NE Trending Research Articles

Structure and emplacement model for late-orogenic Paleoproterozoic granitoids: the Tenkodogo–Yamba elongate pluton (Eastern Burkina Faso)

The Tenkodogo–Yamba (TY) elongate pluton is made of apparently isotropic biotite-bearing granites that form a continuous, 125 km long and NE trending, and 15–20 km wide, succession of granite bodies that intruded the so-called batholith of eastern Burkina Faso dominated by foliated granitoids and associated volcano-sedimentary belts. Geochemically, the granitoids of the batholith have well-defined TTG affinities that characterize the “gneiss-granitoids” of the Paleoproterozoic basement of West Africa. The biotite granites of the TY-elongate pluton, that display the so-called “basin” affinity, seem to be derived from the (partial) remelting of the batholith. The internal microstructures of the TY-elongate pluton are mostly purely magmatic, contrasting with the magmatic to high-temperature solid-state microstructures of the batholith. The systematic anisotropy of magnetic susceptibility study undertaken in the TY-elongate pluton reveals that the magmatic foliations cross-cut the foliations of the batholith and locally define concentric trajectories. The magmatic lineations have predominant steep plunges and well-defined subareas ascribed to magma feeders which can be delineated. The overall NE- to NNW-trending trajectories of both foliations and lineations, independent of structures of the batholith, form contacts with it and form subdivisions into subplutons inside the alignment, clearly depict alignment-scale dextral sigmoids. The latter are interpreted as being formed during a dextral, NE-trending regional shearing parallel to the alignment, that occurred during emplacement of the biotite granites concerned. This study suggests that the ≈2.2 Ga TTGs, which form most of the Birimian terrains of this part of West Africa, were rapidly cooled and reached a brittle behaviour before being passively intruded, a few tens of million years later, by a new generation of granites, derived from partial remelting of the deep basement, during a regional-scale dextral wrench event. The present picture of the alignment is concluded to result from subsequent dissection into subareas along a set of late E-trending dextral faults.

Hydrogeological investigations on Balçova geothermal system in Turkey

The Balçova geothermal system is situated within the confines of Izmir City, Turkey. It is located along the Izmir Fault zone, trending E–W. It has formed a hot water reservoir that is currently exploited for district heating of the nearby Balçova-Narlidere area. Geoscientific studies on the area around the Balçova geothermal field indicate that regional tectonics coupled with a major fault system has created NW–N–NE trending permeable pathways for a geothermal system, starting from a delimited intake area on the Seferihisar Horst in the south to the outflowing area in the north. The Balçova geothermal system is a fracture zone system in which hot water ascends over an area of about 2 km along a major fracture zone associated with the Agamemnon Fault, reaching almost boiling temperatures close to the surface. The hot water discharges via two concealed horizontal flows, one in the alluvium (upper 100 m) and another deeper one in more permeable, ill-defined layers in the flysch formation between 400 and 700 m depth. The two concealed outflows and the steeply dipping fracture zone constitute the presently exploited reservoir, which extends for as much as 1.5 km from the feeding fracture zone. The natural direct discharge of the system was 2.5 kg/s; the surface heat discharge of the system is estimated at a minimum of 9.5 MW t. The average max. total production rate of all the production wells was 135 kg/s during the 2000–2001 heating season. The contribution of recharge to Balçova geothermal field production was estimated at about 50 kg/s.

Fault plane solutions of the january 26th, 2001 bhuj earthquake sequence

A 12-station temporary microearthquake network was established by the Geological Survey of India for aftershock monitoring of the January 26th, 2001 Bhuj earthquake (M w 7.6) in the Kutch district of Gujarat state, western India. The epicentres of the aftershocks show two major trends: one in the NE direction and the other in the NW direction. Fault-plane solutions of the best-located and selected cluster of events that occurred along the NE trend, at a depth of 15–38 km, show reverse faulting with a large left-lateral strike-slip motion, which are comparable with the main-shock solution. The NW trending upper crustal aftershocks at depth <10 km, on the other hand, show reverse faulting with right-lateral strike-slip motion, and the mid crustal and lower crustal aftershocks, at a depth of 15–38 km, show pure reverse faulting as well as reverse faulting with right-lateral and left-lateral strike-slip motions; these solutions are not comparable with the main-shock solution. It is inferred that the intersection of two faults has been the source area for stress concentration to generate the main shock and the aftershocks.

