Wrench Fault Research Articles

Analysis of some fault striations in the Proterozoic southern McArthur Basin, Northern territory, with reference to pre‐and post‐Roper group stress fields

Pre‐ and post‐Roper Group regional far field stress patterns in the southern McArthur Basin were related to north‐south/northwest‐southeast and east‐northeast‐west‐southwest compressions respectively. A number of stress states (up to six in the northern Mallapunyah Dome) reflect local stress conditions that were subject to variation around a fault intersection and ‘pop‐out’ in the northern Mallapunyah Dome. Contrasting extensile stress states, due to stress relaxation or rebound effects, were responsible for normal faulting in areas that suffered compression. These stress states represent periods of crustal extension that may have coincided, in part, with Roper Group sedimentation. The northern Mallapunyah Dome, lying at the intersection of the Mallapunyah and Tawallah Faults, had its origins in a combination of processes that included: (i) the emplacement of a sill or laccolith complex of Settlement Creek Volcanics to higher than usual crustal levels; (ii) uplift caused by wrench faulting and west‐ or east‐directed thrusting, giving the dome its present north‐south elongation. Uplift may have been aided by early (pre‐Masterton Sandstone) east‐side‐up movements on the Tawallah Fault contemporaneous with magma emplacement. The fault striation method works best with data sets of 200 or more; however, fewer data (<15) can be used to adequately define the reduced stress tensor in selected areas as long as a wide range of fault orientations is used.

Wrench Faulting in the Northern Pakistan Foreland

The acquisition, processing, and interpretation of reflection seismic, gravity, and magnetic data, in conjunction with detailed geological mapping throughout the Kohat Plateau, have resulted in a reinterpretation of the structural style of this portion of the northern Pakistan foreland. Our interpretation of the recently acquired geologic and geophysical data, as well as a critical reevaluation of previously acquired data and regional geology, indicates that the Kohat Plateau has undergone major episodes of strike-slip faulting, basement-involved reverse faulting, and minor thrust faulting consistent with transpressional tectonics. Our interpretation contrasts with previous models that invoked only southward-directed thrust faulting. The principal key to the reinterpretation of seismic data was depth migration before stack (MBS) seismic processing, which was performed on five seismic lines to resolve the structural geology. MBS-processed seismic data show complex deformation and steep dips more accurately than other methods, and allowed us to identify the high-amplitude structures in the Kohat Plateau. Surface geological mapping in the Kohat Plateau and adjacent areas indicates tight anticlines, consistent steep dips, horizontal motion fabrics, and a conspicuous lack of low-angle thrust faults. Aeromagnetic and gravity data indicate that Precambrian basement is involved in the deformation. Modern seismicity indicates that faults mapped at the surface and subsurface are dominated by strike-slip displacement. The Kohat Plateau is located near the intersection of the transcurrent Chaman fault system, a 1000-km-long system of north-south-striking sinistral strike-slip faults, and east-west-striking contractual structures that form the grain of the Himalayan mountain chain in this area of northwest Pakistan. Surface and subsurface structures fit the expected regional patterns that result from transpressional deformation in this complex zone of intersection. Our reinterpretation has applicability to other deformed belts worldwide that exhibit analogous characteristics and criteria of transpressional tectonics, but are interpreted as being dominated by low-angle thrust tectonics.

Permo-Carboniferous volcanism in Europe and northwest Africa: a superplume exhaust valve in the centre of Pangaea?

The Permo-Carboniferous transition in the European-northwest African province was characterised by widespread volcanism (calc-alkaline with crustal and/or mantle lithospheric characteristics followed by alkaline/subalkaline with HIMU-type signature) with a maximum in the Stephanian-Autunian (Late Pennsylvannian to Early Permian). Extrusion of volcanics was accompanied by massive S-type synextensional granitic intrusions and hydrothermal mineralisation. The geotectonic framework involved the gravitational collapse of the Variscan Belt by extensional detachment tectonics, and its final disruption by wrench faulting. The characteristics of this volcanism are explained within progressively evolving extensional processes. It is suggested that this Permo-Carboniferous Pangaean volcanic province might be interpreted in terms of a superplume impinging on the base of the lithosphere. This model envisages that magmatism acted as an exhaust valve releasing the heat accumulated beneath the Pangaean supercontinent by insulation and blanketing processes which triggered large-scale mantle-wide upward convection and general instability of the supercontinent.

