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USGS1981

Geology of the Jabal Al Qarah Quadrangle, Sheet 20/43 C, Kingdom of Saudi Arabia

The Jabal al Qarah quadrangle (sheet 20/43 C) is a 30-minute quadrangle mapped at 1:100,000 scale in the southern Najd Province of Saudi Arabia. The quadrangle is located between lat 20°00'N. and 20°30'N. and long 43°00'E. and 43°30'E. It is underlain by volcanic, volcaniclastic, and plutonic rocks of late Precambrian age. Most of the quadrangle consists of a large, asymmetric gneiss dome that extends far to the north and south of the quadrangle. Parallel with the eastern border of the quadrangle, part of the extensive, serpentinite-bearing, Nabitah fault zone probably represents a major suture between crust of Jiddah age to the west and younger crust of Halaban age to the east. An excellent section of the Halaban-age crust is exposed in and adjacent to the gneiss dome. At the eroded top of the section to the east, calc-alkalic volcanic rocks of the Halaban group are dominantly andesite but range from basalt to dacite. Beneath the volcanic rocks, comagmatic, subvolcanic plutonic rocks include predominantly diorites but range from gabbro to trondhjemite. These rocks were formed during island-arc volcanism and plutonism from about 780 m.y. to 720(?) m.y. ago. About 725 m.y. ago this primitive crust was thickened by emplacement of large intermediate-crustal depth plutons of hornblende tonalite and hornblende granodiorite. Compressional tectonism produced folds and faults with northern trends and accompanying greenschist metamorphism. Granitic plutonism began 625 m.y. ago, perhaps in response to a major continental collision between the Halaban crust and a continental mass to the east. Granodiorite-monzogranite comprising the Wadi Musayrah batholith formed the core of the Jabal al Qarah gneiss dome as the dome rose. The low-density, intermediate-depth, plutonic rocks of the Halaban crust were converted to orthogneisses as they rose. Strong compression resulted in a large west-directed thrust, the Nabitah fault zone, containing serpentinite. Subsequently, plutons of monzogranite, syenogranite, and alkali-feldspar granite intruded the eroding crust at progressively shallower levels until about 600 m.y. ago. After this time the polymictic conglomerate of the Murdama group was deposited over the youngest granite--the alkali-feldspar granite at Jabal ash Shawhatah al Hamrah; the conglomerate was subsequently deformed. Continued compression of the now relatively thick continental crust resulted in the Najd faulting event about 570 m.y. ago. Two northwest-trending faults in the quadrangle have left-lateral displacement; one has 7 km of offset. These two faults are small but are characteristic of the large Najd fault zones farther north. Throughout the quadrangle, northwest-trending Najd fracturing was pervasive and many of these secondary fractures were filled with diabase dikes of Najd age. The diabase dikes and the deeper, non-exposed mafic plutons induced small-scale, but widespread hydrothermal convection and alteration that produced small quartz veins locally mineralized with gold. Quartz-vein gold was mined on a small scale by ancient miners in several areas. An extensive gold placer was worked about 2,600 years ago in a 50 km 2 area at Jabal Mokhyat. None of these deposits are economical by present-day standards.

Jabal Al Qarah Quadrangle↗

USGS2010

Geochemistry of southern Pagan Island lavas, Mariana arc: The role of subduction zone processes

New major and trace element abundances, and Pb, Sr, and Nd isotopic ratios of Quaternary lavas from two adjacent volcanoes (South Pagan and the Central Volcanic Region, or CVR) located on Pagan Island allow us to investigate the mantle source (i.e., slab components) and melting dynamics within the Mariana intra-oceanic arc. Geologic mapping reveals a pre-caldera (780-9.4ka) and post-caldera (<9.4ka) eruptive stage for South Pagan, whereas the eruptive history of the older CVR is poorly constrained. Crystal fractionation and magma mixing were important crustal processes for lavas from both volcanoes. Geochemical and isotopic variations indicate that South Pagan and CVR lavas, and lavas from the northern volcano on the island, Mt. Pagan, originated from compositionally distinct parental magmas due to variations in slab contributions (sediment and aqueous fluid) to the mantle wedge and the extent of mantle partial melting. A mixing model based on Pb and Nd isotopic ratios suggests that the average amount of sediment in the source of CVR (~2.1%) and South Pagan (~1.8%) lavas is slightly higher than Mt. Pagan (~1.4%) lavas. These estimates span the range of sediment-poor Guguan (~1.3%) and sediment-rich Agrigan (~2.0%) lavas for the Mariana arc. Melt modeling demonstrates that the saucer-shaped normalized rare earth element (REE) patterns observed in Pagan lavas can arise from partial melting of a mixed source of depleted mantle and enriched sediment, and do not require amphibole interaction or fractionation to depress the middle REE abundances of the lavas. The modeled degree of mantle partial melting for Agrigan (2-5%), Pagan (3-7%), and Guguan (9-15%) lavas correlates with indicators of fluid addition (e.g., Ba/Th). This relationship suggests that the fluid flux to the mantle wedge is the dominant control on the extent of partial melting beneath Mariana arc volcanoes. A decrease in the amount of fluid addition (lower Ba/Th) and extent of melting (higher Sm/Yb), and an increase in the sediment contribution (higher Th/Nb, La/Sm, and Pb isotopic ratios) from Mt. Pagan to South Pagan could reflect systematic cross-arc or irregular along-arc melting variations. These observations indicate that the length scale of compositional heterogeneity in the mantle wedge beneath Mariana arc volcanoes is small (~10km).

