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USGS1990

Tectonic history of the northern Nabitah fault zone, Arabian Shield, Kingdom of Saudi Arabia

Geologic mapping combined with structural, petrographic, and geochemical investigations were used to evaluate the motion and paleogeographic setting of the northern Nabitah fault zone. The orientation and asymmetry of small structures in three areas along the fault zone suggest that motion was primarily left-lateral strike slip. East of the fault zone, the Afif terrane is underlain by a north-trending granodioritic batholith that is interpreted to be the plutonic core of a volcanic arc (herein named the Nabitah arc) whose activity was coincident with Nabitah faulting. Available geochronologic data indicate that the Nabitah fault zone and arc were active at 710 Ma and that activity may have continued until about 670 Ma. Relicts of a prebatholithic island-arc ophiolite crop out as roof pendants and septa east of the fault zone. Analysis of petrologic, geochemical, and geochronologic data suggests that these rocks may have formed a terrane older than 800 Ma that was continuous from the Nabitah fault zone to the Nuqrah belt. These rocks may have been moved northward by Nabitah faulting from an original position opposite the Asir terrane. Scale-model comparisons with Recent and Phanerozoic volcanic arcs suggest that the Nabitah fault zone may have been a transform fault that developed as a result of sinistral oblique subduction east of the Al Amar fault. According to this model, the Abt schist would have formed in an accretionary prism and the Murdama group would have been deposited in a fore-arc basin. Analogs for thrust faults in the northern Arabian Shield are found in modern arcs developed behind oblique subduction zones. The orientations of the Najd faults may reflect initial formation as normal faults in the fore-arc region of such an arc system. Based on the presence of similar lithologies, similar structure, and analogous tectonic setting, the Mother Lode District in California is reviewed as a model for gold occurrences near the Nabitah fault zone in this report.

Open-File Report↗

USGS2009

Implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary (non-collisional) and collisional (suturing) orogens

Arc magmatism at subduction zones (SZs) most voluminously supplies juvenile igneous material to build rafts of continental and intra-oceanic or island arc (CIA) crust. Return or recycling of accumulated CIA material to the mantle is also most vigorous at SZs. Recycling is effected by the processes of sediment subduction, subduction erosion, and detachment and sinking of deeply underthrust sectors of CIA crust. Long-term (>10-20 Ma) rates of additions and losses can be estimated from observational data gathered where oceanic crust underruns modern, long-running (Cenozoic to mid-Mesozoic) ocean-margin subduction zones (OMSZs, e.g. Aleutian and South America SZs). Long-term rates can also be observationally assessed at Mesozoic and older crust-suturing subduction zone (CSSZs) where thick bodies of CIA crust collided in tectonic contact (e.g. Wopmay and Appalachian orogens, India and SE Asia). At modern OMSZs arc magmatic additions at intra-oceanic arcs and at continental margins are globally estimated at c. 1.5 AU and c. 1.0 AU, respectively (1 AU, or Armstrong Unit,= 1 km 3 a -1 of solid material). During collisional suturing at fossil CSSZs, global arc magmatic addition is estimated at 0.2 AU. This assessment presumes that in the past the global length of crustal collision zones averaged c. 6000 km, which is one-half that under way since the early Tertiary. The average long-term rate of arc magmatic additions extracted from modern OMSZs and older CSSZs is thus evaluated at 2.7 AU. Crustal recycling at Mesozoic and younger OMSZs is assessed at c. 60 km 3 Ma -1 km -1 (c. 60% by subduction erosion). The corresponding global recycling rate is c. 2.5 AU. At CSSZs of Mesozoic, Palaeozoic and Proterozoic age, the combined upper and lower plate losses of CIA crust via subduction erosion, sediment subduction, and lower plate crustal detachment and sinking are assessed far less securely at c. 115 km 3 Ma -1 km -1 . At a global length of 6000 km, recycling at CSSZs is accordingly c. 0.7 AU. The collective loss of CIA crust estimated for modern OMSZs and for older CSSZs is thus estimated at c. 3.2 AU. SZ additions (2.7 AU) and subtractions (23.2 AU) are similar. Because many uncertainties and assumptions are involved in assessing and applying them to the deep past, the net growth of CIA crust during at least Phanerozoic time is viewed as effectively nil. With increasing uncertainty, the long-term balance can be applied to the Proterozoic, but not before the initiation of the present style of subduction at c. 3 Ga. Allowing that since this time a rounded-down rate of recycling of 3 AU is applicable, a startlingly high volume of CIA crust equal to that existing now has been recycled to the mantle. Although the recycled volume (c. 9 ?? 10 9 km 3 ) is small (c. 1%) compared with that of the mantle, it is large enough to impart to the mantle the signature of recycled CIA crust. Because subduction zones are not spatially fixed, and their average global lengths have episodically been less or greater than at present, recycling must have contributed significantly to creating recognized heterogeneities in mantle geochemistry. ?? The Geological Society of London 2009.

