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David W. Scholl

Publications and source records attributed to David W. Scholl.

At least 19 recordsLinked to original sources
USGS2025

Did the Aleutian Basin form by plate capture or backarc basin opening?

The origin of the Aleutian Basin is unresolved because its crust is deeply buried beneath sediments. It has been interpreted as forming in the Eocene when the Beringian convergent margin jumped seaward to south of the Aleutian arc, thereby capturing a large sector of Cretaceous Pacific crust. Alternatively, it may have formed by backarc spreading. We present new magnetic and seismic reflection data compilations and review other pertinent data to evaluate these two possibilities. Arguments for entrapment are: 1) Palaeomagnetic and geologic data document that the Aleutian arc formed approximately in place and as a seaward, on-strike continuation of the Alaska Peninsula; 2) basin-central spreading anomalies trend N-S, normal to the Aleutian arc and exhibit amplitudes and lengths typically formed at mid-ocean ridges; 3) seismic reflection profiles document that cross-basin depositional sequences are like those expected of filling a fixed-width basin. Arguments for a backarc spreading origin include: 1) The perpendicular orientation of magnetic anomalies in the Aleutian Basin and the northern Pacific Plate suggest different origins; 2) the sub-parallel orientation of Aleutian Basin spreading magnetic fabric and Palaeogene rift basins on the Bering Shelf suggest a common extensional regime; 3) thinner continental crust beneath outer-shelf basins is consistent with extension; 4) lineated magnetic fabrics on the margins of the basin are similar to early rifting magnetic fabrics found in other backarc basins; 5) basin heat flow is consistent with Palaeogene seafloor spreading; 6) its marginal basin setting is analogous to marginal basins elsewhere that formed by extension and seafloor spreading, not entrapment; and 7) entrapment requires formation of the Aleutian subduction zone by transference, a mode of subduction initiation that has not yet been documented. We present evidence for both interpretations and underscore the need for continued data collection (e.g. scientific drilling) to test competing hypotheses. Similar approaches may be useful for understanding thickly sedimented marginal basins elsewhere.

Alaska↗
USGS2017

Tectonic evolution

The middle Paleozoic through Present tectonic evolution of the Northern Cordillera is portrayed in a series of 15 Tectonic Model figures. These figures depict the successive stages of formation and accretion of orogenic collages along the western margin of the North American Plate (craton and craton margin) and adjacent marine areas. The orogenic collages are composed of terranes and overlap assemblages. The terranes are tectonic (fault-bounded) fragments of island arcs and continental-margin arcs, subduction-zones, oceanic lithosphere, craton margins, and cratons. The overlap assemblages are sequences of mainly late Mesozoic and Cenozoic continental-margin-arc and sedimentary-basin assemblages that were deposited on subjacent terranes and on subjacent craton margin assemblages, following their tectonic assembly.

Book chapter↗
USGS2015

Great (≥Mw8.0) megathrust earthquakes and the subduction of excess sediment and bathymetrically smooth seafloor

Using older and in part flawed data, Ruff (1989) suggested that thick sediment entering the subduction zone (SZ) smooths and strengthens the trench-parallel distribution of interplate coupling. This circumstance was conjectured to favor rupture continuation and the generation of high-magnitude (≥Mw8.0) interplate thrust (IPT) earthquakes. Using larger and more accurate compilations of sediment thickness and instrumental (1899 to January 2013) and pre-instrumental era (1700–1898) IPTs (n = 176 and 12, respectively), we tested if a compelling relation existed between where IPT earthquakes ≥Mw7.5 occurred and where thick (≥1.0 km) versus thin (≤1.0 km) sedimentary sections entered the SZ. Based on the new compilations, a statistically supported statement (see Summary and Conclusions) can be made that high-magnitude earthquakes are most prone to nucleate at well-sedimented SZs. For example, despite the 7500 km shorter global length of thick-sediment trenches, they account for ∼53% of instrumental era IPTs ≥Mw8.0, ∼75% ≥Mw8.5, and 100% ≥Mw9.1. No megathrusts >Mw9.0 ruptured at thin-sediment trenches, whereas three occurred at thick-sediment trenches (1960 Chile Mw9.5, 1964 Alaska Mw9.2, and 2004 Sumatra Mw9.2). However, large Mw8.0–9.0 IPTs commonly (n = 23) nucleated at thin-sediment trenches. These earthquakes are associated with the subduction of low-relief ocean floor and where the debris of subduction erosion thickens the plate-separating subduction channel. The combination of low bathymetric relief and subduction erosion is inferred to also produce a smooth trench-parallel distribution of coupling posited to favor the characteristic lengthy rupturing of high-magnitude IPT earthquakes. In these areas subduction of a weak sedimentary sequence further enables rupture continuation.

