Sinistral Wrench Faults Research Articles

Geology and Genesis of the Cerro la Mina Porphyry-High Sulfidation Au (Cu-Mo) Prospect, Mexico

The Cerro la Mina Au (Cu-Mo) porphyry-high sulfidation prospect is located in Chiapas State, southeastern Mexico, outside of the major metallogenic provinces of Mexico. The prospect is hosted by Pleistocene alkaline volcanic rocks of the Chiapanecan volcanic arc that formed in a complex triple-junction tectonic setting. Cerro la Mina's stratigraphy comprises pyroclastic flows that were intruded by monzodiorites and diorites at 1.04 ± 0.04 Ma (U-Pb, zircon), and that were overlain by debris flows and synvolcanic trachyandesite domes. The volcanic stratigraphy of Cerro la Mina is dominated by pyroclastic flows and rare basalts that are cut by the Cerro la Mina breccia pipe, a matrix-rich granular, vertically oriented, downward-tapering, polymict lithic rock unit that is host to all of the significant alteration and mineralization. A NW-trending sinistral wrench fault, which was active throughout the history of Cerro la Mina, is responsible for dismembering the prospect after mineralization. The magmatic hydrothermal system was composed of early porphyry-style potassic veins (quartz + K-feldspar ± biotite) and stage 1 pyrite that are preserved in clasts within the breccia pipe, suggesting that brecciation disrupted an embryonic porphyry system. Late potassic alteration occurred after the formation of the breccia pipe, as its matrix is strongly K-feldspar altered. Hydrothermal fluids then produced phyllic alteration composed of quartz, muscovite, illite, illite-smectite, and chlorite that is associated with stage 2 pyrite ± chalcopyrite ± molybdenite ± quartz veins. An unusual zoned pattern of advanced argillic-argillic alteration overprinted potassic and phyllic alteration. This zoning included a low-temperature ( 120°C) quartz + dickite ± kaolinite ± pyrophyllite ± alunite that occurs from 250 m to the present-day surface. The advanced argillic-argillic altered rocks host the most significant Au-Cu mineralization, which is associated with stage 3 marcasite, sphalerite, galena, and barite, and stage 4 arsenian pyrite ± enargite ± covellite. The magmatic hydrothermal system at Cerro la Mina began sometime between monzodiorite emplacement (1.04 ± 0.04 Ma; zircon U-Pb) and the precipitation of porphyry stage 2 molybdenite at 780 ± 10 ka (Re-Os). 40Ar/39Ar dating of biotite (689 ± 13 ka) records the age at which the hydrothermal system cooled below the biotite closure temperature of ~300°C and provides a maximum estimate for the onset of advanced argillic-argillic alteration. Sulfur isotope results of sulfides (–2.5 to +4.9‰; mean +0.7‰; n = 20) and a sulfate (barite; +10.5‰; n = 1) suggest a magmatic source of sulfur for all four stages of mineralization. The lack of residual quartz, rare alunite, and anomalous halloysite-kaolinite alteration may be explained by the high acid-buffering capacity of alkaline volcanic host rocks, high CO2 contents of the alkaline magma, and/or potentially by a highly reduced magmatic hydrothermal fluid. At the regional metallogenic scale, the Cerro la Mina prospect along with the nearby Santa Fe mine and Campamento deposit represent parts of a porphyry copper system—specifically, a porphyry/high-sulfidation, proximal skarn and intermediate sulfidation deposit, respectively. The characteristics of Cerro la Mina (i.e., anomalous halloysite-kaolinite alteration) broaden the window for additional discoveries to be made in the porphyry-epithermal environment.

The role of inherited structures in the evolution of the Meknassy Basin, Central Tunisia, based on geological–geophysical transects

This paper uses seismic data, well data, and surface geologic data to present a detailed description of the Meknassy Basin in the Atlas fold and thrust belt of central Tunisia. These data reveal that the Meknassy Basin is bounded by major faults, along which Triassic evaporites have been intruded. The anticlines and synclines of the basin are delimited by two N–S main faults (the North–South Axis and the Sidi Ali Ben Oun fault) and are subdivided by associated N120° and N45° trending fault-related anticlines.The Meknassy Basin is characterized by brittle structures associated with a deep asymmetric geometry that is organized into depressions and uplifts. Halokinesis of Triassic evaporites began during the Jurassic and continued during the Cretaceous period. During extensional deformation, salt movement controlled the sediment accumulation and the location of pre-compressional structures. During compressional deformation, the remobilization of evaporites accentuated the folded uplifts. A zone of decollement is located within the Triassic evaporites.The coeval strike-slip motion along the bounding master faults suggests that the Meknassy Basin was initiated as a pull-apart basin with intrusion of Triassic evaporites. The lozenge structure of the basin was caused by synchronous movements of the Sidi Ali Ben Oun fault and the North–South Axis (sinistral wrench faults) with movement of NW–SE first-order dextral strike-slip faults. Sediment distribution and structural features indicate that a major tectonic inversion has occurred at least since Late Cretaceous and Cenozoic. The transpressional movements are marked by reverse faults and folds associated with unconformities and with remobilization of Triassic evaporites. The formation of different structural features and the evolution of the Meknassy Basin and its neighboring uplifts have been controlled by conjugate dextral and sinistral strike-slip movements and thrust displacement.

