Bay Fault Research Articles

Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska

Megathrust splay fault systems in accretionary prisms have been identified as conduits for long-term plate motion and significant coseismic slip during subduction earthquakes. These fault systems are important because of their role in generating tsunamis, but rarely are emergent above sea level where their long-term (million year) history can be studied. We present 32 apatite (U-Th)/He (AHe) and 27 apatite fission-track (AFT) ages from rocks along an emergent megathrust splay fault system in the Prince William Sound region of Alaska above the shallowly subducting Yakutat microplate. The data show focused exhumation along the Patton Bay megathrust splay fault system since 3–2 Ma. Most AHe ages are younger than 5 Ma; some are as young as 1.1 Ma. AHe ages are youngest at the southwest end of Montague Island, where maximum fault displacement occurred on the Hanning Bay and Patton Bay faults and the highest shoreline uplift occurred during the 1964 earthquake. AFT ages range from ca. 20 to 5 Ma. Age changes across the Montague Strait fault, north of Montague Island, suggest that this fault may be a major structural boundary that acts as backstop to deformation and may be the westward mechanical continuation of the Bagley fault system backstop in the Saint Elias orogen. The regional pattern of ages and corresponding cooling and exhumation rates indicate that the Montague and Hinchinbrook Island splay faults, though separated by only a few kilometers, accommodate kilometer-scale exhumation above a shallowly subducting plate at million year time scales. This long-term pattern of exhumation also reflects short-term seismogenic uplift patterns formed during the 1964 earthquake. The increase in rock uplift and exhumation rate ca. 3–2 Ma is coincident with increased glacial erosion that, in combination with the fault-bounded, narrow width of the islands, has limited topographic development. Increased exhumation starting ca. 3–2 Ma is interpreted to be due to rock uplift caused by increased underplating of sediments derived from the Saint Elias orogen, which was being rapidly eroded at that time.

Multi-stage hydrothermal processes involved in “low-sulfide” Cu(–Ni)–PGE mineralization in the footwall of the Sudbury Igneous Complex (Canada): Amy Lake PGE zone, East Range

The Amy Lake PGE zone is a “low-sulfide-type” Cu-(Ni-)PGE mineralization in the East Range footwall of the 1.85 Ga Sudbury Igneous Complex occurring in a 100-m-wide Sudbury Breccia belt that coincides with an impact-related major fracture zone (Bay Fault zone). Detailed hydrothermal alteration mapping, fluid inclusion, trace element, and stable isotope studies revealed a complex alteration and mineralization history in a multi-source, multi-stage Sudbury-related hydrothermal system. The two major stages of syn-Sudbury hydrothermal activity are characterized by similarly high-salinity, high-temperature fluids that are (1) locally derived from footwall granophyre bodies, and typified with high Ni/Cu and PGE/S ratios and high REE contents (magmatic–hydrothermal stage), and (2) a more voluminous Cu–Ni–PGE-rich fluid flux probably originated from the Sudbury Igneous Complex/footwall contact (hydrothermal stage). The second hydrothermal flux was introduced by brittle fractures in the area and resulted in a complex zonation of alteration assemblages and mineralization governed by local footwall composition. The Sudbury-related hydrothermal event was overprinted by shear-related epidote veining and calcite–chlorite replacement, both regionally present in the Sudbury structure. Based on analogies, the most important factors involved in the formation of hydrothermal low-sulfide mineralization are proposed to be (1) accumulation of PGE-enriched fluids, (2) large-scale brittle structures as conduits to these fluids, and (3) adequate host rock composition as a chemical trap resulting in sulfide and PGM precipitation. In environments meeting these criteria, hydrothermal PGE mineralization is known to have formed not only in the Sudbury footwall but also from mafic–ultramafic intrusions associated with primary magmatic PGE from several locations around the world.

The Northwest Trending North Boqueron Bay-Punta Montalva Fault Zone; A Through Going Active Fault System in Southwestern Puerto Rico

The North Boqueron Bay-Punta Montalva fault zone has been mapped crossing the Lajas Valley in southwest Puerto Rico. Identification of the fault was based upon detailed analysis of geophysical data, satellite images, and field mapping. The fault zone consists of a series of Cretaceous bedrock faults that reactivated and deformed Miocene limestone and Quaternary alluvial fan sediments. The fault zone is seismically active (local magnitude greater than 5.0) with numerous locally felt earthquakes. Focal mechanism solutions suggest strain partitioning with predominantly east-west left-lateral displacements with small normal faults striking mostly toward the northeast. Northeast-trending fractures and normal faults can be found in intermittent streams that cut through the Quaternary alluvial fan deposits along the southern margin of the Lajas Valley, an east-west-trending 30-km-long fault-controlled depression. Areas of preferred erosion within the alluvial fan trend toward the west-northwest parallel to the onland projection of the North Boqueron Bay fault. The North Boqueron Bay fault aligns with the Punta Montalva fault southeast of the Lajas Valley. Both faults show strong southward tilting of Miocene strata. On the western end, the Northern Boqueron Bay fault is covered with flat-lying Holocene sediments, whereas at the southern end the Punta Montalva fault shows left-lateral displacement of stream drainage on the order of a few hundred meters.

