Oceanic Fault Research Articles

SMatStack to Enhance Noisy Teleseismic Seismic Phases: Validation and Application to Resolving Depths of Oceanic Transform Earthquakes

AbstractThe depths of most moderate‐sized oceanic earthquakes are poorly constrained because of the lack of local recording stations and noisy teleseismic recordings. This hampers our understanding of slip behaviors along oceanic faults and the mechanical properties of the oceanic lithosphere. In this study, we develop a new method to enhance the weak body‐wave signals, particularly the depth phases, associated with earthquakes on oceanic transform faults using large‐aperture arrays in the teleseismic range. We simulate the enhanced teleseismic signals to refine the centroid depths of moderate‐sized earthquakes. We validate the new approach using synthetic waveforms and show it outperforms conventional methods when dealing with noisy signals. We obtain the depth estimates for three moderate‐sized earthquakes on the Chain transform fault in the equatorial Atlantic Ocean and find two of them are consistent with a local catalog derived using oceanic bottom seismometers. Application of the new method to the past decades of teleseismic recordings of moderate‐sized earthquakes on the large and slow‐slipping transform faults will provide significantly improved constraints on the width of the seismogenic zone, thus advancing our understanding of the rheology of oceanic lithosphere and earthquake processes in oceanic settings and, by comparison, their more dangerous continental counterparts. The new method is not limited to oceanic transform earthquakes, but can be easily adapted to other seismological studies in which noisy but coherent signals could be revisited for better usage.

Magma and Mineral Composition Response to Increasing Slab-Derived Fluid Flux: Nevado de Longaví Volcano, Southern Chilean Andes

Nevado de Longaví volcano (NLV), in the Southern-Central Chilean Andes, has erupted during the Holocene magmas with compositions that are in several ways atypical for the region. These characteristics include elevated La/Yb ratios in evolved magmas, in an area of only moderately thick crust, coupled with low concentrations of K, Th, and other incompatible elements and elevated ratios of fluid-mobile (B, Cs, Li, Sb) to fluid-immobile elements. Samples have an unusual mafic mineralogy dominated by amphibole. The petrology of the Holocene products of NLV have been related to the influence of an oceanic transform fault (Mocha Fracture Zone; MFZ) that supplies the mantle wedge with unusually high amounts of fluids via dehydration of serpentinite bodies hosted by the subducted oceanic lithosphere. Because the trace of this transform fault is oblique to the convergence vector, its position along the arc has varied through time, as has the magnitude of its influence on the nature of the magmas erupted at NLV. The whole-rock and mineral chemistry of volcanic products from NLV, tied to a simplified stratigraphy, documents the secular changes in the magmatic system as the oceanic fault approached its current position. Magmas erupted ∼1–0.6 Ma are relatively low in water (as inferred from mineralogy and chemical proxies) and reduced (NNO-1 to NNO+0.5), and are similar to compositions found in neighboring volcanoes. From 0.25 Ma to the present, magmas are water-rich and oxidized (NNO-0.5 to NNO+1.7). In the intervening 0.6–0.25 Ma, mafic magmatism acquired a transient crustal component, which we identify as subducted sediment melts, on the basis of radiogenic isotopes and Pb, Th, and U abundances. Fluids released from serpentinite in the fracture zone were rich in Li, B, Sb, Cs and Ba, but not in K, Th, U and Sr. The fluid addition led to enhanced melting, particularly hydrous magmas that stabilized amphibole early during fractionation, higher oxygen fugacities, and distinctive chemical compositions.

Accessibilty analysis of tsunami evacuation route to self supported shelter in sub-district Pasie Nan Tigo, Padang City

