Search USGS⌕ Search

Geology topics

M. S. Marlow

Publications and source records attributed to M. S. Marlow.

At least 19 recordsLinked to original sources
USGS2010

Evidence for cenozoic crustal extension in the Bering Sea region

Geophysical and regional geologic data provide evidence that parts of the oceanic crust in the abyssal basins of the Bering Sea have been created or altered by crustal extension and back‐arc spreading. These processes have occurred during and since early Eocene time when the Aleutian Ridge developed and isolated oceanic crust within parts of the Bering Sea. The crust in the Aleutian Basin, previously noted as presumably Early Cretaceous in age (M1–M13 anomalies), is still uncertain. Some crust may be younger. Vitus arch, a buried 100‐ to 200‐km‐wide extensionally deformed zone with linear basement structures and geophysical anomalies, crosses the entire west central Aleutian Basin. We suggest that the arch and the inferred fracture zones in the Aleutian Basin are early Cenozoic structures related to the early entrapment history of the Bering Sea. These structures lie on trend with known early Cenozoic structures near the Bowers‐Shirshov‐Aleutian ridge junction and on the Beringian continental margin (with possible continuation into Alaska); the structures may have coeval and cogenetic(?) histories for early Cenozoic and possibly younger times. Cenozoic deformation within parts of the Bering Sea region is principally extensional, although the total amount of extension is not known. As examples, the Komandorsky basin formed by back‐arc seafloor spreading, the Aleutian Ridge has been extensively sheared, and extensional block faulting is common. Sedimentary basins of the Bering shelf have formed by extension associated with wrench faulting. The Cenozoic deformation throughout the Bering Sea region probably results from the interaction of major lithospheric plates and associated regional strike‐slip faults. We present models for the Bering Sea over the past 55 m.y. that show oceanic plate entrapment, back‐arc faulting and spreading along Vitus arch, breakup of the oceanic crust in the Aleutian Basin at fracture zones, and back‐arc spreading in Bowers Basin.

Tectonics↗
USGS1999

Development of San Leandro synform and neotectonics of the San Francisco Bay block, California

High-resolution, 24-channel seismic-reflection data show a stratified synform beneath south San Francisco Bay. These seismic-reflection data reveal an eastward-dipping bedrock surface that is about 40 m deep (subbottom) beneath the western south bay, and that reaches a maximum observed depth of 500-800 m (subbottom) below the eastern half of the south bay. An angular unconformity cuts both the synform and underlying bedrock. The age of the unconformity is unknown but may be Pleistocene, when these strata forming the synform were presumably exposed subaerially during lowered sea levels. The synformal strata, the unconformity, and some generally flat-lying and overlying strata are folded near the eastern shore of the bay. This folding may result from movement on the Hayward fault (fault interactions and localized strain partitioning) or from compressional deformation in the East Bay Hills related to NE-SW ('fault-normal') convergence between the Pacific and North American plates. In general, reflections from sediment overlying the unconformity are flat lying (except near the eastern shore of the bay), whereas reflections beneath the unconformity dip eastward. The overlying, flat sediment section fills a shallow basin that is coincident with an elliptical residual gravity low. This low appears to be related to the deeper sedimentary, synformal section based on the spatial correlation between the east-dipping reflections and the gravity anomaly. Projecting the east-dipping reflections to the center of the gravity low suggests that the total section of flat-lying and dipping reflections in the synform may exceed 1000 m. Modeling of the gravity low suggests a total low-density section, about 1.5 km thick, at the center of the synform relative to the surrounding bedrock of presumed Franciscan Complex.High-resolution, 24-channel seismic-reflection data show a stratified synform beneath south San Francisco Bay. These seismic-reflection data reveal an eastward-dipping bedrock surface that is about 40 m deep (subbottom) beneath the western south bay, and that reaches a maximum observed depth of 500-800 m (subbottom) below the eastern half of the south bay. An angular unconformity cuts both the synform and underlying bedrock. The age of the unconformity is unknown but may be Pleistocene, when these strata forming the synform were presumably exposed subaerially during lowered sea levels. The synformal strata, the unconformity, and some generally flat-lying and overlying strata are folded near the eastern shore of the bay. This folding may result from movement on the Hayward fault (fault interactions and localized strain partitioning) or from compressional deformation in the East Bay Hills related to NE-SW (`fault-normal') convergence between the Pacific and North American plates. In general, reflections from sediment overlying the unconformity are flat lying (except near the eastern shore of the bay), whereas reflections beneath the unconformity dip eastward. The overlying, flat sediment section fills a shallow basin that is coincident with an elliptical residual gravity low. This low appears to be related to the deeper sedimentary, synformal section based on the spatial correlation between the east-dipping reflections and the gravity anomaly. Projecting the east-dipping reflections to the center of the gravity low suggests that the total section of flat-lying and dipping reflections in the synform may exceed 1000 m. Modeling of the gravity low suggests a total low-density section, about 1.5 km thick, at the center of the synform relative to the surrounding bedrock of presumed Franciscan Complex.

