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USGS1992

Map showing bottom topography of the Pacific continental margin, Point Conception to Point Loma

All contours, geographic outlines, and political boundaries shown on this map of the bottom topography, or bathymetry, of the Pacific continental margin between 34° and 41° N. latitudes were plotted from digital data bases in the library of the U.S. Geological Survey (USGS)-National Oceanic and Atmospheric Administration (NOAA) Joint Office for Mapping and Research (JOMAR). These digital data were obtained and compiled from many sources; consequently, data quality varies within particular data bases as well as from one data base to another. Bathymetric contours north of 32°N. were digitized from a map by Chase and others (1981) and from the unpublished large scale versions of that map (T. E. Chase, unpublished maps, 1981). South of 32°N, the contours are from unpublished maps (T.E. Chase and B.A. Seekins, unpublished maps, 1989). Chase and others (1981 and unpublished maps) obtained the data for the area seaward of the continental slope (-2000 m depth) primarily from the U.S. Coast and Geodetic Survey (C&GS) 1955 Pacific Exploratory Survey, a systematic and detailed (~8-nmi-trackline spacing) survey between Mexico and Canada. The USGS provided data from cruises S3-78-NC, L2-77-NC, L10-76-NC, K-73-NC, and Bartlett 72. Data were also obtained from Scripps Institution of Oceanography cruises Blue Flash, Kayak B, Scan I, and Seven Tow (Chase and Menard, 1971; Chase and others, 1975). The 200-m contour was derived from the National Ocean Survey charts 1206N-16 (1975a) and 1306N-20 (1975b), and C&GS charts 1206N-15 (1967a) and 1306N-19 (1967b). Sea-floor depths were corrected for sound velocity in sea water using Matthew's (1939) tables. Onshore topographic contours were generated by computer from a modified version of 3-arc-second elevation data provided by the Defense Mapping Agency. The United States digital shoreline was obtained from the NOAA, NOS, Nautical Charting Division, National Atlas files. The coastline of Canada was digitized from Canadian Hydrographic Service bathymetric maps. The primary source of names of the sea floor features was the "Gazetteer of Undersea Features" (Defense Mapping Agency, 1990). Acknowledgments Christopher Hines assisted in the construction of the digital data bases. Reviews and suggestions by Edward C. Escowitz and Florence Wong and advice provided by Will Stettner regarding the cartographic design substantially improved the quality of this map. References Cited Chase, T.E., and Menard, H.W., 1971, Bathymetric atlas of the northeastern Pacific Ocean: U.S. Naval Oceanographic Office Publication 1303, scale 1:2,000,000 at 33° latitude, 48 p. Chase, T.E., Wilde, Pat, and Normark, W.R., 1975, Oceanographic data of the Monterey Deep Sea Fan: San Diego, University of California, Institute of Marine Resources Publication TR 58, scale 1:898,524 at 35° latitude. Chase, T.E., Wilde, Pat, Normark, W.R., Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32° and 49° North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38° latitude, 2 sheets. Coast and Geodetic Survey, 1967a, Santa Barbara to Huntington Beach : Coast and Geodetic Survey Bathymetric Map 1206N-15, scale 1:250,000. Coast and Geodetic Survey, 1967b, Southwest of Santa Rosa Island: Coast and Geodetic Survey Bathymetric Map 1306N-19, scale 1:250,000. Defense Mapping Agency, 1990, Gazetteer of undersea features (4th ed.): Washington, D.C., Defense Mapping Agency. Matthews, D.J., 1939, Tables of the velocity of sound in pure water and sea water (2d ed.): London, Admiralty, Hydrographic Department, H.D. 282, 52 p. National Ocean Survey, 1975a, Hungington Beach to Punta Sal Si Puedes: National Ocean Survey Bathymetric Map 1206N-16, scale 1:250,000. National Ocean Survey, 1975b, Cape San Martin to Point Conception: National Ocean Survey Bathymetric Map 1306N-20, scale 1:250,000. index map for I-2089-A

California↗

USGS2015

Provenance and detrital zircon geochronologic evolution of lower Brookian foreland basin deposits of the western Brooks Range, Alaska, and implications for early Brookian tectonism

The Upper Jurassic and Lower Cretaceous part of the Brookian sequence of northern Alaska consists of syntectonic deposits shed from the north-directed, early Brookian orogenic belt. We employ sandstone petrography, detrital zircon U-Pb age analysis, and zircon fission-track double-dating methods to investigate these deposits in a succession of thin regional thrust sheets in the western Brooks Range and in the adjacent Colville foreland basin to determine sediment provenance, sedimentary dispersal patterns, and to reconstruct the evolution of the Brookian orogen. The oldest and structurally highest deposits are allochthonous Upper Jurassic volcanic arc–derived sandstones that rest on accreted ophiolitic and/or subduction assemblage mafic igneous rocks. These strata contain a nearly unimodal Late Jurassic zircon population and are interpreted to be a fragment of a forearc basin that was emplaced onto the Brooks Range during arc-continent collision. Synorogenic deposits found at structurally lower levels contain decreasing amounts of ophiolite and arc debris, Jurassic zircons, and increasing amounts of continentally derived sedimentary detritus accompanied by broadly distributed late Paleozoic and Triassic (359–200 Ma), early Paleozoic (542–359 Ma), and Paleoproterozoic (2000–1750 Ma) zircon populations. The zircon populations display fission-track evidence of cooling during the Brookian event and evidence of an earlier episode of cooling in the late Paleozoic and Triassic. Surprisingly, there is little evidence for erosion of the continental basement of Arctic Alaska, its Paleozoic sedimentary cover, or its hinterland metamorphic rocks in early foreland basin strata at any structural and/or stratigraphic level in the western Brooks Range. Detritus from exhumation of these sources did not arrive in the foreland basin until the middle or late Albian in the central part of the Colville Basin. These observations indicate that two primary provenance areas provided detritus to the early Brookian foreland basin of the western Brooks Range: (1) local sources in the oceanic Angayucham terrane, which forms the upper plate of the orogen, and (2) a sedimentary source region outside of northern Alaska. Pre-Jurassic zircons and continental grain types suggest the latter detritus was derived from a thick succession of Triassic turbidites in the Russian Far East that were originally shed from source areas in the Uralian-Taimyr orogen and deposited in the South Anyui Ocean, interpreted here as an early Mesozoic remnant basin. Structural thickening and northward emplacement onto the continental margin of Chukotka during the Brookian structural event are proposed to have led to development of a highland source area located in eastern Chukotka, Wrangel Island, and Herald Arch region. The abundance of detritus from this source area in most of the samples argues that the Colville Basin and ancestral foreland basins were supplied by longitudinal sediment dispersal systems that extended eastward along the Brooks Range orogen and were tectonically recycled into the active foredeep as the thrust front propagated toward the foreland. Movement of clastic sedimentary material from eastern Chukotka, Wrangel Island, and Herald Arch into Brookian foreland basins in northern Alaska confirms the interpretations of previous workers that the Brookian deformational belt extends into the Russian Far East and demonstrates that the Arctic Alaska–Chukotka microplate was a unified geologic entity by the Early Cretaceous.

