Bay Fault Research Articles

Geology of the southern Zambales Ophiolite Complex, Luzon, Philippines

Abstract Field, geochemical and geophysical evidence show that the southern Zambales Ophiolite Complex attained its present‐day configuration through the juxtapositioning of an arc terrane (San Antonio massif) to a back‐arc crust (Cabangan massif). The San Antonio massif manifests island arc‐related characteristics (i.e. spinel XCr [Cr/(Cr + Al)] >0.60; mostly plagioclase An92–95; pyroxene crystallizing ahead of plagioclase; orthopyroxene as an early, major crystallizing phase) which cannot be directly parental to the Cabangan massif transitional mid‐ocean ridge basalt to island arc tholeiitic volcanic carapace. The two massifs are believed to be separated by a left‐lateral strike–slip fault, the Subic Bay Fault Zone. Apart from the presence of highly sheared, allochthonous outcrops, the Subic Bay Fault Zone is generally defined by northwest–southeast trending magnetic and bouguer anomalies. The San Antonio massif was translated southward from the northern part of the Zambales Ophiolite Complex through the Subic Bay Fault Zone. This resulted into its suturing with the Cabangan massif and could have led to the formation of the present‐day Subic Bay.

Seismic risk assessment of an active fault system: the example of the Tsurugawan–Isewan tectonic line

The 185km long Tsurugawan–Isewan tectonic line (TITL) is one of the major active fault systems in central Japan. The TITL, oriented in a NW–SE-trend, passes through the western part of central Japan, extending from the Sea of Japan to the Pacific ocean. It is composed mainly of five large active faults; from the northwest to southeast, the Kaburagi, Yanagase, Sekigahara, Yoro, and Ise Bay faults. As a case study of seismic risk assessment of an active fault system using the parameter of the average moment-release rate, the seismic risk of the TITL is evaluated in the present paper. The average moment-release rate of an active fault system can be calculated from slip-rates and lengths of constituent active faults. In addition, the latest earthquake of the active fault system if unknown can be inferred from data of historical earthquakes and trench excavations. The total moments, which are accumulated over a duration time from the latest earthquake to the present, are estimated by multiplying the average moment-release rate by the duration time. The total accumulated moments can be used to empirically determine the magnitude of an earthquake which would presently occur. The calculated magnitude for each segment or active fault which constitutes the TITL presently is determined as 7.1 at the maximum. Further, the maximum magnitude of a possible earthquake which would occur after a 100year durability period of concrete-made structures is estimated as 7.2. This implies that the TITL has the potential of producing earthquakes of M7.1 and 7.2 at the present and after 100years, respectively. The method of seismic risk assessment is important in order to obtain input magnitude data of possible earthquakes for an active fault system, to be used in selecting sites and earthquake-proof designs of large structures in seismically unstable regions.

Constraining Continental Margin Seismicity by Extending On-Shore Seismograph Stations to Critical Off-Shore Sites

The westernmost North American plate margin in Central California includes major faults with both onshore and offshore segments. Prior to deployment during 1997 and 1998 of a suite of ocean-bottom seismometers within Monterey Bay, the distribution and focal mechanisms of offshore earthquakes were poorly constrained. The use of ocean-bottom seismic stations improves the accuracy of locations for events in Monterey Bay, especially for those far offshore, and permits more robust focal mechanism solutions by reducing uncertainties in strike, dip, and rake. These initial results, using U. S. Geological Survey crustal velocity models and location parameters, reduce the apparent scatter of seismic events and focal mechanisms, and the more consistent solutions provide valuable information on fault characteristics for the San Gregorio fault (SGF) and Monterey Bay fault zone (MBFZ). Most seismic events are relocated to the northern section of the SGF defining a zone of compressional deformation and relatively high microseismicity. The MBFZ is the source for fewer, but important and well-located events. Surprisingly, no events were observed on the southern SGF offshore of Carmel and Pebble Beach, despite the well-defined trace of the San Gregorio along Carmel Canyon.