Contributions to Mineralogy and Petrology↗

USGS1994

Circum-North Pacific tectonostratigraphic terrane map

The companion tectonostratigraphic terrane and overlap assemblage of map the Circum-North Pacific presents a modern description of the major geologic and tectonic units of the region. The map illustrates both the onshore terranes and overlap volcanic assemblages of the region, and the major offshore geologic features. The map is the first collaborative compilation of the geology of the region at a scale of 1:5,000,000 by geologists of the Russian Far East, Japanese, Alaskan, Canadian, and U.S.A. Pacific Northwest. The map is designed to be a source of geologic information for all scientists interested in the region, and is designed to be used for several purposes, including regional tectonic analyses, mineral resource and metallogenic analyses (Nokleberg and others, 1993, 1994a), petroleum analyses, neotectonic analyses, and analyses of seismic hazards and volcanic hazards. This text contains an introduction, tectonic definitions, acknowledgments, descriptions of postaccretion stratified rock units, descriptions and stratigraphic columns for tectonostratigraphic terranes in onshore areas, and references for the companion map (Sheets 1 to 5). This map is the result of extensive geologic mapping and associated tectonic studies in the Russian Far East, Hokkaido Island of Japan, Alaska, the Canadian Cordillera, and the U.S.A. Pacific Northwest in the last few decades. Geologic mapping suggests that most of this region can be interpreted as a collage of fault-bounded tectonostratigraphic terranes that were accreted onto continental margins around the Circum- . orth Pacific mainly during the Mesozoic and Cenozoic (Fujita and ewberry, 1983; 1987; Parfenov, 1984, 1991; Howell, 1985; Watson and Fujita, 1985; Parfenov and Natal'in, 1984; Jones and others. 1987; Monger and Berg, 1987, Fujita and Cook. 1990; Zonenshain and others. 1990; Natal'in, 1991, 1993; Moore and others, 1992; Silberling and others, 1992; Nokleberg and others. 1992, 1993, 1994a; Parfenov and others. 1993; Plaflcer and Berg, 1994; Tabor, 1994). This map is the result of extensive geologic mapping and associated tectonic studies in the Russian Far East, Hokkaido Island of Japan, Alaska, the Canadian Cordillera, and the U.S.A. Pacific Northwest in the last few decades. Geologic mapping suggests that most of this region can be interpreted as a collage of fault-bounded tectonostratigraphic terranes that were accreted onto continental margins around the Circum- . orth Pacific mainly during the Mesozoic and Cenozoic (Fujita and ewberry, 1983; 1987; Parfenov, 1984, 1991; Howell, 1985; Watson and Fujita, 1985; Parfenov and Natal'in, 1984; Jones and others. 1987; Monger and Berg, 1987, Fujita and Cook. 1990; Zonenshain and others. 1990; Natal'in, 1991, 1993; Moore and others, 1992; Silberling and others, 1992; Nokleberg and others. 1992, 1993, 1994a; Parfenov and others. 1993; Plaflcer and Berg, 1994; Tabor, 1994). On the companion map and in the descriptions bel?w· terranes are interpreted according to inferred tectonic environments. These environments are (I) cratonai; (2) passive continental margin; (3) metamorphosed continental margin; (4) continental-margin arc; (5) island arc; (6) oceanic crust, seamount, and ophiolite; (7) accretionary wedge and subduction zone: (8) turbidite basin; and (9) metamorphic for terranes that are too highly-deformed and metamorphosed to de~ermine the original tectonic environment. For terranes with complex geologic histories, the chosen color indicates the tectonic environment most prevalent during this history of the terrane. The tectonic environments inf~rred for igneous rocks are both temporal and genetic. The temporal environments are preaccretion and postaccretion. The genetic environments are subduction-related, rift-related, and collisional (anatectic)- related. In addition to terranes, the map also depicts postaccretion units that include: (I) Cenozoic and Mesozoic overlap assemblages of sedimentary and volcanic rocks that are deposited across two or more terranes that formed generally after accretion of most terranes in the region; (2) Cenozoic and Mesozoic basinal deposits that occur within a terrane or on the craton; (3) plutonic rocks. The postaccretion igneous units are identified by age-lithologic abbreviations and by name. These overlap assemblages and basinal deposits formed mainly during sedimentation and magmatism that occurred after accretion of terranes to each other or to a continental margin. Overlap assemblages provide minimum ages on the timing of accretion of terranes. Some Cenozoic and Mesozoic overlap assemblages and basinal deposits, as well as fragments of terranes, are extensively offset by movement along postaccretion faults. In addition, in onshore areas, the map depicts major preaccretion plutonic rocks that are limited to individual terranes. and in offshore areas. the map depicts major oceanic plates,-ocean floor magnetic lineations. oceanic spreading ridges, and seamounts. The map consists of five sheets. Sheets I and 2 depict, at a scale of I :5.000.000. the tectonostratigraphic terranes. preaccretion plutonic rocks, and postaccretion Cenozoic and Mesozoic overlap sedimentary, volcanic. and plutonic assemblages, and basinal deposits for the Circum- orth Pacific including the Russian Far East, northern Hokkaido Island of Japan, Alaska. the Canadian Cordillera, part of the U.S.A. Pacific Northwest. and adjacent offshore areas. Sheet 3 provides the list of map units for Sheets I and 2. Sheet 4 is a index map showing generalized onshore terranes and overlap assemblages for onshore parts of the Circum-North Pacific at a scale of I: I 0,000,000. Sheet 4 is a guide to the more complicated onshore features depicted on Sheets I and 2. Sheet 5 is an index map showing the major geographic regions for the Circum-North Pacific. Significant differences exist between the representation of onshore and offshore geology on Sheets I and 2. These are: (I) compared to the onshore part of the map, the offshore part is depicted in a more schematic fashion because of more limited data and because the offshore terranes and early Cenozoic and older overlap assemblages generally are obscured by extensive late Cenozoic sedimentary cover that is not shown unless thicker than two kilometers; (2) marginal contacts of offshore Cenozoic and Cretaceous sedimentary basins do not match contacts of onshore Cenozoic and Cretaceous sedimentary units because offshore basins are limited to those regions with sediment thicknesses greater than two kilometers; (3) stratigraphic columns, included at the end of this explanation. are provided only for onshore terranes because the geology of offshore terranes is generally less well-known; and (4) for simplicity, the major onshore Cenozoic sedimentary basins are generally not defined and described separately because the onshore part of the map is designed to emphasize terranes and overlap volcanic assemblages that are crucial for both for tectonic and metallogenic analyses published elsewhere (Nokleberg and others, 1993, 1994a). Several key geologic sources were used in the compilation of the map. For Alaska. the basic outcrop pattern for the map is from Beikman (1980), Gehrels and Berg (1992, 1994). Barker and others ( 1994). Brew (1994), and Moli-Stalcup and others ( 1994b). The distribution of terranes is from Jones and others (1987) and Monger and Berg (1987), with modification by Grantz and other (1991 ). Worall (199 1 ), okleberg and others (1993, 1994a), the cited references, and the Alaskan co-authors of this report. For the Canadian Cordillera. the basic outcrop pattern is from Monger and Berg ( 1987), Wheeler and other (1988). and Wheeler and McFeeley ( 1991) with modifications by the Canadian authors. For the northern part of the Russian Far East. the basic outcrop pattern is from So unov (1985) with modifications by the Russian authors. For the outhern part of the Russian Far East, the basic outcrop pattern is from Krasny (1991) and Bazhanov and Oleinik ( 1986) with modification by the Russian authors. The Russian Far East part of the map is the first attempt to define and delineate terranes in that region. In their compilation. the Russian authors utilized the methodology of U.S.A. and Canadian geologists. Because this map is the first attempt to display the terranes. Cenozoic and Mesozoic overlap assemblages. basinal deposit , and plutonic belts of the Russian Far East. the Russian author will appreciate constructive sugge tions for improving the map.