Geological Society Special Publication↗

USGS2009

Morphotectonics of the central Muertos thrust belt and Muertos Trough (northeastern Caribbean)

Multibeam bathymetry data acquired during the 2005 Spanish R/V Hesp??rides cruise and reprocessed multichannel seismic profiles provide the basis for the analysis of the morphology and deformation in the central Muertos Trough and Muertos thrust belt. The Muertos Trough is an elongated basin developed where the Venezuelan Basin crust is thrusted under the Muertos fold-and-thrust belt. Structural variations along the Muertos Trough are suggested to be a consequence of the overburden of the asymmetrical thrust belt and by the variable nature of the Venezuelan Basin crust along the margin. The insular slope can be divided into three east-west trending slope provinces with high lateral variability which correspond to different accretion stages: 1) The lower slope is composed of an active sequence of imbricate thrust slices and closed fold axes, which form short and narrow accretionary ridges and elongated slope basins; 2) The middle slope shows a less active imbricate structure resulting in lower superficial deformation and bigger slope basins; 3) The upper slope comprises the talus region and extended terraces burying an island arc basement and an inactive imbricate structure. The talus region is characterized by a dense drainage network that transports turbidite flows from the islands and their surrounding carbonate platform areas to the slope basins and sometimes to the trough. In the survey area the accommodation of the ongoing east-west differential motion between the Hispaniola and the Puerto Rico-Virgin Islands blocks takes place by means of diffuse deformation. The asymmetrical development of the thrust belt is not related to the geological conditions in the foreland, but rather may be caused by variations in the geometry and movement of the backstop. The map-view curves of the thrust belt and the symmetry of the recesses suggest a main north-south convergence along the Muertos margin. The western end of the Investigator Fault Zone comprises a broad band of active normal faults which result in high instability of the upper insular slope. ?? 2009 Elsevier B.V.

Marine Geology↗

USGS2012

The crustal structure of the Wrangellia Terrane along the East Glenn Highway, eastern‐southern Alaska

Recently acquired seismic refraction data from eastern‐southern Alaska provide new information on the structure and composition of the Wrangellia and adjacent terranes. The data comprise a 160‐km‐long refraction profile along the East Glenn (Tok‐Cutoff) Highway that was collected as part of the U.S. Geological Survey's multidisciplinary Trans‐Alaska Crustal Transect program. The upper 3 km of the Wrangellia terrane and associated rocks is characterized by low compressional wave velocities (V p = 1.9, 3.3, 4.6, 5.6 km s −1 ) and high‐velocity gradients common to most onshore seismic refraction profiles. There is also clear seismic expression of the West Fork fault system as a steep, down‐to‐the‐southwest fault that separates the Peninsular terrane to the southwest and the metamorphic complex of Gulkana River to the northeast. In contrast, no seismic expression occurs for the Paxson Lake fault, which separates the Wrangellia terrane from the metamorphic complex of Gulkana River. Adjacent to the Denali fault, within the Wrangellia terrane, two high‐velocity bodies (V p = 6.6 km s −1 ) occur in the upper crust. One of these extends to ∼10‐km depth and correlates with a late Paleozoic dioritic complex, suggesting that the Wrangellia terrane is at least 10 km thick in this part of Alaska. From 5 to 23 km depth, the crust appears seismically homogeneous, with velocity increasing from V p = 6.2 to V p = 6.6 km s −1 . Beneath this level, the crust is less well resolved, although evidence exists for a low‐velocity zone between 23 and 26 km and a possible southwest dipping interface at 35 km. No identifiable mantle refraction or reflection is observed, possibly indicating a crust as thick as 55 km. The relatively low seismic velocities in the upper 23 km of the crust compare favorably with laboratory‐measured velocities on pelitic schists and intermediate‐composition plutonic rocks (granites and granodiorites), both of which are recognized in Wrangellia. We interpret the seismic velocities to indicate that silicic‐to‐intermediate‐composition rocks are important constituents of the basement of this part of Wrangellia. Geologic evidence indicates that the Alaskan part of the Wrangellia terrane is a Paleozoic and Mesozoic island arc: our seismic evidence indicates it may have been built mostly on continental crust as opposed to the fragment of Wrangellia from Vancouver Island which was probably built on oceanic crust.