知多半島西岸沖の伊勢湾断層

Construction of the Chubu International Airport has started off Tokoname City (Chita Peninsula) in Ise Bay, Central Japan. Various surveys such as bathymetry, seafloor drilling, sonic prospecting, and geologic examinations of recovered borehole core samples have been performed to reveal soil engineering characteristics, submarine topography, submarine geology, and the precise locality and mode of movement of the Ise Bay Fault at the bay area (Chubu Kukou Chousakai=The Chubu International Airport Research Foundation, 1994). Many fruitful results on the Ise Bay Fault have been gained, as follows:(1) The bay area is underlain by the A (Holocene), B (upper Pleistocene), C1-C2 (upper to middle Pleistocene), and T (basement) Formations, in descending order. The A, B, C1, and C2 Formations are correlated with the Nan'yo, Nohbi-First Gravel-Toriimatsu Gravel, Atsuta (lower part), and Ama Formations below the Nohbi Plain, respectively. The basement T Formation corresponds to the Mio-Pliocene Tokoname Group.(2) The Ise Bay Fault extends several kilometers off the west coast of Chita Peninsula, parallel with the coast line, downthrown on the Ise Bay (west) side.(3) Mode of faulting is divided into three types, namely, a) flexure type in the northern part.b) steeply westward dipping type with small-scaled thrusts downthrown Chita Peninsula (east) side in the middle part.c) high-angle faulting type in the southern part.(4) Mean vertical slip of the Cl Formation is measured 46, 42, and 35 meters in the southern, middle and northern part, respectively. Vertical slip of the B Formation measures 2-11 and 2-5 meters in the southern and middle part, respectively, and is not clearly detected in the northern part because of horizontal bedding. Accordingly, the fault is inferred to be more active in the southern and middle parts.Based on the above mentioned results, the Ise Bay Fault is evaluated as follows, (5) The average slip rate of vertical displacement ranges from 0.23 to 0.45 meters per 1000 years, which ranks Class B (lower) in the degree of activity.

Devonian (Givetian) miospores from the Walls Group, Shetland

Abstract Miospores demonstrate that the Walls Group of West Shetland is of early and possibly in part late Givetian age (Late Mid-Devonian age). This is both substantially younger in age and of a shorter duration than previously estimated. The correlative rocks of the Walls Group are those of southeast Shetland, Fair Isle, the Eday Group of Orkney and the John O’Groats Sandstone Group of Caithness. The two formations of the Walls Group are, at least in part, time equivalent rather than a stacked sequence. The total thickness of sediment is likely to be much less than the generally cited 12 km, given the short time duration represented by the single miospore assemblage identified throughout the succession. The low diversity of the miospore assemblage and its fluctuating dominant species is interpreted as reflecting a local vegetation source in a low preservation environment. The age and thermal maturity contrast between the Walls Group and the sedimentary successions to the west (Melby, Papa Stour and Foula) provide further constraints on the movement history along the St Magnus Bay Fault system.

Subic Bay fault zone: its role in the geologic history of the Zambales Ophiolite Complex, Philippines

Several models have been forwarded to explain the distribution of the three massifs, Masinloc, Cabangan and San Antonio, of the Zambales Ophiolite Complex. Available information support a model that calls for the separation and southward translation of the San Antonio massif from the Acoje block of the Masinloc massif. Translation occurred along the left-lateral West Luzon Shear/ Subic Bay Fault Zone. This would explain the presence of clinopyroxenite-gabbronorite allochthonous hills scattered along the western edge of the ophiolite complex where the fault zone is thought to have passed. The recognition of the existence of the Subic Bay Fault Zone helps elucidate our understanding on how this crust-mantle sequence had evolved.

