Dextral Transcurrent Fault Research Articles

Transcurrencia y <em>mélange</em> tectónica en el área de Sierra Arana (Cordillera Bética, NE de Granada)

En el área de Sierra Arana las fallas transcurrentes dextrorsas forman un conjunto de largas y estrechas bandas tectónicas de dirección E-O, varias de las cuales están rellenas por sedimentos oligo-aquitanienses y cretácicos encajados entre carbonatos jurásicos. Estas bandas forman una “estructura en flor”, en cuya parte superior, materiales cortados por las fallas cayeron sobre sedimentos oligo-aquitanienses, formándose una mélange tectónica. La zona de crestas de Sierra Arana corresponde a un gran anticlinal invertido vergente al sur. Los materiales situados directamente al S de Sierra Arana, bien se les denomine Subbético muy interno, bien Dorsal, parecen mostrar la transición paleogeográfica del Subbético al Complejo Maláguide.

Geology of the orogenic Cheminis gold deposit along the Larder Lake – Cadillac deformation zone, Ontario

The Larder Lake – Cadillac deformation zone (LLCDZ) is one of two major, auriferous, deformation zones in the southern Abitibi subprovince of the Archean Superior Province. It hosts the Cheminis and the giant Kerr Addison – Chesterville deposits within a strongly deformed band of Fe-rich tholeiitic basalt and komatiite of the Larder Lake Group (ca. 2705 Ma). The latter is bounded on both sides by younger, less deformed, Timiskaming turbidites (2674–2670 Ma). The earliest deformation features are F1 folds affecting the Timiskaming rocks, which formed either during D1 extensional faulting or during early D2 north–south shortening related to the opening and closure, respectively, of the Timiskaming basin. Continued shortening during D2 imbricated the older volcanic rocks and turbidites and produced regional F2 folds with an axial planar S2 cleavage. D2 deformation was partitioned into the weaker band of volcanic rocks, producing the strong S2 foliation, L2 stretching lineation, and south-side-up shear sense indicators, which characterize the LLCDZ. Gold is present in quartz–carbonate veins in deformed fuchsitic komatiites (carbonate ore) and turbiditic sandstone (sandstone-hosted ore), and in association with disseminated pyrite in altered Fe-rich tholeiitic basalts (flow ore). All host rocks underwent strong mass gains in CO2, S, K2O, Ba, As, and W, during sericitization, carbonatization, and sulphidation of the host rocks, suggesting that they interacted with the same hydrothermal fluids. Textural relationships between alteration minerals and S2 cleavage indicate that mineralization is syn-cleavage. Thus, gold was deposited as hydrothermal fluids migrated upward along the LLCDZ during contractional, D2 south-side-up shearing. The gold zones were subsequently modified during D3 reactivation of the LLCDZ as a dextral transcurrent fault zone.

Microstructures and K-Ar illite fine-fraction ages of the cataclastic rocks associated to the Camburu Shear Zone, Ribeira Belt, Southeastern Brazil

The Camburu Shear Zone (CSZ) is recognized as a NE-SW trending dextral transcurrent fault and is related to the evolution of the Ribeira Belt, southeastern Brazil. The CSZ ductile regime dates from the Precambrian to the Cambro-Ordovician and it is characterized by a meter to hundreds of meters wide belt of quarzto-feldspathic mylonitic rocks organized in a positive-flower structure. Previous studies postulated that the fault underwent normal reactivation, probably during the Cenozoic, developing metric to centimetric thick intercalations of cemented cataclastic rocks. We concluded that these cataclastic rocks maintained their primary cohesion and were syntectonically silicified by feldspar-to-mica reactions. Cataclasis was shown by the fractal distribution of grains and fragments, dilatancy, rigid body rotation, cataclastic flow, microcracking and microfaulting. Two cataclastic events are described: (i) during the Lower Devonian (402 ± 6 Ma) in a ductile-to-brittle regime with temperatures up to 400°C and development of foliated protocataclasites, and (ii) during the Upper Carboniferous (310.9 ± 8 Ma) in a brittle regime with temperatures up to ~300°C and development of protocataclasites and crushed breccias. It is then suggested that the CSZ cataclasites were formed during two Paleozoic reactivation events associated to a far-field orogenic effect induced by the Precordillera and Chanica orogenic cycles in the western margin of the Gondwana Paleocontinent.