Study of the epicentral trends and depth sections for aftershocks of the 26th january 2001, Bhuj earthquake in Western India

The Geological Survey of India (GSI) established a twelve-station temporary microearthquake (MEQ) network to monitor the aftershocks in the epicenter area of the Bhuj earthquake (M w7.5) of 26th January 2001. The main shock occurred in the Kutch rift basin with the epicenter to the north of Bhachao village, at an estimated depth of 25 km (IMD). About 3000 aftershocks (M d ≥ 1.0), were recorded by the GSI network over a monitoring period of about two and half months from 29th January 2001 to 15th April 2001. About 800 aftershocks (M d ≥ 2.0) are located in this study. The epicenters are clustered in an area 60 km × 30 km, between 23.3‡N and 23.6‡N and 70‡E and 70.6‡E. The main shock epicenter is also located within this zone.Two major aftershock trends are observed; one in the NE direction and other in the NW direction. Out of these two trends, the NE trend was more pronounced with depth. The major NE-SW trend is parallel to the Anjar-Rapar lineament. The other trend along NW-SE is parallel to the Bhachao lineament. The aftershocks at a shallower depth (<10km) are aligned only along the NW-SE direction. The depth slice at 10 km to 20 km shows both the NE-SW trend and the NW-SE trend. At greater depth (20 km–38 km) the NE-SW trend becomes more predominant. This observation suggests that the major rupture of the main shock took place at a depth level more than 20 km; it propagated along the NE-SW direction, and a conjugate rupture followed the NW-SE direction.A N-S depth section of the aftershocks shows that some aftershocks are clustered at shallower depth ≤ 10 km, but intense activity is observed at 15–38 km depth. There is almost an aseismic layer at 10–15 km depth. The activity is sparse below 38 km. The estimated depth of the main shock at 25 km is consistent with the cluster of maximum number of the aftershocks at 20–38 km. A NW-SE depth section of the aftershocks, perpendicular to the major NE-SW trend, indicates a SE dipping plane and a NE-SW depth section across the NW-SE trend shows a SW dipping plane.The epicentral map of the stronger aftershocksM ≥ 4.0 shows a prominent NE trend. Stronger aftershocks have followed the major rupture trend of the main shock. The depth section of these stronger aftershocks reveals that it occurred in the depth range of 20 to 38 km, and corroborates with a south dipping seismogenic plane.

Supracrustal faults of the St. Lawrence rift system, Québec: kinematics and geometry as revealed by field mapping and marine seismic reflection data

The St. Lawrence rift system from the Laurentian craton core to the offshore St. Lawrence River system is a seismically active zone in which fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus ocean. The rift-related faults fringe the contact between the Grenvillian basement to the NW and Cambrian–Ordovician rocks of the St. Lawrence Lowlands to the SE and occur also within the Grenvillian basement. The St. Lawrence rift system trends NE–SW and represents a SE-dipping half-graben that links the NW–SE-trending Ottawa–Bonnechère and Saguenay River grabens, both interpreted as Iapetan failed arms. Coastal sections of the St. Lawrence River that expose fault rocks related to the St. Lawrence rift system have been studied between Québec city and the Saguenay River. Brittle faults marking the St. Lawrence rift system consist of NE- and NW-trending structures that show mutual crosscutting relationships. Fault rocks consist of fault breccias, cataclasites and pseudotachylytes. Field relationships suggest that the various types of fault rocks are associated with the same tectonic event. High-resolution marine seismic reflection data acquired in the St. Lawrence River estuary, between Rimouski, the Saguenay River and Forestville, identify submarine topographic relief attributed to the St. Lawrence rift system. Northeast-trending seismic reflection profiles show a basement geometry that agrees with onshore structural features. Northwest-trending seismic profiles suggest that normal faults fringing the St. Lawrence River are associated with a major topographic depression in the estuary, the Laurentian Channel trough, with up to 700 m of basement relief. A two-way travel-time to bedrock map, based on seismic data from the St. Lawrence estuary, and comparison with the onshore rift segment suggest that the Laurentian Channel trough varies from a half-graben to a graben structure from SW to NE. It is speculated that natural gas occurrences within both the onshore and offshore sequences of unconsolidated Quaternary deposits are possibly related to degassing processes of basement rocks, and that hydrocarbons were drained upward by the rift faults.