Recent tectonic activity in the South Barbados prism Deep-towed side-scan sonar imagery

Data collected during the DIAPISAR Cruise (Nov. 1990) using the French deep-towed side-scan-sonar (S.A.R.) are presented. These data provide clear evidence of recent tectonic activity in the southern part of the Barbados accretionary prism. Many markers as linear lineaments or ‘sigmoidal features’ which are observed on diagenetically indurated sea-floor are the surficial traces of faults or folds and networks of conjugate fractures where fluids are released through the structures and provide very active venting. These markers can serve as a basis for detailed mapping of some of the tectonic structures and for an analysis of their kinematic evolution. Their geometry suggests the occurrence of reworked folds and shear zones. This domain is affected by abundant clay-diapiric tectonics due to the interaction between: (1) the sedimentological context characterized by the important terrigenous supplies coming from the Orinoco River and building a wide prodelta in which levels of clay supersaturated in water can act as decollement layers, and (2) the geodynamic context of the area situated in the connection zone of three different types of margins. The data presented show that, on the eastern end of the South Caribbean wrench fault system, shear stresses can rework either ‘en echelon’ folds associated with this system or compressional structures due to subduction. The most recent structures seem largely due to the influence of shearing and correspond to transpressive or transtensive deformations rather than pure compressional structures.

Abstract: Chronology of Oil Generation in Louisiana's Fractured Austin Chalk Deep Horizontal Drilling Trend

ABSTRACT Chronologically, the thermal maturation of potential hydrocarbon source rocks beneath basal Austin and pre-Austin strata across central and southeastern Louisiana has yielded a continuum of oil expulsion from Upper Jurassic and younger source beds since the early Cretaceous. Estimation of volume contributions by these maturing strata below the main fractured chalk drilling trend are made by applying regional modeling and numerical simulation analysis. Horizontal drilling targets are situated in a depth band between 12500 and 17500 feet (3800 and 5300 m) that has a width and length of approximately 20 236 miles (32 380 km). In-trend structural and stratigraphic drilling targets are found in areas of enhanced porosity and permeability resulting from fracture development caused by basin evolutionary processes. The primary processes of sedimentary loading, down-warping, and basinward gravity sliding over subjacent Lower Cretaceous barrier reef trends have combined with divergent wrench faulting and hydrocarbon generation/expulsion from underlying source beds to provide the most important zones of fracturing. Although oil generation began in the area before the Eocene, it was not until after development of Upper Cretaceous and Paleocene seal rocks and post-Paleocene fracturing of the chalk that oil began to fill the present reservoirs. Peak generation of oil occurred from Lower Cretaceous source rocks during Paleocene-Eocene time. Another lesser pulse of generation from Upper Cretaceous (mostly Tuscaloosa) source rocks occurred during early and middle Miocene. Hydrocarbon generation from Upper Cretaceous strata continues to the present time.

Geology and genesis of the Aznalcóllar massive sulphide deposits, Iberian Pyrite Belt, Spain

