Northeast Trending Research Articles

Location of the Focus and Tectonics of the Focal Region of the California Earthquake of 18 April 1906

Modern relocation constrains the hypocenter of the 18 April 1906 California earthquake within an ∼8×24 km, southwest–northeast trending zone to the north of San Francisco that intersects the offshore San Andreas fault system west of San Francisco. Here we further constrain the location of the 1906 focus and examine the tectonics of the focal region using recent microseismicity relocated in a 3D model along with other seismological, geological, and marine evidence. In agreement with previous work, we associate the 1906 focus with an extensional, right stepover in the San Andreas fault system—a zone of sea floor subsidence and complex, normal, and strike-slip faulting. We define a likely focal volume between Lake Merced and offshore of the Golden Gate centered near 37.77° N, 122.55° W, and 10-km depth. We hypothesize possible 1906 rupture in this region by normal deformation around a 10-km-long, steeply west-dipping structure trending about 20° clockwise to the San Andreas fault zone, and bilateral, strike-slip fault rupture along the San Andreas fault system away from this structure: rupture to the northwest on a vertical, currently aseismic faulting structure under the Golden Gate fault and rupture to the southeast under the San Francisco peninsula either along a steeply southwest-dipping structure showing present-day extensional tectonism, or along a near-vertical, currently aseismic fault zone. All three of these structures extend from the near surface to about 10-km depth. We also identify a deeper, 15–20-km-long linear trend of clustered seismicity at 10–13-km depth, rotated about 6° clockwise relative to the strike of the San Andreas fault, which connects the north end of the focal volume with the south end of the seismicity defining the southwest-dipping structure under the San Francisco peninsula. We propose that this trend shows faulting at the base of a brittle, upper crust in response to underlying shear in a ductile, lower crust. These interpretations on the focal region of the 1906 earthquake have implications for seismotectonic understanding, earthquake monitoring, and seismic hazard assessment in the San Francisco Bay Area.

The 2001 Bhuj earthquake (MW7.7) in western India: 3D velocity structure and seismotectonic processes

More than 5000 high precision seismic phases of 560 selected aftershocks ( M ≥ 2.0) of the January 26, 2001 Bhuj earthquake ( M W 7.7) in western India are used for joint determination of the hypocentral parameters and for 3D inversion of P-wave velocity and V p / V s structures in the source area. The aftershocks are located with an average rms of 0.19 s, and average error estimates of latitude, longitude and depth are 1.2, 1.1 and 2.3 km respectively. Most of the aftershocks occurred in an area 70 × 35 sq km; the intense activity was observed at a depth range 12–37 km. A bimodal distribution of aftershocks indicates that the main shock rupture propagated in upward and downward directions. Further, the best located aftershocks show two trends, one in northeast, parallel to Anjar Rapar Lineament, and the other in northwest parallel to the Bhachau Lineament. Fault-plane solutions of the northeast trending aftershocks indicate reverse faulting with left-lateral strike-slip component. These solutions are com...

High-resolution gravity study of the Gray Fossil Site

The Gray Fossil Site, Washington County, Tennessee, has produced a remarkable Mio-Pliocene fauna and flora with no known correlative in the Appalachian region. After its discovery in 2000, a series of auger holes were drilled by the Tennessee Department of Transportation (TDOT) to determine the areal extent of the site. Drilling indicated that the fossils occurred in fill material within a paleokarst basin, but the distribution of boreholes does not permit details of sinkhole topography, and therefore its formation and fill history, to be adequately resolved. To better image the sinkhole basin, a high-resolution gravity survey, which included 1104 gravity measurement stations, was conducted. These data were used to create complete Bouguer and residual gravity anomaly maps and a 3D density model via inversionmethods. The residual gravity anomaly map compares favorably with 29 TDOT auger holes drilled to basement, but contains significantly more detail. The residual gravity anomaly map reveals the presence of seven separate sinkholes. However, 3D inverse modeling constrained by drill-hole depths and density data indicates that there are 11 separate sinkholes formed within the Knox Group carbonates. These sinkholes, which range between 20 and [Formula: see text] in depth, are aligned along northwest and northeast trending linear features that correlate to structural features formed during the Appalachian orogenies. It is possible that the overall sinkhole basin formed as the result of partial coalescence of multiple sinkhole structures controlled by a joint system and that the sinkholes then acted as a natural trap for the Gray Fossil Site fauna and flora.