Alaska↗

USGS1990

Remagnetization and northward translation of Mesozoic red chert from Cedros Island and the San Benito Islands, Baja California, Mexico

Samples for paleomagnetic analysis were collected from red radiolarian ribbon chert in a Franciscan-like subduction complex (Western Baja terrane) exposed along the western margin of the Baja California Peninsula. A 40-m-thick section of chert was sampled on Cedros Island, and a 30-m-thick and two smaller sections of chert were sampled on the San Benito Islands to the west. A site in pillow basalt underlying the chert also was sampled on Cedros Island. The sections of chert on Cedros Island and the San Benito Islands contain radiolaria ranging in age from Late Triassic to Early Cretaceous. All of these rocks have similar normal-polarity direction of magnetization, indicating that they were remagnetized long after deposition. Results from a chert site, including an intrastratal fold, also indicate that the magnetization is postdepositional. A significantly improved grouping (95% confidence level) of directions after correction for the structural attitudes of the Cedros and San Benito chert sections, however, implies that the remagnetization predates breaking up and incorporation of the chert into the subduction complex. Studies of modern subduction zones imply that the attitude of the ancient subducting slab, including the chert sections, was near horizontal (dip angles <10° at the time of remagnetization. Thus, assuming paleohorizontality, a paleolatitude of 12° ± 5° N can be estimated for the Western Baja terrane in mid-Cretaceous time. The paleomagnetic directions also indicate a subsequent poleward translation of 25° ± 7° in latitude (2,800 km) and a clockwise rotation of 56° ± 7° of the Western Baja terrane with respect to North America. The paleolatitude error limits at the 95% confidence level represent minimum values, because deviations of the bedding planes from horizontal during remagnetization would contribute to the error. A 10° NE dip of the subducting slab, however, would account for only 4° of the paleolatitude anomaly, and a ≃60° NE dip would be needed to account for the entire anomaly. Modern analogs, in combination with plate-reconstruction models, imply that the northward translation of the Western Baja terrane along the ancient convergent margin of North America could have been accomplished by arc-parallel strike-slip faulting that was driven by oblique subduction.

Baja California↗

USGS2012

Eruptive history of Mount Katmai, Alaska

Mount Katmai has long been recognized for its caldera collapse during the great pyroclastic eruption of 1912 (which vented 10 km away at Novarupta in the Valley of Ten Thousand Smokes), but little has previously been reported about the geology of the remote ice-clad stratovolcano itself. Over several seasons, we reconnoitered all parts of the edifice and sampled most of the lava flows exposed on its flanks and caldera rim. The precipitous inner walls of the 1912 caldera remain too unstable for systematic sampling; so we provide instead a photographic and interpretive record of the wall sequences exposed. In contrast to the several andesite-dacite stratovolcanoes nearby, products of Mount Katmai range from basalt to rhyolite. Before collapse in 1912, there were two overlapping cones with separate vent complexes and craters; their products are here divided into eight sequences of lava flows, agglutinates, and phreatomagmatic ejecta. Latest Pleistocene and Holocene eruptive units include rhyodacite and rhyolite lava flows along the south rim; a major 22.8-ka rhyolitic plinian fall and ignimbrite deposit; a dacite-andesite zoned scoria fall; a thick sheet of dacite agglutinate that filled a paleocrater and draped the west side of the edifice; unglaciated leveed dacite lava flows on the southeast slope; and the Horseshoe Island dacite dome that extruded on the caldera floor after collapse. Pre-collapse volume of the glaciated Katmai edifice was &sim;30 km 3 , and eruptive volume is estimated to have been 57±13 km 3 . The latter figure includes &sim;40±6 km 3 for the edifice, 5±2 km 3 for off-edifice dacite pyroclastic deposits, and 12±5 km 3 for the 22.8-ka rhyolitic pyroclastic deposits. To these can be added 13.5 km 3 of magma that erupted at Novarupta in 1912, all or much of which is inferred to have been withdrawn from beneath Mount Katmai. The oldest part of the edifice exposed is a basaltic cone, which gave a 40 Ar/ 39 Ar plateau age of 89 ± 25 ka. The seismic record of caldera collapse includes 14 earthquakes of magnitude 6.0–7.0. By combining the times of earthquakes, the hours of downwind plinian-fall episodes from Novarupta, and the stratigraphic record of hydrothermal explosion breccia and phreatic mud layers ejected around the caldera rim and intercalated within the Novarupta pumice-fall sequence, it can be inferred that collapse began in the 11th hour of the 60-h-long eruption and continued fitfully for 3.5 days. Several big landslides and pumiceous debris flows shaken loose by the collapse-related seismicity are bracketed in time by their levels of intercalation within the Novarupta pumice-fall sequence. An intracaldera lake was &sim;10 m deep by 1916, drained away in 1923, and has since deepened progressively to &sim;250 m today. Compositionally, products of Mount Katmai represent an ordinary medium-K arc array, both tholeiitic and calcalkaline, that extends from 51.6% to 72.3% SiO 2 . Values of 87 Sr/ 86 Sr range from 0.70335 to 0.70372, correlating loosely with fractionation indices. The 5–6 km 3 of continuously zoned andesite-dacite magma (58%–68% SiO 2 ) that erupted at Novarupta in 1912 was withdrawn from beneath Mount Katmai and bears close compositional affinity with products of that edifice, not with pre-1912 products of the adjacent Trident cluster. Evidence is presented that the 7–8 km 3 of high-silica rhyolite (77% SiO 2 ) released in 1912 is unlikely to have been stored under Novarupta or Trident. Pre-eruptive contiguity with the andesite-dacite reservoir is suggested by (1) eruption of &sim;3 km 3 of rhyolite magma first, followed by mutual mingling in fluctuating proportions; (2) thermal and redox continuity of the whole zoned sequence despite the wide compositional gap; (3) Nd, Sr, O isotopic, and rare earth element (REE) affinities of the whole array; (4) compositional continuity of the nearly aphyric rhyolite with the glass (melt) phase of the phenocryst-rich dacite; and (5) phase-equilibrium experiments that indicate similar shallow pre-eruptive storage depths (3–6 km) for rhyolite, dacite, and andesite.

