Ophiolitic Fragments Research Articles

West‐northwest directed obduction of the Batain Group on the eastern Oman continental margin at the Cretaceous‐Tertiary boundary

The Batain coast area in eastern Oman is dominated by allochthonous Permian to Late Maastrichtian sedimentary and volcanic rocks (Batain Group), unconformably overlain by neoautochthonous Tertiary sediments. The allochthonous rocks of the Batain coast were previously attributed to the Hawasina complex, the Permian to Coniacian/Santonian sedimentary infill of the neo‐Tethyan Hawasina basin off northern Oman. Previous structural interpretations suggested that the Batain Group, along with the Hawasina complex and the Semail ophiolite, was obducted in the Coniacian to Campanian from NE to SW onto the northern Oman continental margin. Results of our work in the Batain area differ from previous interpretations, with most significant differences concerning timing and direction of obduction. Our results show that WNW directed tectonic movements formed a fold‐and‐thrust belt and led to the obduction of allochthonous rocks onto the east Oman continental margin during latest Maastrichtian/earliest Paleocene times. This is coeval with emplacement of ophiolitic fragments along the eastern coast of Oman (eastern ophiolite belt) but is about 15–20 Myr later than emplacement of Hawasina complex and Semail ophiolite in northern Oman. Postemplacement structural evolution during the Tertiary involved intraplate extension, possibly reflecting the Red Sea/Gulf of Aden opening, and late Tertiary shortening related to convergence between Arabia and Eurasia. Late Tertiary contractional deformation resulted in refolding of the Batain nappes and in folding of the overlying Tertiary sediments. A palinspastic reconstruction of the Batain area indicates that the Permian to Upper Cretaceous sediments were formerly deposited in the Batain basin, a part of the proto‐Indian Ocean, along the present‐day eastern Oman margin. This leads us to propose that Permian breakup of Gondwanaland created both continental margins of Oman and led to the opening of two major basins: the neo‐Tethyan Hawasina basin in the north and the proto‐Indian Ocean Batain basin in the east, the latter separating Arabia from greater India.

MANTLE PERIDOTITES OF GRANULITE - GNEISS COMPLEX AS FRAGMENTS OF ARCHEAN (?) OPHIOLITES IN THE BAIKAL REGION (RUSSIA)

The Sharyzhalgay block, located in the south-western Baikal region, is an exposure of the Precambrian basement at the margin of the Siberian Platform. It chiefly consists of granitoids (80%) that host relic granulite facies rocks (20%). Metamorphism under granulite facies conditions took place about 2.4-2.5 Ga ago. Petrological and geochemical data on the granulites suggest protoliths of predominantly MORB-like basalt (~70%) and minor sedimentary rocks. Granulites also include ultramafic rocks. Relics of protogranular texture, high XMg in rocks and rock-forming minerals, occurrence of depleted harzburgites, mantle compositional trend of spinels, REE patterns, and thermobarometry data allow us to interpret the provenance of Saramta peridotites as mantle material depleted by partial melting in the stability field of spinel. Peridotites are cut by dikes of layered wehrlite, spinel websterite and garnet websterite. Spinel websterites have a chemistry and mineralogy of Cr-Di series. Their origin inferred to be the result of mineral segregations from percolating melt within the upper mantle. Garnet websterites differ from Al-augite pyroxenites by lower contents of TiO2 and Al2O3, and higher SiO2. Incongruent melting of Cr-Di spinel websterites on the scheme Cpx+ Opx+ Sp ® Mg-richer Ol+ Cr-richer Sp+L produced melts and restite wehrlites. Later on the melt crystallized in situ into garnet websterites with magmatic textures. Regional granulite-facies metamorphism at temperatures of 700-800°C and a pressure of 5-6 kb produced exsolution lamellae of spinel in pyroxene, kelyphite Sp+Opx+Pl rims on garnet, and zoning of orthopyroxene in garnet websterite. Tectonitic restite harzburgites with ultramafic dikes are correlative with the mantle parts of ophiolite sections. All these facts allow us to interpret relic granulite-facies rocks as fragments of Precambrian ophiolites.

