WSW Trending Research Articles

The stratigraphy and structure of the Galley Head culmination zone: An area of enhanced shortening related to basin geometry within the Irish Variscides

AbstractIn southwest Ireland an Upper Devonian to Lower Carboniferous clastic succession was deposited in an ENE–WSW trending half‐graben, known as the South Munster Basin. Across the Galley Head peninsula on the south coast, this stratigraphical succession is attenuated due to the presence of a palaeogeographical feature called the Glandore High. Evidence suggests that the Glandore High was an east–west feature, faulted to the north and east, which was part of the southern flank (hangingwall rollover) of the South Munster Basin.During post‐Carboniferous Variscan deformation the relatively thin stratigraphy of Galley Head underwent prolonged folding, causing a local periclinal fold pair to develop within the hinge zone of a regional syncline. The main cleavage then developed parallel to bedding on the overturned south limb of the anticline of this fold pair. The local enhanced shortening caused the development of a structural culmination, and south facing, tight to isoclinal folds. The culmination was enhanced and tightened by a fault system of contractional, strike‐parallel faults linked by cross faults. Secondary folds occur across the hinges of regional anticlines and also on major fold limbs as isolated fold pairs and in monoclinal fold zones, some of which may have nucleated on irregular sandstone bodies. Local crenulation cleavages are related to late fault movements. Syn‐cleavage, conjugate, wrench faults record 10 per cent to 15 per cent strike‐parallel extension in the culmination.The deformation chronology of the Galley Head area is somewhat anomalous for the Irish Variscides in that the folds were well established before the onset of the main cleavage development. The enhanced shortening across the area was compartmentalized by major cross faults and a minor component of north–south sinistral shear was also active across the area causing a swing in strike and a late set of minor cross faults.Structural facing directions in southwest Ireland appear to be directly linked with the geometry of the deformed basins. Hence the southward facing along the south coast is due to the proximity of the southern margin of the South Munster Basin. Structural facing directions fan northwards across the basin and major folds are overturned to the north at the northern margin of the basin.

Tectonic fabric of Sierra Leone, West Africa: implications for Mesozoic continental breakup

A lineament map interpreted from Landsat images has been integrated with available geological and geophysical data from both offshore and onshore regions of Sierra Leone. The lineament patterns are related to intraplate and plate marginal reaction of pre-existing structures during Mesozoic rifting events that resulted in strong tectonic controls on magmatism. Of four main lineament trends, the NNW-SSE to N–S, NNE–SSW, and ENE–WSW trends are directly relatable to Archaean fabric in the Leo Uplift. A NW-SE trend reflects coast-parallel late Mesozoic dykes that follow the Rokelide Pan–African fabric (reactivated Archaean NNW and N–S trending structures). NW–SE trending faults defining offshore basins are segmented and offset by ENE–WSW trending continental extensions of Ocean fracture zones. Both the Guinea and Sierra Leone Fracture Zones have nucleated on ENE–WSW trending sinistral Archaean shear zones in the Leo Uplift. The four lineament trends focused Mesozoic magmatic events through protracted reactivation, Spatial and geometric relationships between the magmatic provinces and tectonic fabric indicate that intraplate deformation occurs far inland during rifting events. The angular relationships between the Permo–Triassic NE trending Guinea Belt, the Jurassic–Triassic NW–SE trending coast-parallel dykes, and the ENE–WSW trending Ocean fracture zones centred on the eariy Jurassic Freetown basic igneous complex, suggest that the Sierra Leone–Liberia continental margin evolved as an obliquely-sheared, rift–rift–transform passive margin during Mesozoic continental breakup.

Sedimentological evidence for a major subsurface fracture system linking the eastern Campsie and the eastern Ochil faults

Synopsis Evidence for intra-Carboniferous and end-Carboniferous right-lateral strike slip within the Midland Valley is concentrated along lineaments which reflect major subsurface fracture systems. One such lineament runs ENE below the Kincardine Basin and joins the eastern Campsie Fault to the eastern Ochil Fault. This moved right-laterally during the deposition of the Passage Group, as proved by contemporaneous movements of its associated oblique folds. The hinge line between the Tulliallan Trough ‘low’ and the Bogside Saddle ‘high’ follows this lineament. The ‘low’ controlled the locus of the major Saline Channel Belt throughout most of the Namurian, whereas unusually thick peats commonly accumulated over the ‘high’. Reversals of apparent downthrow along this hinge reflected movements, probably right-lateral strike slip, along the underlying lineament. Low-angle detachments may explain the absence of any major ENE–WSW trending surface fracture above the lineament in the Kincardine Basin. The lineament, which probably extends WSW below the eastern part of the Milngavie–Kilsyth Fault to underlie the Paisley Ruck, may have been initiated as a Riedel Shear by right-lateral strike-slip along the Highland Boundary Fault, or it may reflect a reactivated Late–Caledonian left-lateral strike-slip fault system.