Alaska↗

USGS2016

Some contrasting biostratigraphic links between the Baker and Olds Ferry Terranes, eastern Oregon

New stratigraphic and paleontologic data indicate that ophiolitic melange windows in the Olds Ferry terrane of eastern Oregon contain limestone blocks and chert that are somewhat different in age than those present in the adjacent Baker terrane melange. The melange windows in the Olds Ferry terrane occur as inliers in the flyschoid Early and Middle Jurassic age Weatherby Formation, which depositionally overlies the contact between the melange-rich Devonian to Upper Triassic rocks of the Baker terrane on the north, and Upper Triassic and Early Jurassic volcanic arc rocks of the Huntington Formation on the south. The Baker terrane and Huntington Formation represent fragments of a subduction complex and related volcanic island arc, whereas the Weatherby Formation consists of forearc basin sedimentary deposits. The tectonic blocks in the melange windows of the Weatherby Formation (in the Olds Ferry terrane) are dated by scarce biostratigraphic evidence as Upper Pennsylvanian to Lower Permian and Upper Triassic. In contrast, tectonic blocks of limestone in theBaker terrane yield mostly fusulinids and small foraminifers of Middle Pennsylvanian Moscovian age at one locality.Middle Permian (Guadalupian) Tethyan fusulinids and smaller foraminifers (neoschwagerinids and other Middle Permian genera) are present at a few other localities. Late Triassic conodonts and bryozoans are also present in a few of the Baker terrane tectonic blocks. These limestone blocks are generally embedded in Permian and Triassic radiolarian bearing chert or argillite. Based on conodont, radiolarian and fusulinid data, the age limits of the meange blocks in the Weatherby Formation range from Pennsylvanian to Late Triassic.

Oregon↗

USGS2010

Seismicity of the Earth 1900-2007, Japan and vicinity

This map shows details of Japan and vicinity not visible in an earlier publication, U.S. Geological Survey Scientific Investigations Map 3064. Japan and its island possessions lie across four major tectonic plates: Pacific plate, North America plate; Eurasia plate; and Philippine Sea plate. The Pacific plate is subducted into the mantle, beneath Hokkaido and northern Honshu, along the eastern margin of the Okhotsk microplate, a proposed subdivision of the North America plate (Bird, 2003). Farther south, the pacific plate is subducted beneath volcanic islands along the eastern margin of the Philippine Sea plate. This 2,200 km-long zone of subduction of the Pacific plate is responsible for the creation of the deep offshore Ogasawara and Japan trenches as well as parallel chains of islands and volcanoes, typical of the Circumpacific island arcs. Similarly, the Philippine Sea plate is itself subducting under the Eurasia plate along a zone, extending from Taiwan to southern Honshu, that comprises the Ryuku Islands and the Nansei-Shonto trench.

Open-File Report↗

USGS2022

Mature diffuse tectonic block boundary revealed by the 2020 southwestern Puerto Rico seismic sequence

Distributed faulting typically tends to coalesce into one or a few faults with repeated deformation. The progression of clustered medium-sized (≥Mw4.5) earthquakes during the 2020 seismic sequence in southwestern Puerto Rico (SWPR), modeling shoreline subsidence from InSAR, and sub-seafloor mapping by high-resolution seismic reflection profiles, suggest that the 2020 SWPR seismic sequence was distributed across several short intersecting strike-slip and normal faults beneath the insular shelf and upper slope of Guayanilla submarine canyon. Multibeam bathymetry map of the seafloor shows significant erosion and retreat of the shelf edge in the area of seismic activity as well as slope-parallel lineaments and submarine canyon meanders that typically develop over geological time. The T -axis of the moderate earthquakes further matches the extension direction previously measured on post early Pliocene (∼>3 Ma) faults. We conclude that although similar deformation has likely taken place in this area during recent geologic time, it does not appear to have coalesced during this time. The deformation may represent the southernmost part of a diffuse boundary, the Western Puerto Rico Deformation Boundary, which accommodates differential movement between the Puerto Rico and Hispaniola arc blocks. This differential movement is possibly driven by the differential seismic coupling along the Puerto Rico—Hispaniola subduction zone. We propose that the compositional heterogeneity across the island arc retards the process of focusing the deformation into a single fault. Given the evidence presented here, we should not expect a single large event in this area but similar diffuse sequences in the future.