Imaging the complexity of an active normal fault system: The 1997 Colfiorito (central Italy) case study

Six moderate magnitude earthquakes (5 &lt; Mw &lt; 6) ruptured normal fault segments of the southern sector of the North Apennine belt (central Italy) in the 1997 Colfiorito earthquake sequence. We study the progressive activation of adjacent and nearby parallel faults of this complex normal fault system using ∼1650 earthquake locations obtained by applying a double‐difference location method, using travel time picks and waveform cross‐correlation measurements. The lateral extent of the fault segments range from 5 to 10 km and make up a broad, ∼45 km long, NW trending fault system. The geometry of each segment is quite simple and consists of planar faults gently dipping toward SW with an average dip of 40°–45°. The fault planes are not listric but maintain a constant dip through the entire seismogenic volume, down to 8 km depth. We observe the activation of faults on the hanging wall and the absence of seismicity in the footwall of the structure. The observed fault segmentation appears to be due to the lateral heterogeneity of the upper crust: preexisting thrusts inherited from Neogene's compressional tectonic intersect the active normal faults and control their maximum length. The stress tensor obtained by inverting the six main shock focal mechanisms of the sequence is in agreement with the tectonic stress active in the inner chain of the Apennine, revealing a clear NE trending extension direction. Aftershock focal mechanisms show a consistent extensional kinematics, 70% of which are mechanically consistent with the main shock stress field.

Colagem paleoproterozóica de placas arqueanas do cráton do São Francisco na Bahia

Recent petrological, geochronological and isotopic researches has identified four important Archean crustal segments in the Sao Francisco Craton, SFC, in the state of Bahia. The oldest Gaviao Block occurs in the WSW part of the SFC is composed essentially of granitic, granodioritic and migmatitic rocks. It includes remnants of TTG suites, considered to represent the oldest rocks in the South American continent (~3,4Ga) and associated Archean greenstone belt sequences. The youngest segment, named the Itabuna-Salvador-Curaca Belt is exposed along the Atlantic Coast, from the SE part of Bahia up to Salvador and then along a NE trend. It is mainly composed of tonalite/trondhjemites, but also includes stripes of intercalated metasediments and ocean-floor/back-arc gabbros and basalts. The Jequie Block, the third segment, is exposed in the SE-SSW area, being characterized by Archean granulitic migmatites with supracrustal inclusions and several charnockitic intrusions. The Serrinha Block (fourth segment) occurs to the NE, composed of orthogneisses and migmatites, which represent the basement of Paleoproterozoic greenstone belts sequences. During the Paleoproterozoic Orogeny, these four crustal segments collided, resulting in the formation of an important mountain belt. Geochronological constrains indicate that the regional metamorphism resulting from crustal thickening associated with the collision process took place c.a. 2.0 Ga.

Seabed morphology and hydrocarbon seepage in the Gulf of Cádiz mud volcano area: Acoustic imagery, multibeam and ultra-high resolution seismic data

Extensive mud volcanism, mud diapirism and carbonate chimneys related to hydrocarbon-rich fluid venting are observed throughout the Spanish–Portuguese margin of the Gulf of Cádiz. All the mud volcanoes and diapirs addressed in this paper lie in the region of olistostrome/accretionary complex units which were emplaced in the Late Miocene in response to NW-directed convergence between the African and Eurasian plates. The study area was investigated by multibeam echo-sounder, high and ultra-high resolution seismic profiling, dredging and coring. The structures observed on multibeam bathymetry, at water depths between 500 and 1300 m, are dominated by elongate mud ridges, mud cones, mud volcanoes and crater-like collapse structures ranging in relief from 50 to 300 m and size from 0.8 to 2 km in diameter. The main morphotectonic features, named the Guadalquivir Diapiric Ridge (GDR) and the Cádiz Diapiric Ridge (CDR), are longitudinally shaped diapirs which trend NE–SW and consist of lower–middle Miocene plastic marly clays. The GDR field and the TASYO field, which consist of mud volcanoes and extensive fluid venting related to diapiric ridge development, are described in this paper. The GDR field is characterised by numerous, single, sub-circular mud volcanoes and mud cones. The single mud volcanoes are cone-shaped features with relatively gentle slopes of 3°–6°, consisting of several generations of mud breccia deposition with indications of gas-saturation, degassing structures and the presence of H 2S. The mud cones have asymmetric profiles with steep slopes of up to 25° and contain large surficial deposits of hydrocarbon-derived carbonate chimneys and slabs. The TASYO field is characterised by an extensive concentration of small, sub-circular depressions, oval and multi-cone mud volcanoes and large sediment slides. Mud volcanoes in this area are characterised by moderate slopes (8°–12°), have bathymetric relief ranging from 100 to 190 m and consist of sulphide-rich mud breccia, calcite chimneys, carbonate crusts and chemosynthetic fauna (Pogonophora tube worms). We propose that all these hydrocarbon seepage structures are related to lateral compressional stress generated at the front of the olistostromic/accretionary wedge. This stress results in the uplifting and squeezing plastic marly clay deposits. Additionally, the compressional stress at the toe of the olistostrome forms overpressurised compartments which provide avenues for hydrocarbon-enriched fluids to migrate.