「中部国際空港」海域（知多半島常滑市沖）の海底地形・地質

Construction of the Chubu International Airport has started off Tokoname City (Chita Peninsula) in Ise Bay. Various surveys such as bathymetry, seafloor drilling, sonic prospecting, and geologic examinations of recovered borehole core samples, have been performed to reveal soil engineering characteristics, submarine topography, and submarine geology at the airport site (Chubu Kuukou Chousakai=The Chubu International Airport Research Foundation, 1994). Many fruitful results on the submarine geology have been obtained in these surveys, as follows : (1) The submarine topography is divided into the inshore “Upper submarine terrace”, “Offshore gently sloping sea floor”, and “Submarine scarp” between the two. A submarine channel is trenched southwards on the inshore submarine terrace.(2) The bay area is underlain by the A, B, C1, C2, and T Formations, in descending order. The T Formation is Mio-Pliocene Tokoname Group, the basement of the bay area. The Tokoname Group forms a buried wave-cut platform of a peninsular shape, which extends southwards. On the east side of the peninsular wave-cut platform, a submarine buried valley stretches southwards under the submarine channel and is mostly filled with the A Formation. On the west side of the wave-cut platform runs the Ise Bay Fault, and the A, B, C1, and C2 Formations thicken offshore.(3) The A Formation, which is divided into the A1 (upper), A2 (middle), and A3 (lower) Members, consists of marine muddy strata of the present bottom surface and is correlated with the Nanyo Formation below the Nohbi Plain (north of the Ise Bay). The A Formation is 25 to 35 meters thick, and the 14C ages range from 9, 400 to 5, 200 y. B.P. of Holocene age.The A3 and A2 Member intercalates the U-Oki and K-Ah tephra, respectively, both are widespread tephra layers in Japan.(4) The B Formation, which is divided into the B1 (upper), B2 (middle), B3 (lower), and B4 (base) Members, consists of sand beds and gravelly beds. The B1, B2, and B3 Members are correlated with the Nohbi and First Gravel Formations below the Nohbi Plain, respectively. The B4 Member forms a buried terrace on the west slope of the peninsular wave-cut platform and is correlated with the Toriimatsu or Ohzone Formations, Lower Terrace Deposits in the eastern margin of the Nohbi Plain. The B Formation is of late Pleistocene age.(5) The C1 Formation consists mainly of marine clay beds about 40 meters thick. The fossil pollen composition indicates that the C1 Formation is correlated with the lower part of the Atsuta Formation in the Nohbi Plain and is of middle Pleistocene age.(6) The C2 Formation consists of sand (upper) and gravelly (lower) beds and exceeds 50 meters thick. The fossil pollen composition indicates that the C2 Formation is correlated with the Ama Formation below the Nohbi Plain and is of middle Pleistocene age.(7) The basement T Formation (Tokoname Group) consists of alternating sand and compact mud beds and intercalates two volcanic ash layers, which are correlated with the Souri (upper, Sr) and Higashitani (lower, Hg) Volcanic Ash Layers from the conformity in index of volcanic glass. The Souri and Higashitani V. A. Ls are intercalated within the upper horizon of the Tokoname Group in the Chita Peninsula.(8) The Ise Bay Fault (faults and flexure zone), trending in the NNW-SSE direction, extends in the western margin of the bay area. The Tokoname Group on the east side of the fault trends northwest and gently dips northeast. The Utsumi Fault, trending in the WNW-ESE dircction, extends in the southern margin of the peninsula.The geological relationship between the Chita Peninsula and airport site in the above-mentioned results brings the following problems for further study.

Translation and docking of an arc terrane: geological and geochemical evidence from the southern Zambales Ophiolite Complex, Philippines

The Zambales Ophiolite Complex is made up of three massifs: the Masinloc, Cabangan and San Antonio Massifs. Field, petrographic and geochemical analyses show that the Cabangan and San Antonio Massifs are genetically related to the Coto (transitional mid-ocean ridge–island arc) and Acoje (island arc) blocks of the Masinloc Massif, respectively. The Subic Bay Fault Zone, a left-lateral fault zone, separates the San Antonio Massif island arc terrane from the transitional mid-ocean ridge–island arc-like sheeted diabase dikes–pillow basalts of the Cabangan Massif. The San Antonio Massif is a rifted terrane from the Acoje block which was translated southward to its present position through the West Luzon Shear–Subic Bay Fault Zone. Tectonized clinopyroxenite and gabbronorite hills, which mimic the physical and geochemical characteristics of the Acoje block and the San Antonio Massif ultramafic–mafic cumulate rocks, were left behind along the western side of the Cabangan Massif during the translation of the arc massif southward. This scenario can account for the present-day configuration of the Zambales Ophiolite Complex.

Geology of the Southern Zambales Ophiolite Complex, (Philippines): juxtaposed terranes of diverse origin

Abstract The Zambales Ophiolite Complex, which is made up of three massifs—Masinloc, Cabangan and San Antonio, was generated in a subduction-related marginal basin. Combined field geological and geochemical evidences show that the Cabangan and San Antonio massifs are genetically related to the Coto (transitional mid-ocean ridge-island arc) and Acoje (island arc) blocks of the Masinloc massif, respectively. A tectonic contact, the Subic Bay Fault Zone, is believed to separate the San Antonio and Cabangan massifs. The San Antonio massif is a displaced terrane rifted from the Acoje block and translated southward to its present position through the West Luzon Shear (?)/Subic Bay Fault Zone. The Zambales Ophiolite Complex could have not formed through the rifting or folding of the arc-related Acoje block-San Antonio massif to generate the transitional mid-ocean ridge-island arc like Coto block-Cabangan massif!