The Role of Sinistral Strike-Slip Faults in the Formation of the Structural Fabric of the Slavonian Mts. (Eastern Croatia)

This paper presents a simplified evolution for the Slavonian Mts. structural fabric during the Neogene, emphasising the importance of the sinistral wrench faults. These faults were formed under the influence of dextral displacements along the faults stretching through the Drava and Sava. They caused the uplift and compression of the general area of the Slavonian Mts. These events were accompanied by folding, reverse faulting and counter-clockwise rotation of the uplifted structures, typical for the transpression model and wrench tectonic processes.

Reaktivitas Sesar Kaligarang, Semarang

http://dx.doi.org/10.17014/ijog.vol3no3.20082Kaligarang Fault which cuts across Semarang City in a north-south direction has been activated for long periods. During Tertiary time, the Kaligarang Fault experienced a dextral wrench movement, which was triggered by a tectonic activity during that time, with a maximum principal stress (σ 1) in a northeast - southwest direction. While on Quaternary time, this fault was reactivated as a sinistral wrench fault. Reactivation of this fault was caused by a principal stress (σ 2) in a north east - south last direction.

Geotechnical mapping for alluvial fan deposits controlled by active faults: a case study in the Erzurum, NE Turkey

Erzurum, the biggest city of Eastern Anatolia Region in the Turkey, is located in Karasu Plain. Karasu Plain, located on the central segment of the Erzurum Fault Zone, is an intermountain sedimentary basin with a Miocene-Quaternary volcanic basement, andesitic-basaltic lava flows and fissure eruptions of basaltic lava. It was filled in the early Quaternary by lacustrine fan-delta deposits. The basin is characterized by NNE-SSW trending sinistral wrench faults on its eastern margin and ENE-WSW trending reverse faults on its southern margin. Both systems of active faults intersect very near to Erzurum, which is considered to be the most likely site for the epicenter of a probable future large earthquake. Historical records of destructive earthquakes, morphotectonic features formed by paleo-seismic events and instrument seismic data of region indicate to a very high regional seismicity. The residential areas of Erzurum are located on thick alluvial fan deposits forming under the control of faults on the central segment of the Erzurum Fault Zone, which is one of the most active fault belts of the East Anatolian Region. Over time, the housing estates of city such as Yenisehir and Yildizkent have been expanded toward to the west and southwest part of Erzurum as a consequence of rapid and massive construction during the last 30 years. Geotechnical investigation has therefore been undertaken the residential areas of city in order to characterize geotechnical properties over the varied lithologies examine the potential for geotechnical mapping and assess the foundation conditions of the present and future settlement areas. The geological field observations and operations have been performed to make the soil sampling and characterize the lateral and vertical changes in thickness of the alluvial deposits in trenches, excavations and deep holes with 6–12 m sections. The soil samples have been subjected to a series of tests under laboratory conditions to obtain physical and mechanical properties. Furthermore, the standard penetration tests have been applied to the soils under field conditions. The geological field observations, geotechnical data and distribution of bearing capacity have been considered for the geotechnical mapping. Based on the geotechnical map, there are five geotechnical zones distinguished in the study area.