Fault structure and related basins of the North Aegean Sea and its surroundings

Fault structure and basin evolution in the northern Aegean Sea and its surroundings have been investigated using bathymetry, available onshore and offshore seismic profiles, onshore fault patterns from NE Greece, NW Turkey, three islands in the Aegean Sea, surface ruptures associated with the, 1978 Thessaloniki earthquakes, GPS measurements from North Aegean Islands and a number of earthquake focal plane solutions, and historic seismicity. Bathymetric and isopach data show that the North Aegean Trough can be divided into the Sporades basin and the Saros Trough. Post‐Miocene sediment thickness differences in these basins, offset of distinct horizons, and frequent seismicity along basin‐bounding faults indicate that sediment accumulation in the area is fault controlled. Offshore seismic profiles show that the basin‐bounding faults are dominantly characterized by apparent normal components as well as strike‐slip components inferred from typical fault zone profiles. Additional evidence for existence of strike‐slip faulting comes from earthquake focal plane solutions. We refined the pattern and kinematics of the faults controlling the basins and classified them into two major groups. The faults in the first group trend NE‐SW and ENE‐WSW and are here interpreted to be dominantly right‐lateral strike slip with normal or reverse components. These faults accommodate a similar amount of deformation like the Ganos fault and thus represent the westernmost extension of the North Anatolian fault zone. The faults in the second group trend NW‐SE and are dominantly normal faults with an occasional left‐lateral component. These faults are believed to manifest the termination of the North Anatolian fault zone. Because of the segmentation of the right‐lateral strike‐slip faults and the preexisting NE trending tectonic grain, the basins are compartmentalized into smaller depocenters. Basins are orientated with their long axes either parallel to the NE‐SW to ENE‐WSW trending echelon strike‐slip faults, or, in the zone north of the strike‐slip faults parallel to the NW‐SE trending normal faults. The interpretations regarding the kinematics of the faults are consistent with the data compiled from the islands near the faults and from the land in NE Greece and NW Turkey. The northern Aegean basins and their spatial relationships to the fault architecture representing the diffused termination of the North Anatolian fault system are consistent with the mechanical principles of the strike‐slip tectonics.

Structure and Cenozoic kinematics of the Eastern Cordillera, southern Bolivia (21°S)

The Eastern Cordillera is a main morphotectonic unit of the central Andes. In southern Bolivia it is predominantly composed of Ordovician sedimentary rocks as thick as 10 km. Revision of the Ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections, improved estimates of crustal shortening, and a new kinematic model for the Eastern Cordillera. New estimates of total shortening in the Eastern Cordillera are up to 95 km (41–44%), higher than previous estimates and sufficient to thicken the Andean crust to its present state between the eastern foreland and the magmatic arc. After initial Tertiary uplift at ∼58 Ma, first thrusts developed in the middle Eocene to early Oligocene (?40–?30 Ma), exhuming the central part of the Eastern Cordillera. Tectonic shortening culminated in late Oligocene and early Miocene time (25–20 Ma) when west facing thrust systems developed in the central and western part of the Eastern Cordillera. Thrusting terminated in the western part by the end of the early Miocene, and middle Miocene tectonic activity was restricted to minor strike‐slip faulting. In the central part, thrusting remained active until the late Miocene. By ∼10 Ma, shortening in the Eastern Cordillera essentially ceased. Kinematic fault analyses indicate clockwise rotation of the shortening direction, from NNE‐SSW or NE‐SW in Eocene‐early Oligocene time over E‐W to WNW‐ESE or even NW‐SE in late Oligocene to middle Miocene time. Nevertheless, WNW‐ESE directed shortening and subvertical extension predominate. Sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the Bolivian orocline. It is proposed that space problems in the oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along NE trending transfer zones.