Seasat altimetry, the North Atlantic geoid, and evaluation by shipborne subsatellite profiles

A geological evaluation of geoid anomalies over the North Atlantic is presented with emphasis on short (40–300 km) wavelength undulations mapped by Seasat radar altimetry. Long (≥3000 km) and intermediate (300–3000 km) wavelength undulations constrain subcrustal processes and are briefly reviewed. Portions of three Seasat revolutions (revs) totaling 9300 km were followed by research ship, which measured gravity, magnetics, topography, and for a 800‐km segment also seismic reflection data, to form a ground truth data set for comparison to the altimetry. The intermediate‐wavelength geoid, the residual upon removal of a (14,14) long‐wavelength surface, correlates with plate age (slope ∼10 cm/m.y.) over Cenozoic crust, and major fracture zones appear as geoid steps. A 3.5‐ to 5‐m high over the Bermuda Rise may reflect thermal rejuvenation created within the plate 40–50 Ma. Short‐wavelength geoid anomalies, typically ±10–50 cm in the open ocean, were analyzed by high‐pass (200‐ or 400‐km cutoff) along‐track filtering and smoothing. The vertical deflection computed from the Seasat geoid compares well with one based on seismic reflection. Shipborne free air gravity, except for a higher‐frequency cutoff, correlates closely with geoid anomalies. Except near seamounts, the principal contribution to geoid anomalies is fracture zone topography. All major and many minor fracture zones (FZ's) appear as geoid lineations, particularly between the 15°20′and Kane FZ, from the ridge axis to anomaly 34. The WSW trend of FZ's between anomaly 21 and 34 is clearly expressed in the altimetry, as is the smooth geoid over mid‐Cretaceous smooth basement formed by fast spreading. Crustal ribbons, 50–150 km wide, of higher or lower geoid, parallel FZ trends. Some geoid lineations are oblique to both isochron and FZ trends; east of the Puerto Rico Trench such anomalies may reflect compressional deformation within Atlantic lithosphere.

Intrusive processes at ocean ridges: Evidence from the sheeted dyke complex of Masirah, Oman

Masirah Island largely consists of a late Mesozoic ophiolite which includes extensive areas of near-vertical, ENE—WSW striking, sheeted dykes. Previously the possibility has been suggested of a correlation between the similarly-aged ophiolites of Masirah and the Semail Complex of the Oman Mts. However, the Masirah ENE–WSW trend contrasts with N—S dyke trends from the Wadi Jizi area of the Semail, possibly suggesting two unrelated spreading centres. The dykes pass up into a pillow lava—minor sediment sequence, down into both layered and unlayered gabbros and are bounded to the west by a major N—S mélange zone which may have originated as a ridge transform fault. Age relations of the dykes and the gabbros are complex: the dykes contain a variable proportion of gabbro screens representing earlier crystallization, but they are also intruded by several small gabbro bodies which are themselves cut by still later dykes. The lava and dyke—gabbro screen sequence shows evidence of metamorphism from zeolite to low amphibolite grade. This metamorphism was caused by ridge hydrothermal activity which appears to have been effective approximately to the lower levels of the dykes. The rapid passage from low-amphibolite dykes to fresh gabbro suggests lithological control of the metamorphism. A combination of structural, geochemical and mineral phase studies may indicate generation in a slow spreading ridge environment and near-ridge metamorphism caused by a geothermal gradient of approximately 200°C/km.

Aulacogenes: Intracratonic Proterozoic Basins That Control Phanerozoic Sedimentation: ABSTRACT

Aulacogenes are long-lived trenchlike basins first described on the Russian and Siberian platforms. Subsidence in known aulacogenes was mainly Proterozoic, but many persisted as moderate negative structures in the Phanerozoic and others rebounded vertically to become linear ridges. Thus, important facies changes in Phanerozoic sediments may be located by projecting the trends of exposed aulacogenes into the subsurface. Two lower Proterozoic aulacogenes have been recognized in the northwestern Canadian shield--one with a WSW trend that dips beneath Phanerozoic cover at Great Slave Lake and a second with a NNW trend that dips beneath the cover of Victoria Island from Bathurst Inlet. Aulacogenes are much longer lived than other intracratonic basins. They are fault bounded, linear, and narrow, commonly 200 mi long but only 25 mi wide. They trend at high angles to the craton margin, where they merge with a contemporaneous orthogeosyncline that borders the craton. Their rock sections thicken outward from the center of the craton to as much as 40,000 ft near the margin. They contain mostly miogeosynclinal and exogeosynclinal facies, but with the addition of basic volcanics and fanglomerate at several levels. Alluvial sediment transport in aulacogenes is mainly parallel with their trends, not transverse as in the geosyncline. The sedimentary beds have mild compressive folds paralleling the boundary faults, but low-angle overthrusts, typical of the geosyncline, are abse t. Aulacogenes, unlike geosynclines, are not accounted for by global plate tectonics. The dipping of laterally adjacent sediments toward aulacogenes, the compressive deformation within aulacogenes, and the abrupt descent of the Mohorovicic discontinuity beneath aulacogenes indicate foundering of a narrow slice of continental crust into the mantle (as opposed to tensional block faulting) to produce aulacogenes. If this interpretation is correct, aulacogenes should be bounded by high-angle reverse faults, rather than by normal faults. Aulacogenes, characteristic of the Proterozoic, may represent a stage in the evolution of the continental portions of global plates intermediate between the pervasive mobility of the Archean and the persistent true cratons of the Phanerozoic in which only relict aulacogenes are found. End_of_Article - Last_Page 627------------

Skiddaw Slates and the Borrowdale Volcanics

A RECENT diagram1 shows the distribution of graptolite zones within the Lake District outcrop of the Skiddaw Group to be disposed about a north-trending anticline. This diagram is erroneous. The actual distribution of zonally significant faunas2–7 (Fig. 1) appears to indicate an ENE–WSW trending anticline, with a marked easterly plunge with respect to the overlying Borrowdale Volcanic Group. It seems likely that older Arenig rocks, yielding Didymograptus extensus and D. hirundo Zone graptolites, lie in the core of this structure and are flanked by younger Llanvirn beds containing D. bifidus and D. murchisoni Zone faunas. This broad picture remains valid, even though the polyphase deformation of the group5,8 doubtless gives a complex local pattern of outcrops.