Thrusting and multiple folding in the Neoproterozoic Pan-African basement of Wadi Hodein area, south Eastern Desert, Egypt

Detailed field mapping and structural studies of the area around the mouth of Wadi Hodein, some 20 km west of Shalatein at the Red Sea coast in the south Eastern Desert of Egypt, revealed four phases of structural deformation (D1–D4) affecting the Neoproterozoic Pan-African basement rocks. D1 is related to arc–arc collision and is represented by ENE–WSW oriented megascopic upright open folds associated with low angle thrusts and mesoscopic tight, overturned and recumbent F1 folds. Kinematic indicators indicate thrusting towards the SSE. D2 is represented by NNW–SSE oriented megascopic and mesoscopic folds, which are tight, verge towards the WSW and display a left-stepping en echelon pattern. D3 includes major NNW–SSE trending sinistral shear zones that show subordinate reverse fault components and dip steeply towards the ENE. These sinistral shear zones are comparable with the Najd Fault System, as they display a similar sense of movement and relationships to earlier structures. Therefore, they are interpreted to be the continuation of the Najd Shear System in southern Egypt. D2 and D3 are related to accretion of east and west Gondwana. D4 is represented by E–W oriented dextral faults with left-stepping segments. The first three deformation events are in agreement with the general evolutionary model for the East African Orogen in the Arabian–Nubian Shield that begins with NNW–SSE shortening, followed by ENE–WSW compression and subsequent deformation by the NNW–SSE striking Najd Fault System. The E–W dextral faults may be the conjugate shear fractures to the D3 NNW–SSE oriented sinistral wrench faults or are related to a subsequent event, D4. NW–SE oriented gold-bearing quartz veins originated during D1 and were subsequently deformed by D2–D4 events.

2D Seismic interpretation of strike-slip faulting, salt tectonics, and Cretaceous unconformities, Atlas Mountains, central Tunisia

The Cretaceous deposits in central Tunisia blocks were studied by sequence stratigraphy, 2D seismic interpretation calibrated to the well and associated outcrop data. The constructing and comparing histories of the northern and southern blocks of the Gafsa master fault was the establishment of platform to basin stratigraphic configuration based on the major unconformity surfaces. Three important basin zones mark subsurface structures: Gafsa to the south, Souinia-Majoura to the northeast and Sidi Aïch-Mèjel Bel Abbès to the northwest. Basin depocenters and upthrown blocks are bounded by the N120° Gafsa and Majoura and N180° Sidi Ali Ben Aoun wrench fault salt-intruded tectonic corridors and subdivided by the associated N60° and N90° trending second-order fault corridors. The Mèjel Bel Abbès block is characterized by brittle structures associated with a deep asymmetric geometry that is organized into depressions and uplifts. Halokinesis of Triassic salt began in the Jurassic and continued during the Cretaceous periods. During extensional deformations, salt movement controlled sedimentation distribution and location of pre-compressional structures. During compressional deformations, salt remobilization accentuated the folded uplifts. The Triassic salt facies constitutes a level of decollement at the base of the Mesozoic deposits during the later displacements. The coeval dextral strike-slip motion along the three northwest–southeast bounding master faults (Gafsa, Sehib-Alima and Majoura-Mech) suggests a pull-apart opening of the Gafsa basin. Synchronous movements of the Gafsa first-order dextral strike-slip fault with the Sidi Ali Ben Aoun sinistral wrench fault caused formation of tectonic obstacles that are shown first by the sealed structures, then by development of the local compressive stress that caused formation of the south overturned folds and the syncline depressions. The transcurrent fault systems caused formation of Turonian and Senonian unconformities and hiatuses on the Sidi Aïch and Souinia-Majoura uplifts that corresponded to the Kasserine Islets. Sedimentary distribution and structural features indicate two events of regional deformation. The Albian–Cenomanian and Turonian transtensional displacements are demonstrated by depressions and grabens, with divergent reflectors striking against dipping growth fault. The Coniacian–Maastrichtian transpressional movements are marked by reverse faults and folds associated with pinch out structures and stratigraphic unconformities.

Fluid Chemistry, Structural Setting, and Emplacement History of the Rosario Cu-Mo Porphyry and Cu-Ag-Au Epithermal Veins, Collahuasi District, Northern Chile