Open-File Report↗

USGS2023

Regional crustal structure of Indonesia from receiver functions

Characterizing the crustal structure of Indonesia is important to gain a better understanding of its geodynamic evolution and improve seismic hazard assessments in the area. However, a unified crustal model of the entire Indonesian region and its surroundings is lacking. We present new maps of crustal thickness and bulk V p /V s ratio in Indonesia and the surrounding area that are obtained using P-wave receiver functions at 36 seismic stations from several permanent regional networks. The measured crustal thickness varies from ∼24 km to ∼38 km. The thickest crust, ∼38 km, is beneath Flores Island, southern Maluku, and neighboring northernmost Australia, whereas the thinnest crust, ∼24 km, is found under eastern Malaysia. Thus, crustal thickness varies by ∼14 km (from ∼24 km to ∼38 km) despite the small changes in elevation at the measurement points. The V p /V s ratios are 1.79 ± "> ± 0.11, with high values (>1.85) found along the Banda-Sunda arc-trench system. We attribute these high values to: (1) the presence of mafic island arc and oceanic crust and (2) partial melting within this volcanic region, which causes a larger decrease in S-wave velocities compared with P-wave velocities. The comparison of the seismic properties of Indonesian island arc crust, particularly the V p /V s ratio, with laboratory measurements and the petrology of the exhumed Talkeetna island arc, Alaska, allows us to infer the crustal composition of Indonesian island arc crust.

Tectonophysics↗

USGS1974

Palynology and stratigraphy of Cretaceous strata in Long Island, New York, and Block Island, Rhode Island

Palynologic analysis of core samples from Fire Island well, S21,091T, in southern Long Island and of surface samples from Garvies Point in northern Long Island and from eastern Block Island indicates that the Cretaceous of this region includes Raritan, Magothy, Matawan, and Monmouth (as previously defined) strata, and ranges in age from Cenomanian to Maestrichtian, in agreement with recent studies in New Jersey. In this study, the oldest Cretaceous, which is equivalent to the Woodbridge day Member of the Raritan Formation of New Jersey, occurs just above sea level at Garvies Point, Glen Cove, and in the Port Washington sand pits in northwestern Long Island, whereas sediments from near the base of the Fire Island well arc correlated with the South Amboy Fire Clay Member. The Magothy and Matawan combined in Long Island are about 1,500 ft (457 m) thick, whereas in New Jersey both units rarely exceed a thickness of 500 ft (152 m). Outcropping Cretaceous sediments in Block Island are equivalent to the South Amboy Fire Clay Member rather than the Magothy, as indicated in prior studies. Palynologic zonation of the Late Cretaceous is tentatively extended to Magothy and younger strata on the basis of the appearance and proliferation of the more advanced, angiosperm pollen forms. This study also provides a basis for extending the zonation and hence the correlation to the sediments of the Continental Shelf.