Alaska↗

USGS2013

Intermediate‐depth intraslab earthquakes and arc volcanism as physical expressions of crustal and uppermost mantle metamorphism in subducting slabs

We elaborate on the well-known spatial association between axc volcanoes and Wadati Benioff zones and explore in detail their genetic relationships as dual physical expressions of slab metamorphism of the oceanic crust and uppermost mantle. At hypocentral depths less than 200 km intra slab Wadati-Benioff earthquakes tend to occur near the top surfaces of slabs. Subduction of very young lithosphere (age < 15-25 Ma) with high heat flow (> 75 mW/m 2) produces mainly shallow earthquakes and spaxse or absent arc volcanism. Subduction of older crust with normal heat flow (50-65 mW/m 2) produces markedly deeper intraslab earthquakes and generally normal volcanic vigor. Seismological observations show that the low-seismic-velocity gabbroic mineralogy of the crust may persist to depths of as much as 150 km in old, cold lithosphere but only to depths of 50-60 km in young, warm slabs. Metamorphic processes in the crust and shallow upper mantle of subducting slabs and the reactivation of faults originally created at shallow depths in the ocean basins probably control the occurrence of intraslab earthquakes to depths of as much as 350 kin. A conceptual model for this metamorphism incorporates the likely effects of water liberated by dehydration. Such dehydration facilitates both brittle faulting by fault reactivation and promotes the kinetics of the transformation of the anhydrous gabbro component of the crust to eclogite. Finite-element modelling shows that densification to eclogite is expected to produce extensional stresses in transformed crust and a smaller compression in the underlying mantle. This model helps explain why most intermediate-depth intraslab earthquakes occur just below the top surfaces of slabs and why many have focal mechanisms indicating down-dip extension. Young, warm slabs have mostly shallow intraslab earthquakes and sparse axc volcanoes because dehydration and eclogite formation largely cease before such slabs axe in contact with asthenosphere. These processes are evidently delayed by kinetic hindrance and the high pressure stability of hydrous phazes at low temperatures in older, colder subducting crust, and thus earthquake activity and asthenospheric-wedge melting tend to be focused at depths of 100-170 kin. Anomalous behavior correlated with the subduction of island-and seamount chains appears to be associated with anomalou shallow intraplate faulting and with perturbations of slab metamorphism by these chains.

Geophysical Monograph↗

USGS1991

Isotopic and chemical constraints on the petrogenesis of Blackburn Hills volcanic field, western Alaska

The Blackburn Hills volcanic field is one of several Late Cretaceous and early Tertiary (75–50 Ma) volcanic fields in western Alaska that comprise a vast magmatic province extending from the Arctic Circle to Bristol Bay. It consists of andesite flows, rhyolite domes, a central granodiorite to quartz monzonite pluton, and small intrusive rhyolite porphyries, overlain by basalt and alkali-rhyolites. Most of the field consists of andesite flows which can be divided into two groups on the basis of elemental and isotopic composition: a group having lower ( 87 Sr 86 Sr "> 87 Sr 86 Sr ) i , higher ( 143 Nd 144 Nd "> 143 Nd 144 Nd ) i , and moderate LREE and HREE contents (group 1), and a group having higher ( 87 Sr 86 Sr "> 87 Sr 86 Sr ) i , lower ( 143 Sr 144 Sr "> 143 Sr 144 Sr ) i , and lower HREE contents. Basalts are restricted to the top of the stratigraphic section, comprise the most primitive part of group 1 [( 87 Sr 86 Sr ) i = 0.7033; ( 143 Nd 144 Nd ) i = 0.5129] "> [( 87 Sr 86 Sr)i= 0.7033; ( 143 Nd 144 Nd)i= 0.5129] , and have trace-element ratios that are similar to those of oceanic island basalts (OIBs). In contrast to the basalts, group 1 andesites have higher ( 87 Sr 86 Sr "> 87 Sr 86 Sr ) i and lower ( 143 Nd 144 Nd "> 143 Nd 144 Nd ) i , and represent interaction of mantle-derived magmas with the lower crust of Koyukuk terrane. Group 2 andesites have ( 87 Sr 86 Sr "> 87 Sr 86 Sr ) i and ( 143 Nd 144 Nd "> 143 Nd 144 Nd ) i that are near bulk-earth values and probably formed by partial melting of the lower crust of Koyukuk terrane. The central pluton and rhyolite porphyries are isotopically uniform ( 87 Sr 86 Sr ) "> ( 87 Sr 86 Sr) i ≈ 0.704, ( 143 Nd 144 Nd "> 143 Nd 144 Nd ) i ≈ 0.51275, and are interpreted to have formed by melting of young mafic to intermediate crustal rocks or by fractionation of group 1 andesites. The rhyolite domes have an isotopic range similar to that of the basalts and andesites [( 87 Sr 86 Sr ) i = 0.70355-0.70499; ( 143 Nd 144 Nd ) i = 0.51263–0.51292] "> [( 87 Sr 86 Sr)i= 0.70355-0.70499; ( 143 Nd 144 Nd)i= 0.51263–0.51292] , which suggests they formed by fractionation of the and site and basalt magmas. Although some workers have suggested that the volcanic field is underlain by old continental crust, none of the data require the presence of Paleozoic or Precambrian continental middle or upper crust under this part of the volcanic field. However, the ultimate source of some of the rocks in the Yukon-Koyukuk province that have high 87 Sr 86 Sr "> 87 Sr 86 Sr and low 143 Nd 144 Nd "> 143 Nd 144 Nd ratios may be old sub-continental mantle and/or lower crust, which was previously subducted beneath the Yukon-Koyukuk province during Early Cretaceous arc-continent collision.