Petrogenetic evolution of an ophiolite fragment in an ensialic marginal basin, northern Aegean (Samothraki Island, Greece)

Petrogenetic evolution of an ophiolite fragment in an ensialic marginal basin, northern Aegean (Samothraki Island, Greece)

Petrologically diverse basalts from a fossil oceanic forearc in California: The Llanada and Black Mountain remnants of the Coast Range ophiolite

Basalts from two geographically separated fragments of the California Coast Range ophiolite are very different petrographically, mineralogically, and geochemically. Pillow lavas from the Llanada Coast Range ophiolite remnant are mostly Cr-rich and TiO 2 -poor basalts in which Ti does not increase with increasing Fe/(Fe + Mg); some Cr-poor siliceous rocks are also present. Most Llanada rocks are depleted in light rare earth elements (REE), although a few show slight light REE enrichment. Ti/V ratios are arc-like. The Llanada basalts appear to be island-arc tholeiites that have limited boninitic affinities—i.e., products of suprasubduction zone magmatism. Basalts from the Black Mountain Coast Range ophiolite remnant near St. Helena are TiO 2 -rich relative to those from most other Coast Range ophiolite fragments, and TiO 2 increases with increasing Fe/(Fe + Mg). Black Mountain basalts are light REE depleted, REE enriched overall relative to Llanada lavas, and have mid-oceanic-ridge basalt (MORB)-like Ti/V ratios. These characteristics are similar to those of MORB, but elevated Th/Ta ratios suggest a suprasubduction-zone origin for the Black Mountain lavas. These results show that the Coast Range ophiolite is highly diverse, containing island-arc tholeiites and possibly suprasubduction zone–related MORB-like components. In the Coast Range ophiolite as a whole, the apparent preponderance of island-arc tholeiite lavas such as those at Llanada over MORB-like lavas such as those at Black Mountain favors a forearc over a backarc setting for the origin of the ophiolite. Other data suggest that the Coast Range ophiolite locally contains ocean-island basalts (OIBs). Moreover, serpentinite-rich ophiolitic detritus overlying Coast Range ophiolite serpentinite suggests that, in some areas, the basaltic section of the oceanic crust was missing completely, and serpentinized harzburgite was exposed at the sea floor. These relationships are similar to those in the modern Mariana forearc, which exposes a complex amalgam of boninitic and arc-tholeiitic suprasubduction zone–related crust, MORB and OIB crust, and serpentinized harzburgite.

An Autochton, Recent Lateritic Deposit in Eastern Tourids: The Buyukbelen (Farasa-Yahyali-Kayseri, Turkey) Iron Occurrences

Buynkbelen laterites at 2.5 km southeast of Farasa (Yahyali-Kayseri) village in the west of eastern Tourids are located at the southwest of Attepe (Mansurlu-Feke-Adana) area being the most important hydrothermal iron province of Turkey. In the area, Upper Cretaceous aged Cavdarusagi olisthostrome and Pozanti-Farasa peridotite nappe belonging to Aladag ophiolite sequence, and transgressively overlying Miocene aged conglomerates (Zebil Formation) crop out (Kupeli, 1996). Cavdarusagi olisthostrome contains mainly carbonate and lesser ophiolitic rock blocks and fragments in a matrix of serpentinite, diabase, chert intercalated pelagic limestone-marl-shale, chert and radiolarite, all of which show lateral and vertical transition to each other. Laterites studied are composed of semi-autochton, a blanket of 5 to 15 m thick lateritic iron crust including also karstic pockets, and surrounding red coloured, thin (0.5-1 m) soil cover. These laterites are observed as an autochton recent weathering zone above chert intercalated clayey limestones of the Cavdarusagi olisthostrome. In the weathering zone, hematite and goethite levels are distinguishable. Hematite part is 5 to 10 cm thick as thin bands exhibiting short lateral continuation. The weathering zone is characterised mainly bay yellowish brown coloured, thick goethite level. In addition to hematite and goethite, reddish rust coloured limonite is also common in the north of lateritic crust where is heavily karstic. Karstic cavities range from a few decimetres to 7 m in diameter. Continuing karst formation within ore mass and Dept. of Geol. Selguk University, Konya 42079, Turkey