The city of Padang, the capital of West Sumatra, has 1 million populations and located on the west coast of Sumatra. The city situated at two earthquake sources that are subduction zone in the Indian Ocean and Sumatra fault inland of Sumatra. Since the year 2006, Padang city is often experiencing an earthquake from both sources. In Mentawai island, about 200 km from Padang City westward, there is still what so-called seismic gap that very potential to generate big earthquake followed by a tsunami. To anticipate this, the mitigation measurements are needed to prepare the people on the west coast of Sumatra, especially in Padang city. One of the activities is to prepare the shelter that can cover around 600,000 people prone to tsunami near the beach. The infrastructures for shelter are very limited. There are only 4 shelters that have been built that have capacity about 2000-3000 people. To anticipate this condition, it needs to build what so-called self-supported shelter. The self-supported shelter is an idea or concept of utilizing a mosque or musholla to be a shelter by participating in the community surrounding its construction and management. The shelter will have a function as the ritual of religion and the tsunami evacuation place. Mosque or musholla is selected to shelter since it does not need to buy land, available in the near residence area, and easy to get funds in its construction. There are some aspects need to be prepared in order to function a shelter well, that is, the selection of location, the evacuation system, the construction method, the accessibility, and the structural design. This paper focus on analysis of the accessibility of evacuation routes to self-supported shelter in terms of time estimation, road facilities, and barrier during evacuation. The case study is chosen as the sub-district of Pasie Nan Tigo that has about 12,000 populations. The result can be a model in designing accessibility to self-supported shelters in the other area.

Active transform faults in the Gulf of Guinea: insights from geophysical data and implications for seismic hazard assessment

The seismotectonics of Western Africa show the occurrence of major earthquakes (e.g., 1636 southwestern Ghana, 1855 offshore Monrovia, 1939 offshore Accra, and 1983 Gaoual-Guinea) and prominent offshore transform faults. However, there is no analysis that links the continental active tectonics with the oceanic fault zones of the Gulf of Guinea. We study the active tectonics by firstly mapping the main transform faults using a combination of bathymetric, gravimetric, and magnetic data. The data analysis associates regional seismicity (historical and instrumental) with focal mechanisms as extracted from the recently published seismotectonic map of Africa. We identify active transform faults, the Chain (CFZ), Romanche (RFZ), Saint Paul (SPFZ), and Arkhangelskiy (AFZ) fault zones. We also calculate strain rates on these faults from Late Cretaceous (–85 Ma) to present time using paleomagnetic data and infer slip rates from the seismic moment data. The strain rates show a first stable trend around 2 cm/year and then accelerate to 4 cm/year in the last 10 million years. The comparison of Late Quaternary strain rates with geodetic strain rates shows an accumulation of seismic energy that could lead to the initiation of an Mw 7–7.5 earthquakes on the Saint Paul transform fault. Our seismotectonic analysis clearly links oceanic and continental tectonics, with about a 20°–30° anticlockwise fault trend rotation for CFZ, RFZ, and SPFZ. The potential for the occurrence of large earthquakes in the Gulf of Guinea should be taken into account for a realistic regional seismic and tsunami hazard of the Gulf of Guinea.

INFLUENCE OF RIFT ZONES AND THERMOKARST LAKES ON THE FORMATION OF SUBAQUEOUS PERMAFROST AND THE STABILITY ZONE OF METHANE HYDRATES OF THE LAPTEV SEA SHELF IN THE PLEISTOCENE

This paper presents results of the analysis of the influence of talik zones associated with thermokarst lakes and processes in rift zones on the dynamics of subaqueous permafrost and zones of stability of methane hydrates for conditions of theLaptev Seashelf. The model of thermophysical processes in the bottom sediments together with the scenario of climate change on the Arctic shelf for the last 400 thousand years (kyr) were used. Typical value of geothermal heat flux for the most part of the shelf and for the shallow shelf (with the present-day depth of ≤50 m) is estimated as 60 mW/m2. It is shown that with this value the duration of the interglacials and the corresponding ocean transgression periods is not sufficient for the complete degradation of permafrost and destruction of the gas hydrates. For a deeper shelf, however, the complete disappearance of the stability zone of the methane hydrates is possible during the interglacial periods. In the areas of oceanic faults (rifts), higher values of the deep heat flux increase rates of degradation of the underwater permafrost rocks in the interglacial periods as compared with the condition when the heat flux is 60 mW/m2. Intensification of degradation of the subsea permafrost is manifested in areas where thermokarst lakes arise, but here it is associated with the temperature rise at the upper boundary of the bottom sediments. The presence of the rift zones and/or the thermokarst lakes promotes decreasing of the presentday thickness of the permafrost, and simultaneous impact of these two factors can lead to a through thawing of the shelf in the interglacials (including Holocene).