Fluid regimes, fault-valve behavior and formation of gold-quartz veins — The Morro do Ouro Mine, Ribeira Belt, Brazil

Fluid inclusion studies, petrographic analyses and structural analyses for Neoproterozoic auriferous veins from the Morro do Ouro mine, Ribeira Belt, southeastern Brazil, are presented. The deposit is classified as an orogenic gold deposit. The vein-type Au mineralization at the mine is restricted to structurally controlled domains in a low-grade Calymmian (ca. 1500–1450Ma) metasedimentary sequence that also hosts syngenetic Au mineralization. Two main auriferous quartz vein systems are present: (i) NW-trending subvertical extensional veins and (ii) NE-trending subvertical veins parallel to the axial surface of tight folds. The mineralized veins are adjacent to a major dextral transcurrent fault zone, and their structural relationships indicate that this fault is severely misoriented for frictional reactivation. The NW-trending veins present a fluid inclusion assemblage dominated by CO2N2CH4 inclusions, H2OCO2N2CH4NaClCaCl2 inclusions and H2ONaClCaCl2 inclusions, both with highly variable salinities (0.5 to 52wt.% NaCl equivalent), while the NE-trending veins are dominated by H2OCO2N2CH4NaClCaCl2 and H2ONaClCaCl2 inclusions also of highly variable salinities (1 to 45wt.% NaCl equivalent). The entrapment of fluids occurred at temperatures between 225 and 240°C in the NW-trending veins and at approximately 208°C in the NE-trending veins, involving processes of fluid immiscibility and mixing between fluids of contrasting compositions. The NW-trending extensional veins were formed predominantly under pressure fluctuating between near-lithostatic and strongly supralithostatic conditions (100–250MPa) during pre-seismic failure stages. The NE-trending veins precipitated predominantly under subhydrostatic to lithostatic pressure conditions (10–150MPa), following the discharge of fluids from different reservoirs along the ruptured zone after earthquake rupture stages. The hydrothermal fluids were probably enriched with gold through interactions with the host rocks, and its precipitation is attributed to changes in physicochemical properties due to fluid immiscibility and mixing between fluids of highly contrasting salinities, as a consequence of cyclic fluctuations in the values of fluid pressure and tectonic stresses accompanying episodes of seismogenic fault activity.

Tectonic settings of porphyry Cu–Mo–Au deposits in the Himalayan–Tibetan orogen, East Tethys

Porphyry Cu (Mo–Au) deposits in the Himalayan–Tibetan orogen formed during the Late Triassic, Early Cretaceous, Eocene, Oligocene, and Miocene and can be classified into different metallogenic belts according to their petrologic features, mineralization ages, and tectonic settings. A close spatial relationship to regional strike–slip faults is evident in all five belts. Porphyry Cu (Mo–Au) deposits exist in a wide range of tectonic environments, including island arc, syn-collision, post-collisional convergence, and continental-transform plate boundaries. Porphyry Cu deposits cluster in the southernmost part of the Yidun–Zhongdian Belt, along the N–S-trending Gaze River dextral strike–slip fault. Porphyry Cu deposits in the Lijiang–Jinping Belt lie along the Ailaoshan–Red River continental–transform shear zone and the associated strike–slip faults. The Yulong–Malasongduo porphyry belt is controlled by the Cesuo Fault, a NNW-trending regional dextral transcurrent fault that is associated with Palaeogene westward continental oblique subduction along the Jinsha suture. In the Gangdis Belt, Miocene porphyry Cu deposits are localized along N–S-trending normal faults, which were produced by transpression within the regional NW–SE-trending Karakoram–Jiali fault zone (KJFZ). A close spatial relationship between porphyry Cu deposits and strike–slip faults also exists for the Bangong–Nujiang Belt.