GEOCHEMICAL CHARACTERISTICS OF PALAEOZOIC PETROLEUM SYSTEMS IN NORTHWESTERN AUSTRALIA

Isotopic and biomarker analyses carried out on Cambrian to Permian oils and source rocks in the Arafura, Bonaparte (Petrel Sub-basin) and Canning Basins have been used to geochemically characterise five distinct petroleum systems within the Larapintine and Gondwanan Petroleum Supersystems. The Larapintine 1 Petroleum System is characterised by isotopically light, free hydrocarbons in the Arafura Basin (613Csat = −32 %o Arafura-1) which have been correlated to kerogens of similar isotopic signature within the Middle Cambrian Jigaimara Formation. The richness and maturity of these source rocks indicate that an effective Larapintine 1 Petroleum System may exist in the northern parts of the Arafura Basin. Larapintine 2 oils, with Gloeocapsomorpha prisca-type signatures, are found on the Barbwire- Dampier Terraces and Admiral Bay Fault Zone in the Canning Basin. These oils can be correlated to source rocks in the Lower Ordovician Goldwyer Formation on the Barbwire Terrace and the Bongabinni Formation in the Admiral Bay Fault Zone by their diagnostic odd- carbon-number preference in the C15—CJ9 n-alkanes. Larapintine 3 oils are derived from Upper Devonian marine carbonates in the Canning Basin and Petrel Sub- basin and have a diagnostic biomarker signature which includes a predominance of steranes relative to diasteranes and abundant gammacerane and 30- norhopanes, similar to those observed in the Upper Devonian Gogo and Pillara Formations. Larapintine 4 oils are derived from Lower Carboniferous marine, clay- rich mudstones in both the Petrel Sub-basin and Canning Basin. They are isotopically light (mean δ13C sat = −28 %o) and have a unique terpane signature which has been identified within the Milligans Formation. Gondwanan 1 Petroleum System hydrocarbons, represented here by the Petrel-4 condensate, have a heavy isotopic signature (δ13C sat = −24 %o) which, coupled with an abundance of the diasterane and diahopane biomarkers, indicates derivation from Permian deltaic source facies. Recognition of the diagnostic geochemical components of each Palaeozoic petroleum system has led to the identification of Permian-like isotopic signatres in some hydrocabon accumulations in the Timor Sea that were previously attributed to Mesozoic sources.

Tectonic structure and petroleum potential of TayabasBay southeast Luzon, Philippines

Tayabas Bay is one of four offshore Philippine areas where the Australian GeologicalSurvey Organization and the Philippine Department of Energy conducted a cooperative marine seismic, gravity, magnetic, bathymetry and geochemical survey. The project acquired new seismic data and reprocessed the 1983 World Bank seismic sections which were all integrated with previous oil company data. the absence of wells drilled offshore, interpretation of offshore seismic data was complemented by onshore well log information and stratigraphy of the Bondoc Peninsula. Geochemistry data, both offshore and onshore, were analyzed to confirm the presence of mature source rocks and hydrocarbon migration. A new seismic interpretation has revealed the structure of this tectonically active geologically complex area. A major structural feature interpreted in offshore Tayabas Bay was a N-NW-trending strike-slip fault which is believed to be a northern splay of the Sibuyan Sea Fault. The authors named this fault the Tayabas Bay Fault and due to its association with the Philippine Fault System the movement is assumed to be left-lateral. The present study suggested the presence of a prolific source rock in the Middle Miocene Vigo Formation and/or the Late Oligocene to Early Miocene Panaon Limestone. Oil and gas generation have been and are occurring in the Bondoc Sub-basin. Two groups of reservoirs were identified, the shelf carbonates beneath the Middle Miocene shales on the Marinduque Platform and the early Middle Miocene carbonates and basin-floor clastics near the base of the Vigo Formation. Carbonate reservoirs are believed to be present in traps formed when the Late Oligocene to Early Miocene carbonate reefs and shelf deposits of the Panaon Limestone were buried by the Middle Miocene shales. A later set of traps was formed and possibly superimposed by the intense deformation associated with the Philippine Fault System which has continued from the Late Pliocene up to the present. Evaluation for hydrocarbon reserves of several possible traps identified three significant leads, namely the Yuni Lead in the south, the Mulanay in the central area and the Mabio in the North.