Compositional fingerprints of chromian spinel from the refractory chrome ores of Metalleion, Othris (Greece): Implications for metallogeny and deformation of chromitites within a “hot” oceanic fault zone

The chrome ores of the retired Metalleion mine of the Othris ophiolite are hosted in a small volume of a pervasively serpentinized, tabular harzburgite body. These ores have been studied to determine their geological mode of occurrence, mineralogy and chromian spinel (Cr-spinel) chemistry. The ores consist of massive chromitite (85–95% modal Cr-spinel) with mylonitic fabric in imbricate-shaped pods. Chromian spinel displays a limited range in Cr# [Cr/(Cr+Al)×100=53–63] and Mg# [Mg/(Mg+Fe2+)×100=59–73] and low TiO2 content (≤0.11wt%). Minor- (Ti, Ni, V, Mn and Zn) and trace-element (Sc, Co and Ga) concentrations do not show any significant variations from Cr-spinel cores to boundaries and were not considerably modified by post-magmatic processes. However, Cr-spinel compositions show slight enrichments in Zn and V, and depletions in Ti and Sc when compared to the composition of chromite from the East Pacific Rise mid-ocean ridge (MOR) basalts. The composition of Cr-spinel from the Metalleion chromitites is quite anomalous on a global perspective as these ores are not clearly associated to a suprasubduction zone (SSZ) or MOR chemistry.Field data indicate that the Metalleion chromitites lack remnant dunite envelopes due to prolonged shearing within the deforming harzburgite host and are constrained in location to a ductile-brittle shear zone that correlates with the transforming direction of the Domokos oceanic fault. Compositional data indicate that chromitites equilibrated with ambient harzburgite via a melt-peridotite interaction process followed by melt mixing. Geochemical calculations demonstrate that the parental magmas of the Metalleion chromitites had intermediate affinity between typical mid-ocean ridge basalts (MORB), island arc tholeiites (IAT) and boninites. We conclude that these melts originated within a hydrated and oxidized mantle wedge beneath an infant forearc basin connected by a transform fault to an active MOR.

Spatial instability of the rift in the St. Paul multifault transform fracture system, Atlantic Ocean

The structure of the acoustic basement of the eastern part of the St. Paul multifault transform fracture system hosts rift paleovalleys and a paleonodal depression that mismatch the position of the currently active zones. This displacement zone, which is composed of five fault troughs, is unstable in terms of the position of the rift segments, which jumped according to redistribution of stresses. The St. Paul system is characterized by straightening of the transform transition between two remote segments of the Mid-Atlantic Ridge (MAR). The eastern part of the system contains anomalous bright-spot-like reflectors on the flattened basement, which is a result of atypical magmatism, that forms the standard ridge relief of the acoustic basement. Deformations of the acoustic basement have a presedimentation character. The present-day deformations with lower amplitude in comparison to the basement are accompanied by acoustic brightening of the sedimentary sequence. The axial Bouguer anomalies in the east of the system continue to the north for 120 km from the active segments of the St. Paul system. Currently seismically active segments of the spreading system are characterized by increasing amplitudes of the E–W displacement along the fault troughs. Cross-correlation of the lengths of the active structural elements of the MAR zone (segments of the ridge and transform fracture zones of displacement) indicates that, statistically, the multifault transform fracture system is a specific type of oceanic strike-slip faults.

Geophysical mapping of the inland extension of Deep Ocean Fault zones, south-western Nigeria