The Rosario Cu-Mo-Ag deposit is located in the Collahuasi district of northern Chile. It comprises high-grade Cu-Ag-(Au) epithermal veins, superimposed on the core of a porphyry Cu-Mo orebody. Rosario has mining reserves of 1,094 million metric tons (Mt) at 1.03 percent copper. An additional 1,022 Mt at 0.93 percent copper occurs in the district at the nearby Ujina and Quebrada Blanca porphyry deposits. The Rosario reserve contains over 95 percent hypogene ore, whereas supergene-sulfide ores dominate at Ujina and Quebrada Blanca. Mineralized veins are hosted within Lower Permian volcanic and sedimentary rocks, Lower Triassic granodiorite and late Eocene porphyritic quartz-monzonite. The Rosario fault system, a series of moderate southwest- dipping faults, has localized high-grade Cu-Ag-(Au) veins. At Cerro La Grande, similar high-grade Cu- Ag-(Au) veins are hosted in north-northeast-trending, sinistral wrench faults. Normal movement in the Rosario fault system is interpreted to have been synchronous with sinistral strike-slip deformation at La Grande. Hydrothermal alteration at Rosario is characterized by a K-feldspar core, focused in the Rosario Porphyry that grades out to a secondary biotite-albite-magnetite assemblage. Paragenetic relationships indicate that magnetite was the earliest formed alteration product but has been replaced by biotite-albite. Vein crosscutting relationships indicate that K-feldspar formed during and after biotite-albite alteration. Chalcopyrite and bornite were deposited in quartz veins associated with both K-feldspar and biotite-albite assemblages. The early hydrothermal fluid was a hypersaline brine (40-45 wt % NaCl) that coexisted with vapor between 400 degrees and >600 degrees C. Weakly mineralized illite-chlorite (intermediate argillic) alteration of the early K and Na silicate assemblages was caused by moderate temperature (250 degrees-350 degrees C), moderate-salinity brines (10-15 wt % NaCl). Molybdenite was precipitated in quartz veins that formed between the potassic and intermediate argillic alteration events. These fluids were 350 degrees to 400 degrees C with salinities between 10 and 15 wt percent NaCl. Porphyry-style ore and alteration minerals were overprinted by structurally controlled quartz-alunite-pyrite, pyrophyllite-dickite, and muscovite-quartz (phyllic) alteration assemblages. The quartz-alunite-pyrite alteration formed at 300 degrees to 400 degrees C from fluids with a salinity of 10 wt percent NaCl. The pyrophyllite-dickite assemblage formed between 250 degrees and 320 degrees C from dilute (5 wt % NaCl) fluids. An upward-flared zone of muscovite- quartz-pyrite altered rocks surrounds the fault-controlled domain of advanced argillic alteration. Thick veins (0.5-2 m wide) of fault-hosted massive pyrite, chalcopyrite, and bornite precipitated brines with a salinity of 30 wt percent NaCl at temperatures of 250 degrees to 300 degrees C. Pressure-depth estimates indicate that at least 1 km of rock was eroded at Rosario between formation of the K-Na silicate and advanced argillic assemblages. This erosion was rapid, occurring over a period of 1.8 m.y. The Rosario Porphyry intruded immediately after the Incaic tectonic phase, implying that it was emplaced as the Domeyko Cordillera underwent gravitational collapse, expressed as normal faults in the upper crust. Gravitational sliding potentially accelerated exhumation and helped to promote telescoping of the high-sulfidation environment onto the Rosario Porphyry. The hydrothermal system responsible for porphyry Cu mineralization at Rosario was partially exhumed prior to the formation of high-sulfidation ore and alteration assemblages. This implies that emplacement of a second blind intrusion occurred somewhere beneath the Rosario and Cerro La Grande high-sulfidation vein systems and is supported by the fault geometry and zoning of precious metals and sulfosalts at the district scale.

Persistent fault controlled basin formation since the Proterozoic along the Western Branch of the East African Rift

The Western Branch of the East African Rift System is outlined by elongate sedimentary basins, frequently occupied by Cenozoic rift lakes. The role of the inheritance of the leading rift faults from pre-existing basement structures has often been invoked. Recent studies in western Tanzania confirm the extent of the northwest orientated Palaeoproterozoic Ubende Belt contribution to the Phanerozoic Rift. Attention is drawn here on the occurrence of different Meso- and Neoproterozoic sedimentary basins that developed along the ductile shear belt as a result of repeated sinistral wrench fault reactivation. These basins partly overlap each other and typically bear shallow and weakly evolved sediments. North of the Ubende Belt, the Mesoproterozoic Kibara Belt is inferred to have originated as a basin controlled by the complex termination of the Ubende wrench fault. Phanerozoic rift basins also develop along the northwest orientated Ubende Belt structure. They display the same elongate shape as the Proterozoic basins. In Late Palaeozoic-Early Mesozoic the Karoo rift basins formed from a dextral lateral shear reactivation of the inherited Proterozoic shear faults. During the first phase of development the Lake Tanganyika Basin is belived to bear the same characteristics as all previous basins along the Ubende Shear Belt, mainly controlled by strike-slip movements along pre-existing shear faults. The present Lake Tanganyika Basin is subdivided in two sub-basins, separated by the transverse Mahali Shoal, which is an active structure located on the Ubende Shear. The deep lake basin mainly developed outside the Ubende Belt. The northern sub-basin appears to be structurally controlled by the reactivation of the Mesoproterozoic sinistral wrench fault termination of the Ubende shear faults. Structural control of the Palaeoproterozoic basement is however unclear for the southern sub-basin of Lake Tanganyika: this part of the rift segment is flanked by Palaeoproterozoic basement which has not been affected by the Ubende Shear.