The Chersky Range Ophiolite Belt, Northeast Russia

Mafic and ultramafic rocks are found in isolated complexes along the Chersky Range, eastern Sakha Republic (Ya kutia), Russia. Five of the six exposures are located on the eastern side of a Devonian-Ordovician carbonate platform and appear to be thrust over it; the sixth, the Debin fragment, is located to the west. On the basis of lithology, geochemical composition, and structural position, the ultramafic rocks and their associated mafic volcanics and deep-marine sedimentary rocks are interpreted as ophiolites or ophiolite fragments derived from back-arc or oceanic crust located to the east of the Chersky Range. Faunal and radiometric dating indicate that the oceanic crust has an age of about 370-430 Ma, Early to Middle Devonian. The ages of metamorphic minerals suggest that the ophiolites were obducted and metamorphosed during the amalgamation of the Kolyma-Omolon superterrane, about 170-174 Ma (Callovian, late Middle Jurassic), at which time olistostromes containing fragments of the ophiolite were formed. This age predates the formation of the Uyandina-Yasachnaya volcanic arc, believed to represent the final closure of the ocean basin between the Kolyma-Omolon superterrane and the North Asian craton. It is possible that the southernmost ophiolite, the Debin fragment, has a different history from the other Chersky Range ophiolites and was emplaced as a result of the accretion of the Kolyma-Omolon superterrane.

Opening and Tectonic Evolution of the Paleo-Asian Ocean

The Paleo-Asian ocean is defined by units located between the Russian (East European), Siberian, Tarim, and Sino-Korean (North China) continents. The study of the composition, age, and structural position of island-arc magmatic rocks, ophiolites, and high-pressure meta-morphic assemblages and their mutual correlations made it possible to identify similarities and differences in the evolution of the Paleo-Asian and Paleo-Pacific oceans. The initial stage of the evolution of the Paleo-Asian ocean defined its opening at 900 Ma, whereas opening of the Paleo-Pacific took place at 750 to 700 Ma. Closing of the Paleo-Asian ocean in the Carboniferous (NE branch) and the Permian corresponds to the main stage of reorganization and reopening of the Paleo-Pacific. The maximal opening of the Paleo-Asian ocean occurred after or simultaneously with the first accretion-collision event at 600 to 700 Ma, resulting from the collision of microcontinents and the Siberian continent. Vendian-Early Cambrian boninite-bearing island-arc complexes occur as lavas, sheeted dikes, and sill-dikes associated with gabbro-pyroxenites and ultramafics. These complexes are widely distributed in the Gornyy Altay, East Sayan, and West Mongolian regions and can be considered fragments of a giant boninite-bearing belt. In the late Early Cambrian, collision of seamounts with an island arc caused the squeezing of the subduction zone and return flows within the accretionary wedge. Serpentinite melange within fragments of ophiolites and high-pressure rocks are typical components of the Late Paleozoic accretionary wedges. Because of Middle Cambrian-Early Ordovician collisional events, two new oceans (Junggar-Irtysh-Kazakhstan and Uralian-South Tien Shan-South Mongolian) were formed. The junction of both oceans in East Mongolia opened to the Paleo-Pacific.

Discussion on a reassessment of the early Ordovician 'Celtic' brachiopod province

R. B. Neuman, D. A. T. Harper & C. R. van Staal write: The results of several recent geological studies relevant to early Ordovician palaeogeography are not mentioned in this recent paper by Cocks & McKerrow (1993). Contributions of particular significance include the recognition that the Dunnage Zone of Newfoundland consists of two distinct parts, the Notre Dame and Exploits subzones of contrasting Ordovician stratigraphy, lithology, structure, metamorphism, plutonism, lead isotope signatures, mineralization, geophysics, and faunas ( Williams et al. 1988 ). The boundary between these subzones, named the Red Indian Line in Newfoundland, is probably a suture. Fossiliferous sedimentary rocks are rare in the Notre Dame Subzone, but Arenig-Llanvirn graptolites occur in shales at three places and at one place conodonts have been extracted from carbonate clasts in volcanic breccias. All these fossils are considered to have Laurentian affinities (Nowlan & Thurlow 1984; Williams 1992 ; Williams et al. 1992 ). Fossiliferous Lower Ordovician rocks are more abundant in the Exploits subzone. In addition to the several occurrences of Celtic assemblage brachiopods (Cl-C6 of Neuman & Harper 1992 ; Harper 1992 ; Cocks & McKerrow 1993), Gondwanan trilobites and graptolites (Cyclopyge and Undulograptus) are known in the Exploits subzone of central Newfoundland ( Williams et al. 1992 ) from shale that overlies a Lower Ordovician ophiolite fragment near an Occurrence of Celtic brachiopods (C5 of Neuman & Harper 1992 ). Palaeomagnetic studies of Newfoundland rocks near the Red Indian Line showed that these two subzones were separated by about 30° of latitude (van der Pluijm et al. 1990;