New York, Rhode Island↗

USGS2003

Ophiolite and volcanic arc assemblages on the Vizcaino Peninsula and Cedros Island region, Baja California Sur, Mexico: Mesozoic forearc lithosphere of the Cordilleran magmatic arc

Mesozoic ophiolites in the Vizcaíno Peninsula and Cedros Island region of Baja California Sur are suprasubduction zone Cordilleran-type ophiolites structurally juxtaposed with underlying high pressure-temperature subduction complex assemblages. The region is divided into three separate tectonostratigraphic terranes, but here we recognize stratigraphic, intrusive, and petrologie links between these terranes and interpret the evolution of the entire region within the same Late Triassic to Early Cretaceous tectonic framework. Several phases of extension are recognized, including two major phases that resulted in development of distinct ophiolite assemblages. The Late Triassic Vizcaine Peninsula Ophiolite (221 ± 2 Ma) represents the earliest stage of this history and comprises a complete spreading center sequence with depleted upper mantle and mafie crustal rocks, including sheeted dike complex, Jurassic are magmatic rocks with low-Ti are tholelite and boninite geochemical affinities were intruded through and constructed on the Triassic ophiolite basement. Ultra-depleted are-ankaramites on Cedros Island may represent an initial phase of are rifting that was followed by major Middle Jurassic extension and production of the Cedros Island Ophiolite (173 ± 2 Ma). The Late Jurassic-Early Cretaceous Coloradito and Eugenia Formations contain mudflows and olistostrome blocks intercalated with are volcanogenic sediment and rift-related pillow lavas; these units record extension and/or transtension and provide the earliest definite evidence of are-continent interaction in the region. Middle Jurassic to Early Cretaceous are plutonic rocks (ca. 165-135 Ma) were shallowly intruded into low greenschist-facies ophiolite and are volcanic basement. Plutonic rocks range in composition from gabbro to granodiorite, but tonalite dominates. These intrusions are typical I-type Cordilleran batholithic rocks with relatively primitive are geochemical affinities (initial Sr 87 / 86 Sr range from ~0.704 to 0.706), but they are distinctly calcic in nature, a feature common to the adjacent Cretaceous Peninsular Ranges batholith. The Vizca í no-Cedros region correlates to ophiolitic terranes of the western Sierra Klamath belt and Coast Ranges of California and Oregon that were constructed in part across the North American margin. Age, stratigraphic, and petrochemical data from the Vizca í no-Cedros region support previously proposed forearc rifting models developed for the U.S. sector of the Cordilleran orogen that interpret the ophiolite assemblages as autochthonous or parautochthonous forearc lithosphere constructed outboard of the Mesozoic continental margin arc.

GSA Special Papers↗

USGS1998

Catalog of the historically active volcanoes of Alaska

Alaska hosts within its borders over 80 major volcanic centers that have erupted during Holocene time (< 10,000 years). At least 29 of these volcanic centers (table 1) had historical eruptions and 12 additional volcanic centers may have had historical eruptions. Historical in Alaska generally means the period since 1760 when explorers, travelers, and inhabitants kept written records. These 41 volcanic centers have been the source for >265 eruptions reported from Alaska volcanoes. With the exception of Wrangell volcano, all the centers are in, or near, the Aleutian volcanic arc, which extends 2500 km from Hayes volcano 145 km west of Anchorage in the Alaska-Aleutian Range to Buldir Island in the western Aleutian Islands (fig. 1). The volcanic arc, a subduction-related feature associated with underthrusting of the Pacific plate beneath the North American plate is divided between oceanic island arc and continental margin segments, the boundary occurring at about 165° W longitude (fig. 1). An additional 7 volcanic centers in the Aleutian arc (table 2; fig. 1 A) have active fumarole fields but no reported historical eruptions. This report discusses the location, physiography and structure, eruptive history, and geology of those volcanoes in Alaska that have experienced one or more eruptions that have been recorded in the written history (i.e., in historical time). It is part of the group of catalogs entitled Catalogue of Active Volcanoes of the World published beginning in 1951 under the auspices of the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI). A knowledge of the information contained in such catalogs aids in understanding the type and scale of activity that might be expected during a particular eruption, the hazards the eruption may pose, and even the prediction of eruptions. The catalog will thus be of value not only to the inhabitants of Alaska but to government agencies concerned with emergency response, air traffic operations, and weather, as well as to industry and scientists. The combination of the hazard posed by volcanic ash to jet aircraft and the heavy use of international air routes located parallel to, and on either side of, the Aleutian volcanic arc means that even remote volcanoes in Alaska now pose significant hazards to life and property. Although this report is concerned with historical eruptions from Alaskan volcanoes, other volcanoes in Alaska have erupted in the past 10,000 years and might therefore be expected to erupt again. Several Holocene volcanic centers in the Aleutian arc have no reported historical activity. Elsewhere in Alaska the Bering Sea basalt fields cover large areas of the Yukon Delta, Seward Peninsula, and several of the islands of the Bering Sea. Holocene centers also occur in the Wrangell Mountains and in isolated occurrences in the interior and southeastern Alaska. Eruptions from these centers have occurred within the past several hundred years but none were transcribed in the written record. Moodie and others (1992), however, report oral traditions among the Northern Athapaskan Indians of the southwestern Yukon Territory that may record the second and younger deposition of the White River Ash circa A.D. 720. This lobe of the White River Ash was deposited during the paroxysmal eruption of Churchill volcano in the Wrangell Mountains of eastcentral Alaska (McGimsey and others, 1992; Richter and others, 1995).