Gold mineralization in the Paleozoic Appalachian Orogen; constraints from geologic, U/Pb, and stable isotope studies of the Hammer Down Prospect, Newfoundland

The Hammer Down gold prospect occurs in an accretionary tectonic setting in the Paleozoic Appalachian orogen. It has many geologic and geochemical features in common with other shear-hosted, mesothermal gold deposits. The gold prospect is located about 25 km to the east of an exposed continental-oceanic suture zone and is near a major fault (the Green Bay fault) of regional extent. The main mineralized zones consist of structurally controlled quartz veins with abundant pyrite, chalcopyrite, sphalerite, and carbonate, hosted by volcanic and sedimentary rocks of the Ordovician Catchers Pond Group. Gold-bearing veins postdate a regional subgreenschist metamorphism and an early phase of deformation. Mineralization occurred synchronously with a locally intense S 2 fabric within narrow brittle-ductile high strain zones in, and adjacent to, Early Silurian felsic porphyry dikes. Carbonate and chlorite alteration is widespread and, in part, predates the mineralization; however, there is increasing chloritization near the gold-bearing zones and the mineralized veins contain abundant carbonate. Strong sericite-pyrite-rutile hydrothermal alteration is restricted to narrow zones adjacent to veins. Isotopic compositions of quartz (delta 18 O =11.4 + or - 0.7%) and carbonate (delta 13 C =-6.2 + or - 0.7%) from gold-bearing veins at Hammer Down are typical of shear-hosted, mesothermal gold deposits of Archean and younger age. Oxygen isotope thermometry using quartz, chlorite, and muscovite yields temperatures of vein formation between 245 degrees and 320 degrees C. The ore-forming fluids have calculated delta 18 O and delta D values between 1.1 and 5.8 and -36 and -54 per mil, respectively, and are interpreted to be of mixed origin, dominantly from a metamorphic reservoir with a small component of low 18 O meteoric water. In the Hammer Down area, felsic tuff in the host arc-related sequence, the Catchers Pond Group, has an igneous U-Pb zircon age of 480 (super +4) (sub -3) Ma. This allochthonous sequence is intruded by felsic dikes which are correlative with granodiorite of the Burlington Intrusive Suite, a major regional unit of the eastern Baie Verte...

Sm–Nd isotopic geochemistry of Precambrian to Paleozoic granitoid suites and the deep-crustal structure of the southeast margin of the Newfoundland Appalachians

In the Eastern Central Mobile Belt of the Newfoundland Appalachians, late Precambrian basement inliers have εNd from −3 to +2, but Cambro-Ordovician metasedimentary rocks have initial εNd below −7. This region is inferred to have an "inverted" crustal residence structure, which influenced subsequent Appalachian-cycle magmatism. Ordovician and Silurian granitoid suites have εNd of −8 to −2, bracketing both basement and cover, but peraluminous, "S-type" granites have the lowest εNd. Devonian granites have initial εNd values from −5 to +1, and low εNd is associated with peraluminous character. These Paleozoic granites show geographic trends, with lowest εNd values in areas where metasedimentary rocks are abundant. They are suggested to contain anatectic material from both Precambrian basement and metasedimentary cover, but some "I-type" suites probably also include a mantle-derived component. In the adjacent Avalon Zone, Precambrian plutonic suites mostly have εNd from +1 to +6, but there are negative εNd values (−8 to −4) in the westernmost Avalon Zone. Devonian plutonic suites mostly have εNd from +2 to +5. Thus, the Precambrian crust of the Avalon Zone is largely "juvenile," except at its westernmost edge. Contrasts across the Eastern Central Mobile Belt–Avalon Zone boundary, defined by the Dover–Hermitage Bay fault system, indicate a major, crustal-scale structure, and suggest an isotopically distinct "central block" beneath the central Appalachian Orogen, rather than a simple extension of "Avalonian" crust. Similar geographic–isotopic patterns have been reported in Nova Scotia and New Brunswick, suggesting that this pattern represents a first-order deep-crustal subdivision of the northern Appalachian Orogen.