Anisotropy of magnetic susceptibility study in two classical localities of the Gastre Fault System, central Patagonia

Abstract The Central Patagonian Batholith is a suite of acid and meso-silicic rocks cropping out in central Patagonia. The emplacement of these rocks has been proposed to be related to the activity of a system of dextral transcurrent faults, the NW-SE Gastre Fault System. This fault system has been ascribed a transcontinental magnitude and a ∼500 km dextral displacement during Gondwana dismembering in Jurassic times. However, the timing, kinematics and amount of displacement of the Gastre Fault System are still controversial. In this work we have visited two localities which were subject of controversial observations, in order to perform petrographical, microstructural and anisotropy of the magnetic susceptibility studies to contribute to the ongoing discussion. The results mostly agree with the findings von Gosen and Loske (2004) in that rocks spatially and temporally associated to the Gastre Fault System do not show evidence supporting the existence of a major dextral fault system active during Jurassic times.

Chapter 7 Tertiary stratigraphy

The purpose of this account is to review the complex terminology of the Tertiary stratigraphic units in Sumatra and propose a revised and a simplified terminology based on the significance of formations for the tectono-stratigraphic development of the island. Formations are classified in terms of Pre-Rift, Horst and Graben, Transgressive, and Regressive tectono-stratigraphic stages. The island of Sumatra lies along the southwestern margin of the SE Asian continent (Sundaland) beneath which the Indian Ocean Plate is currently being subducted at a rate of about 7 cm a -1 in the Sunda Trench (Fig. 7.1). The continental margin of SE Asia is of Andean type, with active and inactive Quaternary volcanoes rising to over 3000 m above a Pre-Tertiary basement, exposed towards the west coast of the island in the Barisan Mountains. Tertiary sedimentary basins occur both to the SW and the NE of the mountains and small basins also occur within the mountain range itself. These basins are described with relationship to the present-day subduction system as forearc, backarc and intra-arc or intramontane basins (Fig. 7.1). The Barisan Mountains are transected by the Sumatran Fault System, a major dextral transcurrent fault zone which extends along the length of the island from the Sunda Strait to the Andaman Sea. Stratigraphic research in the Tertiary sedimentary basins commenced in the last decades of the nineteenth century when oil was discovered in the Telaga Tiga (1883) and Telaga Said (1885) wells near Pangkalan Brandan in North Sumatra. Initially, wildcat drills were sited near oil

Break Up of Australia-India-Madagascar Block, Opening of the Indian Ocean and Continental Accretion in Southeast Asia With Special Reference to the Characteristics of the Peri-Indian Collision Zones