Puerto Rico↗

USGS1985

Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.

Alaska↗

USGS1993

Large-scale deformation associated with ridge subduction

Continuum models are used to investigate the large-scale deformation associated with the subduction of aseismic ridges. Formulated in the horizontal plane using thin viscous sheet theory, these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction. Modelling was used to compare the Tonga arc and Louisville ridge collision with the New Hebrides arc and d'Entrecasteaux ridge collision, which have disparate arc-ridge intersection speeds but otherwise similar characteristics. Models of both systems indicate that diffuse deformation (low values of the effective stress-strain exponent n ) are required to explain the observed deformation. Deformation is somewhat insensitive to the vertically integrated strength of the arc (inversely proportional to the Argand number Ar ), but indicates that the arc lithosphere is not extremely weak ( Ar < 100). Low values of both Ar and n suggest that the thermal structure is typical of ‘cold’ or ‘normal’ arcs and that deformation is dominated by flow in the lower crust and mantle. In addition, low values of n (approaching Newtonian flow) may indicate that specific deformation mechanisms dictate deformation of the arc lithosphere. Possible mechanisms include low-stress, grain-size dependent creep, pyroxenite-controlled rheology and mechanisms associated with water weakening. Changes in the boundary conditions greatly affect deformation within island arcs. High rates of arc-ridge intersection speed (Tonga-Louisville system) yield arc-parallel tension and crustal thickening in the wake of ridge subduction. In contrast, low rates of arc-ridge intersection speed (New Hebrides-d'Entrecasteaux system) yield compressional deformation directly arcward of the collision zone and transverse strike-slip faulting adjacent to the region of compressional deformation. Localized regions of extensional deformation along the frontal part of the arc adjacent to the collision zone may contribute to the formation of re-entrants.

Geophysical Journal International↗

USGS1983

Crustal structure of southwestern Saudi Arabia

The southwestern Arabian Shield is composed of uplifted Proterozoic metamorphic and plutonic rocks. The Shield is bordered on the southwest by Cenozoic sedimentary and igneous rocks of the Red Sea paar and on the east by the Arabian Platform, an area of basin sedimentation throughout Phanerozoic time. The Shield appears to have been formed by successive episodes of island arc volcanism and sea-floor spreading, followed by several cycles of compressive tectonism and metamorphism. An interpretation and synthesis of a deep-refraction seismic profile from the Riyadh area to the Farasan Islands, and regional gravity, aeromagnetic, heat flow, and surface geologic data have yielded a self-consistent regional-scale model of the crust and upper mantle for this area. The model consists of two 20 km-thick layers of crust with an average compressional wave velocity in the upper crust of about 6.3 km/s and an average velocity in the lower. crust of about 7.0 km/s. This crust thins abruptly to less than 20 km near the southwestern end of the profile where Precambrian outcrops abut the Cenozoic rocks and to 8 km beneath the Farasan Islands. The data over the coastal plain and Red Sea shelf areas are fit satisfactorily by an oceanic crustal model. A major lateral velocity inhomogeneity in the crust is inferred about 25 km northeast of Sabhah and is supported by surface geologic evidence. The major velocity discontinuities occur at about the same depth across the entire Shield and are interpreted to indicate horizontal metamorphic stratification of the Precambrian crust. Several lateral inhomogenities in both the upper and lower .crust of the . Shield are interpreted, to indicate bulk compositional variations. The subcrustal portion of the model is composed of a hot, low-density lithosphere beneath the Red Sea which is systematically cooler and denser to the northeast. This model provides a mechanism which explains the observed topographic uplift, regional gravity pattern, heat flow, and mantle compressional wave velocities. Such a lithosphere could be produced by upwelling of hot asthenosphere beneath the Red Sea which then flows laterally beneath the lithosphere of the Arabian Plate.