Multi-Stage Deformation in the Nallamalai Fold Belt, Cuddapah Basin, South India - Implications for Mesoproterozoic Tectonism Along Southeastern Margin of India

The common occurrence of small-scale refolded folds, deformed lineations and overprinting of foliation in the northern Nallamalai Fold Belt (NFB) within the intracratonic Cuddapah Basin, South India, suggest three phases of deformation, D 1, D 2 and D 3 during the late Paleoproterozoic to Neoproterozoic. D 1 structures are represented by tight to isoclinal folds, a slaty cleavage and local development of mylonites. D 2 structures include NE trending tight to open folds (F 2) with variable plunge indicating control of large domal structures and a steep crenulation cleavage developed in phyllites and other schistose rocks. The above structures are overprinted by E-W trending D 3 folds and cleavage, which affect Mesoproterozoic rocks of the Srisailam Formation (uppermost Nallamalai Group) in the E-W trending Vami Konda range and metasediments of the Palnad area, thought to be equivalent of the Neoproterozoic Kurnool Group. Taking into account the ages of intrusive Vellaturu Granite (∼1575 Ma) which is post-D 2 deformation in the NFB and the Chelima Lamproite (∼1400 Ma) which intrudes the folded Nallamalai rocks, we suggest that the earlier episodes of multiple deformation in the NFB represent either late Paleoproterozoic or early Mesoproterozoic NW-SE compression in this part of the East Gondwana. The E-W fold-thrust structures affecting the Srisailam Quartzite and adjacent metasediments in the Palnad area represent late Mesoproterozoic and/or Neoproterozoic event. The granite gneisses and schists of the Nellore Schist Belt, bordering the NFB are affected by both early and later events and thrust over rocks of the Nallamalai Group. We propose that early deformation in the NFB lying southwest of the Eastern Ghats and roughly parallel to the present southeastern coast of India, was unrelated to, and occurred prior to the main deformation in the Eastern Ghats, which is generally correlated with the ∼1000 Ma old Grenville event. Alternatively, in view of the complex, protracted evolution of the Eastern Ghats and published geochronological data supporting pre-Grenvillian tectonism particularly in the Western Charnockite Zone of the Eastern Ghats, D 1 and D 2 deformation in the NFB may be linked to this older (1600-1400 Ma) tectonic disturbance in the Eastern Ghats.

Geological features and the Paleoproterozoic collision of four Archean crustal segments of the São Francisco Craton, Bahia, Brazil: a synthesis

Recent geological, geochronological and isotopic research has identified four important Archean crustal segments in the basement of the São Francisco Craton in the State of Bahia. The oldest Gavião Block occurs in the WSW part, composed essentially of granitic, granodioritic and migmatitic rocks. It includes remnants of TTG suites, considered to represent the oldest rocks in the South American continent (~ 3,4Ga) and associated Archean greenstone belt sequences. The youngest segment, termed the Itabuna-Salvador-Curaçá Belt is exposed along the Atlantic Coast, from the SE part of Bahia up to Salvador and then along a NE trend. It is mainly composed of tonalite/trondhjemites, but also includes stripes of intercalated metasediments and ocean-floor/back-arc gabbros and basalts. The Jequié Block, the third segment, is exposed in the SE-SSW area, being characterized by Archean granulitic migmatites with supracrustal inclusions and several charnockitic intrusions. The Serrinha Block (fourth segment) occurs to the NE, composed of orthogneisses and migmatites, which represent the basement of Paleoproterozoic greenstone belts sequences. During the Paleoproterozoic Transamazonian Orogeny, these four crustal segments collided, resulting in the formation of an important mountain belt. Geochronological constrains indicate that the regional metamorphism resulting from crustal thickening associated with the collision process took place around 2.0 Ga.

Properties of the Aftershock Sequence of the 1999 Mw 7.1 Hector Mine Earthquake: Implications for Aftershock Hazard

We investigate the spatial and temporal seismicity parameters and the related probabilistic aftershock hazard for the aftershock sequence of the 1999 M_w 7.1 Hector Mine mainshock and compare it with the neighboring 1992 M_w 7.3 Landers sequence. Using a catalog of 11,000 earthquakes, we determine the earthquake size distribution (b-value), the aftershock decay rate (p-value), and the seismic activity rate (a-value). The b-values are high (b > 1.2) within the rupture area, significantly lower (b ≈ 0.7) north of the rupture area, and increase with time since the mainshock. Probabilistic aftershock hazard maps, computed automatically as early as 4 days after the mainshock, identified the northernmost part of the sequence as the highest-hazard region. These maps show a good agreement between the forecasts and the recorded large aftershocks. Based on the asymmetrical b-value and hazard patterns for both the Hector Mine and Landers sequences, we hypothesize that the mainshock rupture directivity and slip distribution influence aftershock hazard. Current static or dynamic stress triggering models cannot resolve this spatial and temporal evolution of the hazard. Stress tensor inversions of 1400 relocated first-motion focal mechanisms show predominantly a strike-slip stress state with a SW–NE trend of the greatest principal stress. The heterogeneity of the stress field is unusually high near the Hector Mine and Landers mainshock ruptures, particularly near patches of large slip.