Alaska↗

USGS2007

Hazards from hydrothermally sealed volcanic conduits

The 17 March 2006 eruption from Raoul Island (Kermadec arc, north of New Zealand) is interpreted as a magmatic‐hydrothermal event triggered by shaking associated with a swarm of local earthquakes. The eruption, which tragically claimed the life of New Zealand Department of Conservation Ranger Mark Kearney, occurred without significant volcanic seismicity or any of the precursory responses the volcanic hydrothermal system exhibited prior to a similarly sized eruption in 1964. Preliminary evidence suggests that the absence of precursory behavior is probably the consequence of hydrothermal sealing of the volcanic conduit since the 1964 eruption, and points to potential hazards associated with quiescent oceanic island volcanoes.

Eos, Transactions, American Geophysical Union↗

USGS2012

Recent explosive eruptions and volcano hazards at Soputan volcano—a basalt stratovolcano in north Sulawesi, Indonesia

Soputan is a high-alumina basalt stratovolcano located in the active North Sulawesi-Sangihe Islands magmatic arc. Although immediately adjacent to the still geothermally active Quaternary Tondono Caldera, Soputan’s magmas are geochemically distinct from those of the caldera and from other magmas in the arc. Unusual for a basalt volcano, Soputan produces summit lava domes and explosive eruptions with high-altitude ash plumes and pyroclastic flows—eight explosive eruptions during the period 2003–2011. Our field observations, remote sensing, gas emission, seismic, and petrologic analyses indicate that Soputan is an open-vent-type volcano that taps basalt magma derived from the arc-mantle wedge, accumulated and fractionated in a deep-crustal reservoir and transported slowly or staged at shallow levels prior to eruption. A combination of high phenocryst content, extensive microlite crystallization and separation of a gas phase at shallow levels results in a highly viscous basalt magma and explosive eruptive style. The open-vent structure and frequent eruptions indicate that Soputan will likely erupt again in the next decade, perhaps repeatedly. Explosive eruptions in the Volcano Explosivity Index (VEI) 2–3 range and lava dome growth are most probable, with a small chance of larger VEI 4 eruptions. A rapid ramp up in seismicity preceding the recent eruptions suggests that future eruptions may have no more than a few days of seismic warning. Risk to population in the region is currently greatest for villages located on the southern and western flanks of the volcano where flow deposits are directed by topography. In addition, Soputan’s explosive eruptions produce high-altitude ash clouds that pose a risk to air traffic in the region.

Bulletin of Volcanology↗

USGS2004

Isotopic, geochemical, and temporal characterization of Proterozoic basement rocks in the Quitovac region, northwestern Sonora, Mexico: Implications for the reconstruction of the southwestern margin of Laurentia

A detailed geochemical characterization of 19 representative Proterozoic basement rocks in the Quitovac region in northwestern Sonora, Mexico, has identified two distinct Paleoproterozoic basement blocks that coincide spatially with the previously proposed Caborca and "North America" blocks. New U-Pb zircon geochronology revises their age ranges, the Caborca (1.78-1.69 Ga) and "North America" (1.71-1.66 Ga) blocks at Quitovac, and precludes a simple age differentiation between them. In addition, Grenvillian-age granitoids (ca. 1.1 Ga), spatially associated with the Caborca block have been identified at Quitovac. Nd isotopes and major- and trace-element geochemistry support the distinction of these Paleoproterozoic blocks. Granitoids of the "North America" block are characterized by depleted εNd values (3.4-3.9) and younger Nd model ages (1800-1740 Ma) and have lower K2O, Y, Rb, Ba, Th, REE, and Fe/Mg values than coeval rocks of the Caborca block. The Caborca block granitoids are likewise characterized by slightly less depleted εNd (0.6-2.6) and older Nd model ages (2070-1880 Ma). Despite the subtle differences, granitoids from both the Caborca and "North America" blocks exhibit island arc-like affinities. We propose that the Proterozoic basement rocks from the Quitovac region are an extension of the Proterozoic crustal provinces in the southwestern United States. Specifically, rocks of the Caborca block exhibit an affinity to rocks of either the Yavapai province or the Mojave-Yavapai transition zone, whereas rocks of the "North America" block have signatures similar to those of the Mazatzal province or possibly the Yavapai province of Arizona. The new isotopic ages and geochemical data do not support the existence of the Late Jurassic Mojave-Sonora megashear at Quitovac, as originally proposed. However, the Quitovac region accounts only for a small fraction of the Proterozoic basement in Sonora, so these findings do not eliminate the possibility of a megashear elsewhere in northern Sonora. Our new data create the possibility of alternative hypotheses for the distribution of Paleoproterozoic crustal provinces in southwestern North America that affect reconstructions of the original southwestern margin of Laurentia, and reduce uncertainties in the configuration, timing, and existence of the Proterozoic supercontinent, Rodinia.