The Aznalcollar mining district is located on the eastern edge of the Iberian Pyrite Belt (IPB) containing complex geologic features that may help to understand the geology and metallogeny of the whole IPB. The district includes several ore deposits with total reserves of up to 130 Mt of massive sulphides. Average grades are approximately 3.6% Zn, 2% Pb, 0.4% Cu and 65 ppm Ag. Mined Cu-rich stockwork mineralizations consist of 30 Mt with an average grade of 0.6% Cu. Outcropping lithologies in the Aznalcollar district include detrital and volcanic rocks of the three main stratigraphic units identified in the IPB: Phyllite-Quartzite Group (PQ), Volcano-Sedimentary Complex (VSC) and Culm Group. Two sequences can be distinguished within the VSC. The Southern sequence (SS) is mainly detritic and includes unusual features, such as basaltic pillow-lavas and shallow-water limestone levels, the latter located in its uppermost part. In contrast, the Aznalcollar-Los Frailes sequence (AFS) contains abundant volcanics, related to the two main felsic volcanic episodies in the IPB. These distinct stratigraphic features each show a different palaegeographic evolution during Upper Devonian and Lower Carboniferous. Massive sulphides occur in association with black shales overlying the first felsic volcanic package (VA1) Palynomorph data obtained from this black shale horizon indicate a Strunian age for massive sulphides, and consequently an Upper Devonian age for the VA1 cycle. Field and textural relationships of volcanics suggest an evolution from a subaerial pyroclastic environment (VA1) to hydroclastic subvolcanic conditions for the VA2. This evolution can be related to compartmentalizing and increasing depth of the sedimentary basin, which may also be inferred from changes in the associated sediments, including black shales and massive sulphides. Despite changes in the character of volcanism, the same dacitic to rhyolitic composition is found in both pyroclastic and subvolcanic igneous series. The main igneous process controlling chemical variation of volcanics is fractional crystallization of plagioclase (+accessories). This process took place in shallow, sub-surface reservoirs giving rise to a compositional range of rocks that covers the total variation range of felsic rocks in the IPB. The Hercynian orogeny produced a complex structural evolution with a major, ductile deformation phase (F1), and development of folds that evolved to thrusts by short flank lamination. These thrusts caused tectonic repetition of massive and stockwork orebodies. In Aznalcollar, some of the stockwork mineralization overthrusts massive sulphides. These structures are cut by large brittle overthrusts and by late wrench faults. The original geometric features of massive sulphide deposits correspond to large blankets with very variable thicknesses (10 to 100 m), systematically associated with stockworks. Footwall rock alteration exhibits a zonation, with an inner chloritic zone and a peripheral sericitic zone. Silicification, sulphidization and carbonatization processes also occur. Hydrothermal alteration is considered a multi-stage process, geochemically characterized by Fe, Mg and Co enrichment and intense leaching of alkalies and Ca. REE, Zr, Y and Hf are also mobilized in the inner chloritic zones. Three ore types occur, both in stockworks and massive sulphides, named pyritic, polymetallic and Cu-pyritic. Of these, Cu-pyritic is more common in stockworks, whereas polymetallic is prevalent in massive sulphides. Zoning of sulphide masses roughly sketches a typical VHMS pattern, but many alternating polymetallic and barren pyritic zones are probably related to tectonics. Although the paragenesis is complex, several successive mineral associations can be distinguished, namely: framboidal pyritic, high-temperature pyritic (300 °C), colloform pyritic, polymetallic and a late, Cu-rich high-temperature association (350 °C). Fluid inclusion data suggest that hydrothermal fluids changed continuously in temperature and salinity, both in time and space. Highest Th and salinities correspond to inner stockworks zones and later fluids. Statistic population analysis of fluid inclusion data points to three stages of hydrothermal activity, at low (<200 °C), intermediate (200–300 °C) and high temperatures (300–400 °C). 34S values in massive sulphides are lower than in stockwork mineralization suggesting a moderate bacterial activity, favoured by the euxinoid environment prevailing during black shale deposition. The intimate relation between massive sulphides and black shales points to an origin of massive sulphides by precipitation and replacement within black shale sediments. These would have acted both as physical and chemical barriers during sulphide deposition. Hydrothermal activity started during black shale deposition, triggered by a rise in thermal gradient due to the ascent of basic magmas. We suggest a three-stage genetic model: (1) low temperature, diffuse fluid flow, producing pyrite-bearing lenses and disseminations interbedded with black shales; locally, channelized high-T fluid flow occurs; (2) hydrothermal cyclic activity at a low to intermediate temperature, producing most of the pyritic and polymetallic ores, and (3) a late high-temperature phase, yielding Cu-rich and Bi-bearing mineralization, mainly in the stockwork zone.