Geochimica et Cosmochimica Acta↗

USGS2013

Tectonic evolution and Cretaceous gold metallogenesis of southwestern Alaska

Cretaceous gold metallogenesis in southwestern Alaska comprises three distinct episodes related to the accretionary evolution of northwestern North America. The oldest mineralizing event is characterized by 112 Ma Cu-Au-Bi-Te porphyry-type(?) veining in the zoned Bonanza and adjacent plutons that intruded rocks of the Nyac terrane. Tectonic reconstructions and limited geological and geochemical data suggest that Cu-Au mineralization in the Nyac district may be related to terminal subduction during accretion of the Togiak-Koyukuk arc. The subsequent 100 to 89 Ma metallogenic event is a product of subduction-related magmatism immediately following accretion of the Peninsular-Alexander-Wrangellia superterrane and includes formation of the giant Pebble porphyry Cu-Au-Mo deposit. Pebble underwent a complex history of highly oxidized magmatism that is isotopically linked to enriched lithosphere or metasomatized mantle sources. Pebble and other porphyryrelated plutons were emplaced as a consequence of changes in plate motion and onset of dextral transpression along the continental margin to the southeast of their present-day locations. The final 75 to 65 Ma metallogenic event is regionally the most extensive. It followed a ~15-m.y.-long magmatic lull and is related to enigmatic subduction-related magmatism in the western part of the Alaska Range and the Kuskokwim basin. This event resulted in the formation of porphyry, reduced pluton-related, orogenic, and possible epithermal Au deposits. In the better-studied relatively low-relief areas of the Kuskokwim basin, many of the mineralized systems are spatially associated with ilmenite-series monzonite to quartz monzonite composite plutons that have isotopic signatures consistent with derivation from crustally contaminated mantle sources. These pluton-hosted Au deposits comprise low-sulfide stockworks, sheeted veins, and/or breccias in the cupolas of moderately differentiated intrusions that contain high Au; anomalous As, Bi, Sb, and/or Te; and have variable, but not uneconomic, Cu concentrations. In addition, Au, Sb, and/or Hg deposits hosted by competent NE-trending granite porphyry dike-sill complexes or along faults in flysch are widespread in the Kuskokwim basin. These deposits, including the giant Donlin Creek Au deposit, formed at shallow crustal levels and are classified here as epizonal orogenic Au deposits and related Hg and Sb lodes. Mesozonal orogenic Au deposits were formed along the margins of the uplifting Willow Creek pluton that is now exposed along the southern side of the Talkeetna Mountains batholith. Recent discoveries in the rugged Alaska Range comprise less well-studied porphyry Au-Cu (Whistler, Island Mountain), epithermal(?) Au (Terra), and reduced pluton-hosted Au deposits (Estelle). The cause of the broad latest Cretaceous magmatism associated with the final metallogenic event is enigmatic, but indicates widespread high heat flow possibly related to flat-slab subduction, lithospheric mantle delamination, or escape tectonics. Plutonism postdated regional folding and coincided with periods of movement along regional faults and formation of the NE-trending structural fabric. Magmatism resulted from initiation of transpressional faulting in more landward positions during oblique subduction of the Kula plate. Crustal contamination of mantle melts, either near their source or during ascent through the thick flysch of the Kuskokwim basin, produced low oxidation state magmas and controlled much of the metallogeny, particularly for those deposits that display similarities to both reduced porphyry Au-Cu deposits and, to a lesser degree, reduced intrusion-related Au systems. High heat flow also induced crustal melting and metamorphic devolatilization to form orogenic Au deposits. A transition to extension at ~60 Ma recorded elsewhere in Alaska temporally corresponds to termination of gold deposit formation in southwestern Alaska. The multiple periods of gold metallogenesis in southwestern Alaska offer a wide variety of targets for exploration within discrete parts of the region.

Society of Economic Geologists Special Publication↗

USGS2010

Geologic structure of the northern New Caledonia ridge, as inferred from magnetic and gravity anomalies

Bathymetric, gravity, and magnetic data collected in the southwest Pacific Ocean over the northern New Caledonia ridge show that the main geological units known from the island of New Caledonia extend northward from this island, beneath the Grand Lagon Nord, the Grand Passage, and the d'Entrecasteaux reefs. These data support the model of tectonic evolution of the New Caledonia region proposed by Kroenke [1984]. We interpret a linear axial gravity low that extends from southern New Caledonia to the Grand Passage as evidence for the continuity of the thick pre-Permian to Jurassic core of the island. The Belep magnetic pattern, which covers the southwestern half of the Grand Lagon Nord, shows linear, high relief (1000–2000 nT) anomalies that are locally associated with a 120-mGal gravity high. These magnetic and gravity anomalies resemble anomalies measured over the west coast of New Caledonia, suggesting that the Cretaceous to Eocene basaltic complex of the coastal area is overlain by ophiolite remnants as far north as the western d'Entrecasteaux reefs. The similarity between the Belep magnetic pattern and a highly magnetic province evident 200 km southwest across the New Caledonia basin, along the Fairway ridge, indicates that volcanic rocks lie symmetrically on both sides of the New Caledonia basin. We suggest that part of these volcanic rocks were emplaced during the middle Cretaceous when the New Caledonia ridge rifted from the Australian margin. Simple gravity models of an elongated gravity high, having peak values in the range of +130 to +150 mGal, suggest that the ophiolite, which was thrust over New Caledonia during the Upper Eocene, extends along the east side of the Grand Lagon Nord and as far north as the d'Entrecasteaux reefs. Gravity and magnetic models suggest that in the area of the d'Entrecasteaux reefs, the ophiolite belt includes two subunits. The first subunit underlies the Huon-La Surprise platform and may include highly serpentinized ultramafic rocks without basaltic oceanic crust. The second subunit, which lies beneath the Guilbert ridge, may include ultramafic rocks as well as thin imbricate slices of oceanic crust. At the northern termination of the New Caledonia ridge, seismic and geopotential data evidence a major east-west trending tectonic zone that separates the basement of the New Caledonia ridge from the d'Entrecasteaux zone, an arcuate oceanic feature extending northward from the ridge. Differences in structure, geophysical signatures and morphology evident between areas north and those south of the Grand Passage, together with the nearness of the Le Noroit massif west of the Grand Passage, suggest that contemporaneously with Eocene to early Oligocene subduction along the western New Caledonia margin, an arc-ridge collision may have occurred near the northern termination of this subduction zone.

Tectonics↗

USGS2022

A case of Te-rich low-sulfidation epithermal Au-Ag deposits in a calc-alkaline magmatic arc, NE China