Geochemical and Nd isotope characteristics of pre-Variscan ophiolites and meta-igneous rocks from the Struma Diorite Formation in SW Bulgaria

Recent research carried out in SW Bulgaria has distinguished two ophiolite blocks along the Variscan Thracian suture. These ophiolite fragments are closely associated with the Struma Diorite Formation (SDF), with which they are tectonically imbricated. The ophiolite fragments are formed by a dyke complex considered as dismembered part of a sheeted dyke unit. The SDF is a metamorphosed igneous complex built up of rocks with variable composition (mafic to acid), multistage origin (intrusive, volcanic) and calc-alkaline character. In many localities dykes of SDF rocks intersect the ophiolites. Combined trace element and Sm-Nd isotopic data document different origins for both complexes. The ophiolite association is formed by transitional oceanic basalts, extracted from depleted mantle. For the SDF an origin from less depleted source material in an ensimatic island arc setting is favoured.

Implications of Palaeozoic ophiolites from Western Junggar, NW China, for the tectonics of central Asia

Western Junggar, NW China, contains fragments of several Palaeozoic island arcs, accretionary prisms and ophiolites which were accreted to each other, and possibly to the southern margin of Eurasia, by the end of the Palaeozoic. The ophiolites of Western Junggar are important in reconstructions of these events. Four ophiolite belts are recognized, from south to north these are: Tangbale, Mayila, Dalabute and Barleik/Honggulueleng/Hebukesair. They record the production and destruction of oceanic tracts in this region from the Cambrian to the Carboniferous. The following characteristics are representative of them all. (1) The ophiolitic successions are strongly deformed. (2) Sheeted-dyke swarms are not preserved. (3) Basic igneous rocks show a range of chemical characteristics, from depleted, island arc tholeiite to ocean-island basalt. Altered N-type mid-ocean basalts may be represented. (4) Large-scale thrusts, typically directed towards the southeast, lie at the base of many individual ophiolitic bodies. (5) The ophiolites rarely mark the position of possible sutures between arcs; they are normally highly deformed and imbricated with other lithologies. Accretion of Palaeozoic arcs and obduction of ophiolites was complete by the end of the Carboniferous, but the timing and sequence of events remain obscure. No continental collisions occurred during this period of assembly; crustal growth occurred by the amalgamation of non-continental blocks.

Concordant paleolatitudes from ophiolite sequences in the northern California Coast Ranges, U.S.A.

Paleomagnetic data have been obtained from two ophiolite sequences in the northern California Coast Ranges: from Mount Diablo in the San Francisco Bay area and from Potter Valley, north of Clear Lake. The ophiolite exposed at Mount Diablo is part of the late Middle to Late Jurassic Coast Range ophiolite, and that exposed near Potter Valley is Late Jurassic to perhaps Early Cretaceous in age and occurs within the Franciscan assemblage. Data from the sheeted-dike complex at Mount Diablo show these rocks to be strongly overprinted, probably following uplift and erosion of the ophiolite. Samples whose primary remanent magnetization seems to be recovered yield a mean paleomagnetic pole at 30.7°N, 159.5°E with α 95 = 5.6°. A comparison of this pole with the Jurassic apparent polar wander path for North America indicates that the ophiolite has rotated 45° ± 7° counterclockwise relative to the craton and has not been latitudinally displaced. The diabase and pillow basalt in Potter Valley have not been strongly overprinted and data from those rocks yield a paleomagnetic pole at 79.0°N, 61.5°E with α 95 = 6.4°. This result indicates that the ophiolite at Potter Valley has rotated approximately 29° ± 8° clockwise, and has undergone little or no latitudinal displacement. Because of the predominantly northeastward transport of oceanic plates converging with the western margin of North America since middle Mesozoic time, the absence of appreciable northward displacement of either ophiolite fragment indicates that both formed close to the continental margin.