Late Paleozoic transpression in Buenos Aires and northeast Patagonia ranges, Argentina

Paleozoic sediments are present in three regions in eastern central Argentina: (1) the Sierras Australes of Buenos Aires, (2) Sierras Septentrionales of Buenos Aires and (3) Northeast Patagonia. All of these deposits share a common deformational imprint imparted by late Paleozoic Gondwanan deformation. Exposures of these rocks are scattered, variably deformed, and isolated by younger sediments deposited in basins related to the Mesozoic through Tertiary opening of the South Atlantic such as the offshore Colorado Basin. The Sierras Australes of Buenos Aires outcrops are the best preserved. They are mostly located along the Sierras Australes foldbelt, with minor outliers distributed in the adjacent Claromec-basin. The Tunas Formation (early-early late? Permian) is the uppermost unit of the Pillahuincó Group (late Carboniferous-Permian) and is crucial to the understanding of the tectono-sedimentary evolution of the region during the late Paleozoic. The underlying units of the Pillahuincó Group (Sauce Grande, Piedra Azul and Bonete Formations) exhibit a depositional and compositional history characterized by glaciomarine sedimentation and postglacial transgression. They are also characterized by rather uniform quartz-rich compositions indicative of a cratonic provenance from the La Plata craton to the NE. In contrast, the sandstone-rich Tunas Formation has low quartz contents, and abundant volcanic and metasedimentary fragments; paleocurrents are consistently from the SW. Glassrich tuffs are interbedded with sandstone in the upper half of the Tunas Formation. The age of the deformation in the Sierras Australes is Permian and early-middle Triassic. This is based on metamorphic events indicated by formation of illite at 282 ± 3 Ma, 273 ± 8 Ma, 265 ± 3 Ma, and 260 ± 3 Ma ( K Ar illite) in the Silurian Curamalal Group. Evidence of syntectonic magmatism is provided by a radiometric date of 245 ± 12 Ma ( K Ar hornblende) for the López Lecube Granite, immediately west of the Sierras Australes. In the Sierras Septentrionales of Buenos Aires, Precambrian through early Paleozoic deposits of La Tinta, Sierras Bayas, Las Aguilas and Balcarce Formations rest on Precambrian crystalline basement of the La Plata craton. These exposed rocks are affected by subordinate, right lateral wrench faulting; some thrusting indicates tectonic transport toward the NE. In northeast Patagonia (Sierra Grande region) synkinematic deformation of early Permian (261 ± 5 Ma, Rb Sr whole rock) age has been identified in Silurian metasediments of the Sierra Grande Formation. Bands of deformation in Sierra Grande quartzites indicate right lateral wrenching in a N-S direction. Contraction in a NE-SW direction is evidenced by folding. Three stages of tectonic evolution can be discerned for the above regions: (1) Early Paleozoic platform sedimentation, punctuated by episodes of accelerated subsidence during the Silurian and early Devonian, as shown by transgressive episodes, (2) late Paleozoic sedimentation and deformation, and (3) Meso-Cenozoic extensional inversion due to the South Atlantic opening. The late Paleozoic sedimentation and deformation (stage 2) includes late Carboniferous-earliest Permian glacial deposits of the Sierras Australes and Colorado offshore basin, deposited during an initial phase of extension, and cratonward foreland subsidence triggered sedimentation of the synorogenic deposits of the Permian Tunas Formation. Tuffs are intercalated in the upper half of this unit. These tuffs are associated with the silicic volcanism along the Andes and Patagonia (Choiyoi magmatic province) that peaked between the late early Permian and late Permian. Likewise, the first widespread appearance of tuffs in the Karoo basin is in the Whitehill Formation, of late early Permian (260 Ma) age. The deformation described in this paper can be considered as part of a large scale intracontinental deformation in SW Gondwanaland inboard of an Andean-type compressive margin. This deformation is characterized by transpression (right lateral wrenching) combined with overthrusting to the NE and N-S horizontal contraction.