Open-File Report↗

USGS2013

Geochemistry, petrography, and zircon U-Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon-Tanana terrane

We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

Alaska, British Columbia, Yukon↗

USGS1991

The Briggs Creek Amphibolite, Klamath Mountains, Oregon: Its origin and dispersal

The Briggs Creek Amphibolite (BCA) in southwestern Oregon is an allochthonous slice of metamorphosed oceanic crust emplaced during the Nevadan orogenic event. Even though the evidence is not clear, the authors prefer to have the protolith for the BCA formed in a back‐arc basin contemporaneously as part of the Josephine Ophiolite during late Callovian to early Oxfordian. Although major rearrangement of some of the chemical elements took place during metamorphism, it is possible, by use of major and rare earth elements, to suggest that the BCA formed in an oceanic spreading centre rather than an island arc setting. Meta‐morphic mineral assemblages and composition of the hornblendes indicate that metamorphism attained amphibolite facies grade. Widespread occurrence of allochthonous blocks of BCA in Western Oregon indicate that its dispersion following metamorphism can be related to the accretionary history of the continental margin. Westward thrusting of these blocks of BCA over the Chetco Arc during the Nevadan orogenic event provided blocks that slid into the outboard active Early Cretaceous trench. These blocks were then incorporated into the downgoing subduction complex and overprinted by high P/T metamorphic assemblages. Tectonic blocks within the Early Cretaceous Otter Point trench melange (Franciscan Complex equivalent) contain amphibolites correlated with the BCA and overprinted by high P/T mineral assemblages.

Oregon↗

USGS2010

Further considerations of the Ce/Yb vs. Ba/Ce plot in volcanology and tectonics

A plot of Ce/Yb vs. Ba/Ce, for locality averages, effectively separates mid-ocean ridge basalts (MORB) (Ce/Yb <10, Ba/Ce 1-4.2), oceanic island volcanics (OIV) (Ce/Yb >10, Ba/Ce <6), which are generally hotspot related, and island arc volcanics (IAV) (Ce/Yb <23, Ba/Ce >4.2). The conventional interpretation is that these three types of volcanic environments involve oceanic rift-related, large-volume partial melts (˜20-30%) of a depleted source (MORB), small volume melts (˜5% for alkalic volcanics) of enriched sources related to plumes (OIV), and melts of hydrous-enriched sources during subduction, especially for Ba (IAV). Three OIV sites, however, have average ratios that fall in the MORB field (e.g., Krafla Volcano, Iceland), and these localities also tend to have other geochemical data similar to MORB. Average ratios of Hawaiian tholeiitic shield basalts of Mauna Kea and Koolau volcanoes occupy a restricted field on a plot of Ce/Yb vs. Ba/Ce of 10-18 for Ce/Yb and 2.8-3.1 for Ba/Ce, a field toward which other shield basalts and cone-building volcanics regress. In general, post-shield alkalic rocks have higher values of Ce/Yb than do tholeiites. Peralkalic basalts (basanites, melilitites, and phonolites) have even higher values of Ce/Yb, reflecting smaller degrees of partial melting (perhaps 1-2%) and melting of sources containing phlogopite that were enriched by CO 2 -dominated fluids. The minor post-erosion nephelinitic suites of Hawaii (e.g., the Honolulu Series on Oahu, and the Koloa suite on Kauai) generally have values both greater than IAV for Ce/Yb and greater than other kinds of OIV for Ba/Ce in a part of the plot previously not found to be occupied by data. Alkali basalts of both these nephelinitic series have the lowest and similar ratios (Ce/Yb ˜ 25; Ba/Ce ˜ 10). In the Hawaiian Islands, there are two trends. One (a), where phlogopite has been interpreted to remain in the source, generally has Ba/Ce decrease away from the alkali basalts as Ce/Yb increases. The other (b), where phlogopite has been interpreted to enter the melt, occupies a field that is high in both Ce/Yb (>30) relative to IAV and in Ba/Ce (>8) relative to the OIV field. There are some exceptions, also, for IAV that plot outside the IAV field. The values of Ce/Yb in Mariana Islands samples, for example, are exceptionally low for the IAV (Ce/Yb <5 with many samples <2). Examples of two cross-chain Kasuga Islands, however, have average values of Ce/Yb considerably greater than for any other Mariana Islands data, and individual samples extend from within the IAV field into the OIV field, which may indicate a mixture of IAV and OIV sources (rather than involvement of a hotspot, these island volcanics have been interpreted as magma of OIV entrapped "plums" in an IAV "pudding" by Stern et al., 1993). Not surprisingly, continental arc volcanics (CAV) are generally similar to IAV, but with somewhat greater dispersion in Ce/Yb, perhaps representing a larger contribution of continental materials to the volcanics. Continental rift volcanics (CRV) are complex. The Antarctic rift data fall in the OIV field, and clearly define a hotspot origin for the rift with little contamination in the continental lithosphere, but most CRV data fall in the IAV field (Rio Grande rift tholeiites, Yellowstone Plateau basalts, Columbia River basalts, East African rift basalts). The Yellowstone basalt samples judged to be least crustally contaminated from other considerations (e.g., through Pb and Sr isotopes) approach closest to the OIV or hotspot field in the Ce/Yb vs. Ba/Ce plot, compatible with a hotspot origin with variable continental lithosphere interactions. The data from the Rio Grande rift have no such trend in Ce/Yb vs. Ba/Ce. Other trace element and isotopic data are suggestive of a different kind of origin, perhaps melting in the continental lithosphere from pressure release or other causes as suggested in the literature. Carbonatites, kimberlites, and ultrapotassic rocks form extreme end members for the peralkalic rocks on the continents with Ce/Yb values in the hundreds and even exceeding 1,000 in natrocarbonatite. Carbonatites and kimberlite type I, however, have Ba/Ce <8 with few exceptions. Ultrapo tassic rocks and kimberlite type II also have Ce/Yb values in the hundreds but with Ba/Ce >9. These rocks, although rare in the ocean basins (e.g., carbonatite on São Vicente Island in the Cape Verdes archipelago, Indian Ocean) plot similarly to their continental cousins. For Hawaii, the nephelinitic suites of both the Honolulu and Koloa series trend from alkali and alkali olivine basalt ratios toward higher signatures for Ce/Yb for other rock types. The Honolulu series, however, progresses towards smaller values of Ba/Ce for nephelinite-melilitite (Ce/Yb ˜ 85; Ba/Ce ˜ 5-7) near the low end of Ce/Yb found in carbonatite/kimberlite type I, whereas the Koloa series progresses toward higher Ba/Ce (Ce/Yb ˜ 65; Ba/Ce ˜ 14-15) for nephelinite-melilitite with Ce/Yb values near the lower end of kimberlite type II/ultrapotassic rocks. Carbonated phlogopitic sources have been proposed for peralkalic rocks of both oceans and continents. Carbonatites and/or kimberlites are suggested to possibly be present at depth under the Hawaiian nephelinitic series and in other OIV environments containing peralkalic suites.