Tellurium-bearing low-sulfidation epithermal Au-Ag deposits are significant producers of gold, silver, and potentially strategic elements if mineral processing methods are optimized for recovery. Although these deposits are generally related to alkaline magmatism, our study documents an unusual occurrence of Te-rich low-sulfidation epithermal systems in the North Heilongjiang Belt in northeast China that is spatially and temporally associated with calc-alkaline magmatism and tectonic extension in a continental arc‐setting. In the North Heilongjiang Belt, Te-bearing Au-Ag deposits are usually sited in dilatant zones, mostly along extensional NW structures or at their intersections with deep‐seated NE-striking faults such as the Nenjiang-Heihe and Jiayin-Mudanjiang lineaments. Of these deposits, Sandaowanzi is well‐known for its bonanza gold grades. These faults localized andesitic to rhyolitic volcano-plutonic centers that evolved from mantle‐derived mafic melts to intermediate compositions due to differentiation and crustal assimilation as documented by their mineralogy and Sr, Nd, and Pb isotope compositions. During assimilation of country rocks, fluids containing 3 He and other volatiles derived from mantle magmas displaced or mixed with external groundwater containing radiogenic 4 He derived from country rocks. In this belt, the bulk metal content of Au, Ag and Te was probably introduced by deep mantle‐derived mafic‐intermediate calc‐alkaline intrusions. However, our study does not exclude input of metals leached from underlying metasedimentary rocks and older Te‐bearing Au mineralization. The noble gas, hydrogen, oxygen and lead isotope compositions of fluid inclusions and ore minerals suggest that the deposits formed by mixing between magmatic fluids and convecting meteoric ground waters containing lead leached from surrounding country rocks. At deeper levels, isotopic evidence suggests that Au-Ag-Te (Bi) precipitated with pyrite, quartz, sericite, and carbonate minerals due to mixing with Te-rich fluids. At shallower levels, Au and Ag precipitated contemporaneously with base‐metal sulfides, hydrothermal quartz and sericite during episodic boiling as evidenced by silica morphology and mineral textures. The relatively uniform spacing of epithermal systems along NW- and NE-trending structures in the North Heilongjiang Belt was used to identify unexplored spaces within this mineral belt that are prospective for concealed low-sulfidation epithermal gold deposits. Although some of the Au-Ag deposits in this belt are enriched in Te, there is a lack of coeval alkaline igneous rocks that are commonly associated with Te-bearing gold deposits elsewhere in the world (e.g., Cripple Creek, Emperor, and Lihir Island).

North Heilongjiang Belt↗

USGS2020

Generation of calc-alkaline magmas during crystallization at high oxygen fugacity: An experimental and petrologic study of tephras from Buldir Volcano, western Aleutian Arc, Alaska, USA

Despite agreement that calc-alkaline volcanism occurs at subduction zones and is responsible for the genesis of continental landmasses, there is no consensus on the source of the Fe-depleted signature hallmark to calc-alkaline volcanism. In this study, we utilize mafic tephras collected from Buldir Volcano to address the genesis of strongly calc-alkaline volcanic rocks (those with a low Tholeiitic Index; ≤0·7) in a segment of the western Aleutian Arc to determine if the eruptions are plausibly part of a liquid line of descent, if they are mixtures of crustal melts and parental magmas, or if they are mixtures of melts of the mantle and the subducting slab. We conducted a series of H 2 O-saturated phase equilibrium experiments (1175–1000°C; 100 MPa) in a rapid-quench cold-seal (MHC) apparatus on the most primitive natural lava from Buldir (9·34 wt % MgO) at oxidizing conditions near the Re–ReO 2 buffer. We confirmed that all experiments equilibrated 0·3 ± 0·23 log units above the Re–ReO 2 buffer (ΔQFM ∼ +2·8) using X-ray Absorption Near Edge Structure (XANES) spectroscopy. Chromite is the liquidus phase, followed by olivine, then plagioclase, then clinopyroxene, and finally hornblende. Once clinopyroxene saturates, spinel composition shifts to magnetite. We compared our experimental results to the major element geochemistry and petrology of six tephras (51·9–54·8 wt % SiO 2 ) from Buldir collected during the 2015 field season of the GeoPRISMS shared platform. Tephras contain olivine + plagioclase + clinopyroxene + spinel ± hornblende; plagioclase comprises most of the crystalline volume, followed by either olivine or hornblende. Spinel is ubiquitous; with Cr-rich spinel inclusions in olivine and hornblende, and magnetite in the groundmass. Variations in phenocryst assemblages and compositions between samples can be attributed to differences in pre-eruptive temperatures, where hotter samples are devoid of hornblende, and contain Fo-rich olivine and plagioclase with lower An-contents, owing to the position of the mineral-in curves at fluid-saturated conditions. Experimental glasses match the depletion in FeO T observed in the tephra whole rock compositions. The continuous depletion in FeO T is attributable to saturation of spinel as a liquidus phase (initially as chromite) and continuous crystallization through the experimental series (changing to magnetite at colder temperatures). In contrast to the natural samples, the experiments show enrichment in TiO 2 with decreasing MgO, suggesting that differentiation did not occur at 100 MPa on Buldir. The TiO 2 depletion in volcanic rocks from Buldir can be accounted for if hornblende crystallization occurs close to the liquidus of a parental magma; a condition that is met at higher pressures and hydrous conditions. The emerging picture for Buldir Island is that (1) oxidizing conditions are required to drive the observed depletions in FeO T via crystallization of spinel, and (2) elevated H 2 O contents and high pressures are required to saturate hornblende close to the liquidus to reproduce the entire suite of major elements. Our study provides a mechanism to generate the calc-alkaline trends observed at Buldir without requiring mixing of slab and mantle melts. We conclude that calc-alkaline volcanic rocks with extremely low Tholeiitic Indices (0·7), like those from Buldir, cannot be generated in absence of high oxygen fugacity, even at high pressure and/or elevated water pressures.

Alaska↗

USGS1995

Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: A review

The Taupo Volcanic Zone (TVZ) in the central North Island is the main focus of young volcanism in New Zealand. Andesitic activity started at c. 2 Ma, joined by voluminous rhyolitic (plus minor basaltic and dacitic) activity from c. 1.6 Ma. The TVZ is c. 300 km long (200 km on land) and up to 60 km wide, as defined by vent positions and caldera structural boundaries. The total volume of TVZ volcanic deposits is uncertain because a sub-volcanic basement has not been identified, but present data suggest bulk volumes of 15–20,000 km 3 , and that faulted metasediments form most of the immediate subvolcanic basement. Rhyolite (≥15,000 km 3 bulk volume, typically 70–77% SiO 2 ) is the dominant magma erupted in the TVZ (mostly as calderaforming ignimbrite eruptions), andesite is an order of magnitude less abundant, and basalt and dacite are minor in volume (< 100 km 3 each). The history of the TVZ is here divided into ‘old TVZ’ from 2.0 Ma to 0.34 Ma, and ‘young TVZ’ from 0.34 Ma onwards, separated by the Whakamaru eruptions, which obscured much of the evidence for older activity within the zone. The TVZ shows a pronounced segmentation into northeastern and southwestern andesite-dominated extremities with composite cones and no calderas, and a central 125-km-long rhyolite-dominated segment. Eight rhyolitic caldera centres have so far been identified in the central segment, of which two (Mangakino and Kapenga) are composite features, and more centres will probably be delineated as further data accumulate. These centres account for 34 inferred caldera-forming ignimbrite eruptions, in the c. 1.6-Ma lifetime of the central TVZ. The modern central TVZ is the most frequently active and productive silicic volcanic system on Earth, erupting rhyolite at c. 0.28 m 3 s −1 , and available information suggests this has been so for at least the past 0.34 Ma. The rhyolites show no major compositional changes with time, though the extent of magma chamber zonation may have changed with the incoming of rifting and crustal extension in the past c. 0.9 Ma. Within the central TVZ, non-rhyolitic compositions have been erupted apparently irregularly in time and space; in particular there is no evidence for a geographic separation of basalts from andesites. Between 0.9 and 0.34 Ma, a major episode of uplift affected areas around the TVZ, while at the same time the main focus of activity may have migrated eastwards within the TVZ accompanying rifting along the axis of the zone. The modern TVZ is rifting at rates between 7 and 18 mm a −1 and restoration of the thin (15km) ‘crust’ ( V p ≤ 6.1 km s −1 ) beneath the central TVZ to its pre-rifting thickness (25 km) implies that rifting at such rates may have begun only at c. 0.9 Ma. The TVZ is a rifted arc, but its longitudinally segmented nature, high thermal flux and voluminous rhyolitic volcanism make it unique on Earth.