Linear belts of Gondwana basins developed in the Indian continent since Late Palaeozoic along favoured sites of Precambrian weak zones like cratonic sutures and reactivated mobile belts. The Tibetan and Sibumasu - West Yunnan continental blocks, that were located adjacent to proto-Himalayan part of the Indian continent, rifted and drifted from the northern margin of the East Gondwanic Indo-Australian continent, during Late Palaeozoic, when the said northern margin was under glacial or cool climatic condition and rift-drift tectonic setting. The Indo-Burma-Andaman (IBA), Sikule, Lolotoi blocks were also rifted and drifted from the same northern margin during Late Jurassic. This was followed by the break-up of the Australia-India-Madagascar continental block during the Cretaceous. The activity was associated with hot spot related volcanism and opening up of the Indian Ocean. The Late Cretaceous and Tertiary phases of opening of the Arabian Sea succeeded the Early Cretaceous phase of opening of the Bay of Bengal, part of the Indian Ocean. The Palaeo- and Neo-Tethyan sutures in Tibet, Yunnan, Laos, Thailand and Vietnam reveal the complex opening and closing history of the Tethys. The IBA block rotated clockwise from its initial E-W orientation because of 90°E and adjacent dextral transcurrent fault movements caused due to faster northward movement of the Indian plate relative to that of Australia. The India-Tibet terminal collision during Early-Middle Eocene initiated Himalayan orogenesis and contemporaneously there was foreland basin development that was accompanied with sporadic but laterally extensive continental-flood-basalt (CFB) type and related volcanism. The Paleogene rocks of the Himalayan foreland basin are involved in tectonism and are mostly concealed under older rocks. The Mesozoic-Early Eocene ophiolite terrane on IBA does not represent the eastern suture of the Indian plate but occurs as klippe on IBA, caused due to oblique collision between Sibumasu and IBA during Late Oligocene. Post-collisional indentation of Y-shaped Indian continent into the Asian collage produced Himalayan syntaxes, clockwise rotation of the Sibumasu block which was then sutured to the Tibetan and SE Asian blocks, and tectonic extrusion of the Indochina block along the Ailao Shan Red River (ASRR) shear zone. Highly potassic magmatic rocks were emplaced during Late Palaeogene at the oroclinally flexed marginal parts of the South China continental lithosphere. These magmatic bodies were dislocated by the ASRR left lateral shear zone soon afterwards. Petrogenetic and tectonic processes that generated the Eocene CFB volcanics at the Himalayan foreland basin may have also produced Late Palaeogene magmatism from outer parts of the Namche-Barwa Syntaxis. Their site-specific location and time sequence suggest them to be genetically related to the India-Asia collision process and Indian continent's indentation-induced syntaxial buckling. Deep mantle-reaching fractures were apparently produced during India-Asia terminal collision at the strongly flexed leading brittle edge of the Indian continental lithosphere, and possibly later in time at the outer oroclinally bent marginal parts of the rigid South China continental lithosphere, generating typical magma. The subduction zone that developed along the western margin of IBA due to oblique convergence between the IBA and the Indian plate is still active. The northern end of IBA ultimately collided with the NE prolongation of the Indian continent and was accreted to it during Mio-Pliocene. The Shillong massif was uplifted and overthrust over the Bengal Basin located over its passive margin to the south, whereas, the Eocene distal shelf sediments of IBA were overthrust over the Tertiary shelf of the Indian continent.

Break-up of the Greater Indo-Australian Continent and accretion of blocks framing South and East Asia