Timing of Late Carboniferous/Permian Granite and Granite Porphyry Intrusions in the Ruhla Crystalline Complex (Central Germany), New Constraints from SHRIMP and 207Pb/ 206Pb Single Zircon Dating

Abstract The Ruhla Crystalline Complex (RCC) forms part of the NW trending Thuringian Forest horst block, which was affected by complex block faulting during Late Carboniferous/Early Permian times accompanied by an extensive magmatism. Block faulting results from dextral transtensive movements along the NW trending Franconian fault system, which separates the Saxo-Bohemian massif to the north from the South German block to the south. In the RCC granite bodies and a few magmatic dykes intruded along NE trending structures, whereas the majority of the magmatic dykes trend W, NW and ENE. SHRIMP zircon dating of the NE trending Trusetal Granite yielded a weighted mean age of 301 ±5 Ma, identical to a formerly presented age of 298 ±2 Ma. 207Pb/ 206Pb single zircon evaporation dating of a ductily deformed NE trending granite porphyry dyke yielded an age of 294 ±4 Ma, which is significantly older than an age of 285 ±5 Ma obtained from a WNW-trending felsite porphyry dyke. Zircon xenocrysts in the felsite porphyry gave ages of 325 ±9, 338 ±4 and 543 ±4 Ma, which indicate that crustal rocks were assimilated during magma generation and/or ascent. The presented geochronological data provide evidence that emplacement of granites and porphyry dykes along NE trending structures occured contemporaneously between 301 and 294 Ma, whereas magmatism along W to NW trending structures took place at about 285 Ma.

Auriferous arsenopyrite-pyrite and stibnite mineralization from the Siflitz-Guginock area (Austria): indications for hydrothermal activity during Tertiary oblique terrane accretion in the Eastern Alps

Abstract Polymetamorphic schists and marbles of the Austroalpine Kreuzeck-Goldeck Complex are the host to auriferous arsenopyrite-pyrite as well as stibnite mineralization. In the Siflitz-Guginock area both types of mineralization are closely related spatially, but restricted to different lithologies. The auriferous arsenopyrite-pyrite mineralization is either disseminated or bound to quartz veins and strongly silicified fault-zones hosted in phyllites to (garnet-) micaschists. Similar disseminations within marbles are of minor importance. A stockwork-like mineralization of stibnite-filled fractures with weak metasomatic replacement is limited to marbles. Both types of mineralization in the Kreuzeck-Goldeck Complex are intimately related to roughly east-west-trending semi-ductile to brittle strike-slip faults which formed during orogen-parallel wrenching. Semi-ductile to brittle kinematic indicators point to dextral, as well as sinistral, modes of fault movements, coeval with the formation of pyrite-arsenopyrite-bearing quartz veins, as well as the intrusion of Oligocene lamprophyre dykes. Mineralizing fluids are suggested to be derived from devolatilization of the subducted Penninic upper crust. Fluid ascent and ore precipitation is controlled by a transpressive strike-slip regime related to oblique terrane accretion during the Late Eocene to Oligocene. Subsequent development of a Late Oligocene-Early Miocene pure shear regime with contraction trending (N)NE-(S)SW led to the development of conjugate NW-SE dextral and NE-SW sinistral brittle strike-slip faults, and overprinting by ESE-WNW oriented extension. These later events are related to the formation of auriferous mineralization from within the metamorphic core complex of the Tauern Window to the north of the Kreuzeck-Goldeck Complex, hence implying a significant change in spatial, temporal and structural control of Tertiary auriferous mineralization in the Eastern Alps.