Taupo Volcanic Field↗

USGS1979

Resource report for proposed OCS lease sale no. 70, St. George Basin, shelf area, Alaska

St. George basin is a long (300 km), narrow (30-50 km) graben whose long axis strikes northwestward, parallel to the continental margin of the southern Bering Sea. Located near the Pribilof Islands, and beneath the virtually featureless Bering Sea shelf, the basin is filled with more than 10 km of sedimentary deposits. These sedimentary rocks are ruptured by normal faults associated with the sides of the graben; these ruptures commonly correlate with offsets in the basement surface. Offset along these faults increases with depth implying that they are growth-type structures. Basement rocks, that floor and flank St. George basin are part of an assemblage of Mesozoic eugeosynclinal rocks that extends from southern Alaska to eastern Siberia beneath the Bering Sea margin and outer shelf. A parallel belt of igneous rocks of late Mesozoic and earliest Tertiary age may also extend from western Alaska to northeastern Siberia beneath the inner Bering Sea shelf. The Bering Sea margin and adjacent shelf were apparently uplifted by the end of Mesozoic time, resulting in deep subaerial erosion• Following uplift, the outer Bering Sea shelf has undergone extensional rifting and regional subsidence. Differential subsidence has resulted in the formation of a series of basement ridges and basins whose axes parallel the Bering Sea margin. Some of these basins are very large to gigantic in size, e.g., St. George basin, and involve crustal subsidence exceeding 10 km. Such large scale crustal collapse suggests deep crustal or upper mantle processes, such as thermal metamorphism or stress-induced crustal migration. Nine wells drilled along the northern coast of the Alaska Peninsula, as well as several onshore Soviet wells in northeastern Siberia, relate directly to the submerged basins of the Bering Sea shelf. Although all of the wells on the Alaska Peninsula were abandoned as dry holes, shows of oil and gas were found. In addition, Soviet drilling resulted in the discovery of oil and gas shows in Oligocene and Miocene sandstone. Regional geologic and geophysical mapping suggests that there are suitable source beds, reservoir rocks, and traps within St. George basin. However, it is not known if hydrocarbons are present or if the possible reservoirs are of commercial size. A resource appraisal of St. George basin out to 200 meters water depth indicates that, at 5 percent probability, 6.4 billion barrels of oil and 18.6 trillion cubic feet of gas may be in the basin; at 95 percent probability 0.8 billion barrels of oil and 4.5 trillion cubic feet of gas may be in the basin. The statistical mean of the appraisal is 2.7 billion barrels of oil and 10.3 trillion cubic feet of gas. A large number of faults, evidence for recent movement along some of the faults, and high seismicity all indicate that faulting is a major environmental concern for the outer continental shelf region of the southern Bering Sea, especially in St. George basin. Most of the faults are potentially active and their movement is probably influenced by the local geology, including basement structures and sediment loading. Unstable sediment masses pose potential threats to resource development in the vicinity of the Pribilof Canyon. Volcanic activity along the Aleutian arc south of St. George basin may also pose an environmental hazard to petroleum development in the area. Another environmental hazard is the presence of shallow gas pockets, which could pose such problems during drilling as blowouts and liquefaction of bottom sediment.

Alaska↗

USGS1969

Tectonics of the March 27, 1964, Alaska earthquake

The March 27, 1964, earthquake was accomp anied by crustal deformation-including warping, horizontal distortion, and faulting-over probably more than 110,000 square miles of land and sea bottom in south-central Alaska. Regional uplift and subsidence occurred mainly in two nearly parallel elongate zones, together about 600 miles long and as much as 250 miles wide, that lie along the continental margin. From the earthquake epicenter in northern Prince William Sound, the deformation extends eastward 190 miles almost to long 142° and southwestward slightly more than 400 miles to about long 155°. It extends across the two zones from the chain of active volcanoes in the Aleutian Range and Wrangell Mountains probably to the Aleutian Trench axis. Uplift that averages 6 feet over broad areas occurred mainly along the coast of the Gulf of Alaska, on the adjacent Continental Shelf, and probably on the continental slope. This uplift attained a measured maximum on land of 38 feet in a northwest-trending narrow belt less than 10 miles wide that is exposed on Montague Island in southwestern Prince William Sound. Two earthquake faults exposed on Montague Island are subsidiary northwest-dipping reverse faults along which the northwest blocks were relatively displaced a maximum of 26 feet, and both blocks were upthrown relative to sea level. From Montague Island, the faults and related belt of maximum uplift may extend southwestward on the Continental Shelf to the vicinity of the Kodiak group of islands. To the north and northwest of the zone of uplift, subsidence forms a broad asymmetrical downwarp centered over the Kodiak-Kenai-Chugach Mountains that averages 2½ feet and attains a measured maximum of 7½ feet along the southwest coast of the Kenai Peninsula. Maximum indicated uplift in the Alaska and Aleutian Ranges to the north of the zone of subsidence was l½ feet. Retriangulation over roughly 25,000 square miles of the deformed region in and around Prince William Sound shows that vertical movements there were accompanied by horizontal distortion, involving systematic shifts of about 64 feet in a relative seaward direction. Comparable horizontal movements are presumed to have affected those parts of the major zones of uplift and subsidence for which retriangulation data are unavailable. Regional vertical deformation generated a train of destructive long-period seismic sea waves in the Gulf of Alaska as well as unique atmospheric and ionospheric disturbances that were recorded at points far distant from Alaska. Warping resulted in permanent tilt of larger lake basins and temporary reductions in discharge of some major rivers. Uplift and subsidence relative to sea level caused profound modifications in shoreline morphology with attendant catastrophic effects on the nearshore biota and costly damage to coasta1 installations. Systematic horizontal movements of the land relative to bodies of confined or semiconfined water may have caused unexplained short-period waves—some of which were highly destructive—observed during or immediately after the earthquake at certain coastal localities and in Kenai Lake. Porosity increases, probably related to horizontal displacements in the zone of subsidence, were reflected in lowered well-water levels and in losses of surface water. The primary fault, or zone of faults, along which the earthquake occurred is not exposed at the surface on land. Focal-mechanism studies, when considered in conjunction with the pattern of deformation and seismicity, suggest that it was a complex thrust fault (megathrust) dipping at a gentle angle beneath the continental margin from the vicinity of the Aleutian Trench. Movement on the megathrust was accompanied by subsidiary reverse faulting, and perhaps wrench faulting, within the upper plate. Aftershock distribution suggests movement on a segment of the megathrust, some 550–600 miles long and 110–180 miles wide, that underlies most of the major zone of uplift and the seaward part of the major zone of subsidence. According to the postulated model, the observed and inferred tectonic displacements that accompanied the earthquake resulted primarily from (1) relative seaward displacement and uplift of the seaward part of the block by movement along the dipping megathrust and subsidiary faults that break through the upper plate to the surface, and (2) simultaneous elastic horizontal extension and vertical attenuation (subsidence) of the crustal slab behind the upper plate. Slight uplift inland from the major zones of deformation presumably was related to elastic strain changes resulting from the overthrusting; however, the data are insufficient to permit conclusions regarding its cause. The belt of seismic activity and major zones of tectonic deformation associated with the 1964 earthquake, to a large extent, lie between and parallel to the Aleutian Volcanic Arc and the Aleutian Trench, and are probably genetically related to the arc. Geologic data indicate that the earthquake-related tectonic movements were but the most recent pulse in an episode of deformation that probably began in late Pleistocene time and has continued intermittently to the present. Evidence for progressive coastal submergence in the deformed region for several centuries preceding the earthquake, in combin1ation with transverse horizontal shortening indicated by the retriangulation data, suggests pre-earthquake strain directed at a gentle angle downward beneath the arc. The duration of strain accumulation in the epicentral region, as interpreted from the time interval during which the coastal submergence occurred, probably is 930–1,360 years.