Evolution of the Rodgers Creek-Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California

The Rodgers Creek–Maacama fault system in the northern California Coast Ranges (United States) takes up substantial right-lateral motion within the wide transform boundary between the Pacific and North American plates, over a slab window that has opened northward beneath the Coast Ranges. The fault system evolved in several right steps and splays preceded and accompanied by extension, volcanism, and strike-slip basin development. Fault and basin geometries have changed with time, in places with younger basins and faults overprinting older structures. Along-strike and successional changes in fault and basin geometry at the southern end of the fault system probably are adjustments to frequent fault zone reorganizations in response to Mendocino Triple Junction migration and northward transit of a major releasing bend in the northern San Andreas fault. The earliest Rodgers Creek fault zone displacement is interpreted to have occurred ca. 7 Ma along extensional basin-forming faults that splayed northwest from a west-northwest proto-Hayward fault zone, opening a transtensional basin west of Santa Rosa. After ca. 5 Ma, the early transtensional basin was compressed and extensional faults were reactivated as thrusts that uplifted the northeast side of the basin. After ca. 2.78 Ma, the Rodgers Creek fault zone again splayed from the earlier extensional and thrust faults to steeper dipping faults with more north-northwest orientations. In conjunction with the changes in orientation and slip mode, the Rodgers Creek fault zone dextral slip rate increased from ∼2–4 mm/yr 7–3 Ma, to 5–8 mm/yr after 3 Ma. The Maacama fault zone is shown from several data sets to have initiated ca. 3.2 Ma and has slipped right-laterally at ∼5–8 mm/yr since its initiation. The initial Maacama fault zone splayed northeastward from the south end of the Rodgers Creek fault zone, accompanied by the opening of several strike-slip basins, some of which were later uplifted and compressed during late-stage fault zone reorganization. The Santa Rosa pull-apart basin formed ca. 1 Ma, during the reorganization of the right stepover geometry of the Rodgers Creek–Maacama fault system, when the maturely evolved overlapping geometry of the northern Rodgers Creek and Maacama fault zones was overprinted by a less evolved, non-overlapping stepover geometry. The Rodgers Creek–Maacama fault system has contributed at least 44–53 km of right-lateral displacement to the East Bay fault system south of San Pablo Bay since 7 Ma, at a minimum rate of 6.1–7.8 mm/yr.

The Northeast Arran Trough, the Corrie conundrum and the Highland Boundary Fault in the Firth of Clyde, SW Scotland

AbstractTwo strikingly different successions of Lower Carboniferous (mainly Tournaisian) sedimentary rocks are closely juxtaposed on the NE coast of the island of Arran, SW Scotland. Near the village of Corrie a thin succession (~ 17 m) of Tournaisian rocks is preserved, whereas in the neighbouring Fallen Rocks–Laggan area, correlative rocks are > 300 m in thickness. These contrasting successions are separated by the Laggan Fault, which is a landward extension of the submarine Brodick Bay Fault, marking the SW boundary of the Northeast Arran Trough. The contrasting thickness and stratigraphy of the two sequences of sedimentary rocks result from juxtaposition of shoulder and trough deposits along the Laggan–Brodick Bay Fault. Although originally a normal, basin-defining fault, later sinistral movements caused significant displacement of the NE Arran Trough, together with a segment of the Highland Boundary Fault, from their original positions. The most northerly occurrence of the Highland Boundary Fault on Arran is thought to be the truncated northern end of the Corloch Fault. To the SW the surface trace of the Highland Boundary Fault is largely obscured by a Palaeogene granite body but it is present on the west side of the island, near Dougrie. The Highland Boundary Fault appears to be displaced to the south, in Kilbrannan Sound, by a series of NW-trending sinistral transcurrent faults. Thus the ‘anomalous’ trend of the Highland Boundary Fault and narrowing of the Midland Valley of Scotland in the Firth of Clyde area may be explained by later fault movements and intrusion of the Palaeogene North Arran Granite Pluton.