Timing of hydrocarbon generation and accumulation in fault-bounded compartments in the Norphlet Formation, offshore Alabama

Analysis of the variation in depth of the gas-water contacts in the Bon Secour Bay and Lower Mobile Bay-Mary Ann Fields of offshore Mobile Bay, Alabama, together with fault seal analysis, implies that some of the pre-Cretaceous faults in these fields must be sealing. It is also likely that the large, post-Cretaceous Lower Mobile Bay Fault is dip-leaking. The gas currently reservoired in the Norphlet Formation has most likely been produced by thermal degradation of oils generated in the overlying Jurassic lower Smackover Formation. For a range of heat flow models appropriate for this area of the Gulf Coast, the onset of gas generation (defined as the time at which 10% of the maximum possible amount of gas has been generated) occurs at 104 −5 +10 Ma. The error bounds in this estimate are the 90% confidence limits, which were calculated by assuming that the times of onset of gas generation are log-normally distributed. The time of onset of gas generation is younger than the age of the sealing faults in the area, hence it is likely that gas generation occurred in situ in the Norphlet Formation, with re-migration occurring during the development of the post-Cretaceous Lower Mobile Bay Fault.

Depositional Setting, Sequence Stratigraphy, Diagenesis, and Reservoir Potential of a Mixed-Lithology, Upwelling Deposit: Upper Triassic Shublik Formation, Prudhoe Bay, Alaska

The Upper Triassic Shublik Formation within the Prudhoe Bay field unit, North Slope, Alaska, is a potentially economic hydrocarbon reservoir comprised of mixed lithology and mineralogy. Its composition includes limestone, phosphate, shale, siltstone, and sandstone, as well as accessory amounts of siderite, glauconite, pyrite, kaolinite, and dolomite. Within the Prudhoe Bay field unit, the Shublik has been subdivided into four zones, lettered from base to top, D through A, which become thinner and show evidence of deposition under higher energy conditions toward the northeast. The formation is truncated to the east by the regional Lower Cretaceous unconformity. Zones within the Shublik comprise a basal transgressive systems tract (conglomerate lag at the Shublik Formation/Ivishak Formation contact through basal zone C shales) and two highstand shallowing-upward parasequences (zones C through B, and zone A, respectively). The parasequences are bounded by shales interpreted to represent deposition during periods of marine flooding. The contact between the Shublik and the overlying Sag River Formation juxtaposes comparatively deeper marine Shublik with shallower water glauconitic sandstones of the Sag River Formation. The contact is unconformable and is interpreted to represent a regional sequence boundary. Lithofacies of the Shublik are interpreted to have been coeval depositional facies of an upwelling system. Relative sea level changes durin Shublik deposition are interpreted to have caused the observed vertical and lateral variability in lithofacies via systematic changes between anaerobic, dysaerobic, and aerobic upwelling conditions. Dissolution of carbonate allochems resulted in the creation of moldic porosity that positively affected reservoir quality (i.e., permeability) in the carbonate packstone/grainstone facies. Areas of highest porosity are in the northern and northeastern parts of the field, which correspond to a combination of facies-controlled reservoir quality improvement toward the northeast and carbonate dissolution along the Lower Cretaceous unconformity and the North Prudhoe Bay fault zone. Oil in place for the Shublik within the Prudhoe Bay unit is estimated to be between 250 and 500 million bbl. Although permeabilities are generally low throughout the field area, the Shublik Formation has the potential to add significant reserves to the Prudhoe Bay field unit.

Roles of plutonism, midcrustal flow, tectonic rafting, and horizontal collapse in shaping the Miocene strain field of the Lake Mead area, Nevada and Arizona

The Lake Mead area contains a well‐exposed record of highly contrasting Neogene structural development that includes major magmatism and extension in its southern part and large‐magnitude lateral translations (to 65 km) of amagmatic structural blocks in its northern part. These highly contrasting parts are joined at the Hamblin Bay fault, a major crustal boundary that forms the south margin of the Lake Mead fault system and the north margin of the lower Colorado River extensional corridor. Intrusive rocks adjacent to this crustal boundary on the south record major crustal rifting normal to an 350° axis. They also display steep‐axis disharmonic folds that formed by post‐magmatic extension‐related horizontal collapse of highly weakened crust. The rocks directly north of the crustal boundary not only display large lateral translations, but they also are cut by fault systems suggestive of diverse paleostress conditions and formed into diversely oriented coeval folds collectively producing a complex Neogene deformation field that defies interpretation in terms of a uniform system of remote stresses. Because structures forming this complex deformation field range from outcrop scale to 65 km of lateral displacement and can not be uniformly partitioned according to relative age, we prefer tectonic models based on a protracted uniform dynamic process or super‐posed dynamic processes to models based on time‐varying states of stress. We suggest that the large lateral displacements and extreme structural complexity record synextensional rafting of structural blocks atop a flowing undermass and concomitant contraction of the zone of flowage. Structural blocks north of the main zone of flowage (the Great Basin sector of the Basin and Range) moved southward and occluded to the block south of the zone (the Colorado River extensional corridor) as a large west‐ward widening wedge of upper crust was carried to the west–southwest toward California on the flowing undermass. Where occlusion is almost complete in the northern Black Mountains, north–south shortening is estimated at 55 km. The narrow zone of strong rifting, plutonism, crustal softening, and horizontal collapse that developed at the north margin of the Colorado River extensional corridor is interpreted to result from contact with the zone of crustal flowage.