The Tibetan and ‘Sibumasu’ continental blocks rifted apart from the northern margin of Gondwanan Indo-Australia during the Permo-Carboniferous whereas, the IndoBurma-Andamans (IBA), Sikuleh, Lolotoi micro-continents did so during the Late Jurassic. This continental margin experienced glacial or cool conditions during the Permo-Carboniferous. The Tibetan and Sibumasu blocks drifted northward during mid-late Permian initiating opening of the Neo-Tethys. The arm of the Palaeo-Tethys located to the north of these blocks closed as these blocks were accreted to the South China and Indochina blocks which had separated from the same Gondwanan margin during early Palaeozoic. All these blocks were amalgamated to form the Cathaysialand. The Sibumasu block was accreted to the Indochina block, and the Tibetan Changtang block to Eurasia during the late Permian-mid Triassic. Contemporaneously Mesozoic Neo-Tethys expanded between the Indian, Lhasa and Changtang blocks. The Indian and Australian continents separated during the Cretaceous leading to the opening up of the Indian Ocean and closing of the Tethyan ocean. The Palaeo- and Neo-Tethyan sutures in Tibet, Yunnan, Myanmar, Laos-Thailand and Vietnam reveal the complex opening and closing history of the Tethys. IBA rotated clockwise from its earlier E-W orientation, because of dextral transcurrent fault movements which ensured faster northward movement of the Indian plate relative to Australia during late Cretaceous-early Eocene. Contemporaneous to India-Tibet terminal collision during early-mid Eocene there was thloeiitic-alkalic foreland volcanism (Abor and equivalents) at the leading edge of the Indian continent. Sustained post-collisional movement of the Indian plate, caused southward propagation of the Himalayan crystalline and frontal foreland thrust sheets. It also produced E-W trending folds and thrusts even in distal Central Indian Ocean areas as well as a clockwise rotation of the amalgamated SE Asian Cathaysian composite block. Dislocations of the 90 °E Ridge, indicate that the main compression occurred during early Eocene which was followed by movement since late Miocene. The ophiolite trail on IBA does not represent the eastern suture of the Indian continent. As a result of Late Oligocene oblique terminal collision between Sibumasu and 1BA, Mesozoic early Eocene ophiolites. their mid-Eocene cover and trench sediments occur as klippen on IBA. Oblique convergence between IBA and the Indian plate in turn produced an active subduction regime along the western margin of the Indo-Burma mobile belt and the Andaman-Java trench. The activity produced Neogcne-Quaternary volcanism. dextral strike-slip movement in Central Burma-Andaman-Sumatran region, and opened up the Andaman sea. Northward, the Shillong massif, representing NE prolongation of the Indian sub-continent, is technically juxtaposed against the northern end of IBA. The collision of these blocks might have uplifted the Shillong massif during Mio-Pliocene. Far eastward, the convergence was orthogonal between the Australian continent and the Indonesian Arc resulting emplacement of the Lolotoi continental rocks. Maubissi exotic blocks and ophiolitic rocks as nappe over the Timor shelf which possibly remained attached to the Australian continent.

Goringe-Alboran-Tell tectonic zone: A transpression system along the Africa-Eurasia plate boundary

Neotectonic structures distributed from the Goringe bank to the Tell Atlas mountains show that the Africa-Eurasia plate boundary in the western Mediterranean corresponds to an east-west–trending deforming zone (Goringe-Alboran-Tell). Main Pliocene and Quaternary tectonic structures of the northern Atlas Mountains are northeast-southwest–striking fault-related folds, similar to that of El Asnam, and are related to north-south to northwest-southeast compressional movements. Active tectonics in agreement with the main seismicity distribution and focal mechanisms also support an oblique plate convergence and the existence of a 50–100-km-wide transpression zone from Goringe, across Alboran, to the Tell Atlas mountains. A kinematic model shows the interaction between dextral transcurrent faults and left-lateral faults with block rotations and no lateral extension (pinned model). Shortening and lateral movement rates during the Quaternary are estimated to be 1.0–2.3 mm/yr, and block rotation rate is estimated as 2.5°–3.9° /m.y. The plate boundary may correspond to a transpression zone, active thrust faults being controlled by deep-seated dextral transcurrent faults.

Ramp-flat geometry within the central Kapuskasing uplift? Evidence from potential field modeling results

Potential field data constrained by seismic reflection, seismic refraction, geologic, and physical-properties data have been used to study the deep crustal structure of the central Kapuskasing uplift. The Val Rita block is interpreted as a thrust sheet of granulites detached at mid-crustal depths and uplifted along a ramp producing an arched, double granulite layer. A thin thrust sheet of southward-thickening granulites best describes the Groundhog River block, whereas simple thrusting along a ramp characterizes the Chapleau block. From this interpretation, a sequential evolution of the Kapuskasing uplift is proposed.At about 2450 Ma, northwest–southeast compression (σ1) resulted in variable thrusting along the Ivanhoe Lake fault zone. This was followed by a period of normal faulting, resulting in the juxtaposition of high- and low-pressure rocks across the Lepage and Saganash Lake faults, with possible truncation of the Val Rita block arch. Further rotation of σ1 to an east–west direction resulted in the reactivation of the Lepage and Saganash Lake faults as dextral transcurrent or transpressional faults accentuating the Val Rita block as an en echelon fold, and distorting the Matachewan dykes. Later (about 2140 Ma), rotation of σ1 into a southwest–northeast direction brought transpression to a halt and a period of intermittent dyke and carbonatite intrusion was initiated, which lasted to about 1200 Ma or just prior to the Grenville orogeny.