International Geology Review↗

USGS1988

Descriptive and grade-tonnage models of volcanogenic manganese deposits in oceanic environments; a modification

Four types of volcanogenic manganese deposits, distinguished on the basis of geologic, geochemical, and geophysical characteristics, appear to result from a combination of volcanic and hydrothermal processes related to hot-spring activity in oceanic environments. We compare these four desposit types, here called the Franciscan, Cuban, Olympic Peninsula, and Cyprus, with respect to host rocks, associated rocks, minerals, deposit shape, dimensions, volume, tonnage, grade, and mineral-deposit density (number of deposits per unit area). Franciscan-type deposits occur in obducted oceanic ridge and backarc marginal-basin environments, are associated with chert, shale, and graywacke aroun the margins of mafic volcanic centers, and have a median tonnage of 450 t and median grades of 36 weight percent Mn and less than 5.1 weight percent Fe. Cuban-type deposits occur in island-arc environments, are associated with tuff and limestone around domal structures or intrusions inferred to be volcanic centers, and have a median tonnage of 6,400 t and median grades of 39 weight percent Mn and less than 4.4 weight percent Fe. Olympic Peninsula-type deposits occur in obducted oceanic midplate settings, are associated with argillaceous limestone, argillite, and graywacke around mafic volcanic centers (seamounts or islands), and have a median tonnage of 340 t and median grades of 35 weight percent Mn and less than 6.5 weight percent Fe. Cyprus-type deposits occur in the same tectonic environments as Franciscan type but are associated with basalt, marl, chalk, silt, and chert off the ridge-axis position and have a median tonnage of 41,000 t and median grades of 33 weight percent Fe and 8 weight percent Mn. All these deposits are thin ellipsoids, concordant to the host rocks, but Cyprus-and Cuban-type deposits are larger than Franciscan- and Olympic Peninsula-type deposits. Except for Cyprus-type deposits, which are manganiferous iron (umber) deposits composed of hydrated iron and manganese oxides, all volcanogenic manganese deposits contain manganese oxides, silicates, and carbonates. Mineral-deposit densities, along with grade and tonnage information, are useful for estimating the number, size, andgrades of these deposits in resource assessments.