Regional gas geochemistry in an active tectonic zone, Erzurum Basin, eastern Turkey

Abstract The Karasu Plain is an intermontane sedimentary basin with a Miocene—Quaternary volcanicbasement, andesitic-basalticlava flows and fissure eruptions of basaltic lava. It was filled in the early Quaternary by lacustrine fan-delta deposits, and later by fluvial-alluvial fan deposits. The basin is characterized by NNE-SSW trending sinistral wrench faults on its eastern margin and ENE-WSW trending reverse faults on its southern margin. Both systems of active faults intersect very near to Erzurum, which is considered to be the most likely site for the epicentre of a probable future large earthquake. Regional gas geochemistry has therefore been undertaken adjacent to the city of Erzurum, in order to characterize radon levels over the varied lithologies, examine the potential for geological fault mapping and assess the status of fault activity. Initially,41 sites were sampled and analysed for Rn, CO 2 , O 2 and CH 4 in soils along three traverses to the east (8.26km and 4.5km length) and to the west (6.8km length) of Erzurum. During the first investigation, 495 Rn determinations were made from 165 samples at 41 sites over a ten-day period, during which a small earthquake (M=2.95) occurred. Another 211 sites have been subsequently used for Rn investigations including one traverse through the centre of the city. Additionally,water samples were degassed for Rn and soils tested at five key sites around the Erzurum Plain. Radon was readily detected in soils at all sites. Both 222 Rn and 220 Rn were present at most. Soils over major lithologies gave characteristic signals and, in all traverses, anomalously high values coincided with active faults. Little correlation was found between spring gases (thermal and cold) and adjacent soils. De-gassed spring water Rn values proved highest along the southern margin of the basin and CO 2 values were highest along the northern edge.

Cadomian terranes, wrench faulting and thrusting in the central Europe Variscides: geophysical and geological evidence

Sixty five per cent of the Paleozoic basement of western and central Europe is hidden by a sedimen- tary cover and/or sea. This work aims to remove that blanket to detect new structures which could used to build a more comprehensive model of the Variscan or- ogeny. It is based on the interpretation of various forms of data: (a) published gravity maps corrected for the ef- fects of the crust-mantle boundary topography and light sedimentary basins; (b) aeromagnetic maps; (c) measurements of densities; and (d) induced and rema- nent magnetizations on rocks from Paleozoic outcrops of the upper Rhenish area. From the northern B ohem- ian Massif to the eastern Paris Basin, the Saxothurin- gian is characterized by a 500 km long belt of gravity highs, the most important being the Kraichgau high. Most of the corresponding heavy bodies are buried un- der a post-early Visdan cover. They are interpreted as relics of Late Proterozoic terranes overlain by an Early to Middle Paleozoic sequence, equivalent to the Bo- hemian t errane in the Bohemian Massif. The most probable continuation of these dense Bohemian ter- ranes toward the west is the Southern Channel-North- ern Brittany Cadomian terrane. The gravity lows are correlated with Variscan granites and pre- and early Variscan metagranites. Gravity and magnetic maps demonstrate large-scale displacement in Devonian-Early Carboniferous times along the parallel and equidistant, NW-SE striking, Vistula, Elbe, Bavarian, Bray and South Armorican dextral wrench faults. In the Vosges-Schwarzwald and Central Massif the faults continue with the east-west striking Lalaye-Lubine-Ba den-Baden and Marche faults and with south vergent thrusts. The Bavarian faults shift the Kraichgau terrane by 150 km relative to the Bohemian terrane, whereas the offset of the North- ern Brittany Cadomian relative to the Northern Vosges-Kraichgau terranes is estimated at 400km along the Bray fault. Sinistral wrench faults are the NE-SW striking Sillon Houiller, Rheingraben, Rodl, Vitis and Diendorf faults. The southern Vosges- Schwarzwald Devonian-Dinanti an basin is interpreted as a pull-apart basin at the south-easterly extremity of the Bray fault. The Bohemian and Kraichgau body form allochthonous terranes which were thrust over the Saxothuringian crust. Thrusting to the north-west was accompanied by back-thrusting and led to the forma- tion of pop-up structures. Contemporaneous dextral and sinistral wrench faulting resulted in transpressive strain during collision. The zonal structure of the Varis- cides in the sense of Kossmat (1927) is relevant only to the Rhenohercynian Foreland Belt. Kossmat (1927) al- ready spoke of a Moldanubian Region because it dis- plays no real zonal structure. The Saxothuringian Zone was formed by terrane accretion. Their apparent zonal structure is not a pre-collisional feature, but only the result of accretion and collision.