Contrasting accreted terranes in the southern Appalachian Orogen, basement beneath the Atlantic and Gulf Coastal Plains, and West African orogens

A variety of lithostratigraphic units may be outlined within the pre-Mesozoic crystalline basement of the Atlantic and Gulf Coastal Plains. These include: (1) a group of metamorphic rocks of variable grade together with deformed and retrogressed granite in southwestern Alabama and southeastern Mississippi (Wiggins Uplift). Hornblende and biotite within higher-grade units record 40Ar/ 39Ar post-metamorphic cooling ages of ∼310−300 Ma; (2) a suite of contrasting igneous rocks (granite, basalt, and agglomerate) and serpentinite which occurs along the Brunswick-Altamaha Magnetic Anomaly in southwestern Alabama; and (3) an extensive, apparently coherent tectonic element (the Suwannee Terrane) comprised of undeformed granite (in which biotite records 40Ar/ 39Ar post-magmatic cooling ages of ∼530−525 Ma), low-grade felsic metavolcanic rocks, a suite of high-grade metamorphic lithologies (St. Lucie Metamorphic Complex in which hornblende yields 40Ar/ 39Ar post-metamorphic cooling ages of ∼515−510 Ma), and a succession of Lower Ordovician-Middle Devonian sedimentary rocks (characterized by Gondwanan fauna). The West African Orogens (Mauritanides, Bassarides, and Rokelides) record a locally complex, polyphase tectonothermal evolution. The earliest event corresponded to westward rifting of a continental fragment from the West African Craton at ∼700 Ma. This led to development of a rift-facies lithotectonic succession which included sedimentary units and intracontinental igneous sequences. Rifting was limited, and a western ensialic arc began to develop by ∼680 Ma. The associated convergence culminated in an episode of folding and metamorphism at ∼650 Ma (Pan-African I orogenesis). Following widespread deposition of late Paleozoic glacial and flyschoid sediments, a second tectonothermal event occurred between ∼550 and 500 Ma (Pan-African II orogenesis). The West African Orogens were marked by general tectonic quiescence throughout most of the early and middle Paleozoic. Late Paleozoic (∼300 Ma) collision of Gondwana and Laurentia resulted in eastward translation of previously tectonized Mauritanide units over their foreland, and emplacement at highest structural levels of previously imbricated nappes which include sequences with uncertain palinspastic origins. The late Paleozoic (Hercynian) transport was largely intracontinental, and only westernmost portions of the exposed Mauritanides record an associated penetrative tectonothermal overprint. Characteristics of pre-Mesozoic basement units which comprise the Suwannee Terrane in the Florida subsurface suggest they represent extensions of the Bassaride-Rokelide Orogen. There is no apparent record of Paleozoic tectonothermal activity in the Suwannee Terrane. This contrasts markedly with penetrative late Paleozoic reworking of basement units in the Wiggins Uplift. This suggests that a major dextral transcurrent fault was active during the late Paleozoic, and that proximal basement units were directly involved in collisional aspects of Pangea assembly. The various basement units of the Coastal Plain are not correlative with any of the non-Laurentian terranes exposed in the southern Appalachian Orogen (e.g., Carolina terrane of the eastern Piedmont) which accreted in the Ordovician-Devonian to exterior positions along the eastern margin of the North American craton. These exotic Appalachian terranes were transported into their present structural positions during late Paleozoic collision of Gondwana and Laurentia. They are separated from Gondwanan elements of the Suwannee Terrane by a suture approximately marked by the Brunswick-Altamaha Magnetic Anomaly.