Structural evolution of the Paleoproterozoic Rio Itapicuru granite-greenstone belt (Bahia, Brazil): the role of synkinematic plutons in the regional tectonics

The Paleoproterozoic structural evolution of the Rio Itapicuru greenstone belt (RIGB, Bahia, Brazil) involves two tectonic events of different natures, both associated with magmatism. The first event (D1) developed in response to NW-SE shortening and was responsible for subhorizontal foliation, NW-SE trending lineation and SE-directed thrusts. D1 was coeval with the emplacement of rare 2130 Ma granodioritic plutons. The D2 tectonic event was characterized by left-lateral strike-slip tectonics, developed on steeply dipping D1 foliation and within numerous NS elongated ca 2080 Ma plutons. Quartz c-axis analysis within the granites shows that they experienced progressive left-lateral shearing from magmatic to solid-state conditions under decreasing temperatures. This fact, in addition to microstructural arguments, indicates syntectonic emplacement of these plutons. A zone of maximum deformation (the Main Shear Zone) has been identified in the centre of the RIGB on the basis of strain measurements. A tectonic model in which D1 thrusting event initiated basin closure before being replaced by D2 strike-slip motion is proposed. The Main Shear Zone corresponds successively to the main thrust zone and the main wrench fault during D1 and D2 events, respectively. Such a switch from orogen-normal thrusts to orogen-parallel transcurrent movements may represent a common progression during evolution of mountain belts in order to accommodate excess crustal thickness. However, because low-grade metamorphism and low intensity of the deformation demonstrate that excess crustal thickness was not attained, it is argued that granite emplacement and ascent played a significant role in triggering strike-slip tectonics in the RIGB.

Divergent Wrench Faulting in the Belize Southern Lagoon: Implications for Tertiary Caribbean Plate Movements and Quaternary Reef Distribution: Discussion

Analysis of 500 km of multichannel seismic reflection data in the Southern Lagoon of Belize reveals that its Tertiary structural evolution has been punctuated by two major transtensional episodes. First, left-lateral strike-slip motion parallel to the coast, together with a major component of extension, took place during the Paleocene, producing a series of southwest-northeast-trending en echelon normal faults, which created a topographic relief of ridges and troughs. Both the types of structures and their timing of formation are consistent with models that propose that Cuba swept past the eastern coast of Belize and the adjacent Yucatan peninsula, moving along a left-lateral transform boundary, until it collided with the Bahamas platform. The second tectonic event occurred during the Pliocene and/or later, revealing very recent activity along the North American-Caribbean left-lateral transform boundary. High-angle normal faults with a southwest-northeast orientation dominate the style. A comparison between the deep structure and the present bathymetry of the lagoon reveals that portions of the barrier platform and elongate to rhombohedral platform atolls, such as Laughing Bird Cay, are located over ancient structural highs, whereas bathymetric lows, such as the Victoria Channel, are long-lived troughs. This suggests that Quaternary reef foundations are deeper than previously thought, and that structure exercised a major control on sedimentation throughout the Tertiary evolution of the Southern Lagoon.

Divergent Wrench Faulting in the Belize Southern Lagoon: Implications for Tertiary Caribbean Plate Movements and Quaternary Reef Distribution: Reply

Divergent Wrench Faulting in the Belize Southern Lagoon: Implications for Tertiary Caribbean Plate Movements and Quaternary Reef Distribution: Reply

Deformation and stress states along the central segmentof the Philippine Fault: implications to wrench fault tectonics

This paper discusses the space-time evolution of deformation and states of stress along the central segment of the left-lateral Philippine Fault from Bondoc Peninsula to Leyte Island. This segment is characterized by deformations and stress states which do not always conform to predictions of classical wrench tectonics. Directions of the maximum principal stress axis σ1 are, in some cases, oriented perpendicular to the fault trace, contradicting the classical fault strike σ1 angle of 30–45°. In other cases, it is the minimum axis σ3 that orients perpendicular to the fault or, in other cases, parallel to it. Rotations of σ1 directions as functions of time and space are observed all along the fault. More complex stress regimes are observed in areas where the Philippine Fault intersects with two other major left-lateral strike-slip faults, namely the Legaspi Lineament in the Ragay Gulf area and the Sibuyan Sea Fault in Masbate. These observations strongly underline the urgence of prudence and precaution in applying wrench fault tectonic principles in natural examples.