Geophysical framework of the northern San Francisco Bay region, California

We use geophysical data to examine the structural framework of the northern San Francisco Bay region, an area that hosts the northward continuation of the East Bay fault system. Although this fault system has accommodated ∼175 km of right-lateral offset since 12 Ma, how this offset is partitioned north of the bay is controversial and important for understanding where and how strain is accommodated along this stretch of the broader San Andreas transform margin. Using gravity and magnetic data, we map these faults, many of which influenced basin formation and volcanism. Continuity of magnetic anomalies in certain areas, such as Napa and Sonoma Valleys, the region north of Napa Valley, and the region south of the Santa Rosa Plain, preclude significant (>10 km) offset. Much of the slip is partitioned around Sonoma and Napa Valleys and onto the Carneros, Rodgers Creek, and Green Valley faults. The absence of correlative magnetic anomalies across the Hayward–Rodgers Creek–Maacama fault system suggests that this system reactivated older basement structures, which appear to influence seismicity patterns in the region.

Age and provenance of the Paleoproterozoic Thluicho Lake Group based on detrital zircon U–Pb SHRIMP geochronology: new insights into the protracted tectonic evolution of the southwestern Rae Province, Canadian Shield

The middle Paleoproterozoic Thluicho Lake Group comprises coarse-grained continental clastic rocks deposited in fault-controlled alluvial basin(s) in the southwestern Rae Province of the Canadian Shield. The group nonconformably overlies crystalline basement that records late-Taltson high-grade metamorphism, ca. 1.93 Ga, and is folded and metamorphosed to greenschist facies, providing an important window of insight into tectonic activity in this region. This paper reports ∼100 detrital zircon ages for two samples from the lower and upper parts of the Thuicho Lake Group. Four analyses of one of the youngest detrital zircons from the stratigraphically higher member define a weighted mean 207Pb/ 206Pb age of 1922 ± 6 Ma, establishing a maximum depositional age for the group. This result accords with regional geological relationships indicating that the group is intermediate in age between development of two major tectonic fronts; an older, ca. 1.93 Ga east-southeast-trending front that formed at the height of Thelon-Taltson orogeny, and a younger, ca. 1.91–1.90 Ga northeast-striking front expressed by the Black Bay Fault and related northeast-striking structures affiliated with the Snowbird tectonic zone. The younger age limit of the group may therefore be older (≥1.91–1.90 Ga) than the previously reported minimum age, constrained by cross-cutting dykes dated at ca. 1.82 Ga. In terms of isotopic provenance, the bulk of analyses of the combined dataset lie within two principal age-ranges: 2750–2500 Ma and 2475–2250 Ma. The older of these is dominated by ca. 2600 Ma zircons, whereas the younger contains two statistically distinct subgroups of 2410 and 2310 Ma. This signature is consistent with derivation almost exclusively from immediately surrounding basement rocks of the Nolan, Taltson, Zemlak, Ena and Beaverlodge domains. Abundant 2475–2250 Ma zircons provide indirect evidence for the 2.45–2.3 Ga Arrowsmith orogeny. Despite proximity to the Taltson magmatic zone, 1990–1960 Ma zircons are nearly completely absent, suggesting that Thluicho Lake basins formed in the hinterland of the orogen, possibly separated from the foreland (Taltson arc) by a continental divide. Alternatively, Taltson arc plutons may not have been unroofed at the time of deposition of Thluicho Lake strata. The data presented herein support the previously proposed broad correlation between the Thuicho Lake and Nonacho groups; however, there are apparent differences in detail between respective basin settings and controls, suggesting that this correlation requires further investigation.

The Carribuddy Group and Worral Formation, Canning Basin, Western Australia: reassessment of stratigraphy and petroleum potential