Faults in Cook Strait and their bearing on the structure of central New Zealand

Abstract Seismic reflection data reveal that the major dextral transcurrent faults of the North and South Islands, with the possible exception of the Wairau Fault, do not link directly across Cook Strait. Off Marlborough, the extension of the Wairau Fault is a series of northeast-trending fractures, one of which runs west of Kapiti Island towards the mouth of the Rangitikei River. En route, the Wairau Fault defines the western margins of the prominent Wairau and Narrows sedimentary basins. The southeastern edge of the Wairau Basin is the Awatere Fault which bends sharply to the east to merge with the steep topography of Cook S trait Canyon. The neighbouring Hog Swamp Fault follows a similar trend. The Clarence Fault has not been identified offshore whereas the Kekerengu and Hope Faults can be traced only a few kilometres beyond the coast. On the Wellington side of Cook Strait, the offshore extensions of the Ohariu and Wellington Faults terminate in the thick sedimentary pile of the Wairau Basin. Further a...

East African rift system: offset, age and tectonic significance of the Tanganyika-Rukwa-Malawi intracontinental transcurrent fault zone

The 1000 × 200 km Tanganyika-Rukwa-Malawi fractures area is shown to be a right lateral intracontinental transform zone belonging to the East African Rift System. Using the results carried out from complementary techniques (remote sensing, field structural analysis and K-Ar isotopic dating) it appears that tectonics in this area have acted at different times, the oldest being at 2000-1400 m.y., the youngest being still active today, and are mainly due to the reactivation of NW-SE structures. The area consists mainly of large NW-SE “en échelon” dextral transcurrent faults, along which the Southern Tanganyika, Rukwa and Northern Malawi troughs are opening. NE-SW transverse faults showing a curved horizontal shape end these troughs axially. Eocene to recent volcanic activity is associated with the intersection of the northeastern main NW-SE transcurrent fracture (the Livingstone fault).

The origin of the Apenninic arcs

The Apennine area is composed of a variety of arcs, some of them forming the mountain chain and some of them being submerged below the Po Plain and the Adriatic Sea. The structure of the Apennine mountain belt is that of a succession of northeast-, east- and southeastward directed arcs corresponding to single thrust sheets. Each arc is limited by N-S oriented dextral transcurrent faults. Data from volcanology, gravimetry and reflection seismic surveys show that an important section of the upper mantle is involved in these features. The geodynamic reconstruction during Neogene times i.e., the period of Apenninic up-building, suggests that a unique arc existed from Oligocene to Middle Miocene times as a result of the rotation of the Sardo-Corso continental block. This arc was successively disrupted in several segments which advanced and shortened differentially as a function of the resistance to the orogenic expansion opposed by the rigid Adriatic foreland. This new tectonic phase, which is responsible for most of the Apenninic features, has been triggered by the differential evolution of anomalous mantle domes in the Tyrrhenian area from post-Middle Miocene times onwards.

The role of the Cruachan Lineament during Dalradian evolution

Synopsis Geological and geophysical evidence is reviewed for the existence of a trans-Caledonoid (NW–SE) lineament, the Cruachan Lineament, in the Scottish Caledonides, separating a high-density SW Highland block dominated by abundant late Precambrian Dalradian mafic rocks in which the Moine is absent from a low density Moine plus Dalradian Central Highland block poor in mafic rocks. Contrasted metamorphic P-T-time histories are predicted on opposite sides of the lineament. An alternative model of early Dalradian evolution is proposed in which basaltic volcanism and sedimentation in the SW Scottish Highlands occurred in a quasi-oceanic trans-Caledonoid pull-apart basin bounded along its NE side by a major fault now identified as the Cruachan Lineament. The Dalradian basin(s) may have experienced right-lateral translation relative to the crustal block to the SE along a dextral transcurrent fault zone possibly in the vicinity of the Highland Boundary Fault during late Precambrian time. In this model, the Dalradian terrain may provide no evidence regarding the early history of the NW side of the lapetus Ocean.