Stress-induced late-stage subsidence anomalies in the Pannonian basin

Subsidence, sedimentation and tectonic quiescence of the Pannonian basin was interrupted a few million years ago by tectonic reactivation. This recent activity has manifested itself in uplift of the western and eastern flanks, and continuing subsidence of the central part of the Pannonian basin. Low- to medium-magnitude earthquakes of the Carpathian-Pannonian region are generated mostly in the upper crust by reverse and wrench fault mechanisms. There is no evidence for earthquakes of extensional origin. 2-D model calculation of the subsidence history shows that a recent increase in magnitude of horizontal compressional intraplate stress can explain fairly well the observed Quaternary uplift and subsidence pattern. We propose that this stress increase is caused by the overall Europe/Africa convergence. In Late Pliocene, consumption of subductible lithosphere at the eastern margin of the Pannonian basin was completed, and the lithosphere underlying the Pannonian basin became locked in a stable continental frame. Consequently extensional basin formation has come to an end, and compressional inversion of the Pannonian basin is in progress.

Mechanisms of inheritance of rift faulting in the western branch of the East African Rift, Tanzania

The western branch of the East African Rift system is commonly cited as a result of Phanerozoic reactivation of the Paleoproterozoic Ubendian belt in western Tanzania. Geological evidence is provided to show that prominent mechanical anisotropies successively appeared during Proterozoic evolution of the Precambrian basement and that their different reactivation behavior contributed to the Phanerozoic rift pattern. The Ubende belt (1950–1850 Ma) is a NW oriented, amphibolite facies ductile lateral shear belt in which older (2100–2025 Ma) and complex granulite facies terranes are included along trend. Retrograde multiphase sinistral strike‐slip mylonites developed along the NW oriented ductile shear belt. They reflect persistent Proterozoic wrench fault reactivation of the latter. Shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the Proterozoic wrench fault reactivation. Neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone, which appears as an elongate and NW trending positive flower structure, locally enhanced by late Proterozoic contraction. Phanerozoic rifting is demonstrated by others to occur in three distinct episodes, during which the complex rift segment formed upon the multiphase Proterozoic wrench fault zone. The evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered. The Proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern, which became selectively reactivated by sinistral transpressional ductile‐brittle mylonites. Proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults. According to the position of these mylonites in the center or in the external parts of their NW oriented Neoproterozoic transpression, they reactivate as complex and multiphase rift faults or as normal and recent faults, respectively. The Paleoproterozoic NW oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy. Its reactivation in Phanerozoic stress fields is likely dextral oblique transtension, considered as a leading mechanism of the pluriphase and NW oriented deep rift basins.

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.

Early Miocene thin‐skinned tectonics and wrench faulting in the Pongaroa district, Hikurangi margin, North Island, New Zealand

Abstract The Pongaroa‐Akitio area, Northern Wairarapa, North Island, New Zealand, is part of the exposed East Coast Deformed Belt at the obliquely convergent plate boundary of the Hikurangi margin. The sedimentary succession includes an allochthonous unit of Early Cretaceous greywacke basement resting on latest Cretaceous rocks. Since the unit's basal contact is subparallel to the bedding of the strata it overlies, the allochthon is inferred to be an unrooted gliding nappe similar to allochthonous outliers described in Northland and the Raukumara Peninsula. The southward emplacement of this “Greywacke Nappe” is supported by structural markers in the body of the nappe and is well dated as earliest Miocene by the youngest rocks involved, which are earliest Miocene (Waitakian; c. Aquitanian), and because Otaian‐Altonian (c. Burdigalian) faults postdate nappe emplacement. This thin‐skinned tectonic phase immediately preceded inception of dextral strike‐slip faulting along northeast‐trending Otaian‐Altonian (Burdigalian) faults. The present 300 km offset of similar allochthonous outliers on both sides of the faults of the western coastal ranges results from cumulative dextral strike‐slip movement on these faults through the Miocene.