High‐resolution aeromagnetic data over central Australia assist Grenville‐era (1300–1100 Ma) Rodinia reconstructions

High resolution aeromagnetic data covering the polydeformed and poorly exposed Musgrave Province of Australia reveals the Grenville‐aged crustal architecture. A combination of upward continuation and the potential field tilt filter out magnetic anomalies relating to a late Neoproterozoic orogenic event, and emphasize the more subtle magnetic structural grain formed during the ca. 1320–1150 Ma Musgravian Orogeny. The resulting images indicate that crustal architecture is defined by the distribution of ca. 1150 Ma magnetic granitoids within less magnetic basement and is dominantly northeast trending, continuous beneath the Amadeus and Officer basins, and defines an orogenic belt that connects Mesoproterozoic provinces. The orogenic belt is truncated within Australia and precludes a direct east trending connection between the Musgrave Province and contemporaneous orogens in Laurentia. Clockwise rotation of the South Australian Craton and subsequent collision with the North and West Australian cratons is our preferred model to explain the Grenville‐aged architecture observed in Australia.

The Bedrock Surface of the Western Lake Ontario Region: Evidence of Reactivated Basement Structures?

Lower Paleozoic bedrock strata, in south-central Ontario and the adjacent part of New York State are covered by a thick (100m+) blanket of Pleistocene glacial and interglacial sediments. The form of the buried bedrock surface has been reconstructed from 70,000 waterwell boreholes that extend through the entire Pleistocene cover using GIS data processing techniques. The sub-drift bedrock surface shows linear channels that connect the basins of lakes Huron, Ontario and Erie and which form part of an ancestral mid-continent Great Lake drainage system prior to modification and infilling during successive Pleistocene glaciations. This relict drainage system is cut across Lower Paleozoic carbonates and elastics up to 500 m thick, but the position of several channels is aligned above terrane boundaries, faults and other deep-seated and poorly understood geophysical anomalies in underlying mid-Proterozoic Grenville basement rocks. Other channels are controlled by a dominant northwest and northeast trending regional joint system. A close relationship among deeply seated geophysical lineaments, basement structures and topographic lineaments cut across thick Paleozoic cover strata suggests a history of Phanerozoic reactivation and upward propagation of fractures from the Precambrian basement. Several basement structures and lineaments are seismically active suggesting ongoing neotectonic activity across the 'stable' craton of south-central Ontario.

Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier

The Selwyn Zone is characterized by multiple ductile deformation and metamorphic events. There is general consensus that early in the Isan Orogeny there were both extensional and thrust events that produced bedding-parallel foliation (S bp). We propose that these were followed by the D 1 event that resulted in a regional steep east–west foliation, and that the Snake Creek Anticline and the dominant S 2a regional foliation formed as upright north–south structures. Local overturning of D 2 structures was due to subsequent localized deformation (D 2b) that resulted in structures with flat-lying axial planes. D 3 and D 4 produced steep upright folds and crenulations with north–northwest and northeast trending axial surfaces. Metamorphism, largely controlled by the emplacement of mafic rocks, is dominantly low pressure, but the P–T–t path is complex. Early growth of cordierite (M 1) followed by andalusite, was succeeded by a clockwise medium pressure loop of ca. 500–600 MPa in the northern Snake Creek Anticline, as indicated by early D 2a kyanite growth. The metamorphic peak during D 2 was around 580 °C and 400 MPa at the sillimanite isograd and reached a maximum of around 670 °C and 400–500 MPa in the migmatitic sillimanite K-feldspar zone at the Cannington and Osborne mines. Previous age determinations for the metamorphic peak of ∼1580–1600 Ma are confirmed with new electron microprobe dating of monazite, whereas an age grouping of 1600–1630 Ma may represent the D 1 (M 2) event. Additional ages in the range 1640–1680 Ma are recorded in albitites from the Soldiers Cap Group, and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills. This is supported by SHRIMP dating of zircon from a trondhjemite lens in a largely mafic sill complex in the Snake Creek Anticline that provided an age of 1686 ± 8 Ma, also indicating that at least part of the Soldiers Cap Group is older than previously indicated. A zircon SHRIMP age of 1527 ± 4 Ma was obtained for a sample of albitized Saxby Granite. This age also represents the D 4 (M 6) event, and is the timing of the extensive brecciation and albitization of calc-silicate rocks of the Corella Formation in the Selwyn Zone.