Reassessment of stratigraphic relationships and biostratigraphic data pertaining to the Carribuddy Group and Worral Formation in all relevant petroleum wells and many mineral drill holes across the southern Canning Basin has led to the following important results. The Carribuddy Group is restricted to the Late Ordovician to earliest Silurian. The overlying Worral Formation is mostly of Silurian age and does not intertongue with the Middle Devonian Tandalgoo Formation, as previously thought. A thin basin-wide chronostratigraphic marker—the Pegasus Dolomite Member (previously referred to as dolomite spike or dolomite marker) of the Sahara Formation—allows improved correlation between salt-bearing sub-basins and adjacent condensed Carribuddy Group successions. The Mallowa Salt is not as extensive as previously thought; rather the Minjoo Salt thickens to become the only salt seal in the eastern and southern Kidson Sub-basin. The Carribuddy Group forms the regional seal to the prospective Larapintine 2 petroleum system, but also contains local source and reservoir facies. The Bongabinni Formation contains extremely rich oil-prone source rocks in local lagoonal facies along the Admiral Bay Fault Zone; these rocks have been linked by other studies to migrated oil in that area. The distribution of the source facies is poorly known, but it may extend down-dip into more mature parts of the Willara Sub-basin, and west into offshore areas. Other local source units may be present in the Mallowa Salt, and possibly the Nibil Formation, but are not well documented. Aeolian sandstone with excellent reservoir potential is locally present in the Nibil Formation, but is more extensive in the lower Worral Formation, particularly the Elsa Sandstone Member.

Evidence for enrichment of subcontinental lithospheric mantle from Paleoproterozoic intracratonic magmas: Geochemistry and U–Pb geochronology of Martin Group igneous rocks, western Rae Craton, Canada

The Paleoproterozoic Martin Group, located in the Rae Craton of northwestern Saskatchewan, is a sequence of intercalated siliciclastic deposits and mafic igneous rocks that were deposited in a fault-controlled intracratonic basin. Based on field relationships, mafic volcanic rocks and sills of the Martin Group have historically been viewed as genetically related to a suite of regionally extensive mafic dikes. In this study, the geochemical characteristics of the three mafic suites (volcanic rocks, sills, and dikes) were used to interpret their respective magmatic histories, to assess the possibility of a genetic relationship between the suites, and to deduce the nature of the magma source. Geochemically, these rocks range from subalkaline (tholeiitic) basalts to alkali basalts or trachyandesites and contain trace element compositions consistent with emplacement in an intracontinental setting. All three suites are highly enriched in light rare earth elements but have anomalously low concentrations of high field strength elements (e.g. Nb, Ta, Hf, Zr) relative to other trace elements. The trace element compositions suggest derivation of all three suites from a common subcontinental lithospheric mantle source that had experienced a complex enrichment history, including modification of depleted mantle by ancient subduction-related components and by small degree asthenospheric melts. A common, enriched source for these magmas is confirmed by their identical initial Nd isotopic compositions ( ɛ Nd at the time of emplacement of −3.7 to −5.3), likely indicative of input of isotopically evolved crust to the source region during the subduction-related enrichment. The overall similarity of chemical compositions for the three suites indicates that regional emplacement of the mafic dikes and (localized?) magmatism within the Martin Group was essentially part of the same mafic magmatic event in the area, though slight intersuite compositional variations might reflect differences in magma evolution histories. A U–Pb baddeleyite date from one dike broadly dates this magmatic event at ca. 1818 Ma, though it must have occurred as a series of magmatic pulses that alternated with periods of sedimentary deposition within the Martin basin. The dike magmas infiltrated pre-existing fractures, whereas the intrabasinal suites might have utilized major faults associated with basin genesis (e.g. the Black Bay fault) as magma conduits.

Proterozoic anorthosite–granite Nain batholith: structure and intrusion processes in an active lithosphere-scale fault zone, northern Labrador

We present field evidence of the structure and evolution of a typical Proterozoic massif-type anorthosite. The anorthosite forms a batholith composed of numerous plutons and ring complexes intruded episodically in two main sequences, each producing anorthositic rocks and monzonite–ferrodiorite followed by granite. The batholith is located where a lithosphere-scale fault zone, ∼50 km wide and extending over 1000 km from Greenland to Labrador, intersected an older continental suture. Intermittent episodes of mainly sinistral transtension along this Gardar – Voisey’s Bay fault zone were associated with the generation of a number of pulses of magmatism and controlled the rise of magmas to middle and upper crustal levels between ∼1363 and 1289 Ma. Both the large- and small-scale structures and magma intrusion processes are similar to those of many cordilleran batholiths, with tabular plutons associated with ring dykes emplaced by cauldron subsidence, and basin-shaped plutons associated with cone sheets. These structures are well known in subvolcanic situations but little known at the mid-crustal depth of the Nain batholith. The location of these structures within the batholith was controlled by intersecting, externally generated faults and shear zones. We conclude that this typical massif-type anorthosite is an intracratonic structural equivalent of cordilleran batholiths of continental margins.