Alaska↗

USGS1991

Magmatic evolution of the Gulf of California rift

Magmatic events in the Baja peninsula-Gulf of California region are closely related to sequential tectonic regimes of: (1) late Tertiary subduction beneath the continental margin until ~16 to 12.5 Ma; and (2) continental to oceanic rifting that began about 13 Ma. Orogenic calcalkaline volcanic rocks formed subparallel belts of rhyolite ignimbrite of Oligocene age (~34 to 27 Ma) east of the Gulf in the Sierra Madre Occidental and andesite of Miocene age (~24 to 11 Ma) along the eastern Baja peninsula. Shutoff of the Miocene andesitic arc broadly corresponds to migration of the Pacific-Farallon (Guadalupe)-North America triple junction along the Baja peninsula. Orogenic andesitic volcanism ended at ~16 Ma in northern Baja, and at ~11 Ma in southern Baja, within ~1 to 2 Ma of cessation of subduction. Waning orogenic magmatism persisted along the southern Baja peninsula during the initial stages of rifting. Paleogeographic implications of the distribution of circum-Gulf volcanic rocks and their inferred easterly source areas suggest that, from about 13 to 8 Ma, the rift consisted of a narrow seaway along the eastern side of the present Gulf. By about 6 Ma, the Gulf had broadened westward to approximately its present width. The period from 13 to 10 Ma was a time of tectonic transition and magmatic diversity. During this interval, medium-K calcalkaline, high-K calcalkaline, alkalic, and tholeiitic magmas erupted from the central to southern part of the Baja peninsula-Gulf region, and from about 14 to 8 Ma, rhyolite ignimbrite erupted in the northern Gulf region. Since 10 Ma, volcanism on the western Baja peninsula has been dominated by alkalic magmas, the Gulf margins by calcalkaline magmas, and the Gulf by tholeiitic magmas. Postsubduction calcalkaline andesite to rhyolite erupted sporadically and locally along the northern Gulf margins from ~9 Ma through Holocene time. Alkalic magmas associated with rifting in the Gulf of California are unlike alkalic magmas from intraplate settings. They comprise nepheline- to quartz-normative, basaltic to andesitic rocks characterized by diverse trace element ratios. They are broadly similar to intraplate alkalic rocks in having high K, P, Ba, Sr and the light REE (rare earth elements), but have some trace element characteristics typical of orogenic magmas (low Nb and Ta relative to K, Ba, Sr, and La) that distinguish these alkalic rocks from intraplate or cratonic rift alkalic rocks. Rb abundances (mostly <25 ppm, as low as 5 ppm) are unusually low in these alkalic rocks, both in contrast to the high K (~1.7-4.1 wt% K 2 O), Ba (~800-2200 ppm), and Sr (~1000-3700 ppm), and in comparison to Rb abundances in mafic lavas in general; many samples have Rb at levels typically found in low-K tholeiites. The high K/Rb ratios, mostly ~1000 to 5000, are among the highest reported for any lavas. Samples ranging in SiO 2 from 48 to 61% have Mg’ ≥ 65, indicating that a broad range of compositions may represent primary melts. The generally high Mg’, and the low Rb, Th, Nb, and Ta in some lavas, suggest that depleted refractory mantle equivalent to a mid-ocean ridge basalt (MORB) source contributed to these melts. The distinctive trace element enrichments are attributed to the stabilization in the source of amphibole and apatite from metasomatic fluids related to prior subduction beneath the peninsula. In discrimination diagrams, these alkalic rocks typically cluster outside previously defined fields for various magma types. The common tectonic setting among these and geochemically similar alkalic suites is the occurrence in a continental margin that is undergoing extension, located along a recently or currently active convergent plate boundary. Tholeiitic basalts have erupted since the earliest stages of continental rifting through development of oceanic spreading centers. Gulf of California tholeiites exhibit an evolution distinguished particularly by differences in rare-earth-element (REE) abundances. Early-rift magmas are similar in trace element geochemistry to some intraplate tholeiites from ocean islands and continental flood basalts, and have convex-upward REE patterns. The most recent tholeiites from the East Pacific Rise in the mouth of the Gulf are equivalent to N-MORB (normal mid-ocean ridge basalt) from other mid-ocean rifts, and are depleted in the light-REE. The transition in tholeiite geochemistry from intraplate tholeiite to MORB is attributed to progressive fusion of more refractory mantle components. Incipient-rift tholeiites are derived from selective fusion of clinopyroxenite veins, and with continued melting, N-MORB are formed by fusion of peridotite. This tholeiite sequence reflects the evolution of mantle source regions in rifts that sustain an ensialic to oceanic transition.