Research activities close to the south western edge of the coastal line has shown inland extension of deep oceanic features identified as regional fault zones. This research work focused on mapping of inland extension of Chain Fault zones into the south-western part of Nigeria using both satellite images and aeromagnetic intensity map over a 55 km by 55 km Ifon Basement-Sedimentary area. The area is a transition zone between the south-western basement complex and sedimentary basin of the south-western Nigeria. Interpretations of magnetic field over the area revealed intra-sedimentary features identified as fault zone which form an alignment and inland extension pattern with the offshore Chain fault zone. On the account of the qualitative and quantitative interpretation of the acquired aeromagnetic data, the Landsat-ETM+ multi-spectral bands and Digital Elevation Model (DEM) of the topography of the area, the evidence of inter-sedimentary tectonic events are recorded in the geomorphic characteristics of the study area’s terrain in relation to the underlying influence of geology and structural trends revealed by extracted lineaments. From geophysical and geomorphologic interpretations, the observed geomorphic characteristics show three physiographical areas: the northern uplifted block (connection of a series of hills cut by deep-sided valleys), the central SW-NE trending subsided block (or graben) and the southern uplifted block. This geomorphic pattern of a series of flat and highly undulating terrain broken by a series of deep valleys and hills is believed to be related to rifting arising from the influence of the transform fault zones, which subdivide the area into smaller units. The rift valleys in the area are long, deep valleys bounded by parallel faults, or fractures. In addition, the predominant SW/NE, SE/NW and E-W trending linear structures mapped as fracture/faults cut the basement and penetrated the overlying Cretaceous and Tertiary sediments in the study area. Key word: Chain Fault zones, transform faults, rifting, inter-sedimentary tectonics, south-western Nigeria Embayment.

Caught in the act

Faults break under the stress of plate tectonic forces, but the processes immediately preceding rupture are enigmatic. Monitoring of a remote oceanic fault that breaks regularly indicates that rupture is controlled by physical properties of the fault zone.

Gray Fuzzy Cluster Method and its Application in Development Scheme of Fault Block Reservoirs

Fault-block oil and Gas Field is one of special complicated reservoirs. For its development, No matter at home or aboard, the instances are not too many, and most of them are terrestrial oil reservoir. For ocean fault-block oil reservoirs, there are no instances that can be consulted. This paper aims at some ocean fault block oil reservoir .Gray relation model and fuzzy cluster analysis are combined to set gray fuzzy cluster analysis .Through a case analysis ,gray fuzzy cluster method is used to clustering of reservoirs. with the experiences that has been obtained, so that the foundation are provided for the development of some ocean fault block oil reservoir.

Interaction of Gravity Flows With a Rugged Mid-Ocean-Ridge Seafloor: An Outcrop Example from Macquarie Island

Rugged bathymetry along slow-spreading mid-ocean ridges has the potential to impinge a strong control on gravity flows derived from oceanic fault scarps. Sedimentary lithofacies from the Macquarie Island ophiolite supports this hypothesis by displaying systematic variations that correspond with volcanic substrate differences and proximity to rift-related faults. Pillow-basalt terranes are associated with tightly confined bedrock corridors that funnel gravity flows into one direction. Vertical lithofacies variations formed from high-density to low-density turbidity currents record successive fill stages of the corridor axis. During initial stages tight confinement in the axis suppressed flow dilution and fluid turbulence. With continued corridor-axis filling, more dilute gravity flows predominated and formed lateral gradations from axial coarse-grained turbidites into thinly interbedded overbank lithofacies along corridor margins. These gravity-flow lithofacies converge into very thin muddy condensed intervals along inter-corridor highs where significant bottom-current reworking occurred. Conversely, partly confined tabular-basalt-floored basins promoted lateral expansion and dilution of gravity flows throughout the duration of basin filling. Variable paleocurrent-indicator directions record multiple reflections of single gravity-flow events off basin-bounding fault barriers. Coarsening-upward trends in these partly confined basins from thinly interbedded pelagic chert and ripple-bedded sandstones up into stacked turbidites preserves the ponding of sediment-starved submarine fans. In general, these ponded basins preserve the fine-grained distal ends of a gravity-flow continuum from coarse-grained fault-proximal en masse failures and cohesionless debris flows into medial high-density turbidity flows and distal dilute turbidity flows.