Geomorphology, site distribution, and Paleolithic settlement dynamics of the Ma'aloula region, Damascus Province, Syria

AbstractThis survey of the geology, geomorphology, and Paleolithic archaeology of a 300 km2 area in Damascus Province, Syria, focused around the villages of Ma'aloula and Jaba'deen. The study resulted in the definition of seven geomorphological zones that trend northeast–southwest, parallel to the prevailing geological features of the region. The zones span a broad range of elevations, from a dry lake bed in the Jeiroud Basin (ca. 800 m) to the peaks of the Anti‐Lebanon Mountains (ca. 2350 m). The research focused on assessing the geological and paleoenvironmental history of the region as a backdrop for 500,000 years of Paleolithic settlement. Among the 618 archaeological sites documented thus far, all major archaeological periods from the Lower Paleolithic through the Epipaleolithic are represented. Most abundant are occurrences of the Levalloisian Middle Paleolithic, with 255 new sites documented. Nine Neolithic sites were also included in the survey. The distribution of sites varies during the Paleolithic and reflects the availability of resources, especially of flint and water, as well as the spatial patterns of erosion and deposition on the ancient land surfaces. © 2007 Wiley Periodicals, Inc.

Alboran Basin, southern Spain—Part II: Neogene tectonic implications for the orogenic float model

We infer that the Alboran Basin, the first western Mediterranean Basin found after crossing Gibraltar, is an orogenic float underlained by a décollement system, a multi-layered ductile shear extending from 10 km to between 30 and 40 km below sea level. This float was formed as consequence of the collision of the African–Eurasian plates in the Oligocene–late Miocene. Synchronous with this compression the float experienced basin wide crustal thinning and subsidence about 25 m/year ago by subcrustal processes. Since latest Miocene the float has undergone compression due to the continuous convergence of Eurasia and Africa. The faults created as a result of this compression are dominated by a conjugate system of northeast trending left-lateral and northwest right-lateral strike–slip faults. This deformation is taking place under a simple shear mechanism. Associated with the northwest and northeast lateral faults are zones of compression trending west and east of north extending from the base of the basin's north upper slope to the Alboran Ridge. The initial morphology of the Alboran Ridge on the southern side of the Alboran Basin was due to the construction of a volcanic edifice at the northeast end of the ridge and igneous activity along northeast trending fractures southwest of the edifice. At the northeast end of the Alboran Ridge motion along a right-lateral fault cutting across the ridge led to sediment collapse and the creation of a prominent embayment on the ridge's northwest flank. Deformation is more subdued in the western than in the eastern part of the Alboran Basin, a tectonic style due either to differences in sediment rheology or that the accommodation of the convergence of Africa and Iberia is more diffused and attenuated in the west than in the east.

Early structural development of the Okavango rift zone, NW Botswana

Aeromagnetic and gravity data collected across the Okavango rift zone, northwest Botswana are used to map the distribution of faults, provide insights into the two-dimensional shallow subsurface geometry of the rift, and evaluate models for basin formation as well as the role of pre-existing basement fabric on the development of this nascent continental rift. The structural fabric (fold axes and foliation) of the Proterozoic basement terrane is clearly imaged on both gravity and magnetic maps. The strike of rift-related faults (030–050° in the north and 060–070° in the south) parallels fold axes and the prominent foliation directions of the basement rocks. These pre-existing fabrics and structures represent a significant strength anisotropy that controlled the orientation of younger brittle faults within the stress regime present during initiation of this rift. Northwest dipping faults consistently exhibit greater displacements than southeast dipping faults, suggesting a developing half-graben geometry for this rift zone. However, the absence of fully developed half-grabens along this rift zone suggests that the border fault system is not fully developed consistent with the infancy of rifting. Three en-echelon northeast trending depocenters coincide with structural grabens that define the Okavango rift zone. Along the southeastern boundary of the rift, developing border faults define a 50 km wide zone of subsidence within a larger 150 km wide zone of extension forming a rift-in-rift structure. We infer from this observation that the localization of strain resulting from extension is occurring mostly along the southeastern boundary where the border fault system is being initiated, underscoring the important role of border faults in accommodating strain even during this early stage of rift development. We conclude that incipient rift zones may provide critical insights into the development of rift basins during the earliest stages of continental rifting.