Subalkaline andesite from Valu Fa Ridge, a back-arc spreading center in southern Lau Basin: petrogenesis, comparative chemistry, and tectonic implications

Tholeiitic andesite was dredged from two sites on Valu Fa Ridge (VFR), a back-arc spreading center in Lau Basin. Valu Fa Ridge, at least 200 km long, is located 40–50 km west of the active Tofua Volcanic Arc (TVA) axis and lies about 150 km above the subducted oceanic plate. One or more magma chambers, traced discontinuously for about 100 km along the ridge axis, lie 3–4 km beneath the ridge. The mostly aphyric and glassy lavas had high volatile contents, as shown by the abundance and large sizes of vesicles. An extensive fractionation history is inferred from the high SiO 2 contents and FeO∗ MgO ratios. Chemical data show that the VFR lavas have both volcanic arc and back-arc basin affinities. The volcanic arc characteristics are: (1) relatively high abundances of most alkali and alkaline earth elements; (2) low abundances of high field strength elements Nb and Ta; (3) high U/Th ratios; (4) similar radiogenic isotope ratios in VFR and TVA lavas, in particular the enrichment of 87 Sr 86 Sr relative to 206 Pb 204 Pb ; (5) high 238 U 230 Th , 230 Th 232 Th , and 226 Ra 230 Th activity ratios; and (6) high ratios of Rb/Cs, Ba/Nb, and Ba/La. Other chemical characteristics suggest that the VFR lavas are related to MORB-type back-arc basin lavas. For example, VFR lavas have (1) lower 87 Sr 86 Sr ratios and higher 143 Nd 144 Nd ratios than most lavas from the TVA, except samples from Ata Island, and are similar to many Lau Basin lavas; (2) lower Sr/REE, Rb/Zr, and Ba/Zr ratios than in arc lavas; and (3) higher Ti, Fe, and V, and higher Ti/V ratios than arc lavas generally and TVA lavas specifically. Most characteristics of VFR lavas can be explained by mixing depleted mantle with either small amounts of sediment and fluids from the subducting slab and/or an older fragment of volcanic arc lithosphere. The eruption of subalkaline andesite with some arc affinities along a back-arc spreading ridge is not unique. Collision of the Louisville and Tonga ridges probably activated back-arc extension that ultimately led to the creation and growth of Valu Fa Ridge. Some ophiolitic fragments in circum-Pacific and circum-Tethyan allochthonous terranes, presently interpreted to have originated in volcanic arcs, may instead be fragments of lithosphere that formed during early stages of seafloor spreading in a back-arc basin.

Magnetic anomaly pattern of Central Europe

The magnetic anomalies represented on our map of Central Europe reflect the geological units and these anomalies are tentatively correlated with known geological structures. In Scandinavia strong magnetic anomalies caused by crystalline rocks of the East European Platform have been observed. Their southern boundary corresponds to the Elbe Line, the border between the mobile part of the East European Platform and the Dutch-German Platform. Here only some broader anomalies can be found and are caused by a magnetic basement (magnetization M = 2 A/m) under a 12–15 km thick layer of sediments. A southern margin of the Dutch-German Platform can be assumed along the W-E “Northern German Line” which is marked by local magnetic anomalies. Younger magmatic events are presumed to be the sources of these anomalies. The Rhenohercynian is located south of the Dutch-German Platform. Small NE-striking anomalies indicate a weak magnetic basement ( M = 0.35 A/m). A magnetic relief varying between 9 and 15 km is obtained by relief calculations. West of the Rhenohercynian anomalies of relatively strong amplitudes prevail. These correspond to the Brabant Massif, the eastern boundary of which presumably influences the tectonically controlled anomalies of Erkelenz and Lingen. There is no magnetic indication of a continuation of the Brabant Massif into the Rhenohercynian. The Mid-German Crystalline Rise and the Saxothuringian zone of the Variscan belt are situated south of the Rhenohercynian. Both areas show strong magnetic anomalies. The southwestern border forms an arch around the Brabant Massif and ends at the Paris Basin Anomaly. This magnetic zone finds its equivalent in a magnetic belt, the Donau Anomaly, which can be followed to the Massif Central in the southwest. This is located in the middle of the Moldanubian, which is characterized by a relatively undisturbed magnetic field. The strong anomalies in the Saxothuringian and Donau magnetic belts point to an accumulation of magmatic rocks. These might be fragments of oceanic crust or ophiolites marking previous subduction or obduction zones. Another explanation may be the intrusion of predominantly basic material during former crustal thinning or stretching. With the exception of the Saxothuringian and Donau Anomaly magnetic belts there is no indication of oceanic crust in the magnetic anomaly map.