Fission track and Ar/Ar study of the scotian basin: A report on progress

This paper uses seismic data, well data, and surface geologic data to present a detailed description of the Meknassy Basin in the Atlas fold and thrust belt of central Tunisia. These data reveal that the Meknassy Basin is bounded by major faults, along which Triassic evaporites have been intruded. The anticlines and synclines of the basin are delimited by two N–S main faults (the North–South Axis and the Sidi Ali Ben Oun fault) and are subdivided by associated N120° and N45° trending fault-related anticlines.The Meknassy Basin is characterized by brittle structures associated with a deep asymmetric geometry that is organized into depressions and uplifts. Halokinesis of Triassic evaporites began during the Jurassic and continued during the Cretaceous period. During extensional deformation, salt movement controlled the sediment accumulation and the location of pre-compressional structures. During compressional deformation, the remobilization of evaporites accentuated the folded uplifts. A zone of decollement is located within the Triassic evaporites.The coeval strike-slip motion along the bounding master faults suggests that the Meknassy Basin was initiated as a pull-apart basin with intrusion of Triassic evaporites. The lozenge structure of the basin was caused by synchronous movements of the Sidi Ali Ben Oun fault and the North–South Axis (sinistral wrench faults) with movement of NW–SE first-order dextral strike-slip faults. Sediment distribution and structural features indicate that a major tectonic inversion has occurred at least since Late Cretaceous and Cenozoic. The transpressional movements are marked by reverse faults and folds associated with unconformities and with remobilization of Triassic evaporites. The formation of different structural features and the evolution of the Meknassy Basin and its neighboring uplifts have been controlled by conjugate dextral and sinistral strike-slip movements and thrust displacement.

The Sredne-Amursky Basin: A Migrating Cretaceous Depocenter for the Amur River, Eastern Siberia

Recently acquired seismic, well, and regional geological data imply favorable conditions for the accumulation of oil and gas in the 20,000 km[sup 2] Sredne-Amursky basin. Major graben and northeast-trending sinistral wrench-fault systems are recognized in the basin. Lower and Upper Cretaceous sediments are up to 9000 and 3000 m thick, respectively. Paleogeographic reconstructions imply that during the Late Triassic-Early Cretaceous the Sredne-Amursky basin was part of a narrow marine embayment (back-arc basin), which was open to the north. During the Cretaceous, the region was part of a foreland basin complicated by strike-slip, which produced subsidence related to transtension during oblique collision of the Sikhote-Alin arc with Eurasian margin. Contemporaneous uplift also related to this collision migrated from south to north and may have sourced northward-directed deltas and alluvial fans, which fed northward into the closing back-arc basin between 130 and 85 Ma. The progradational clastic succession of the Berriasian-Albian and the Late Cretaceous fluvial, brackish water and paralic sediments within the basin may be analogous to the highly productive late Tertiary clastics of the Amur River delta in the northeast Sakhalin basin. Cretaceous-Tertiary lacustrine-deltaic sapropelic shales provide significant source and seal potential and potential reservoirs occur in the Cretaceous andmore » Tertiary. Structural plays were developed during Cretaceous rifting and subsequent strike-slip deformation. If the full hydrocarbon potential of the Sredne-Amursky basin is to be realized, the regional appraisal suggests that exploration should be focused toward the identification of plays related to prograding Cretaceous deltaic depositional systems.« less

A serpentinite ridge in a collisional paleomargin setting: the Beni Malek massif, External Rif, Morocco

In the Eastern Rif Belt of northern Morocco the Beni Malek serpentinised peridotite massif is included in a low-grade metasedimentary tectonic pile, which was thrust southward onto the Atlas foreland, and which belongs to the external zones of the Maghrebides belt. We describe serpentinite clasts within calcareous deposits that are transgressive onto the ultramafic (lherzolitic) massif and detrital serpentinite layers that occur within greenschist formations. Reworking of the serpentinite clasts occurred at the beginning of the Late Jurassic-Early Cretaceous accumulation of the overlying, marginal sedimentary prism. We connect the ultramafic outcrop with a regional, NE-SW trending magnetic anomaly (the Temsamane anomaly), and conclude that the source of the serpentinite clasts extended roughly parallel to the local, NE-SW trend of the North African margin. By comparison with the submarine serpentinite ridge of the Galician margin, we suggest that the Beni Malek ultramafics were part of a similar ridge of serpentinised mantle rocks exhumed by the Tethyan rifting off northwestern Africa. The extensional emplacement of this ridge is correlated with the early stage of uplift of the Ronda-Beni Bousera lherzolite massifs under the Alboran thinned continental block, and with ocean-floor spreading on both sides(?) of the Alboran block. The seismic Nekor fault parallels the Temsamane anomaly to the southeast and is interpreted as a former extensional fault of the North African paleomargin that was reactivated as a sinistral wrench fault during the Miocene by the Iberian-African collision, which caused the obduction of the Beni Malek massif itself.