Investigation of late Quaternary fault block uplift along the Motagua/Swan Islands fault system: Implications for seismic/tsunami hazard for the Bay of Honduras

Uplifted and warped coastal landforms (fossil coral reef and beachrock, wave-cut and beach terraces) on the western part of Roatan Island off the northern Honduran coast record at least two late Holocene earthquakes that we estimate to have had magnitudes of > M7. Uplift has been primarily related to a fault that follows the southern coast of western Roatan, herein termed the “Flowers Bay fault”, a subsidiary fault of the Motagua/Swan Islands Fault System which marks the boundary between the North American and Caribbean plates. Using electron spin resonance (ESR) and radiocarbon ages of calcium carbonate samples and a late Quaternary sea level elevation curve that is compatible with Caribbean sea level data, we constrain the ages and long-term uplift rates of the displaced landforms on Roatan caused by the vertical component of slip on the Flowers Bay fault. The fossil reef that is uplifted along the fault grew between 43 and 34 ka, and the beachrock horizon and lowest uplifted terrace along the southern and western coasts developed between 1000 and 1700 AD. We describe evidence of one earthquake that raised the south coast ~ 3 m (as much as 5 m locally) and that post-dates 1700 AD. We interpret this event to be the great earthquake of August 1856 that generated a tsunami which ran as much as 24 km onto the mainland. Another earthquake circa 900 AD produced a similar amount of uplift as the 1856 event and likely generated a similar tsunami. The age and elevation of the fossil reef suggest a long-term uplift rate of 3 mm/year, consistent with a recurrence interval of ~ 1000 years for these large earthquakes.

Neoproterozoic and Cambrian U–Pb (zircon) ages from Grand Manan Island, New Brunswick: implications for stratigraphy and northern Appalachian terrane correlations

Pre-Mesozoic rocks are exposed on the southeastern third of Grand Manan Island and adjacent offshore islands in the Bay of Fundy. U–Pb (zircon) ages reported here demonstrate that most of these rocks are late Neoproterozoic to Early Cambrian. The oldest dated unit is the Ingalls Head Formation, from which two felsic tuff samples yielded ages of 617.6 ± 3.2 and 618.3 ± 2.8 Ma. The Three Islands granite, exposed only on offshore islands, is younger than the Ingalls Head Formation at 611.1 ± 2.4 Ma. It provides a minimum age for marble of the Kent Island Formation, which occurs as large xenoliths in the granite. The High Duck Island granite yielded an age of 547.3 ± 1.1 Ma, providing a minimum late Neoproterozoic age for the host Long Island Bay Formation. An age of 539.0 ± 3.3 Ma from a dacitic lithic-crystal tuff demonstrates that the Priest Cove Formation is earliest Cambrian; this supersedes a Silurian age previously suggested on the basis of poorly documented fossil evidence. The Long Pond Bay Formation is considered to be the youngest pre-Mesozoic unit on Grand Manan Island, based on petrological features, but a felsic intrusive sample from the unit yielded ca. 588 Ma xenocrystic zircon grains that provide only a maximum age for the unit. Although lack of exposed contacts or U–Pb ages from a few units precludes a complete stratigraphic picture, the ages reported here deomonstrate that most pre-Mesozoic units on Grand Manan Island are not Paleozoic as previously assumed. Some units on Grand Manan Island show similarities in rock types and ages to components in the Islesboro block in Penobscot Bay in coastal Maine. Overall, the range of ages and rock types suggests that they correlate with the New River and Mascarene terranes of southern New Brunswick, requiring an offset of at least 40 km between southern New Brunswick and adjacent Maine along the Grand Manan – Oak Bay fault system. Hence, both the Ganderia–Avalonia and Avalonia–Meguma boundaries lie farther offshore than Grand Manan Island.

Rupture process of the 1964 Prince William Sound, Alaska, earthquake from the combined inversion of seismic, tsunami, and geodetic data