Bulletin↗

USGS2012

Seismic hazard assessment for Guam and the Northern Mariana Islands

We present the results of a new probabilistic seismic hazard assessment for Guam and the Northern Mariana Islands. The Mariana island arc has formed in response to northwestward subduction of the Pacific plate beneath the Philippine Sea plate, and this process controls seismic activity in the region. Historical seismicity, the Mariana megathrust, and two crustal faults on Guam were modeled as seismic sources, and ground motions were estimated by using published relations for a firm-rock site condition. Maps of peak ground acceleration, 0.2-second spectral acceleration for 5 percent critical damping, and 1.0-second spectral acceleration for 5 percent critical damping were computed for exceedance probabilities of 2 percent and 10 percent in 50 years. For 2 percent probability of exceedance in 50 years, probabilistic peak ground acceleration is 0.94 gravitational acceleration at Guam and 0.57 gravitational acceleration at Saipan, 0.2-second spectral acceleration is 2.86 gravitational acceleration at Guam and 1.75 gravitational acceleration at Saipan, and 1.0-second spectral acceleration is 0.61 gravitational acceleration at Guam and 0.37 gravitational acceleration at Saipan. For 10 percent probability of exceedance in 50 years, probabilistic peak ground acceleration is 0.49 gravitational acceleration at Guam and 0.29 gravitational acceleration at Saipan, 0.2-second spectral acceleration is 1.43 gravitational acceleration at Guam and 0.83 gravitational acceleration at Saipan, and 1.0-second spectral acceleration is 0.30 gravitational acceleration at Guam and 0.18 gravitational acceleration at Saipan. The dominant hazard source at the islands is upper Benioff-zone seismicity (depth 40–160 kilometers). The large probabilistic ground motions reflect the strong concentrations of this activity below the arc, especially near Guam.

Open-File Report↗

USGS1999

Chapter 2: Geological and tectonic setting of Santorini

Santorini lies in an area of complex extensional and subduction-related tectonics in a continental environment (Jackson 1994). The region has had a long geological history with large changes occurring during the Alpine orogeny and in the Late Tertiary as a consequence of post-collisional extension and major re-adjustments of the plate boundaries. The Aegean region is thought to be moving towards the southwest where the Aegean microplate overrides the Eastern Mediterranean sea floor. The collision has created the Hellenic trench to the south of Crete where the eastern Mediterranean sea floor is subducting beneath the Aegean Sea at 5-6 cm a-1. Santorini is one of several Quaternary volcanoes that define the present day active volcanic arc related to this subduction zone (Fig. 2.1). We here provide a synopsis of the broader geological and tectonic context of Santorini within the Aegean area. The geology, isotope geochemistry and geochronology of the crystalline basement of the southern Aegean is of particular relevance as it is likely that the Santorini magmas have interacted extensively with the continental crust. Geochemical evidence presented in Chapters 6 and 7 confirms this prediction. We also give a synopsis of the Quaternary Aegean Island Arc, of which Santorini is the most active centre.