PGE exploration targets to the west of the Pilanesberg, South Africa

The main approach to PGE target selection west of the Pilanesberg was to use the geological models for the setting of PGE mineralisation (together with their associated geophysical and geochemical signatures) in the southwestern lobe of the Bushveld Complex where abundant information is available. A small gravity anomaly to the west of the Pilanesberg could represent stratigraphic thickening of the Bushveld stratigraphy or a local magma feeder zone. If the Merensky Reef is present in this area then it could be expected to be of the narrow, pegmatoidal, potholed (including contact type) and high-grade variety, as in the southwestern and northwestern lobes of the Bushveld Complex. High precision, airborne magnetic and radiometric surveys over the area, carried out by the Council for Geoscience, demarcate Upper Zone stratigraphy and a range of dykes, faults and probable scattered bodies of iron-rich ultramafic pegmatites (IRUPS). Of particular significance are a set of northeasterly trending dykes and/or faults of the Vlakfontein swarm, which could host Ni (PGE) sulphide bodies of the type already known from the area. Anomalously low radiometric signatures were used to demarcate areas of unequivocal mafic/ultramafic rock outcrops or sub-outcrops. Higher radiometric signatures reflect Transvaal floor rocks while extremely high signatures occur over the Pilanesberg alkalic rocks. In terms of soil geochemistry, chromium is a good indicator of the Lower and Lower Critical zones of the Bushveld Complex. The transition from a high to a low chromium signature in the Rustenburg area is broadly coincident with the base of the Upper Critical Zone, which contains the important UG2 and Merensky Reef platiniferous layers. The same high to low chrome transition is seen to the west of the Pilanesberg and is considered to be an important PGE target indicator. Anomalously high nickel values occur over the Lower Zone rocks to the west of the Pilanesberg, although in the Rustenburg sector the most pronounced nickel geochemical anomalies are shown to be related to the impact of mining including tailings dams, smelters etc. The expansive nickel anomaly west of the Pilanesberg probably reflects the greater abundance of altered (including birbiritized) olivine-bearing rocks (harzburgites) and the presence of nickeliferous pipes. The transition area from a high-nickel (>220ppm) to a low-nickel signature, as in the case of chromium, defines a good PGE target area. Vanadium and titanium as expected show an anomalous build-up in the Upper Zone rocks of the Bushveld Complex. V also shows anomalous values over the upper chromitite zone and over IRUPS. The zone east of the vanadium anomalies to the west of the Pilanesberg therefore emerges as one with PGE potential. By integrating the interpretations of the above data sets, six main PGE target areas emerge to the west of the Pilanesberg. The most important targets include a well-defined zone of probable Critical Zone rocks as well as east-northeasterly trending linear targets along the Vlakfontein and related fault/dyke sets. Possible fault-, basal- and offset-style mineralisations are identified in the northwestern part of the area.

Geology of the submarine Kenn Plateau off northeast Australia: a rifted continental block

New seismic reflection data and dredged rock samples confirm that the submarine Kenn Plateau is a thinned continental fragment. It is twice the size of Tasmania and lies 400 km east of central Queensland beyond Cato Trough. East and northeast trending faults separate thinly sedimented basement highs from sedimentary basins with >2 km of Cretaceous and younger strata. In the Late Cretaceous the future plateau fitted against Australia south of Marion Plateau, and consisted of Late Triassic and younger basins unconformably overlying the New England Fold Belt. After Lower Cretaceous rift volcanism, the Kenn Plateau itself formed by Late Cretaceous extension and breakup. Paleocene drifting moved it northeast and rotated it 30° anticlockwise, leaving behind Tasman Basin oceanic basalts. Siliciclastic sediments poured into the basins from the Australian mainland and local highs. After a regional Paleocene-Eocene boundary unconformity, siliciclastic sedimentation resumed nearshore. In deep water, deposition of radiolarian chalks ended at the regional Eocene-Oligocene boundary unconformity, and pure biogenic carbonates accumulated in warming surface waters. From the Middle Eocene, calcarenite formed on the shelves of the subsiding plateau. Some seismic profiles show Middle to Late Eocene compression, probably related to New Caledonian obduction to the east. Hotspots formed parts of two volcanic chains as the plateau moved northward: in the west, Upper Eocene and younger volcanics of the Tasmantid chain; and in the east Upper Oligocene and younger volcanics of the Lord Howe chain. The subsiding volcanoes were fringed by reefs, some of which have persisted until now; others have subsided to form guyots. The plateau has subsided 2000 m or more since its breakup and is now subject solely to pelagic carbonate sedimentation.