Tertiary evolution of the Sivas Basin, central Turkey

The Sivas Basin is one of several basins in Turkey formed during closure of the northern branch of Neotethys in early Tertiary times. Cretaceous ophiolitic fragments and Eocene platform carbonates and volcaniclastics, transported northwards into the basin as olistoliths and grain-flow aprons, were incorporated into autochthonous Eocene turbidites and bioclastic limestones. The sequence as a whole was thrust northwards in late Eocene times. A southward-sloping terrestial foreland basin, related to northward-directed thrusting, developed during Oligocene times. A piggy-back basin developed on top of this thrust system. During the late Oligocene, the Eocene thrusts were reactivated, probably resulting in northward propagation of thrusts in the subsurface. In early and mid-Miocene times, the basin was floored by a thrust sheet which had been cut by N-S trending tear faults or oblique culminations as a result of non-uniform thrust advance in pre-Miocene times. These N-S faults were subsequently reactivated as extensional faults, radial to the thrust front, during early to mid-Miocene alluvial and shallow marine sedimentation. Later strike-slip displacement along the N-S faults was associated with the development of the Northern Boundary Fault of the Sivas Basin in late Miocene times, which is regarded as a left-lateral transpressive fault related to the North Anatolian Fault Zone.

Seismic reflection and gravity study of proposed Taconic suture under the New Jersey Coastal Plain: Implications for continental growth

Research Article| March 01, 1991 Seismic reflection and gravity study of proposed Taconic suture under the New Jersey Coastal Plain: Implications for continental growth R. E. SHERIDAN; R. E. SHERIDAN 1Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903 Search for other works by this author on: GSW Google Scholar R. K. OLSSON; R. K. OLSSON 1Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903 Search for other works by this author on: GSW Google Scholar J. J. MILLER J. J. MILLER 2U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information R. E. SHERIDAN 1Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903 R. K. OLSSON 1Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903 J. J. MILLER 2U.S. Geological Survey, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1991) 103 (3): 402–414. https://doi.org/10.1130/0016-7606(1991)103<0402:SRAGSO>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. E. SHERIDAN, R. K. OLSSON, J. J. MILLER; Seismic reflection and gravity study of proposed Taconic suture under the New Jersey Coastal Plain: Implications for continental growth. GSA Bulletin 1991;; 103 (3): 402–414. doi: https://doi.org/10.1130/0016-7606(1991)103<0402:SRAGSO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A vibroseis seismic reflection line near Buena, New Jersey, crosses the linear positive Salisbury Bouguer gravity and magnetic anomaly that follows the proposed Taconic suture. Two reflections at 1.4 and 1.7 sec under the positive gravity anomaly and beneath the Coastal Plain basement reflections at 1.1 sec are from interpreted Taconic suture rocks with ophiolitic crustal fragments (7.1-7.9 km/sec). A gently southeast-dipping reflection (2.0-2.7 sec) is interpreted as the detachment thrust below the allochthonous suture rocks. A stratified sequence (5.4-6.3 km/sec) is interpreted as remnants of parautochthonous lower Paleozoic drift-stage sedimentary rocks beneath the detachment thrust. Below 3.0 sec (8 km depth), the reflections are subhorizontal or gently dipping (11°) all the way to the Moho reflection at 11.0 sec. On the northwest end of the vibroseis line, a northwest-tilted Mesozoic rift-basin basement reflection is seen from 1.1 to 1.5 sec. These Mesozoic rocks (4.5-4.8 km/sec) are at least 1 km thick. The denser suture rocks with ophiolitic fragments and the less dense Mesozoic rocks explain almost all of the Salisbury Bouguer anomaly. No significant lateral density variations are needed below 8 km depth. These results are compatible with the Taconic suture under the New Jersey Coastal Plain being allochthonous and underlain by parautochthonous Grenville-age continental crust. A deep-seated Taconic suture at the edge of the Grenville North American basement may be southeast of the New Jersey Coastal Plain. This implies a greater amount of Taconic overthrusting and lesser amounts of accretion of the North American crust than previously proposed. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Formation and emplacement ages of the Masirah ophiolite, Sultanate of Oman