Age estimation of the Cottage Grove Fault System (Illinois, U.S.A.) from high-resolution seismic reflection studies of coal measures

Geologic and high-resolution seismic reflection studies of Pennsylvanian coal deposits have provided new insights concerning the Cottage Grove Fault System (CGFS), a major east-west oriented wrench fault system in southern Illinois. The age of faulting has previously been estimated to be late Pennsylvanian to early Permian, based upon KAr dating of igneous intrusive rocks associated with subsidiary faults. Although these dates are accurate for intrusions and faulting at certain locations, the duration of Cottage Grove master fault zone or subsidiary faulting is still unclear. High-resolution seismic reflection profiles were collected approximately 6.4 km north of the master fault zone in the vicinity of mapped subsidiary faulting to study coal deposits. Several small-offset, high-angle normal and reverse faults were interpreted from the seismic records. Two small-offset fault zones observed on the seismic records, interpreted to be subsidiary faults of the CGFS, bound a gentle asymmetric anticline. Several small sandstone channels located on the down-thrown sides of these faults are interpreted from the seismic data. Vertical stacking of these sandstone channels and other sands within the coal measures is also observable in the correlated well data. Location of channel deposits suggests depositional control by these faults, indicating that subsidiary faulting was contemporaneous with sand deposition. Thus, subsidiary faulting of the CGFS occurred as early as middle Pennsylvanian time.

A pull apart volcanic related tertiary basin, an example from the Patagonian Andes

The Oligocene — Miocene Ñirihuau basin is located along the eastern foothills of the Patagonian Andes (41 °–42 ° S.L.) to the east of an Oligocene volcanic belt formed during a slow period of oblique convergence between the South American and the Farallón plates. The basin is a pull-apart wrench fault furrow depression located to the east of the active (Pacific) continental margin, between the volcanic chain (magnetic arc) and the cratonic area, the sedimentary fill was mainly controlled by tectonism and synchronous volcanic activity. Most sediments in the Ñirihuau Basin are continental volcaniclastics, associated to primary pyroclastites, lava flows and carbonate sediments. Three tectosedimentary stages of evolution have been defined for the Ñirihuau Basin infill: 1) Initial stage, characterized by a deep, narrow and elongated trough formed by rotation and translation along very active strike-slip fault-systems. The sedimentary record is composed of two (fan-delta and lake) facies associations. 2) Intermediate stage, with increasing and more uniform subsidence, and consequent enlargement of the basin along its major axis. A longitudinal fluvial system prograding from the north, and associated to proximal laharic faciés and distal lacustrine facies is inferred. 3) Final stage, characterized by slight subsidence and enlargement of the basin both along and across its major axis. The sedimentary fill is composed of primary and reworked pyroclastics deposited as aeolian dust (loess) sediments as well as in high sinuosity fluvial systems, flood-plains and shallow lakes.

Lithospheric extension northwest of the Central African Shear Zone in Sudan from potential field studies

The Central African Shear Zone (CASZ) is a 4000-km-long, NE-striking wrench fault system, along which motion occurred during the break-up of Gondwanaland, but its location in central Sudan was poorly defined. Prior to this study, it was thought that Mesozoic extension was restricted to Sudan south of the mapped and proposed location of the CASZ. We use nearly all gravity data from Sudan, including 1894 unpublished gravity points in eastern Sudan to (1) clarify the position of the CASZ in central and eastern Sudan and to (2) interpret anomalies northwest of the CASZ with similar trends and amplitudes as those basins known to occur southeast of the CASZ. Considering the dimensions of these gravity minima and existing geological, aeromagnetic, magneto-telluric, seismic and well data, we suggest that fault-bounded sedimentary basins lie north of the CASZ in central and eastern Sudan. We construct geological models of the Bara, Bagbag, Humar, Gilif and Abu Dulu rift basins to investigate the subsurface geometry of these previously undetected basins, constraining models with existing geological and geophysical data. The detrended gravity profiles can be fit by 40–60-km-wide basins bounded on one or both sides by normal fault systems, and with depths of 1.5–3.5 km. These patterns suggest that the CASZ in central and eastern Sudan is a broader zone of deformation than had previously been considered. The < 10 km cumulative Mesozoic extension within the African plate north of the CASZ implied by these studies, however, will not change significantly current plate models for the break-up of Africa and South America.