Methane seepage and its relation to slumping and gas hydrate at the Hikurangi margin, New Zealand

Dissolved methane and high resolution bathymetry surveys were conducted over the Rock Garden region of Ritchie Ridge, along the Hikurangi margin, eastern New Zealand. Multibeam bathymetry reveals two prominent, northeast trending ridges, parallel to subduction along the margin, that are steep sided and extensively slumped. Elevated concentrations of methane (up to 10 nM, 10× background) within the water column are associated with a slump structure at the southern end of Eastern Rock Garden. The anomalous methane concentrations were detected by a methane sensor (METS) attached to a conductivity‐temperature‐depth‐optical backscatter device (CTDO) and are associated with elevated light scattering and flare‐shaped backscatter signals revealed by the ship's echo sounder. Increased particulate matter in the water column, possibly related to the seepage and/or higher rates of erosion near slump structures, is considered to be the cause of the increased light scattering, rather than bubbles in the water column. Methane concentrations calculated from the METS are in good agreement with concentrations measured by gas chromatography in water samples collected at the same time. However, there is a c. 20 min (c. 900 m) delay in the METS signal reaching maximum CH4 concentrations. The maximum methane concentration occurs near the plateau of Eastern Rock Garden close to the edge of a slump, at 610 m below sea level (mbsl). This is close to the depth (c. 630 mbsl) where a bottom simulating reflector (BSR) pinches out at the seafloor. Fluctuating water temperatures observed in previous studies indicate that the stability zone for pure methane hydrate in the ocean varies between 630 and 710 mbsl. However, based on calculations of the geothermal gradients from BSRs, we suggest gas hydrate in the study area to be more stable than hydrate from pure methane in sea water, moving the phase boundary in the ocean upward. Small fractions of additional higher order hydrocarbon gases are the most likely cause for increased hydrate stability. Relatively high methane concentrations have been measured down to c. 1000 mbsl, most likely in response to sediment slumping caused by gas hydrate destabilisation of the sediments and/or marking seepage through the gas hydrate zone.

Active fault survey on the Tanlu fault zone in Laizhou Bay

Shallow-depth acoustic reflection profiling survey has been conducted on the Tanlu fault zone in Laizhou Bay. It is found that the Tanlu fault zone is obviously active during the late Quaternary and it is still the dominating structure in this region. The Tanlu fault zone consists of two branches. The KL3 fault of the western branch is composed of several high angle normal faults which had been active during the period from the the latest Pleistocene to early Holocene, dissected by a series of northeast or approximate east-west trending fault which leaped sediment of the late Pleistocene. The Longkou fault of the eastern branch consists of two right-laterally stepped segments. Late Quaternary offsets and growth strata developed along the Tanlu fault zone verify that the fault zone retained active in the latest Pleistocene to the early Holocene. The Anqiu-Juxian fault that passes through the middle of Shandong and corresponds to the Longkou fault is composed of a series of right-laterally stepped segments. The active faults along the eastern branch of the Tanlu fault zone from the Laizhou bay to the north of Anqiu make up a dextral simple shear deformation zone which is characterized by right-lateral strike-slip movement with dip-slip component during the late Quaternary.