Four great earthquakes (1952, 1960, 1964, and 2004) have occurred since seismic monitoring began and only two since the installation of a global seismic network. A reexamination of the 1964 (M 9.2) Prince William Sound (PWS), Alaska, earthquake is timely due to the 2004 Sumatra earthquake because it adds constraints to the potential range of source parameters for these types of infrequent events and aids in the ability to predict the ground motions for other subduction zone including Cascadia. We first measured the durations of high‐frequency energy radiation from teleseismic P wave codas recorded by short‐period World Wide Standardized Seismographic Network (WWSSN SP) instruments to put constraints on the earthquake rupture length. The durations ranged between 250 and 600 s and are shorter in time between azimuths of 190° and 260°. The fault length is estimated to range from 540 to 740 km, rupturing at average speeds of 1.4–2 km/s in a 220°–238° direction. We developed a multiple time window kinematic rupture model for the PWS earthquake from the least squares inversion of teleseismic P waves, tsunami tide gauge records, and geodetic leveling survey observations based on the Green's function technique. We assume three major fault segments for the subduction zone dipping 6‐12° based on geologic data. The subfault size on the megathrust was set to 50 × 50 km. The Patton Bay fault (PBF) imbricate thrust was assigned a 60 dip with a subfault size of 20 × 20 km. We estimated a seismic moment of 5.52 × 1022 N m (Mw9.12). We identified three areas of major moment release, on the PWS segment near Montague and Middleton Islands extending out to the trench, beneath the Portlock anticline on the Cook segment between 58°N fracture zone and the Kodiak‐Bowie seamount chain and finally on the Katmai segment off the coast of Kodiak Island extending to the trench. Our preferred rupture model has a peak slip of 14.9 m along the megathrust and 17.4 m along the PBF. The addition of the PBF resulted in an 8% improvement in residuals and also had a significant effect on the overall slip distribution. Moment release occurred as deep as 50 km depth slab contour near the 350°C isotherm and mantle fore‐arc wedge, which is about 10–20 km deeper than previous coupling depths.

Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland

ABSTRACTThe main phase (∼400 Ma) emplacement of the central and northern part of the reversely zoned Galway Granite was incremental by progressive northward marginal dyke injection and stoping of the 470–467 Ma Connemara metagabbro-gneiss country rock. The space was provided by the synchronous ESE-opening, along the strike of the country rocks, of extensional fractures generated successively northward by a releasing bend in the sinistrally moving Skird Rocks Fault or an equivalent Galway Bay Fault. This fault is a prolongation of the Antrim–Galway (a splay off the Highland Boundary Fault) and Southern Upland Faults. The ESE-strike of the spalled-off rocks controlled the resultant ESE-elongated shape of the batholith. The magma pulses (∼5–30 m in thickness) were progressively more fractionated towards the northern margin so that the coarse Porphyritic (or Megacrystic) Granite (GP; technically granodiorite) in the centre was followed outwards by finer grained, drier and more siliceous granite, until the movements opening the fractures ceased and the magma became too viscous to intrude. ‘Out-of-sequence’ pulses of more basic diorite-granodiorite (including the Mingling–Mixing Zone) and late main phase, more acid, coarse but Aphyric Granite, into the centre of the batholith, complicated the outward fractionation scheme. The outward expansion, caused by the intrusions into the centre, caused a foliation and flattening of cognate xenoliths within the partly crystallised northern marginal granite and in the Mingling–Mixing Zone to the south.Late phase (∼380 Ma) central intrusions of the newly-discovered aphyric Shannapheasteen Finegrained Granite (technically granodiorite), the Knock, the Lurgan and the Costello Murvey Granites, all more siliceous and less dense than the GP, were emplaced by pushing up the already solid and jointed GP along marginal faults. This concentration of lighter granites plus compression shown in thrusting, caused overall fault uplift of the Central Block of the Galway batholith so that the originally deepest part of the GP is exposed where there is the most late phase granite. Chemical analyses show the main and late phase magmas, including late dykes, were very similar, with repetition of the same fractionation except that the late phase magmas were drier and more quickly cooled, giving finer grained rocks.

Coulomb Stress Transfer and Tectonic Loading Preceding the 2002 Denali Fault Earthquake

Pre-2002 tectonic loading and Coulomb stress transfer are modeled along the rupture zone of the M 7.9 Denali fault earthquake (dfe) and on adjacent segments of the right-lateral Denali–Totschunda fault system in central Alaska, using a three-dimensional boundary-element program. The segments modeled closely follow, for about 95°, the arc of a circle of radius 375 km centered on an inferred asperity near the northeastern end of the intersection of the Patton Bay fault with the Alaskan megathrust under Prince William Sound. The loading model includes slip of 6 mm/yr below 12 km along the fault system, consistent with rotation of the Wrangell block about the asperity at a rate of about 1°/m.y. as well as slip of the Pacific plate at 5 cm/yr at depth along the Fairweather–Queen Charlotte transform fault system and on the Alaska megathrust. The model is consistent with most available pre-2002 Global Positioning System (gps) displacement rate data. Coulomb stresses induced on the Denali–Totschunda fault system (locked above 12 km) by slip at depth and by transfer from the M 9.2 Prince William Sound earthquake of 1964 dominated the changing Coulomb stress distribution along the fault. The combination of loading (∼70–85%) and coseismic stress transfer from the great 1964 earthquake (∼15–30%) were the principal post-1900 stress factors building toward strike-slip failure of the northern Denali and Totschunda segments in the M 7.9 earthquake of November 2002. Postseismic stresses transferred from the 1964 earthquake may also have been a significant factor. The M 7.2–7.4 Delta River earthquake of 1912 (Carver et al. , 2004) may have delayed or advanced the timing of the dfe, depending on the details and location of its rupture. The initial subevent of the 2002 dfe earthquake was on the 40-km Susitna Glacier thrust fault at the western end of the Denali fault rupture. The Coulomb stress transferred from the 1964 earthquake moved the Susitna Glacier thrust fault uniformly away from thrust failure by about 100 kPa. The initiation of the Denali fault earthquake was advanced by transfer of 30–50 kPa of positive Coulomb stress to the Susitna Glacier fault (Anderson and Ji, 2003) by the nearby M 6.7 Nenana Mountain foreshock of 23 October 2002. The regional tectonic loading model used here suggests that the Semidi (Alaska Peninsula) segment of the megathrust that ruptured in 1938 ( M 8.2) may be reloaded and approaching failure.