Santorini↗

USGS2009

The 2006-2007 Kuril Islands great earthquake sequence

[1] The southwestern half of a ∼500 km long seismic gap in the central Kuril Island arc subduction zone experienced two great earthquakes with extensive preshock and aftershock sequences in late 2006 to early 2007. The nature of seismic coupling in the gap had been uncertain due to the limited historical record of prior large events and the presence of distinctive upper plate, trench and outer rise structures relative to adjacent regions along the arc that have experienced repeated great interplate earthquakes in the last few centuries. The intraplate region seaward of the seismic gap had several shallow compressional events during the preceding decades (notably an M S 7.2 event on 16 March 1963), leading to speculation that the interplate fault was seismically coupled. This issue was partly resolved by failure of the shallow portion of the interplate megathrust in an M W = 8.3 thrust event on 15 November 2006. This event ruptured ∼250 km along the seismic gap, just northeast of the great 1963 Kuril Island (M w = 8.5) earthquake rupture zone. Within minutes of the thrust event, intense earthquake activity commenced beneath the outer wall of the trench seaward of the interplate rupture, with the larger events having normal-faulting mechanisms. An unusual double band of interplate and intraplate aftershocks developed. On 13 January 2007, an M W = 8.1 extensional earthquake ruptured within the Pacific plate beneath the seaward edge of the Kuril trench. This event is the third largest normal-faulting earthquake seaward of a subduction zone on record, and its rupture zone extended to at least 33 km depth and paralleled most of the length of the 2006 rupture. The 13 January 2007 event produced stronger shaking in Japan than the larger thrust event, as a consequence of higher short-period energy radiation from the source. The great event aftershock sequences were dominated by the expected faulting geometries; thrust faulting for the 2006 rupture zone, and normal faulting for the 2007 rupture zone. A large intraplate compressional event occurred on 15 January 2009 (M w = 7.4) near 45 km depth, below the rupture zone of the 2007 event and in the vicinity of the 16 March 1963 compressional event. The fault geometry, rupture process and slip distributions of the two great events are estimated using very broadband teleseismic body and surface wave observations. The occurrence of the thrust event in the shallowest portion of the interplate fault in a region with a paucity of large thrust events at greater depths suggests that the event removed most of the slip deficit on this portion of the interplate fault. This great earthquake doublet demonstrates the heightened seismic hazard posed by induced intraplate faulting following large interplate thrust events. Future seismic failure of the remainder of the seismic gap appears viable, with the northeastern region that has also experienced compressional activity seaward of the megathrust warranting particular attention.

Journal of Geophysical Research B: Solid Earth↗

USGS1989

The Macon Complex; An ancient accretionary complex in the southern Appalachians

The Macon Complex, which extends from eastern Alabama to northern North Carolina, is a late Precambrian–Middle Cambrian accretionary complex comparable in size to the Franciscan Complex of California and Oregon. Much of the complex is tectonic, sedimentary, and metamorphic chaos, properly termed mélange, where well-rounded to angular fragments, blocks, and slabs of contrasting metamorphic grades, different igneous parentages, drastically different sedimentary facies, and different degrees of deformation “float” in highly imbricated and tectonized matrices, the whole having been intruded by Devonian mafic plutons and associated syenites, and by Carboniferous granitic plutons. We have divided the complex into three mélanges that probably reflect different structural regimes within the accretionary wedge: (1) the Juliette mélange, with two tectonostratigraphic lithofacies, the clastic-rich, partly olistostromal Potato Creek facies and the Gladesville facies, rich in mafic and ultramaflc fragments, blocks, and slabs; (2) the Po Biddy mélange, characterized by metamorphosed manganiferous sediments, metavolcaniclastic rocks, graphitic schists, and locally by metamorphosed thinly bedded pyritiferous limestones, and a wide variety of mineral deposits; and (3) the Falls Lake mélange, which is quite similar to the Juliette mélange and probably represents the same tectonostratigraphic horizon in the accretionary prism. The matrices of the mélanges contain a wide variety of metaigneous and metasedimentary exotic clasts, including mafic and ultramaflc rocks. The Macon Complex is structurally overlain by the late Precambrian–Middle Cambrian Little River Complex, made up of thick piles of mostly felsic calc-alkaline metavolcanic rocks, and lesser amounts of metaplutonic rocks, that originated in a continental-margin volcanic arc (Little River arc). Trilobites from near the top of one of the youngest sections are restricted to the upper two-thirds of the Middle Cambrian and are characteristic of the Atlantic faunal province. The Little River Complex is overlain, beneath the Atlantic Coastal Plain, by the African cratonic Northern Florida platform sequence; the Macon and Little River complexes and the Northern Florida platform sequence make up the Little River thrust stack. The magmas of the Devonian plutons that have intruded the Macon Complex probably formed when the Little River stack was thrust upon the underlying Georgiabama thrust stack, which was itself still being thrust toward the North American craton. The Macon Complex is interpreted to have formed between a trench and the Little River island arc at the oceanward edge of what was either a microcontinent off the African continent or the core of the present African continent. Many mafic and all ultramaflc bodies in the mélange are probably pieces of Iapetus Ocean crust and mantle offscraped from the downgoing slab and imbricated into the accretionary wedge. Rocks of the Macon Complex have previously been assigned to the “Charlotte,” “Kiokee,” “Kings Mountain,” and “Lowndesville” belts and to parts of the “Uchee,” “Raleigh,” “Pine Mountain,” and “Inner Piedmont” “belts.”

Alabama, Georgia, North Carolina, South Carolina↗