Marine and Petroleum Geology↗
USGS1993

Petrology and isotopic composition of Quaternary basanites dredged from the Bering Sea continental margin near Navarin Basin

Quaternary basanites were recovered from shallow water depth from the continental margin of the Bering Sea (58°39.0′N, 177°12.9′W) near Navarin Basin. The basanites are highly vesicular flow rock and hyaloclastites similar to other alkalic volcanic rocks erupted repeatedly during the late Cenozoic on islands in the Bering Sea region and in mainland Alaska. K–Ar ages for the basanites indicate at least two episodes of volcanism at about 1.1 and 0.4 Ma. Similar alkalic volcanism occurred sporadically at geographically widely separated centers in the Bering Sea region for at least the past 6 Ma. Chemically, these alkalic lavas are intraplate basalts similar to those erupted from oceanic islands and in some continental settings. Trace-element data indicate these alkalic lavas have been generated by small, but variable, amounts of partial melting of a meta-somatized lherzolite source. The relatively primitive compositions (MgO > 9%), presence of mantle-derived xenoliths in some alkalic lavas, and presence of forsteritic olivine with low CaO and high NiO suggest that magma rose rapidly from greath depth without spending time in large, long-lived magma chambers. Although lavas from different volcanic centers in the Bering Sea region are similar with respect to major elements and many trace-element ratios, isotopic compositions indicate heterogeneities in the source. The Navarin basanites have higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd values than any other lavas so far reported from this region. The 207 Pb/ 204 Pb isotopic ratios indicate involvement of a crustal component, which may have resulted from metasomatism associated with subduction-related magmatic activity during the Early Eocene in this region. Although some volcanic episodes appear to have occurred roughly synchronously at geographically widely separated centers, no large-scale regional extension nor presence of large mantle plumes are indicated. Instead, alkalic volcanism apparently resulted from upwelling and decompressional melting of small isolated mantle diapirs in response to local lithospheric attenuation associated with jostling of blocks during adjustment to regional stresses.

Alaska↗
USGS1990

Site 783

No abstract available

Conference Paper↗
USGS1989

Petrology and age of volcanic-arc rocks from the continental margin of the Bering Sea: Implications for Early Eocene relocation of plate boundaries

Eocene volcanic flow and dike rocks from the Beringian margin have arc characteristics, implying a convergent history for this region during the early Tertiary. The extrusive rocks are basalt, basaltic andesite, andesite, and minor dacite and rhyolite. The intrusive sample is from a quartz diorite dike intruding serpentinized peridotite. Major-element oxide contents, particularly FeO*/MgO versus SiO 2 , identify both tholeiitic and calc-alkalic basalt; more silicic lavas have calc-alkalic affinities. Consistent with volcanic-arc compositions, spidergrams show pronounced Nb–Ta depletion and alkali enrichment relative to light-rare-earth-element ( LREE ) abundance. Chondrite-normalized REE plots show relatively flat patterns, with only slight LREE enrichment for tholeiitic compositions and greater LREE enrichment and lower heavy-rare-earth-element ( HREE ) abundance for calc-alkalic compositions. The samples, particularly those with calc-alkalic compositions, are rich in plagioclase that is strongly zoned; the more silicic samples contain orthopyroxene, clinopyroxene, and primary amphibole. The quartz diorite dike contains iron-rich almandine phenocrysts that appear to be magmatic, suggesting emplacement at great depth near the base of the crust or upper mantle.Chemical and mineralogical compositions are similar to those of modern Aleutian-arc lavas. They also resemble volcanic-arc compositions from western mainland Alaska, although greater chemical diversity and a stronger continental influence are observed in the Alaskan mainland rocks.Early Eocene ages of 54.4–50.2 Ma for the Beringian samples are well constrained by conventional K–Ar ages of nine plagioclase separates and by concordant 40 Ar/ 39 Ar incremental heating and total-fusion experiments. A concordant U–Pb zircon age of 53 Ma for the quartz-diorite dike is in good agreement with the K–Ar data.Plate motion studies of the North Pacific Ocean indicate more northerly directed subduction prior to the Tertiary and a continuous belt of arc-type volcanism extending from Siberia, along the Beringian margin, into mainland Alaska. Around 56 Ma (chron 25–24), subduction changed to a more westerly direction and subduction-related volcanism ceased for most of mainland Alaska. The increasingly oblique angle of convergence should have ended subduction along the Beringian margin as well. However, consistent ages of 54–50 Ma indicate a final pulse in arc-type magmatism during this period of plate adjustment, which may be explained by three different models: (1) The northern and central part of the Beringian margin maintained a higher angle of convergence, allowing a final pulse of arc-type magmatism. (2) The rocks erupted in an early, or proto, Aleutian arc and were rafted against the continental margin along transform faults. (3) The rocks erupted along a leaky transform fault, analogous to calc-alkalic volcanism in the southern California borderland.

Alaska↗
USGS1989

Site 785

No abstract available

Conference Paper↗
USGS1989

Site 779

No abstract available

Conference Paper↗
USGS1988

Geophysical study of a magma chamber near Mussau Island, Papua New Guinea

Analysis of a 24-channel seismic-reflection data collected near Mussau Island, Papua New Guinea, shows a high-amplitude, negative-polarity reflection that we believe is from the top of a magma chamber. The reflecting horizon lies at a depth of about 4.4 s subbottom and can be traced laterally for 2.6 km. On shot gathers, the reflection demonstrates normal moveout appropriate for an in-place event. The frequency spectrum of the reflection shows a decrease in high-frequency content when compared to the sea floor reflection, as would be expected for a deep subsurface event. The polarity of the reflection event is negative, suggesting that the reflection horizon is the top of a low-velocity zone. Magnetic data indicate that the ridge containing the reflecting horizon is magnetic, and the geology of Massau Island suggests that the ridge is volcanic in its origin. We speculate that the high-amplitude reflection is from the top of a magma chamber some 7-11 km deep.

Mussau Island↗