Changes in tectonic stress field in northern Sunda Shelf basins

Abstract The Tertiary basins of the northern Sunda Shelf are underlain by normal and attenuated continental crust that is characterized by moderate to high average geothermal gradients in excess of 5°C/100 m. In the Malay basin, upper Oligocene and younger sediments are more than 12 km thick; in the other basins, such sediments are between 4 and 8 km thick. The Malay, Penyu and West Natuna basins are aulacogens meeting at a triple junction that marks a Late Cretaceous hot spot in the centre of the Malay Dome. Sub-basins (commonly half-graben) developed as pull-apart basins within regional, north to northwest-striking, wrench fault zones. The NW-striking Three Pagodas fault most probably extends farther southeast as the Axial Malay fault in the basement along the length of the Malay basin. Pre-late Oligocene sinistral slip along this fault developed east-west half-graben that fixed the position and orientation of inverted anticlines that later developed in the basin-filling sediments through strike-slip reversal along the fault. Initial basin subsidence took place during Eocene-Oligocene time. Regional tensional conditions prevailed until the early Miocene. During the middle to late Miocene, regional compressional stresses caused reversals of the sense of motion on the major wrench faults, and structural inversion of the basin-filling sediments. On some of the north-striking wrench faults there are indications of up to 45 km of right-lateral displacement which is possibly post-Miocene. The regional wrench faults have acted as domain boundaries with each tectonic domain characterized by different stress fields. The stress systems evolving during the Cenozoic are attributed to varying degrees of interference of plates coupled with changes in convergent directions and/or rates of motion of the Pacific plate, the Indian Ocean-Australian plate, and continued, differential extrusion of SE Asian crust following the collision of the Indian plate with the Eurasian plate.

Role of pre-Tertiary fractures in formation and development of the Malay and Penyu basins

Abstract Major faults in Sundaland trend NNW to NW, WNW, N and E. Some of the NNW to NW and N-striking faults across Mesozoic areas of the Malay Peninsula were active until mid-Eocene time. Small, fault-bounded Tertiary basins onshore may be pull-apart basins associated with such faults. Mainly from seismic data, NNW to NW, N and E-striking faults have been recognized in the pre-Tertiary basement of the Malay and Penyu basins off the east coast of the peninsula. These faults were reactivated before the Late Oligocene and during the Middle to Late Miocene. N-striking faults in pre-Tertiary areas are common throughout Sundaland. In the field, these faults are found to be the oldest (possibly Jurassic) regional fractures. The regional NNW-NW and WNW fractures are believed to have originated as strike-slip faults when the peninsula was subjected to late Mesozoic deformation. The onshore E-W faults were probably extensional fractures that developed as secondary structures associated with sinistral slip motions along NNW-NW faults. Upper Cretaceous dolerite dykes fill some of the E-W fractures. NW-striking basement faults of the Malay basin continue onshore SE Asia as the Three Pagodas fault zone. Initially these were sinistral basement wrench faults creating secondary E-W extensional fractures. In the Middle to Late Miocene the regional stress field changed, resulting in reversal of slip movement along major wrench faults and structural inversion of the sedimentary basins. This inversion is manifested as E-W anticlines located over half-graben. In the Penyu basin similarly striking half-graben probably developed in the same fashion. There, the NW-striking Rumbia fault divides the basin into two parts. Half-graben in the western part remained orientated E-W, but those in the eastern part became rotated clockwise by continued left-lateral slip along the Rumbia fault. After the Miocene the two basins continued to subside, developing an almost undisturbed blanket of post-Miocene sediments. Locally, residual stress caused some of the structures to grow.