The origin and evolution of complex transfer zones (graben shifts) in conjugate fault systems around the Funan Field, Pattani Basin, Gulf of Thailand

Changes in dip province associated with conjugate fault sets produce complex transfer zones (locally known as graben shifts) between sets of convergent conjugate faults in the Pattani Basin. 3D seismic data over a well-developed graben shift geometry in the Funan Field area of the Pattani Basin has revealed details of the transfer zone development. In map view, the graben shift is seen as west dipping faults forming a wedge-shaped incursion into a zone of east-dipping faults. The wedge boundaries trend NE–SW and NW–SE at a high angle to the N–S striking normal faults. Within the transfer zones, faults only slightly overlap with their neighbours, with individual faults tending to widen upwards. The narrow regions of overlap indicate displacement transfer must occur between more widely separated faults, not neighbouring faults. The initial Oligocene–Early Miocene syn-rift fault pattern appears to have been strongly influenced by pre-existing basement trends as indicated by: (1) the restriction of west dipping secondary faults to the present day area of the graben shift, (2) the line of the NE–SW boundary to the graben shift coinciding with fault segment linkage geometries in two major syn-rift faults, and (3) curvature of minor fault tips into the transfer zone. The subsequent conjugate fault system developed during thermal subsidence shows that the dip asymmetry and map-boundaries of the fault dip panels are strongly influenced by the syn-rift fault geometry. There is no indication of active strike-slip faulting affecting the graben shift geometry. Fault displacement diagrams commonly show linkage between deeper and shallower displacement maxima, indicating early (Late Oligocene–Early Miocene), minor, west-dipping normal faults influenced the location and width of higher Middle–Late Miocene conjugate faults. Hence, two stages of structural inheritance led to the Middle–Late Miocene graben shift geometry.

Southeast Baffin volcanic margin and the North American‐Greenland plate separation

Plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at Moho level and extruded as thick piles of seaward dipping lava formations. The Disko‐Svartenhuk area in west Greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward. We present a quantitative tectonic study of this margin, which we tentatively restored in its geodynamic position during the different stages of plate separation between Greenland and North America. Our structural data are constrained with recently published 39Ar/40Ar and new and coherent 40K/40Ar geochronology in dikes of different orientations. The first‐order structure is that of a tectonically‐driven seaward crustal flexure linked to definitive plate breakup between Greenland and Baffin Island during the Eocene, coeval with the formation of the upper part of the exposed seaward dipping volcanic prism. This flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults. This across‐strike structure is correlated to a fundamental along‐strike segmentation with the three “segments” adopting a “zigzag” strike. There is a clear increase of extensional strain at the extremities of the segments. Eocene extension trended N060 on average in the northern and southern segments but was NW trending in the central NE trending Nugssuaq segment. We discuss the interpretation of such an extension perpendicular to the different margin segments. From a regional point of view the N060 extension is distinct in orientation from the approximately N‐S trend of plate separation between North America and Greenland in the Baffin Bay during the Eocene. However, the extension recorded in the margin is nearly perpendicular to the obliquely spreading Eocene accretion axis in the Baffin Bay. This latter point suggests a mantle or ridge control of the development of the margin over regional far‐field lithospheric stress models.

Interaction entre un chevauchement imbrique et une zone transcurrente; le flanc nord-ouest de Andes venezueliennes