Correlation of the Rhenohercynian Variscides

Recent structural and stratigraphical data from the Rhenohercynian Variscides of Belgium, Germany and South Cornwall indicate tectonic continuity within this zone prior to disruption along a late Carboniferous dextral transcurrent fault. This extends from the Bristol Channel to the Vosges, parallel to similar lineaments of Variscan age in Europe and North America. Consequently, a tectonic model for south Cornwall is extrapolated to Europe and a plate tectonic setting for the evolution of the north European Variscides by the closure of an oceanic basin is discussed.

Evidence for a margin-truncating transcurrent fault (pre-late Eocene) in western Washington

I postulate that a major dextral transcurrent fault truncated the pre-Tertiary continental framework of western Washington and southern Vancouver Island during the Late Cretaceous and early Tertiary. The fault trace trends north and is covered in the Puget Lowland; it corresponds to the west-trending San Juan and northwest-trending Survey Mountain faults on southern Vancouver Island. The geometry of the structure has probably been greatly modified by subsequent deformation. Sedimentary basins that formed marginal to the fault are characterized by rapid fades changes and sediment accumulation rates, abrupt stratigraphic thinning and thickening, irregular basin margins, petrographic mismatches, and other features consistent with origin in a zone of strike-slip faulting. The structure was probably part of a network of intracontinental Late Cretaceous-early Tertiary strike-slip faults in the northern North American Cordillera. As an outboard member of this fault network, it likely served as a major avenue of northward translation along the northeast margin of the Pacific basin.

A description and interpretation of the Proterozoic Kobbefjord Fault Zone, Godthåb district, West Greenland

The Kobbefjord Fault Zone (KFZ) is recognized as the largest of a group of early Proterozoic dextral transcurrent fault zones that cut Archaean gneisses in the vicinity of Godthåb. Detailed fjeld study of the KFZ reveals that it corresponds to a zone of deformation and alteration - 100 m wide, which completely obliterate pre-existing structures in country rock gneisses. Rocks within the KFZ are dominantly quartz-rich laminated mylonites characterized by regionally consistent NE-SW trending subvertical foliations and subhorizontallineations. Such mylonites were produced by intense ductile deformation, although local pseudotachylites and cataclasites occur which indicate minor amounts of brittIe deformation as well. Alteration in the KFZ involved pervasive pink staining and/or bleaching, and development of hydrated mineral assemblages (Chl + Ep + Musc), indicating an important role for fluid flux during the faulting process. Movement on the KFZ was probably initiated in the time period 2000-1800 Ma, as it displaces mafic dikes which appear correlative to dikes to the north of the Godthåb area which yield ages in this range. Deformation in the KFZ most plausibly represents internal deformation of a stable continental block subjected to compression on at least two sides. Data for fabric elements in the KFZ and for associated conjugate shears indicate that the principal compressive stress (o1) for this faulting episode was oriented W-NW, but precise relationships to deformation events on the margin of this block have yet to be deduced.

Geological evidence for post–40 m.y. B.P. large-scale northwestward displacement of part of southeastern Alaska

The Leech River complex on southern Vancouver Island and schistose rocks on southern Baranof Island have very similar geologic histories. Both were derived from probable Upper Jurassic–Cretaceous terrigenous clastic rocks and minor associated volcanogenic rocks and chert. Each underwent (1) at least two penetrative deformations locally involving folding, disruption, and transposition of thinly layered sequences, and (2) synkinematic, low-pressure, Buchan-type metamorphism. Progressive metamorphic zones ranging from greenschist to amphibolite facies are related to granitic intrusive bodies. Deformation, metamorphism, and intrusion culminated about 40 to 42 m.y. ago. Apparently, these units, now separated by more than 1,100 km, were once contiguous and lay at least as far south as the latitude of southern Vancouver Island. Shortly after 40 m.y. B.P., the allochthonous Leech River complex was emplaced at the southern edge of Vancouver Island, and the schists of Baranof Island and perhaps the entire Chugach terrane began their long northwestward journey along a system of dextral transcurrent faults.