CANNING BASIN AND GLOBAL PALAEOZOIC PETROLEUM SYSTEMS—A REVIEW

There are more than 131 giant and super-giant oil and gas fields with Palaeozoic source and reservoir that are similar to the Canning Basin. These include Palaeozoic basins of North America, North Africa, and the North Caspian Basin of Kazakhstan and Russia.The productivity of these Palaeozoic petroleum systems depends on timing of generation and preservation of charge. Thick Ordovician, Permian, and Triassic evaporite deposits played a very important role in creating and preserving the North American, north Caspian, and north African giant oil and gas fields, respectively.The Mesozoic–Tertiary charged Palaeozoic systems are typically more productive than the Palaeozoic charged systems as exemplified by the north African basins.The Ordovician sourced and reservoired giant oil fields of the North American Mid-Continent are also highly productive. Within the Canning Basin, Ordovician sourced oil has been recovered on the Barbwire Terrace (in Dodonea–1, Percival–1 and Solanum–1) on the Dampier Terrace (in Edgar Range–1 and Pictor–1) and along the Admiral Bay Fault Zone (in Cudalgarra–1, Great Sandy–1, and Leo–1).The Canning Basin may be the least explored of the known Palaeozoic basins with proven petroleum systems. The Palaeozoic basins of North America are the most explored with 500-wells/10,000 km2 compared to the Canning Basin with only 4-wells/10,000 km2.The presence of five oil fields, numerous oil and gas shows and the well density in the Canning Basin (200 wells in 530,000 km2) suggests that further exploration is warranted. Critical analysis of the distribution of source rock, reservoir, seal, timing of generation versus trap formation and post accumulation modification for each tectonic unit of the Canning Basin is required.

Basement-involved inversion at the Appalachian structural front, western Newfoundland: an interpretation of seismic reflection data with implications for petroleum prospectivity

Abstract Recent hydrocarbon exploration in western Newfoundland has resulted in six new wells in the Port au Port Peninsula area. Port au Port No.1, drilled in 1994/95, penetrated the Cambro-Ordovician platform and underlying Grenville basement in the hanging wall of the southeast-dipping Round Head Thrust, terminated in the platform succession in the footwall of this basement-involved inversion structure, and discovered the Garden Hill petroleum pool. The most recent well, Shoal Point K-39, was drilled in 1999 to test a model in which the Round Head Thrust loses reverse displacement to the northeast, eventually becoming a normal fault. This model hinged on an interpretation of a seismic reflection survey acquired in 1996 in Port au Port Bay. This survey is now in the public domain. In our interpretation of these data, the Round Head Thrust is associated with another basement-involved feature, the northwest-dipping Piccadilly Bay Fault, which is mapped on Port au Port Peninsula. Active as normal faults in the Taconian foreland, both these faults were later inverted during Acadian orogenesis. The present reverse offset on the Piccadilly Bay Fault was previously interpreted as normal offset on the southeast-dipping Round Head Thrust. Our new interpretation is consistent with mapping on Port au Port Peninsula and north of Stephenville, where all basement-involved faults are inverted and display reverse senses of motion. It also explains spatially restricted, enigmatic reflections adjacent to the faults as carbonate conglomerates of the Cape Cormorant Formation or Daniel’s Harbour Member, units associated with inverted thick-skinned faults. The K-39 well, which targeted the footwall of the Round Head Thrust, actually penetrated the hanging wall of the Piccadilly Bay Fault. This distinction is important because the reservoir model invoked for this play involved preferential karstification and subsequent dolomitization in the footwalls of inverted thick-skinned faults. The apparent magnitude of structural inversion across the Piccadilly Bay Fault suggests other possible structural plays to the northeast of K-39.