Island Arcs and Convection

Abstract : If island arcs are the outcrops of compressive faults, they cannot be products of a uniform field of compressive stress because their symmetry is lower. The polarity of the curvature, however, is provided if the compression is of convective origin. Incipient faulting is then followed by a convergence of the convective flow towards the fault, causing a convergence of the initially parallel stress trajectories. Since the fault propagates at right angle to the trajectories of maximum compressive stress, its outcrop becomes an arc convex towards the upstream side. In oceanic compressive faults due to convection the two conjugate fault planes are not equivalent because overthrusting of the downstream side by the upstream side would lead to the accumulation of a thick crust which would inhibit the progress of faulting along the plane, and because underthrusting is libricated by decomposing serpentine, fluxed basalt etc. This seems the reason why apparently in all long-lived oceanic compressive faults the upstream (usually the oceanward) side underthrusts the downstream side. (Author)

Tectonic provinces of the Atlantic Ocean

The tectonic structure of the floor of the Atlantic Ocean beyond the continental margins is insufficiently studied. This is also true of its tectonic demarcation. The segmentation of the floor into regional-scale tectonic provinces of several orders proposed in this paper is primarily based on structural and historical geological features. It is shown that deep oceanic basins and fault tectonics are of particular importance in this respect. Tectonic provinces of two orders are distinguished by a set of attributes. The first-order provinces are the North, Central, South, and Antarctic domains of the Atlantic Ocean. They are separated by wide demarcation fracture zones into Transatlantic (transverse) second-order tectonic provinces. Ten such provinces are recognized (from the north southward): Greenland-Lofoten, Greenland-Scandinavia, Greenland-Ireland, Newfoundland-European, North American-African, Antilles-African, Angola-Brazil, Cape-Argentine, North Antarctic, and South Antarctic. This subdivision demonstrates significant differentiation in the geodynamic state of the oceanic lithosphere that determines nonuniform ocean formation and the tectonic features of the ocean floor. The latitudinal orientation of the second-order provinces inherits the past tectonic pattern, though newly formed structural units cannot be ruled out. The Earth rotation exerts a crucial effect on the crust and the mantle.

Complex Faulting Associated with the 22 December 2003 Mw 6.5 San Simeon, California, Earthquake, Aftershocks, and Postseismic Surface Deformation

We use data from two seismic networks and satellite interferometric synthetic aperture radar (InSAR) imagery to characterize the 22 December 2003 M w 6.5 San Simeon earthquake sequence. Absolute locations for the mainshock and nearly 10,000 aftershocks were determined using a new three-dimensional (3D) seismic velocity model; relative locations were obtained using double difference. The mainshock location found using the 3D velocity model is 35.704° N, 121.096° W at a depth of 9.7±0.7 km. The aftershocks concentrate at the northwest and southeast parts of the aftershock zone, between the mapped traces of the Oceanic and Nacimiento fault zones. The northwest end of the mainshock rupture, as defined by the aftershocks, projects from the mainshock hypocenter to the surface a few kilometers west of the mapped trace of the Oceanic fault, near the Santa Lucia Range front and the >5 mm postseismic InSAR imagery contour. The Oceanic fault in this area, as mapped by Hall (1991), is therefore probably a second-order synthetic thrust or reverse fault that splays upward from the main seismogenic fault at depth. The southeast end of the rupture projects closer to the mapped Oceanic fault trace, suggesting much of the slip was along this fault, or at a minimum is accommodating much of the postseismic deformation. InSAR imagery shows ∼72 mm of postseismic uplift in the vicinity of maximum coseismic slip in the central section of the rupture, and ∼48 and ∼45 mm at the northwest and southeast end of the aftershock zone, respectively. From these observations, we model a ∼30-km-long northwest-trending northeast-dipping mainshock rupture surface—called the mainthrust—which is likely the Oceanic fault at depth, a ∼10-km-long southwest-dipping backthrust parallel to the mainthrust near the hypocenter, several smaller southwest-dipping structures in the southeast, and perhaps additional northeast-dipping or subvertical structures southeast of the mainshock plane. Discontinuous backthrust features opposite the mainthrust in the southeast part of the aftershock zone may offset the relic Nacimiento fault zone at depth. The InSAR data image surface deformation associated with both aseismic slip and aftershock production on the mainthrust and the backthrusts at the northwest and southeast ends of the aftershock zone. The well-defined mainthrust at the latitude of the epicenter and antithetic backthrust illuminated by the aftershock zone indicate uplift of the Santa Lucia Range as a popup block; aftershocks in the southeast part of the zone also indicate a popup block, but it is less well defined. The absence of backthrust features in the central part of the zone suggests range-front uplift by fault-propagation folding, or backthrusts in the central part were not activated during the mainshock.