Quitovac↗

USGS1979

Post-orogenic peralkaline and calc-alkaline granites and associated mineralization of the Arabian Shield, Kingdom of Saudi Arabia

Following a long period of island arc-like orogenic activity, post-orogenic granites of Pan-African age (670-550 m.y.) were emplaced throughout the Arabian Shield. Most of these rocks are typical subsolvus calc-alkaline biotite monzogranite. The early Pan-African granites are associated with widespread small tholeiitic gabbro intrusions, many of which are layered. The calc-alkaline granites are not evenly distributed and their area of exposure relative to other rock types increases from southwest to north and northeast. In the northeastern part of the shield, they are associated spatially with rhyolites of similar composition and with molasse derived from granitic terrane. The occurrence of 45 late alkali granites, most of which are peralkaline, is described and their chemistry discussed. Most of these rocks are medium- to coarse-grained hypersolvus alkali granites, but phenocrystalline and micrographic varieties are also common. The mafic silicate minerals of the alkali granites are arfvedsonite, aegirine, aegirine-augite, riebeckite, and sparse biotite, aenigmatite, and barkevikite. The alkali granites do not occur in the southwestern third of the Arabian Shield nor in the southeastern portion of the adjacent Nubian Shield of Africa. A syenite-shonkinite province located in the southwestern part of the Arabian Shield is identified. No economic mineral deposits are known to be associated with the Pan-African granites. Numerous mineral occurrences and geochemical anomalies have been found that contain tungsten, molybdenum, tin, beryllium, zirconium, niobium, thorium, uranium, rare earths, and fluorite. The most important of these occurrences are tabulated and their distribution shown.

Open-File Report↗

USGS1983

Geochronology, geochemistry, and tectonic environment of porphyry mineralization in the central Alaska Peninsula

Porphyry type sulfide systems on the central Alaska Peninsula occupy a transition zone between the Aleutian island magmatic arc and the continental magmatic arc of southern Alaska. Mineralization occurs associated with early and late Tertiary magmatic centers emplaced through a thick section of Mesozoic continental margin clastic sedimentary rocks. The systems are of the molybdenum-rich as opposed to gold-rich type and have anomalous tungsten, bismuth, and tin, attributes of continental-margin deposits, yet gravity data suggest that at least part of the study area is underlain by oceanic or transitional crust. Potassium-argon age determinations indicate a variable time span of up to 2 million years between emplacement and mineralization in a sulfide system with mineralization usually followed by postmineral intrusive events. Finally, mineralization in the study area occurred at many times during the time span of igneous activity and should be an expected stage in the history of a subduction related magmatic center.

Open-File Report↗

USGS2007

New M 40 Ar/ 39 Ar and K/Ar Ages of Dikes in the South Shetland Islands (Antarctic Peninsula)

Eighteen plagioclase 40Ar/39Ar and 7 whole rock K/Ar ages suggest that dikes in the South Shetland Islands (Antarctic Peninsula) are of Paleocene to Eocene age. The oldest dikes are exposed on Hurd Peninsula (Livingston Island) and do not yield 40Ar/39Ar plateaux. Our best estimates suggest dike intrusion at about the Cretaceous/Paleogene boundary. An older age limit for the dikes is established by Campanian nannofossil ages from their metasedimentary host. Dike intrusion began earlier and lasted longer on Hurd Peninsula (Danian to Priabonian) than on King George Island (Thanetian to Lutetian). Arc magmatism on King George Island, possibly accompanied also by hypabyssal intrusions, began in the Cretaceous as indicated by ages from the stratiform volcanic sequence. The dikes on King George Island were emplaced beginning in the late Paleocene and ending 47–45 Ma. The youngest arc-related dikes on Hurd Peninsula were emplaced ~37 Ma

Open-File Report↗

USGS2018

Postglacial eruptive history and geochemistry of Semisopochnoi volcano, western Aleutian Islands, Alaska