Stress Transfer to the Denali and Other Regional Faults from the M 9.2 Alaska Earthquake of 1964

Stress transfer from the great 1964 Prince William Sound earthquake is modeled on the Denali fault, including the Denali-Totschunda fault segments that ruptured in 2002, and on other regional fault systems where M 7.5 and larger earth- quakes have occurred since 1900. The results indicate that analysis of Coulomb stress transfer from the dominant earthquake in a region is a potentially powerful tool in assessing time-varying earthquake hazard. Modeled Coulomb stress increases on the northern Denali and Totschunda faults from the great 1964 earthquake coincide with zones that ruptured in the 2002 Denali fault earthquake, although stress on the Susitna Glacier thrust plane, where the 2002 event initiated, was decreased. A southeasterly- trending Coulomb stress transect along the right-lateral Totschunda-Fairweather- Queen Charlotte trend shows stress transfer from the 1964 event advancing slip on the Totschunda, Fairweather, and Queen Charlotte segments, including the southern Fairweather segment that ruptured in 1972. Stress transfer retarding right-lateral strike slip was observed from the southern part of the Totschunda fault to the northern end of the Fairweather fault (1958 rupture). This region encompasses a gap with shallow thrust faulting but with little evidence of strike-slip faulting connecting the segments to the northwest and southeast. Stress transfer toward failure was computed on the north-south trending right-lateral strike-slip faults in the Gulf of Alaska that ruptured in 1987 and 1988, with inhibitory stress changes at the northern end of the northernmost (1987) rupture. The northern Denali and Totschunda faults, including the zones that ruptured in the 2002 earthquakes, follow very closely (within 3%), for about 90, an arc of a circle of radius 375 km. The center of this circle is within a few kilometers of the intersection at depth of the Patton Bay fault with the Alaskan megathrust. This inferred asperity edge may be the pole of counterclockwise rotation of the block south of the Denali fault. These observations suggest that the asperity and its recurrent rupture in great earthquakes as in 1964 may have influenced the tectonics of the region during the later stages of evolution of the Denali strike-slip fault system.

Controls on timing and amount of right-lateral offset on the East Bay fault system, San Francisco Bay region, California

The distribution of offset, correlated rock units, relative position of northward younging volcanic rocks, and distribution of units not offset together reveal the history of strike-slip deformation along the East Bay fault system. The system accumulated 175 km of right-lateral offset since 12 Ma, but of this amount, only ~25 km was accumulated between 12 and 10 Ma and only ~75 km between 12 and 6 Ma. The westernmost Hayward fault zone is responsible for more than half (100 km) of the total, and it, too, has undergone more offset (60 km) after 6 Ma than before. The activity on any single fault zone in the system has been sporadic, and alternating periods of activity and inactivity have been recorded on at least some of the fault zones.

A Crustal Velocity Model for Locating Earthquakes in Monterey Bay, California

The seismicity of the Monterey Bay displays a sparse distribution of events with a majority in northern Monterey Bay, on the San Gregorio fault. The paucity of near-shore and offshore seismic recording instruments and the use of velocity models from inland regions for earthquake hypocentral locations and focal mechanisms have led to uncertainties and inaccuracies for seismic events on major faults that crosscut the Bay. New three-component seismic data were acquired during 1997–1999 by the Monterey Bay Aquarium Research Institute (MBARI) Margin Seismology project using ocean-bottom digital corehole and standard seismometers, and coastal digital RefTek instruments obtained from IRIS-PASSCAL by the University of California, Santa Cruz. We have analyzed phases from earthquakes in the Monterey Bay vicinity located by these instruments and used them to supplement the adjacent coastal stations of the permanent Northern California Seismic Network. A new one-dimensional velocity model for the region requires slow velocities from 2 to 6 km that we attribute to sheared granites observed in the Salinian Block in the center of Monterey Bay. Velocities in the 10–16 km layer are consistent with continental crustal velocities. A sharp increase in velocity at ∼16 km suggests a boundary that results from underplating of oceanic crust. This underplated zone appears to extend to a depth of ∼27–30 km where we observe normal upper mantle velocities near 8.0 km/sec. New details from the ocean-bottom corehole and coastal RefTek instruments for events along the SGF and Monterey Bay fault zones hint at local fine-scale structures and have implications for tectonic history and plate reconstruction interpretation.