Baja California, Baja California Sur↗

USGS2020

Cascadia Margin cold seeps: Subduction zone fluids, gas hydrates, and chemosynthetic habitats

Priority Geographic Area: The outer continental shelf and upper continental slope from Canada/U.S. border offshore Washington State to the Mendocino Fracture Zone (Northern California), entirely within the U.S. Exclusive Economic Zone (EEZ), from the outermost shelf to at least 2000 m water depth (Figure 1). Description of Priority Area: Since 2015, over a thousand water column gas plumes originating at seafloor gas seeps have been discovered landward of the Cascadia deformation front (e.g., Embley et al., 2016; Johnson et al., 2015, 2019; Merle and Embley, 2016; NA-95 Cruise Report, 2018; Riedel et al., 2018), adding to those that had long been known on Hydrate Ridge (e.g., Heeschen et al., 2003; Tréhu et al., 2004). The recently-discovered seeps stretch from offshore Vancouver Island to the Mendocino Fracture Zone and from the outer shelf to ~2000 m water depth, occurring both landward and seaward of the nominal limit for gas hydrate stability zone on the upper continental slope (Figure 1). Hundreds of seeps likely remain undiscovered. Water column imaging is incomplete both within the target geographic area and farther seaward, between the 2000 m isobath and the deformation front, which is the subject of an imaging study described in a white paper by Watt et al. The recently-discovered Cascadia Margin cold seeps partially overlap an important active margin gas hydrate province (Spence et al., 2001; Tréhu et al., 2003, 2004), as well as an area where sediments on the North American plate are folded and faulted and affected by fluids generated in the subduction complex beneath the Cascadia forearc (e.g., Saffer and Tobin, 2011). Several Ocean Drilling Program expeditions have focused on hydrate systems offshore Vancouver and Oregon (e.g., Riedel et al., 2009; Tréhu et al., 2004) and on the connection between the shallow and deep hydrogeologic systems. Cabled observatories now continuously monitor physical, chemical, and venting processes on south Hydrate Ridge (OOI; e.g., Philip et al., 2016a) and offshore Vancouver Island (NEPTUNE; e.g. Römer et al., 2016). Outside of these well-studied gas hydrate areas, a subset of the recently-discovered Cascadia seeps, including some that we visited with R/V Falkor in 2019 (e.g., https://schmidtocean.org/cruise/methane-seeps-at-edge-of-hydrate-stability/), also likely emit methane associated with shallow subseafloor gas hydrate systems. Other seeps are delivering not only methane, but also deep-derived gases (Baumberger et al., 2018, 2020) to the seafloor. Many Cascadia Margin seeps have also been recognized at water depths too shallow (e.g., 175 m) to be connected to gas hydrate dynamics. These seeps are postulated to be emitting gas and fluids that originated deep in accretionary wedge before migrating up normal faults generated during forearc extension associated with large earthquakes (Johnson et al., 2019). Only a small fraction of the recently discovered U.S. Cascadia Margin water column gas plumes has so far been verified by ROVs (Hercules from E/V Nautilus in 2016 and 2018; SuBastian from R/V Falkor in 2018 and 2019) to correspond to seafloor seeps. Careful scientific mapping, investigation, and sampling at the seeps have also been limited (e.g., Baumberger et al., 2018, 2020; Merle and Embley, 2016; Seabrook et al., 2018; Greinert et al. 2019). This white paper focuses on expanding exploration of already-identified U.S. Cascadia Margin cold seeps through a multipronged and multidisciplinary discovery program that could be accomplished with a variety of NOAA assets. The goals of the proposed exploration activities are to develop high-resolution maps of seep fields from deep ocean vehicles; to verify (and sample) seafloor gas emissions at the locations of water column plumes for compositional and isotopic studies; to map, sample, and conduct analyses on chemosynthetic communities and deep-sea coral habitats near seep sites to document species distributions and habitats as a function of depth and latitude along the margin; to collect seep geologic samples that can constrain the timing of methane emissions through geochronology; and to record environmental data (e.g., CTD) near the seafloor and in the water column above the seeps. Seafloor mapping using shipboard systems (multibeam/backscatter) would be needed to characterize seafloor features near seep sites. Water column imaging (EK60/80 and/or multibeam WCD data) conducted before and after seafloor explorations would capture active methane plumes and constrain temporal variations in seep emissions (e.g., Kannberg et al., 2013; Philip et al., 2016a, 2016b), which are known to vary on time scales as rapid as tidal cycles on this margin (e.g., Römer et al., 2016). What are the characterization and data needs in this area? Check all that apply: __x_ Biology, Geology, Physical Oceanography, Chemistry ___ Marine Archaeology ___ Other Provide a list or brief description of the data needed within this area, from your perspective: 1. Water column backscatter to image active gas plumes 2. High-resolution multibeam bathymetry, seafloor backscatter, and shallow sub-bottom imaging 3. Visual characterization and ground truthing of potential seeps, including high-resolution mapping and photography from near-seafloor vehicles; collection of seep-associated species, corals, sediments, authigenic carbonates, gases, and seawater Describe relevance to national security, conservation, and/or the economy: The Cascadia margin seeps provide significant ecosystem services, including habitat for commercially important fishes and support for diversity along the continental margin. Methane seeps are also biological hotspots for krill, plankton, and crustaceans, which in turn sustain higher trophic levels (e.g., whales). Methane-derived authigenic carbonates serve as a hard substrate for deep-sea corals and sponges on millennial time scales. The studies proposed here will elucidate the relationship among seep environments, deep-sea corals, sponges, fisheries, and other organisms and provide new insight into subduction zone and hydrate-associated fluids in this important seismogenic zone. The studies address fishery management concerns and inform future conservation of sensitive species (e.g., deep-sea corals) and benthic habitats. From your perspective, what makes this area unique? The Cascadia Margin seeps are a critical component of the leaky margin that stretches from Baja California to the Aleutian Arc along the Pacific coastline of North America. Cold seeps have been intensely studied on the Gulf of Mexico and U.S. Atlantic passive margins with a focus on chemosynthetic communities, deep-sea corals, and leakage of microbially-generated and/or thermogenic hydrocarbons; however, the recently-discovered Cascadia Margin seeps, as well as active margin seep systems in general, remain more poorly characterized. Such seeps not only contribute to the ocean carbon cycle (e.g., Pohlman et al., 2011), thereby fueling the base of the food chain in these settings, but also emit subduction zone fluids that provide clues about processes within the seismogenic zone and the accretionary complex. The Cascadia seeps area allows both biological (e.g., benthic habitats, coral distributions) and physical processes (e.g., generation of subduction zone fluids) to be studied along both depth (perpendicular to the deformation front) and latitudinal gradients.

California, Oregon, Washington↗