Semisopochnoi Island, located in the Rat Islands group of the western Aleutian Islands and Aleutian volcanic arc, is a roughly circular island composed of scattered volcanic vents, the prominent caldera of Semisopochnoi volcano, and older, ancestral volcanic rocks. The oldest rocks on the island are gently radially dipping lavas that are the remnants of a shield volcano and of Ragged Top, which is an eroded stratocone southeast of the current caldera. None of these oldest rocks have been dated, but they all are likely Pleistocene in age. Anvil Peak, to the caldera’s north, has the morphology of a young stratocone and is latest Pleistocene to early Holocene in age. The oldest recognized Holocene deposits are those of the caldera-forming eruption, which produced the 7- by 6-km caldera in the center of the island, left nonwelded ignimbrite in valleys below the edifice, and left welded ignimbrite high on its flanks. The caldera-forming eruption produced rocks showing a range of intermediate whole-rock compositions throughout the eruption sequence, although a majority of clasts analyzed form a fairly tight cluster on SiO 2 -variation diagrams at 62.9 to 63.4 weight percent SiO 2 . This clustering of compositions at about 63 weight percent SiO 2 includes black, dense, obsidian-like clasts, as well as tan, variably oxidized, highly inflated pumice clasts. The best estimate for the timing of the eruption is from a soil dated at 6,920±60 14 C years before present underlying a thin facies of the ignimbrite deposit on the island’s north coast. Shortly after the caldera-forming eruption, two scoria cones on the northwest flank of the volcano outside the caldera, Ringworm crater and Threequarter Cone, simultaneously erupted small volumes of andesite. The oldest intracaldera lavas, on the floor of the caldera, are andesitic to dacitic, but are mostly covered by younger lavas and tephras. These intracaldera lavas include the basaltic andesites of small Windy cone, as well as the more voluminous basaltic andesites of three-peaked Mount Cerberus, which takes up most of the west half of the caldera and has erupted lavas that flowed to the sea on the southwestern coast of the island. Apparently active at the same time as Mount Cerberus, extracaldera Sugarloaf Peak at the southern point of the island has exclusively erupted basalts. Its young satellite peak, Sugarloaf Head, has erupted morphologically young lavas and cinder cones and may be the source of the last historical eruption in 1987. Several tephra sections on the east half of the island record as many as 50 tephras, mostly from Mount Cerberus, Sugarloaf Peak, and Sugarloaf Head, over the past several thousand years. Eruptive products of Semisopochnoi Island show an overall compositional range of basalt to dacite, though basaltic andesite and andesite constitute the largest proportions of rock types. They are tholeiitic, low to medium K, and have geochemical characteristics typical of magmatic arcs. The earliest Pleistocene lavas are mostly basalts that show the greatest geochemical diversity, as illustrated by, for example, La N /Yb N ratios of 1.9 to 3.5, suggesting fluctuations in the magma source region over the hundreds of thousands of years recorded by these older lavas. The Holocene rocks, in contrast, follow arrays in compositional space that suggest crystallization differentiation from discrete, subtly different batches of magma under varying pressure and temperature conditions. Increasingly negative Eu anomalies and an only modestly increasing alumina saturation index value with differentiation suggest that plagioclase and mafic silicates (amphibole and pyroxene) were involved to varying degrees in fractional crystallization to produce Semisopochnoi’s magmatic diversity. The crystal-poor, andesitic magmas that erupted during caldera formation likely separated from a plagioclase-, amphibole-, and clinopyroxene-dominated crystal residue in the upper crust at less than 900 °C, possibly following a period of decreased magmatic flux. During the Holocene, basaltic Sugarloaf Peak appears to bypass any upper crustal magmatic storage region and erupt crystal-rich basalts. Recent seismic swarms and long-lived warm springs attest to ongoing magmatic activity. The Holocene eruptive record at Semisopochnoi volcano is one of diverse eruptive styles as well as frequent eruptions from multiple vents located within and outside the caldera. The number and diversity of postcaldera vents means that the sites of future eruptions cannot be predicted with certainty. Future eruptions of ash similar in magnitude to the VEI 3 or less eruptions recorded in the documented tephra deposits would pose a hazard to aircraft in the region.

Alaska↗

USGS2007

Sea birds as proxies of marine habitats and food webs in the western Aleutian Arc

We propose that ocean conditions of the Near Islands in the western Aleutian Arc mimic those of the shallow continental shelf of the eastern Bering Sea to the extent that the marine community, including assemblages of forage fishes and their avian predators, has distinctly coastal characteristics. In contrast, marine avifauna and their prey at neighbouring Buldir Island are distinctly oceanic. For example, at the Near Islands, the ratio of thick-billed to common murres, Vria lomvia and U. aalge , is low and black-legged kittiwakes, Rissa tridactyla , but not red-legged kittiwakes, R. brevirostris , nest there. Diets of murres and kittiwakes are dominated by sand lance, Ammodytes hexapterus , an abundant coastal species. At Buldir Island, thick-billed murres greatly outnumber common murres, red-legged kittiwakes and black-legged kittiwakes are both abundant, and diets of the birds consist primarily of oceanic squid and lantern-fish (Myctophidae). This mesoscale difference in food webs is apparently a consequence of the local physiography. A broad escarpment on the Near physiographic block creates a comparatively expansive, shallow, shelflike habitat around the Near Islands, where a pelagic community typical of coastal regions flourishes. Buldir Island is the only emergent feature of the Buldir physiographic block, with little shallow water surrounding it and, apparently, little opportunity for other than oceanic species to exist. Patterns in the distribution of fishes, and thus of sea birds, throughout the Aleutian Islands might be largely explained by the presence or absence of shelf-like habitat and the relationship between physical environments and food webs. In the larger context of fisheries oceanography, this model for the Aleutian Islands improves our ability to interpret physical and biological heterogeneity in the ocean and its relationship to regional community dynamics and trends in the abundance and productivity of individual species at higher trophic levels.