Hornblende separates from gabbros and biotite separates from crosscutting potassic granites of the Masirah ophiolite have been analyzed by conventional K-Ar techniques. The gabbros have Late Jurassic-Early Cretaceous ages (126-158 Ma), and the granites have broadly similar ages (124-146 Ma). Because the gabbros are likely to give ophiolite formation ages and the granites ophiolite emplacement ages, it can be concluded that the Masirah ophiolite was emplaced very soon after formation. The Late Jurassic-Early Cretaceous ages contrast with the middle Cretaceous ages (90-95 Ma) of the Semail ophiolite, confirming earlier suggestions that the Masirah ophiolite cannot simply be a right-laterally displaced fragment of the Semail ophiolite, but is instead an uplifted block of Indian Ocean crust. The formation and emplacement of the Masirah ophiolite are discussed in the context of the late Mesozoic evolution of the western Indian Ocean.

Three‐dimensional P and S velocity structure in the Coalinga Region, California

The Coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three‐dimensional velocity and hypocenter inversion in an area of complex three‐dimensional structure dominated by folding and blind thrusts. Additionally, since other varied geological and geophysical studies have been completed in this area, the three‐dimensional inversion solution could be compared to other interpretations. Inversion of 7696 P and 1511 S first arrivals from earthquakes and 696 P first arrivals from refraction shots produced a three‐dimensional velocity model with grid spacing of 1–2 km in the hypocentral area. The overall shape and location of velocity features correspond well to the mapped surface geology. The three‐dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower. The amounts of structural relief inferred for the local folds are similar to values inferred from geologic data. The three‐dimensional velocity solution has several distinctive features. There is a linear high‐velocity body (6.1–6.5 km/s), about 25 km long, from 6 to 8 km depth, that may represent a fragment of Coast Range ophiolite. A shallow low‐velocity zone (LVZ), which extends for 20 km along the fold axis at about 6 km depth and correlates with LVZs observed in both the refraction and the reflection data, may indicate high pore pressure caused by lateral compressive strain. Deeper LVZs occur within inferred Franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness, 4–8 km wide and 5–10 km long. These LVZs may represent multiple thrust faults. Their locations and geometry are consistent with thrust faults inferred with seismic reflection data. The three‐dimensional velocity solution compares well to prior two‐dimensional seismic reflection and refraction models and observed gravity. The shape of the inferred sedimentary section agrees well with the reflections from the Cenozoic strata. Compared to the refraction model, the three‐dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data. The gravity computed from the three‐dimensional velocities is similar to the observed gravity in both shape and amplitude. Both a simple one‐dimensional initial model and a complex initial model derived from the refraction interpretation were tried. A simple starting model gave the best results. The S velocity solution has different resolution than the P velocity solution because it uses a different set of stations, and it has lower resolution because it uses fewer arrival times. While the general patterns of velocity variation are similar for both Vp and Vs, the Vs solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features.

A geochemical approach to allochthonous terranes: a Pan-African case study

The recognition of Mesozoic and Cenozoic terranes can best be made from palaeomagnetic, structural and palaeontological studies, but older regions of continental crust require geochemical constraints to evaluate crustal growth through terrane accretion. For Precambrian shields, the pattern of Pb and Nd isotopic provinces may reveal the mechanism of crustal growth. The Afro-Arabian Shield was generated by calc-alkaline magmatism between 900 and 600 Ma ago. This example of Pan-African crustal growth underlies an area of at least 1.2 x 10 6 km 2 , which may extend to 3.5 x 10 6 km 2 beneath Phanerozoic sediments and Tertiary volcanic cover. Field evidence and trace element geochemistry suggest that Pan-African tectonics began as a series of intra-oceanic island arcs that were accreted to form continental lithosphere over a period of 300 Ma. The great majority of Nd and Pb isotope ratios obtained for igneous rocks from the shield are indicative of a mantle magma source. Although many of the dismembered ophiolites cannot be identified with inter-terrane sutures in their present location, the eastern margin of the Nabitah orogenic belt is a major tectonic break that coincides with a critical boundary between Nd and Pb isotopic provinces and is marked by a linear array of ophiolite fragments across the length of the shield. Other terrane boundaries have not been identified conclusively, both because coeval island arcs can not be distinguished readily on isotopic grounds and because many ophiolites are allochthonous. However, the calculated rates of crustal growth (measured as volume of magma, extracted from the mantle per unit time) between 900 and 600 Ma are similar to those calculated for Phanerozoic terranes from the Canadian Cordillera. Such high rates in the Afro-Arabian Shield suggest that island arc terranes have accreted along a continental margin now exposed in NE Africa, together with minor continental fragments. If crustal growth rates during this time were no greater than contemporary rates, ca . 4000 km of arc length are required, which is considerably less than that responsible for crustal growth in the SW Pacific.