Devonian basin initiation in East Greenland: a result of sinistral wrench faulting and Caledonian extensional collapse

Following the Caledonian orogeny, large scale ESE–WNW extensional faulting and N–S directed sinistral wrenching affected East Greenland leading to the initiation of the Devonian basin. The NNE–SSW trending extensional faults have very large downthrows to the east exceeding 10 km and separate up to 90 km wide fault blocks, which during deformation were tilted approximately 12°. The tilting of one of the fault blocks led to uplift of deep-seated hot crystalline rocks in the eastern part causing contact metamorphism in the adjacent downfaulted block to the east. Based on K-Ar cooling ages, the faulting seems to have taken place during the Middle Devonian around 385 Ma, coinciding with the initiation of deposition within the Devonian basin. A N–S trending sinistral wrench fault zone developed in addition to the extensional faults leading to formation of synthetic and antithetic Riedel shears along the present-day western border of the Devonian basin. The timing of the wrench displacement cannot be younger than Middle Devonian. The extension faults and the wrench fault can be followed as regional structures in East Greenland and can be compared to similar Devonian extensional structures in northern Great Britain and to the major wrench faults in Scotland. No clear separation in time is possible between the extension and the wrenching, and a transtensional kinematic history is the most plausible. The basin-forming tectonic evolution in East Greenland during Middle Devonian can be related to an extensional collapse of an overthickened Caledonian crustal welt associated with wrench deformations due to late Caledonian shear displacements along plate boundaries. This evolution is analogous to the Devonian basin formation in northern Great Britain.

The history of movement on the Henty Fault Zone, western Tasmania: An analysis of fault striations

The Henty Fault Zone is a major west‐dipping structure in the economically important Mount Read Volcanics of western Tasmania. It has been active from the Early Palaeozoic to the Tertiary. Detailed investigations of this fault history were carried out using numerical analysis of fault striations. Five generations of fault movement were recognized. The first generation of movement was east‐directed thrusting, which pre‐dates Devonian folding. This phase may correlate with Early Ordovician thrusting on the Great Lyell Fault. High angle reverse faulting took place after the Devonian folding and synchronous with granite intrusion. The last Devonian movement was a sinistral displacement of less than 5 km. The structure was reactivated as a sinistral wrench fault, possibly in the Mesozoic, and a normal fault in the Tertiary.

The onset and timing of Caledonian sinistral shear in County Down

In the south-westerly Irish continuation of the Southern Uplands ample evidence of sinistral transpression includes: (a) the clockwise transection of F 1 folds by S 1 cleavage combined with sub-horizontal extension in the cleavage plane, (b) local development of vein arrays deformed by sinistral simple shear and (c) an abundance of sinistral wrench faults. The sinistral movements are dated relative to the local history of Caledonian deformation and the intrusion of two suites of Caledonian lamprophyre dykes. Clockwise transection of folds by cleavage is not a feature of the Northern Belt or of the northern tracts of the Central Belt in County Down. Its geographic distribution suggests that sinistral transpression was first effective during the accretion of Llandovery-age tracts mid-way south in the Central Belt. Sinistral movements continued episodically during the deformation of early pre- and post-S 1 vein sets and was finally expressed in widespread wrench faulting which continued beyond the date of intrusion of the younger dykes, at about 400 Ma.