The gold–vanadium–tellurium association at the Tuvatu gold–silver prospect, Fiji: conditions of ore deposition

The Tuvatu gold–telluride prospect is one of several epithermal gold systems along the >250 km northeast trending Viti Levu lineament, Fiji, which are genetically associated with alkalic magmatism. Vein structures contain a variety of sulfides, native elements, sulfosalts, and tellurides. Calaverite is intimately associated with various vanadium-bearing minerals: roscoelite, karelianite, vanadian muscovite, Ti-free nolanite, vanadian rutile, schreyerite, and an unnamed vanadium silicate. Thermodynamic calculations for the systems V–Al–K–Si–O–H (Cameron, 1998) and Au–Te–Cl–S–O–H at estimated conditions of formation of the telluride-native gold stage at Tuvatu (∼250 °C, ΣAu = 1 ppb, ΣTe = 1 ppb, ΣS = 0.001 m, ΣV = 0.0001 m, and aK = 0.01), show that the stability fields of calaverite, roscoelite, and karelianite converge in pH-fO2 space near the hematite–magnetite buffer and at neutral to slightly acid conditions. Thermodynamic and textural data suggest that these minerals were deposited together at Tuvatu and likely explain the common coexistence of roscoelite and calaverite in epithermal gold systems elsewhere. The presence of magnetite with up to 0.7 wt.% V2O3 in the Navilawa Monzonite is consistent with the derivation of V from the alkalic intrusive rocks, which are also considered to be the source of Au and Te in the Tuvatu deposit.

Mesoproterozoic Albany – Fraser Orogen-related deformation along the southeastern margin of the Yilgarn Craton

Deformation associated with the Mesoproterozoic Albany – Fraser Orogen of Western Australia extends up to 50 km into the Archaean rocks along the southeastern margin of the Yilgarn Craton. The Albany – Fraser Orogeny represents the Mesoproterozoic continent – continent collision of the Yilgarn Craton and the combined East Antarctic and South Australian Cratons. Albany – Fraser Orogen-related deformation (D5a – d) overprints regional D1 – D4 structures in the Archaean rocks and the onset of this Proterozoic deformation is marked by open northeast – southwest-plunging folds that formed as a result of northwest – southeast compression during D5a. A large drag fold developed during D5b immediately northwest of the Cundeelee Fault, which is the boundary between rocks of Archaean and Proterozoic origin. This fold is associated with the clockwise rotation of D1 – D5a structures from the regional north-northwest Archaean structural trend to a local boundary-parallel northeast trend. Continued Proterozoic convergence resulted in the development of an intense boundary-parallel fabric in the Archaean rocks (S5c). Late cross-cutting conjugate northwest – northeast-striking D5d structures mark the final stage of Albany – Fraser Orogen-related deformation in the Archaean rocks. A gradual increase in the metamorphic grade of Archaean rocks towards the Yilgarn Craton margin, from greenschist to amphibolite facies, may indicate that uplift of the Yilgarn Craton was associated with the collision, with exposure of deeper crustal material adjacent to the Cundeelee Fault. The clockwise rotation of Archaean rocks adjacent to the boundary indicates that a large component of dextral shear was associated with the Mesoproterozoic collision event. This is consistent with dextral shear-sense indicators in gneisses of the Albany – Fraser Orogen in the Fraser Range.

Rejuvenation of The Pre-Cambrien Najd Fault System and Its Importance In The Oil Province of Saudi Arabia

Structural interpretation covering the Central Saudi Arabia was carried out through Landsat, Radar images, topographic maps and seismic data. Existing geology maps provide materialized evidence for this study. A strong northwest trending lineament system has been mapped in the region and is interpreted as being rela- ted to the reactivated Najd Fault System. Elongate hills, strait-going creeks, offset along valley and ridges, and pull-apart basins are evident, indicating rejuvenation of the Najd transtensional sinistral movement. Geological map shows evidence indicating a northwestward dragging on the sedimentary cover along a NW trending regio- nal lineament in the region which extend from South Ghawar to An Nafud Basin in Saudi Arabia. Northeast trend- ing structures in Saudi Arabia are related to the pre-Permian structuring and Oman-Masirah stress regime. The Ghawar (Saudi Arabia), Dukhan (Qatar) and other north-northwest trending anticlines were reshaped by the reju- venated Najd stress regime. The northwest trending sinistral Najd Fault was initiated in Precambrian and was reactivated at various times, probably during Paleozoic and late Jurassic times, and continuously, though of vari- able intensity, from late Cretaceous to the late Tertiary time. The Carboniferous, Late Cretaceous and Tertiary events are overprinted on one another in the region. The Oman stress regime and the stress regime that is rela- ted to oblique obduction of the Masirah Ophiolite are the principal controlling event for the anticline structures in Central Arabia. Dominant structural elements of the area are N-S anticline axis that suggests approximate E-W compressional stress direction, which is consistent with Oman and reactivated Najd tectonic regimes. The effect of the Tertiary orogenic overprint of the Oman-Masirah stress regime in eastern Arabia was to favor a slight renewed movement along the old basement faults such as the Najd Trend. Reactivated Najd system, Oman prin- cipal horizontal stress regime and obduction of the Masirah Ophiolite onto the Arabian continent have produced a combined effect on the structures in Eastern Arabian Plate. The Zagros stress regime may have produced Mineral Res. Exp., Bull., 132, 73-82, 2006 little effect in the region.