Alaska↗

USGS2015

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

Despite the role of the Alaska-Aleutian megathrust as the source of some of the largest earthquakes and tsunamis, the history of its pre–twentieth century tsunamis is largely unknown west of the rupture zone of the great (magnitude, M 9.2) 1964 earthquake. Stratigraphy in core transects at two boggy lowland sites on Chirikof Island’s southwest coast preserves tsunami deposits dating from the postglacial to the twentieth century. In a 500-m-long basin 13–15 m above sea level and 400 m from the sea, 4 of 10 sandy to silty beds in a 3–5-m-thick sequence of freshwater peat were probably deposited by tsunamis. The freshwater peat sequence beneath a gently sloping alluvial fan 2 km to the east, 5–15 m above sea level and 550 m from the sea, contains 20 sandy to silty beds deposited since 3.5 ka; at least 13 were probably deposited by tsunamis. Although most of the sandy beds have consistent thicknesses (over distances of 10–265 m), sharp lower contacts, good sorting, and/or upward fining typical of tsunami deposits, the beds contain abundant freshwater diatoms, very few brackish-water diatoms, and no marine diatoms. Apparently, tsunamis traveling inland over low dunes and boggy lowland entrained largely freshwater diatoms. Abundant fragmented diatoms, and lake species in some sandy beds not found in host peat, were probably transported by tsunamis to elevations of >10 m at the eastern site. Single-aliquot regeneration optically stimulated luminescence dating of the third youngest bed is consistent with its having been deposited by the tsunami recorded at Russian hunting outposts in 1788, and with the second youngest bed being deposited by a tsunami during an upper plate earthquake in 1880. We infer from stratigraphy, 14 C-dated peat deposition rates, and unpublished analyses of the island’s history that the 1938 tsunami may locally have reached an elevation of >10 m. As this is the first record of Aleutian tsunamis extending throughout the Holocene, we cannot estimate source earthquake locations or magnitudes for most tsunami-deposited beds. We infer that no more than 3 of the 23 possible tsunamis beds at both sites were deposited following upper plate faulting or submarine landslides independent of megathrust earthquakes. If so, the Semidi segment of the Alaska-Aleutian megathrust near Chirikof Island probably sent high tsunamis southward every 180–270 yr for at least the past 3500 yr.

Alaska↗

USGS1988

Acoustic stratigraphy and hydrothermal activity within Epi Submarine Caldera, Vanuatu, New Hebrides Arc

Geological and geophysical surveys of active submarine volcanoes offshore and southeast of Epi Island, Vanuatu, New Hebrides Arc, have delineated details of the structure and acoustic stratigraphy of three volcanic cones. These submarine cones, named Epia, Epib, and Epic, are aligned east-west and spaced 3.5 km apart on the rim of a submerged caldera. At least three acoustic sequences, of presumed Quaternary age, can be identified on single-channel seismic-reflection profiles. Rocks dredged from these cones include basalt, dacite, and cognate gabbroic inclusions with magmatic affinities similar to those of the Karua (an active submarine volcano off the southeastern tip of Epi) lavas. ?? 1988 Springer-Verlag New York Inc.

Geo-Marine Letters↗

USGS2010

The role of water in generating the calc-alkaline trend: New volatile data for aleutian magmas and a new tholeiitic index

The origin of tholeiitic (TH) versus calc-alkaline (CA) magmatic trends has long been debated. Part of the problem stems from the lack of a quantitative measure for the way in which a magma evolves. Recognizing that the salient feature in many TH–CA discrimination diagrams is enrichment in Fe during magma evolution, we have developed a quantitative index of Fe enrichment, the Tholeiitic Index (THI): THI = Fe 4 ·0 /Fe 8 ·0 , where Fe 4 ·0 is the average FeO* concentration of samples with 4 ± 1 wt % MgO, and Fe 8 ·0 is the average FeO* at 8 ± 1 wt % MgO. Magmas with THI > 1 have enriched in FeO* during differentiation from basalts to andesites and are tholeiitic; magmas with THI < 1 are calc-alkaline. Most subduction zone volcanism is CA, but to varying extents; the THI expresses the continuum of Fe enrichment observed in magmatic suites in all tectonic settings. To test various controls on the development of CA trends, we present new magmatic water measurements in melt inclusions from eight volcanoes from the Aleutian volcanic arc (Augustine, Emmons, Shishaldin, Akutan, Unalaska, Okmok, Seguam, and Korovin). Least degassed H 2 O contents vary from ∼2 wt % (Shishaldin) to >7 wt % (Augustine), spanning the global range in arc mafic magmas. Within the Aleutian data, H 2 O correlates negatively with THI, from strongly calc-alkaline (Augustine, THI = 0·65) to moderately tholeiitic (Shishaldin, THI = 1·16). The relationship between THI and magmatic water is maintained when data are included from additional arc volcanoes, back-arc basins, ocean islands, and mid-ocean ridge basalts (MORBs), supporting a dominant role of magmatic water in generating CA trends. An effective break between TH and CA trends occurs at ∼2 wt % H 2 O. Both pMELTs calculations and laboratory experiments demonstrate that the observed co-variation of H 2 O and THI in arcs can be generated by the effect of H 2 O on the suppression of plagioclase and the relative enhancement of Fe-oxides on the liquid line of descent. The full THI–H 2 O array requires an increase in fO 2 with H 2 O, from ≤FMQ (where FMQ is the fayalite–magnetite–quartz buffer) in MORB to ∼ΔFMQ +0·5 to +2 in arcs, consistent with inferences from measured Fe and S species in glasses and melt inclusions. A curve fit to the data, H 2 O (wt % ± 1·2) = exp[(1·26 – THI)/0·32], may provide a useful tool for estimating the H 2 O content of magmas that are inaccessible to melt inclusion study.

Journal of Petrology↗