Discussion on a tectonic model for the emplacement of the Main Donegal Granite, NW Ireland

Mr M. J. Arthur writes: Hutton (1982) suggested that the Main Donegal Granite (MDG) was accommodated in a NE–SW zone of tension ('elongate hole'), contemporaneously with NE–SW sinistral shearing around that zone about 400 Ma ago (end Silurian–early Devonian times). Calculations from strain studies suggested that the sinistral displacement was about 20 km at its maximum, across the NW boundary of the MDG, but only about 5 km where the NW and SE boundaries united and continued to the SW (Hutton 1982). No figure was given for the offset across the SE boundary, though it was considered to be a major sinistral shear zone (Hutton 1982). However, Hutton (1982) proposed an unorthodox ‘crack opening model’ for the emplacement, which is adequately explained by the well established pull-apart model. The MDG has the rhomboidal/elliptical shape (Fig. 1) typical of a pull-apart produced at a releasing bend (cf. Crowell 1974, fig. 3), resulting from the intersection of the two throughgoing parts of a left-stepping sinistral wrench fault (cf. Ramsay 1980, fig. 18). The MDG is about 50 km long and 11 km wide and thus has a length/width ratio of about 4.5, whilst the ratio for pull-aparts is commonly between 2 and 5 with an average of about 3 (Aydin & Nur 1982). Emplacement into an ‘elongate hole’ contemporaneously with shearing around it, is precisely a characteristic expected of a pull-apart. Although pull-aparts are perhaps best known for their occupation by sedimentary deposits they also often contain contemporaneous igneous

Mesoscopic structures produced by Plio-Pleistocene wrench faulting in South Sulawesi, Indonesia

The Bantimala Complex and overlying Cretaceous sediments of South Sulawesi, Indonesia, were intensely faulted in the Plio-Pleistocene as a result of the northward movement of the Banada Sea microplate with respect to Western Indonesia. The area is dominated by NNW-striking sinistral wrench faults. The structures in these faults formed at less than 250°C. Zones of intense rotation and faulting, 200 m wide, parallel the major faults. They consist of lenses of a wide range of sedimentary and metamorphic rocks separated by narrow zones of foliated cataclasite. Within the lenses, folding is non-cylindrical and there is a weak stretching lineation parallel to the shear direction. Extension fractures and veins occur at 70° to the stretching lineation. Sinistral and dextral faults within the lenses are symmetrical to the major faults. The fabric within these lenses suggests moderate horizontal extension rather than strong simple shear. Within the cataclasite, the foliation is composed of anastomosing microfaults. The sense of shear is demonstrated by offset on these microfaults and on smaller fractures within the microlithons. Elongate clasts are horizontally aligned within the cleavage and have symmetrical drag patterns. Many of the equidimensional clasts have asymmetrical drag patterns consistent with sinistral shear.

The Orlock Bridge Fault: a major Late Caledonian sinistral fault in the Southern Uplands terrane, British Isles

ABSTRACTA detailed examination of the boundary between Peach and Home's (1899) Northern and Central Belts, in the Lower Palaeozoic rocks of the Southern Uplands of Scotland and their Irish continuation in Longford–Down, demonstrates it to be a major Caledonian sinistral wrench fault. Maps and descriptions of outcrops of the fault at Slieve Glah in County Cavan, at Orlock Bridge on the NE coast of Down, at Cairngarroch on the Rhinns of Galloway and at Garvald, 11 km SW of Dunbar, are presented. A distinctive fault fabric, clearly the result of repeated sinistral slip at the first three localities, characterises these outcrops along the 400 km trace. The fabric elements include: (1) lenticular shearing of arenites in the fault protolith, (2) a fault-associated phyllonitic fabric, with abundant pressure-solution seams, overprinting the regional S1, (3) numerous foliation-parallel quartz segregations, (4) refolding of the regional S1 cleavage, segregation veins and the phyllonitic fabric in at least two generations of steeply plunging, sinistrally-verging folds, (5) a non-penetrative crenulation cleavage, (6) a locally developed sinistral S–C fabric, and (7) typical protomylonite, mylonite and ultramylonite textures in thin sections of the fault rocks. The zone of fault-associated deformation varies from a few metres across at Garvald to over 1 km at Slieve Glah. The fault thus differs from other tract-defining faults in the Southern Uplands, particularly in its clear evidence of ductile, quasi-plastic, deformation at depth, in its lack of associated imbrication, in its significant refolding and overprinting of the regional S1 cleavage and in its uniquely large stratigraphic effects.The main fabric-generating movement postdates the regional, accretion-related, S1 cleavage but predates minor Caledonian (c. 400 Ma) intrusions. For its entire length the fault separates the Northern Belt, with proof only of Upper Ordovician turbidites, and the Central Belt, where turbidite deposition began in the Silurian. The age of the base of the turbidites decreases southward by some six graptolite zones at the fault trace. The fault thus either (a) excises the equivalent of four tracts from a single accretionary-prism terrane, or (b) juxtaposes two distinct terranes, one of late Ordovician and the other of Silurian age. In either event the sinistral slip is probably in excess of 400 km.