Abstract Geological framework; Geological setting: The Venezuela Andes or Merida Andes (fig. 1) extend from the Colombian border in the SW to Barquisimeto in the NE, and constitute a basement uplift exceeding 5,000 m near Merida (Pico Bolivar). This young chain is bordered to the W by the Maracaibo foredeep basin, and to the E by the Barinas-Apure foreland basin. The Bocono fault divides the Andean Belt in two parts along a NE-SW direction. This shows that the uplift of the Andes is contemporaneous with an oblique translation. In the study area, located on the northwestern flank near Maracaibo basin, three major structures are present: in the E, the N-S senestral strike slip Valera-Rio Momboy fault, in the S the E-W dextral strike slip Pinango fault and, in the center, the SW-NE striking Las Virtudes thrust verging toward NW. Lithologic and stratigraphic formations (fig. 4): The Las Virtudes Fault separates two different structural zones. In the SE, overthrust units are made of crystalline basement, Paleozoic substratum and preorogenic sedimentary formations (Cretaceous-Eocene). The foredeep flexural basin, located NW, is filled by synorogenic molasses (Neogene and Quaternary), largely developed within the Betijoque Fm. (Upper Miocene to Pliocene in age) which reaches a thickness of 5000 m. Structure of the northwestern Andean flank; Las Virtudes Fault and its thrust slice zone: Near Las Virtudes village (fig. 5, 6-2), this thrust is systematically associated with a narrow overturned foredeep depobelt (Cretaceous to Neogene in age). These slices are unknown elsewhere in the Andean Chain and represent the terminal faulted part of the thrust drag. However, where this slice zone is missing (central and northeastern part of the study area), the Las Virtudes Fault is not clearly documented: its throw decreases rapidly and it is possible that the fault disappears northeastward. Andean unit: Near the main strike slip faults, NE trending SE verging reverse faults develop (fig. 6-5). In central and northeastern parts, the throw of the reverse faults increases toward the Valera Fault. It seems that reverse faults are horsetail of this major strike slip fault (fig. 5). Internal part of the northwestern Andean foredeep basin: The foredeep sedimentary formations generally dip toward the NW. Associated to the lack of some formations, tilted anticlines toward the SE are observed (fig. 6-3 and 6-7), and indicate the vicinity of decollement levels in the foredeep, located in Luna-Colon, Pauji and Betijoque Fm.. Seismic profiles show (fig. 7) that the major decollement level of the foredeep is located in La Luna and Colon Fms. [Audemard, 1991; De Toni and Kellogg, 1993; Colletta et al., 1997]. Crustal architecture and timing of the deformation: Several stages can be distinguished in the building of the Andes. Development of an intracutaneous thrust wedge: The first effects of the Andean phase during Miocene are the development of an intracutaneous thrust wedge [Price, 1986]. The lower flat is located in the basement and the upper one in Cretaceous formations. The transport direction is NW. The foredeep develops on the forelimb of this structure and collects detrital products coming from erosion of the first (oldest) reliefs. Decollements in the foredeep basin could be contemporaneous with this major overthrust. Their origin could be due to radius of curvature differences within the thick sedimentary formations (fig. 8). Las Virtudes Fault and backthrusting: Las Virtudes Fault is one of the last events of this part of the Andean Belt. During Plio-Pleistocene, the continental crust breaks with a dip of 35 degrees SE. The Andean unit overthrusts the foredeep basin. Some of the foredeep decollements could still be active and form, together with Andean basement, a triangle zone. Las Virtudes Fault throw reaches 5 km between Las Virtudes and Monte Carmelo villages (fig. 8A). It decreases southwestwards and the back thrusts are probably younger. Northeastwards the throw decreases and eventually disappears (fig. 8B). In the same time the back thrust throws increase. Both seem to be contemporaneous. Conclusions: This structural model explains the basement occurrence in front of the Las Virtudes Fault on seismic profiles and allows to restore correctly the different northwestern flank structures of the Venezuela Andes. These structures can be explained by the conjugate movements of a NW verging intracutaneous thrust wedge and strike slip faults which create a SE verging triangular area (fig. 5). The Andean overthrust is transferred in the Falcon zone along the Valera fault. In the northeastern part of the Maracaibo block, the Valera and Bocono strike slip faults limit the Trujillo block (fig. 10) which moves towards the North during Neogene and Quaternary times.

Tectonic evolution and stress field of the Kymi-Aliveri basin, Evia island, Greece

Stress and strain analysis has been used to reconstruct the post-Oligocene geodynamics of the Kymi-Aliveri basin: The Kymi-Aliveri basin occupies the footwall of the Kymi-Thrust, which formed during the Middle Miocene as a large transpressional structure in the late orogenic stages of the Hellenides. Subsequently, in the Upper Miocene the shape of the basin was strongly modified by an orthogonal system of NE and NW trending normal faults as a result of post orogenic collapse. In the Pliocene and Pleistocene time the basin is a part of the back arc basin, which developed behind the Hellenic Arc. WNW trending normal faults and reactivated faults characterized this tectonic phase.

New seismotectonic data from an intraplate region: focal mechanisms in the Armorican Massif (northwestern France)

Focal mechanism solutions are determined for 11 small intraplate earthquakes that occurred between 1990 and 1998 in Normandy and the Channel Islands. These mechanisms are obtained from the P-wave first-motion polarities recorded by stations from local and regional seismic networks. The accuracy of each hypocentral location is closely examined and the quality of each fault plane solution is discussed by considering the influence of the velocity structure. The predominant feature of the computed focal mechanisms is the relatively widespread near-horizontal NE–SW T-axis orientation. Horizontal P-axes strike roughly NW–SE. Mechanism solutions for the earthquakes in the Avranches region show left-lateral strike slip on a NNW–SSE fault zone. For the overall region, it seems that nodal planes of normal faulting solutions trend NW–SE or WNW–ESE, whereas those of thrust faulting solutions trend NE–SW. This is in agreement with the general regional stress pattern. The NE–SW normal fault plane solution of the 1990 Jersey event is unique because it is not consistent with the regional style of faulting.