Determination of fault friction from reactivation of abyssal-hill faults in subduction zones

Abyssal-hill faults are reactivated on the outer slope of trenches when they strike within 25° from trench parallel; otherwise, new faults form parallel to the trench. We use the observed transition angle (25°) and a three-dimensional failure analysis to determine the coeffi cient of friction on reactivated abyssal-hill faults. The stress state in the outer slope is modeled as the superposition of the overburden stress and bending-induced plane strain deformation. If new trench-parallel faults fail according to a linear failure relationship with no cohesion, then determination of the coeffi cient of sliding friction (μs) on reactivated faults is independent of the absolute value of the principal stresses and depends only on the Poisson ratio of the crust and the slope of the failure law for new faults (μf). We fi nd that reactivated faults, dipping at 45°, are ~30% weaker than surrounding crust (e.g., μs = 0.6 for μf = 0.85). These results suggest that the variation in the strength of oceanic crust due to the seafl oor spreading fabric is small, and that the coeffi cient of sliding friction on oceanic faults is consistent with that observed in the laboratory.

Indian Ocean fault line poses threat of further earthquakes

Energy from 26 December quake could hasten the next rupture.

Imaging the source region of the 2003 San Simeon earthquake within the weak Franciscan subduction complex, central California

Data collected from the 2003 Mw6.5 San Simeon earthquake sequence in central California and a 1986 seismic refraction experiment demonstrate that the weak Franciscan subduction complex suffered brittle failure in a region without significant velocity contrast across a slip plane. Relocated hypocenters suggest a spatial relationship between the seismicity and the Oceanic fault, although blind faulting on a nearby, unknown fault is an equally plausible alternative. The aftershock volume is sandwiched between the Nacimiento and Oceanic faults and is characterized by rocks of low compressional velocity (Vp) abutted to the east and west by rocks of higher Vp. This volume of inferred Franciscan rocks is embedded within the larger Santa Lucia anticline. Pore fluids, whose presence is implied by elevated Vp/Vs values, may locally decrease normal stress and limit the aftershock depth distribution between 3 to 10 km within the hanging wall. The paucity of aftershocks along the mainshock rupture surface may reflect either the absence of a damage zone or an almost complete stress drop within the low Vp or weak rock matrix surrounding the mainshock rupture.

Hypersolidus deformation in the lower crust of the Josephine ophiolite: evidence for kinematic decoupling between the upper and lower oceanic crust

New mapping and structural observations in the lower crust of the Josephine ophiolite provide insights into the geometry and kinematics of hypersolidus flow beneath an oceanic spreading center. The lower crust, defined here as the sequence of rocks overlying mantle peridotite and beneath exposure of 100% sheeted dikes, can be divided into lower wehrlite–dunite and upper gabbroic sections. The contact between the two is mutually intrusive where exposed. Hypersolidus fabrics are the dominant structures observed. No pervasive crystal-plastic deformation is observed. Restoration of the hypersolidus foliations and igneous modal layers to their on-axis orientation indicates that they strike approximately perpendicular to the strike of the inferred paleo-spreading center as defined by the orientation of sheeted dikes and on-axis, oceanic faults. Hypersolidus lineations define a dispersed 3-D flow pattern in the lower crust, whereas extension directions in the upper crust (i.e. poles to sheeted dikes, oceanic normal faults) are unidirectional and perpendicular to the paleo-ridge axis. Collectively, these observations are consistent with partitioning of deformation between the upper and lower crust, and local ridge-parallel extension in the partially-molten lower crust due to possible subsidence of the thickened, axial upper crust. However, some component of kinematic coupling between the lower crust and mantle peridotite driven by asthenospheric flow cannot be ruled out.