Autochthonous and allochthonous strata of the El Callao greenstone belt: Implications for the nature of the Paleoproterozoic Trans-Amazonian orogeny and the origin of gold-bearing shear zones in the El Callao mining district, Guayana shield, Venezuela

The tectonic style of the Paleoproterozoic Trans-Amazonian orogeny in the Guayana shield was considered by previous workers to represent non-actualistic tectonics in which rising masses of granitoid magma of the Supamo complex deformed 2.1–2.0Ga supracrustal belts of volcanic and sedimentary rocks to form greenstone belts. Subsequent collision with the dominantly granulite-grade Imataca complex deformed the greenstone belts. However, the base of the greenstone belts is everywhere marked by a major mylonitic shear zone that placed rocks of the greenstone belts atop a regional unit of quartzite that sits unconformably upon both the Imataca and Supamo complexes. The simplest interpretation of these relationships is that the greenstone belts are allochthonous with respect to the Imataca and Supamo complexes. As the quartzite oversteps the contact between the Imataca and Supamo complexes, juxtaposition of the two complexes must predate deposition of the quartzite and deformation of the Trans-Amazonian orogeny.The quartzite and the thrust fault are used as paleohorizontal markers to unfold the greenstone belt in the El Callao area where auriferous shear zones, which clearly predate the thrusting, show reverse sense of movement. When restored to their pre-folding orientation they are normal faults. This, and the tholeiitic to calc-alkaline volcanic packages, are compatible with an extensional forearc to arc setting for both the mineralization and the volcano-sedimentary sequences.East–northeasterly trending folds of both the allochthon and the autochthon, coupled with the northerly thickening of the quartzite, suggest that the allochthon was part of an unknown craton that formerly lay to the north. A second regional fold set, which trends northerly, refolds the earlier structures. It is possibly related to collision of the San Carlos terrane with the amalgamated Imataca–Supamo–Pastora terranes. The unconformably overlying 1.98Ga Cuchivero arc terrane provides a minimum age for the two collisional events.

Zircons, Dikes, and Gold Mineralization at Jundee-Nimary: Post ca. 2.66 Ga Archean Lode Gold in the Yandal Belt, Western Australia

In recent years, there has been considerable controversy concerning the timing of lode gold mineralization in the Yilgarn craton. The majority of published robust direct and indirect geochronological constraints indicate that orogenic gold deposits formed broadly in the period 2655 to 2625 Ma, with most between 2645 and 2630 Ma. Recent studies, however, have suggested that gold mineralization in the Yandal belt may be significantly older, with the implication that gold deposits could be diachronous across the craton over approximately 40 m.y. Critical to this assertion is that essentially unmineralized, broadly north-northeast– to northeast-trending, granodiorite porphyry dikes at the Jundee-Nimary gold deposits are postore intrusions and that the age of one of the dikes, the Moore porphyry, is 2669 ± 7 Ma based on U-Pb SHRIMP geochronology. Reassessment of field relationships indicates that the Moore porphyry and other granodiorite dikes control the position of anomalously oriented ore shoots where the dikes bend from their common north-northeast to northeast orientation into a more east- northeast trend. In the Nim 6 deposit, one such granodiorite dike hosts gold-related veins and alteration and anomalous gold grades. Contrary to previous interpretations, the Moore porphyry is a preore dike that was relatively unaffected by deformation and related ore fluid because it was oriented broadly parallel to σ1 at the time of gold mineralization. Reexamination of zircons from the Moore porphyry indicates that they have volumetrically dominant cores, interpreted to be xenocrysts, with narrow magmatic rims. The previous age of 2669 ± 7 Ma includes at least some core analyses. By specifically targeting magmatic zircon rims, a more robust magmatic age of 2657 ± 6 Ma is obtained. Thus, not only is the Moore porphyry a pre- rather than postore dike, but its mean age is about 10 m.y. younger than that previously recorded. The new data require that the absolute maximum age of mineralization at Jundee-Nimary is 2663 Ma, whereas previous results suggested a minimum age of 2662 Ma. In combination with other indirect constraints on the age of gold mineralization in the Yandal belt, there is no evidence to suggest that the Jundee-Nimary gold deposits did not form in the same broad 2655 to 2625 Ma range as most other Yilgarn gold deposits. This study illustrates that dikes that are unfavorably oriented for failure in the regional stress field may appear to be postore because they are largely unmineralized, but detailed examination at all scales is required for unequivocal interpretation. Similarly, zircons may give unrealistically old ages if their internal morphologies and growth histories are not carefully considered prior to analysis.