Large‐scale structures on Gazelle Peninsula, New Britain: Implications for the evolution of the New Britain arc

Geological mapping of fault systems on the Gazelle Peninsula, eastern New Britain arc, combined with a reinterpretation of existing sea floor data indicate that faults previously thought to be a possible location of the boundary between the North and South Bismarck Plates, do not appear to be directly related to the plate boundary spreading centres and transform faults in the 3.5 Ma Manus Basin. Structure on the Gazelle Peninsula is dominated by the Mediva Fault (new name) and the Wide Bay Fault System, both north‐northwest trending, deep‐seated features. The Mediva Fault, an element of the Baining Mountain Horst and Graben Zone, is an extensional structure which has focused Middle Miocene intrusive activity, controlled Mio‐Pliocene sedimentation in the central Gazelle Peninsula, and displaced Quaternary volcanic deposits. The Wide Bay Fault System has been active since at least the Late Oligocene. One hundred kilometres of sinistral strike‐slip motion is likely on this fault since at least the late Middl...

Coseismic slip in the 1964 Prince William Sound earthquake: A new geodetic inversion

The 1964 Prince William Sound earthquake (March 28, 1964; M w = 9.2) caused crustal deformation over an area of approximately 140,000 km 2 in south central Alaska. In this study geodetic and geologic measurements of this surface deformation were inverted for the slip distribution on the 1964 rupture surface. Previous seismologic, geologic, and geodetic studies of this region were used to constrain the geometry of the fault surface. In the Kodiak Island region, 28 rectangular planes (50 by 50 km each) oriented ~218~ with a dip varying from 8 ~ nearest the Aleutian trench to 9 ~ below Kodiak Island, define the rupture surface. In the Prince William Sound region 39 planes with variable dimensions ( ~40 by 50 km near the trench, ~64 by 50 km inland) and orientation (218~ in the west and 270~ in the east) were used to approximate the complex faulting. Prior information was introduced to constrain offshore dip-slip values, the strike-slip component, and slip variation between adjacent planes. Our results suggest a variable dip-slip component with local slip maximums occurring near Montague Island (up to ~30 m), further to the east near Kayak Island (up to ~ 14 m), and trenchward of the northeast segment of Kodiak Island (up to ~ 17 m). A single fault plane dipping 30~ corresponding to the Patton Bay fault, with a slip value of ~ 8 m modeled the localized but large uplift on Montague Island. The moment calculated on the basis of our geodetically derived slip model of 5.0  1029 dyne cm is 30% less than the seismic moment of 7.5 x 1029 dyne cm calculated from long-pe- riod surface waves (KANAMORI, 1970) but is close to the seismic moment of 5.9  1029 dyne cm obtained by KIKUCHI and FUKAO (1987).

Oil, geology and changing concepts in the southwest Philippines (Palawan and the Sulu Sea)

