quaternary tectonic shortening south of the volcano than to the north of it, with the transcurrent faults acting as transfer structures. Magma emplacement at Cotopaxi in this compressional setting has been favoured by the horizontal σ3 linked to the transfer fault zone.

Separate Eocene–Early Oligocene and Miocene stages of extension and core complex formation in the Western Rhodopes, Mesta Basin, and Pirin Mountains (Bulgaria)

The Rhodope Metamorphic Province in Bulgaria and Northern Greece has been affected by significant extensional tectonics since the Middle to Late Eocene. An important fault system active in the Eocene and Early Oligocene includes the Ribnovo Fault on the eastern side of the Mesta Basin in Bulgaria and the Vertiskos–Kerdilion Fault in Greece. Together with several minor normal fault relicts identified during this study, these represent an originally west-southwest-dipping, low-angle (at least at the end of faulting) normal fault with greenschist-facies mylonites in the footwall and cataclasites along the fault plane, the Mesta–Kerdilion Detachment, exposed over ca. 150 km along strike and about 50 km parallel to the slip direction. During the intrusion of several plutons in the Pirin Mountains at ca. 32 Ma, the footwall of the fault was uplifted to form a large anticline parallel to fault strike, and the fault was offset by a system of antithetic, northeast-dipping normal faults along the northeastern flank of this anticline (Dobrotino and Breznitsa faults). The Mesta–Kerdilion Detachment was later, in the Miocene, again crosscut and offset by the southwest-dipping Strimon Valley Detachment which accommodated important, core complex-like exhumation to the South, strongly diminishing and finally ceasing towards north. This rotational activity of the Strimon Valley Detachment represents the onset of the extension that led to opening of the Aegean Basin. The Mesta–Kerdilion Detachment can be viewed as a precursor of this, but with slightly different kinematics (i.e. not involving significant vertical-axis rotation) and separated in time from the following events by a phase of relative tectonic quiescence in the Late Oligocene.