Albite deformation within a basal ophiolite shear zone

Abstract Albite mylonites at the base of the White Hills Peridotite ophiolite fragment at St. Anthony, Newfoundland formed during obduction at lower crustal P - T conditions. Optical microstructures preserve a history of intracrystalline distortion, recovery and cyclic dynamic recrystallization associated with hot-working. The crystallographic fabric is consistent with shear of the polycrystalline aggregate dominated by glide on (010) planes, as is also indicated by TEM observations. Slip systems implied by trace analysis and invisibility experiments using TEM are b = [101](010), b = [001](010) and b = 1/2[112](201). Dissociated dislocations have separations ranging from 8.3–15 nm. These separations are notably smaller than in more calcic plagioclases and indicate a relatively higher ductility for albite in the case of cross-slip controlled creep. The observed dislocation densities relate to a high stress/strain rate pulse imposed on the dominant high-temperature creep textures.

High-Mg extrusive rocks from the Romeral Zone ophiolites in the southewestern Colombian Andes

In the southwestern Colombian Andes, high-Mg extrusive rocks occur within the Mesozoic (?Late Cretaceous) ophiolitic fragments from the Romeral fault zone, a major suture between the continental domain of the Central Cordillera, and the oceanic domain of the Western Cordillera. The high-Mg rocks include picrites and picritic basalts, and are associated in places with peridotites showing transitional to extrusive textures. Plagioclase-free picrites characterized by quench textures have been found in two localities, and occur as pillow-lava (Rio Boloblanco) or as clasts of monogenic and polygenic volcanic breccias (El Encenillo). They show similar olivine-phyric micro-spinifex textures with groundmass consisting of hopper and plate olivine (or Ca-poor pyroxene at El Encenillo), skeletal clinopyroxene and glass. Hyaloporphyritic texture occurs in the Rio Boloblanco picrites at pillow rims. Chemically, these picrites are characterized by MgO contents of >20%, and are distinguished by the content of TiO 2 and incompatible elements in a high-Ti type (Rio Boloblanco) and a very high-Ti type (El Encenillo), the latter particularly remarkable for the high contents of Ti, Zr and Nb which give evidence of an alkaline tendency. Phase chemistry is consistently different between the two picrite types, but comparably high Al contents (up to 14%) of skeletal pyroxenes suggest that the magmas were erupted and cooled in similar conditions. Olivine-phyric picritic rocks characterized by microlitic non-quenched groundmasses occur as clasts of volcanic breccias at El Encenillo (picritic basalts), and are present with comparatively high frequency within major (Los Azules) and minor (Upper Chapungo Valley) ophiolitic fragments from the Romeral Zone, either as lavas and volcanigenic breccias. These rocks are mostly richer in olivine than the quench-textured picrites, contain modal plagioclase, and relatively low-Al pyroxenes. As regards chemistry, a high-Ti and very low-Al group (Los Azules, Upper Chapungo Valley) and a very high-Ti group (El Encenillo) can also be distinguished within these picrites. A comparison with the ultramafic volcanic rocks from Gorgona Island, in the nearby Pacific domain off Colombia, among which the unique Phanerozoic komatiites were discovered, indicates close similarities between the Rio Boloblanco picrites and some Gorgona Island komatiites (olivine-phyric komatiites with micro-spinifex groundmass) regarding their petrochemical features. It is suggested that they record a unique event related to the same tectonic environment, and consequently that they belong to the same structural unit, developed in the Late Cretaceous and extending from continental Colombia to the Pacific domain. This is in contrast with previous subdivisions of the ophiolitic complexes from southwestern Colombia into distinct age groups progressively younger from the Central Cordillera toward the Pacific Ocean.