Linking mineral and fluid inclusion paragenetic studies: The Batman deposit, Mt. Todd (Yimuyn Manjerr) goldfield, Australia

The quartz–sulphide vein system of the Batman deposit at the Mt. Todd Mine, Australia, formed during east–west extension of a north–northeasterly trending structure system, early in D 2 during retrograde contact metamorphism associated with cooling of the Tennysons leucogranite. Four distinct fluid inclusion types have been identified in gold-bearing quartz–sulphide veins of the deposit: (Type I) low to moderate salinity aqueous primary inclusions (1 to 20 wt.% NaCl equivalent) with small amounts of CO 2; (Type II) high salinity Na–Ca–Cl aqueous secondary inclusions (35 to 50 wt.% total NaCl–CaCl 2), containing halite, carbonate and trona daughter minerals, and minor CO 2–CH 4 vapour; (Type III) liquid and vapour secondary fluid inclusions (28 to 34 or 17 to 18 wt.% NaCl equivalent) with CH 4 vapour and discrete gold and bismuth grains and associated growth zones; (Type IV) vapour-rich CO 2 ± (CH 4) secondary inclusions with minor NaCl liquid (> 20 wt.% NaCl equivalent). A consistency in bulk fluid composition suggests a common parent fluid and a common parent source for the fluid inclusions. Previous paragenetic and textural studies have established a sequence in sulphide and silicate precipitation. The mineral paragenesis has been linked to the trapping chronology of Types I–IV fluid inclusions in order to constrain the fluid conditions during (micro)-tectonism and the mineralising process. The link is possible because two mineral phases exhibit a temporal genetic relationship with the formation of fluid inclusions: (1) Growth zones in simple open space quartz fill contain tourmaline crystals in intimate association with Type I liquid–vapour fluid inclusions; (2) Gold is intimately associated with bismuth in Type III vapour–liquid fluid inclusions, either as discrete inclusions within the trails, or wetting the inclusions. The progressive change in the composition of Types I–IV is consistent with the evolution of a retrograde contact metamorphic fluid in wall rock lithologies.

Late Pleistocene and Holocene Paleoearthquake Activity of the Olinghouse Fault Zone, Nevada

Paleoseismic trenching and fault surface trace mapping indicate that the Olinghouse fault zone, a northeast trending, left-lateral strike-slip fault located in the northern Walker Lane, Nevada, has been the source of multiple latest Pleistocene and Holocene surface-rupturing earthquakes. A trench exposure near the eastern end of the fault records two, and possibly three, earthquakes after 3360 ± 190 cal. yr B.P. and two trenches at the western end of the fault contain evidence for two earthquakes after 19,800 ± 630 cal. yr B.P., with the most recent earthquake occurring after 1935 ± 70 cal. yr B.P. The apparent higher frequency of recent earthquakes at the eastern end of the fault may reflect triggered slip on the Olinghouse fault zone due to earthquakes on the conjugate northwest-trending, right-lateral Pyramid Lake fault. Repeated late Pleistocene and Holocene earthquakes on the Olinghouse fault zone demonstrate that northwest-directed, right-lateral shear of the northern Walker Lane is accommodated in part by northeast-trending left-lateral faults.