Abstract The Southwest Philippines; Palawan, its offshore shelves, Reed Bank, and the southwestern Sulu Sea encompass 350,000 sq km (126,000 sq mi). This region contains the better areas to search for significant oil and gas deposits in the Philippines. The two largest oil fields yet found in the Philippines were discovered and confirmed in 1991-1992. West Linapacan field has more than 100 million barrels of recoverable oil. In June 1992, Shell Oil is reported in the Philippine press to have identified the possible occurrence of a 200 to 400 MMBO field at Camago-Malampaya about 30 km southwest of West Linapacan. The Southwest Philippines’ oil producing trend, about 40 km off the northwest coasts of Palawan and Busuanga is a northerly trending belt 200 km in length. Oil reservoirs in the Philippine fields are Oligocene to Lower Miocene platform limestones, reefs, and both silicic and carbonate turbidite sequences. A 540 km gap occurs between the Philippine production at Nido and the closest oil fields to the southwest in Sabah. Untested prospects and leads are present along the South China Sea coast of Palawan. The southwest Sulu Sea overlies three poorly explored Tertiary sedimentary-structural basins (Balabac, Bancauan, and Sandakan basins). Geology and oil shows suggest the possible presence of commercial oil fields. Prospects and prospect leads are numerous. One well in the Malaysia sector of the Sulu Sea (Nymphe Norde 1) suggests economic potential. Deep-water exploration targets (untested reefs and structural traps) are present in the South China Sea offshore Palawan. Large gas-condensate and oil reserves are indicated at Camago-Malampaya in 2400+ feet of water. A test of a major deep water reef prospect, Sarap-1, provided an exploration disappointment in 1991. Other major reef traps (e.g Cliff Head) in deep water with better source rock association remain to be tested. Besides economic interest, the Southwest Philippines has been the spawning ground of several concepts and theories about the origin and evolution of the overall region. Some theories and concepts are reviewed. The “Ulugan Bay fault” is disputed and recommended for elimination from future maps based on later field work and offshore geophysical studies. A major redefinition is an alternative. Two schools of thought on the origin of much of Palawan are reviewed. Was continental crust from the South China Sea area subducted beneath Palawan, or is Palawan a complicated thrust pile composed in part of protocrust types and early Tertiary sediments that have been thrust or emplaced to the northwest from the Sulu Sea region? Some future exploration areas, plays, and prospects are illustrated.

Geochemistry of deep formation waters in the Canning Basin, Western Australia, and their relationship to Zn‐Pb mineralization

The Canning Basin contains several Mississippi Valley‐type Zn‐Pb sulphide prospects and deposits in Devonian carbonate reef complexes on the northern edge of the Fitzroy Trough, and in Ordovician and Silurian marine sequences on the northern margin of the Willara Sub‐basin. This study uses the ionic composition and 5D, δ18O, δ34S, 87Sr/86Sr isotopic data on present‐day deep formation waters to determine their origin and possible relationship to the Zn‐Pb mineralizing palaeofluids. The present‐day Canning Basin formation waters have salinity ranging from typically less than 5000 mg/L up to 250 000 mg/L locally. The brines are mixtures of highly saline water, formed by seawater which evaporated beyond halite saturation (bittern water), with meteoric water ranging in salinity from low (<5000 mg/L) to hypersaline water (up to about 50 000 mg/L) formed by re‐solution of halite and calcium sulphate minerals. The original marine chemical composition of the bittern‐dominated brines was changed to that of a Na‐Ca‐Cl water by addition of Ca and removal of Mg and SO4, initially by bacterial sulphate reduction and later by dolomitization of carbonate. Other reactions with terrigenous components of the sediment have provided additional Ca and Sr, including a small proportion of 87Sr‐rich material. The δ34S values of the bittern‐containing waters are within the range over which marine sulphate has fluctuated from the Ordovician to the Holocene, although one of the hypersaline waters has a value of +6.8%, indicating SO4 of non‐marine origin. The pH of the bittern‐containing waters is low (about 5) and they contain significant concentrations of dissolved Fe (up to 120 mg/L). The Canning Basin bitterns appear similar in origin and chemical composition to highly saline marine brines in the Mississippi Salt Dome Basin, USA, which are known to be either metal or sulphide‐rich depending on the organic content of the host rock. In the Canning Basin, mixing of the bittern water with the various types of meteoric water has resulted in decreases in salinity, Na, Ca, Mg, K, Sr, Li and Fe, and increases in HCO3, SO4 and pH. Mixing of the bitterns with other types of metalliferous fluids and/or with sulphate‐containing hypersaline meteoric waters formed from the same marine evaporite sequence should produce ore‐precipitating fluids which are relatively hot and saline, and the resulting ore deposit should be of high grade and contain abundant sulphate minerals. In the southern Canning Basin, this type of mixing and the corresponding style of ore deposit is evident in the evaporite‐associated areas of Zn‐Pb mineralization near the Admiral Bay Fault. If the bitterns mix with low salinity HCO3‐waters in near‐surface environments, then the ore‐precipitating fluids should have relatively low salinities and carbonate minerals would precipitate during later stages of mixing. In the Lennard Shelf, the present‐day formation waters, the style of the Zn‐Pb deposits, and range of salinity and temperature of the ore‐forming palaeofluids are consistent with this type of mixing.