Structure, petrology and seafloor spreading tectonics of the Kizildag Ophiolite, Turkey

Abstract The Kizildag ophiolite in southern Turkey is a remnant of the Neo-Tethyan oceanic crust and displays well-preserved magmatic and tectonic structures of seafloor spreading origin. The ophiolite consists of two structurally distinct massifs that are separated by the NW-striking high-angle Tahtaköprü fault. The main massif to the west contains a serpentinized peridotite core adjoined on the southeast by the normal fault-bounded plutonic sequence and sheeted dyke complex in a structural graben. The dyke-gabbro boundary within this graben is in places faulted along a low-angle detachment surface and is locally marked by a transition zone with mutual intrusive relations between the dykes and isotropic gabbros and plagiogranites. This igneous boundary contains numerous proto-dyke intrusions marking a well-preserved root zone of the sheeted dyke complex and may represent the roof of a fossil magma chamber. Mineralized oceanic faults within the dyke complex form two major subsets. Dyke-parallel normal faults form horst and graben structures and locally flatten with depth acquiring a listric geometry. Dyke-perpendicular faults display steep dips and subhorizontal slickenside lineations, suggesting their oblique- to strike-slip nature. The graben structure containing the plutonic sequence and the sheeted dyke complex is analogous to those documented in the Troodos ophiolite and may similarly represent a fossil spreading axis. The second massif east of the Tahtaköprü fault consists mainly of serpentinized peridotites directly overlain by lava flows, rotated dyke blocks, and gabbros. Sulfide mineralization along some fault planes in the extrusive rocks indicates that hydrothermal systems were associated and operated synchronously with magmatic and tectonic extensional processes. Stratigraphic relations and the structural architecture in this massif suggest that the Kizildag oceanic crust underwent crustal denudation and unroofing of the upper mantle as a result of tectonic extension at a spreading centre. The Tahtaköprü fault separating the two massifs is an accommodation zone that permitted differential movements between the adjacent ridge segments during generation of the Neo-Tethyan oceanic lithosphere. The general structure of the ophiolite suggests its evolution via seafloor spreading and an asymmetric simple shear extension along a slow-spreading centre. The sheeted dykes and pillow lavas have relatively high SiO 2 and Al 2 O 3 and low FeO and TiO 2 concentrations and show limited FeO and TiO 2 enrichments with decreasing MgO contents. These compositional properties differ markedly from typical tholeiitic suites from the ocean floor and arc settings, but correspond closely to those documented from the sheeted dyke complex and the lower volcanic suite of the Troodos ophiolite. The major element compositions suggest low pressure and high degree of melting from a depleted mantle source. The trace element concentrations are markedly depleted in both high-field strength and rare-earth elements and relatively enriched in largeion-lithophile elements compared to normal MORB. It is inferred that the Kizildag ophiolite formed in a tectonic setting where melting occurred at relatively low pressures within the stability field of spinel and proceeded to high melt fractions possibly by progressive depletion and melt removal. A limited positive correlation between the extent of melting and light rare-earth enrichment can be related to infiltration by fluids or melts derived from a more fertile source or possibly a subducting slab fragment. Comparison of the structure and petrology of the Troodos and Kizildag ophiolites and the regional geology suggest their evolution along a seafloor spreading system within the Southern Neo-Tethys. Spreading probably occurred in short segments and resulted in development of the Cretaceous Neo-Tethyan seaway as a marginal basin between the Tauride platform in the north and Afro-Arabia in the south.

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, III: faulting and fluid circulation. Constraints from integration of FMS images, geophysical logs and core data

Abstract Downhole geophysical logs and high-resolution electrical images (FMS) from DSDP/ODP Hole 504B are analysed in combination with core data to obtain an integrated description of oceanic faults met in the hole. About 34 500 fractures were mapped from FMS images over 1672 m of basement. The fracture distribution from FMS confirms the presence of a main fault zone between 800 and 1100 mbsf (metres below sea floor), elsewhere detected from seismic data as well as magnetic, acoustic, and electrical resistivity measurements. The fracture density profile reveals the presence of two other highly fractured zones, (1) between 400 and 575 mbsf and (2) close to the bottom of Hole 504B (1700 to 2100 mbsf). Consequently, we infer that Site 504 was submitted first to an extensional stress regime near the ridge axis, with circulation of high-temperature fluids and pervasive alteration of the basalts. This initial phase is associated with the main fault met in Hole 504B. Similar but less developed deformation was generated off-axis, with lower-temperature parageneses, such as that cored between 400 and 575 mbsf. The present compressional to strike-slip stress regime is expressed in subhorizontal fracturing detected in discrete zones, such as within the main fault zone and the lower fracture zone (1700 to 2100 mbsf) in Hole 504B.