Geosphere↗
USGS2015

Deep-Water Acoustic Anomalies from Methane Hydrate in the Bering Sea

A recent expedition to the central Bering Sea, one of the most remote locations in the world, has yielded observations confirming gas and gas hydrates in this deep ocean basin. Significant sound speed anomalies found using inversion of pre-stack seismic data are observed in association with variable seismic amplitude anomalies in the thick sediment column. The anomalously low sound speeds below the inferred base of methane hydrate stability indicate the presence of potentially large quantities of gas-phase methane associated with each velocity-amplitude anomaly (VAMP). The data acquired are of such high quality that quantitative estimates of the concentrations of gas hydrates in the upper few hundred meters of sediment are also possible, and analyses are under way to make these estimates. Several VAMPs were specifically targeted in this survey; others were crossed incidentally. Indications of many dozens or hundreds of these features exist throughout the portion of the Bering Sea relevant to the U.S. extended continental shelf (ECS) consistent with the United Nations Convention on the Law of the Sea.

Bering Sea↗
USGS2012

Application of continuum models to deformation of the Aleutian Island Arc

Continuum models were constructed to describe large‐scale deformation of the Aleutian Island Arc over the past 5 m.y. These models consider the island arc as a continuum in the horizontal plane with the velocity boundary condition at the Pacific edge stated as a fraction of Pacific plate convergence transferred to the arc. First, a simple model of uniformly distributed strain is formulated to illustrate the mechanics of continuous deformation. Lineaments along the arc massif rotated about a vertical axis are matched by small‐element rotation calculated from the model. However, this model does not predict across‐arc variations in deformation and produces an unrealistic amount of crustal thickening after 5 m.y. A physically more meaningful model of deformation is the thin viscous sheet model based on averages of stress and rheology throughout the lithosphere. The amount of motion transferred from the Pacific plate to the arc is constrained by the rotated lineaments, while the effective stress‐strain exponent ( n ) and the ability the lithosphere has to sustain crustal thickness contrasts (the Argand number) are independent variables. Primarily, bathymetry, earthquake focal mechanisms, and styles of faulting are used to evaluate the models. The preferred model is one where the amount of motion transferred from the Pacific plate is greater in an arc‐parallel direction than in an arc‐normal direction, producing stresses consistent with strike‐slip faulting at the far western end of the arc and tensional stresses consistent with transverse normal faulting elsewhere in the arc massif. This model agrees with observations of slip vectors by Ekström and Engdahl (1989), who conclude that a portion of the arc‐parallel component of relative plate motion is taken up in the overriding plate. This model implies that compressive stress transferred to the arc is small in comparison to along‐arc shear stress and that stresses conducive to strikeslip faulting are prevalent throughout the arc.

Alaska↗
USGS2012

Influence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin, central Aleutian subduction zone

During Pliocene to Quaternary time, the central Aleutian forearc basin evolved in response to a combination of tectonic and climatic factors. Initially, along-trench transport of sediment and accretion of a frontal prism created the accommodation space to allow forearc basin deposition. Transport of sufficient sediment to overtop the bathymetrically high Amlia fracture zone and reach the central Aleutian arc began with glaciation of continental Alaska in the Pliocene. As the obliquely subducting Amlia fracture zone swept along the central Aleutian arc, it further affected the structural evolution of the forearc basins. The subduction of the Amlia fracture zone resulted in basin inversion and loss of accommodation space east of the migrating fracture zone. Conversely, west of Amlia fracture zone, accommodation space increased arcward of a large outer-arc high that formed, in part, by a thickening of arc basement. This difference in deformation is interpreted to be the result of a variation in interplate coupling across the Amlia fracture zone that was facilitated by increasing subduction obliquity, a change in orientation of the subducting Amlia fracture zone, and late Quaternary intensification of glaciation. The change in coupling is manifested by a possible tear in the subducting slab along the Amlia fracture zone. Differences in coupling across the Amlia fracture zone have important implications for the location of maximum slip during future great earthquakes. In addition, shaking during a great earthquake could trigger large mass failures of the summit platform, as evidenced by the presence of thick mass transport deposits of primarily Quaternary age that are found in the forearc basin west of the Amlia fracture zone.

Geosphere↗
USGS2012

History of earthquakes and tsunamis along the eastern Aleutian-Alaska megathrust, with implications for tsunami hazards in the California Continental Borderland

During the past several years, devastating tsunamis were generated along subduction zones in Indonesia, Chile, and most recently Japan. Both the Chile and Japan tsunamis traveled across the Pacific Ocean and caused localized damage at several coastal areas in California. The question remains as to whether coastal California, in particular the California Continental Borderland, is vulnerable to more extensive damage from a far-field tsunami sourced along a Pacific subduction zone. Assuming that the coast of California is at risk from a far-field tsunami, its coastline is most exposed to a trans-Pacific tsunami generated along the eastern Aleutian-Alaska subduction zone. We present the background geologic constraints that could control a possible giant (M w ~9) earthquake sourced along the eastern Aleutian-Alaska megathrust. Previous great earthquakes (M w ~8) in 1788, 1938, and 1946 ruptured single segments of the eastern Aleutian-Alaska megathrust. However, in order to generate a giant earthquake, it is necessary to rupture through multiple segments of the megathrust. Potential barriers to a throughgoing rupture, such as high-relief fracture zones or ridges, are absent on the subducting Pacific Plate between the Fox and Semidi Islands. Possible asperities (areas on the megathrust that are locked and therefore subject to infrequent but large slip) are identified by patches of high moment release observed in the historical earthquake record, geodetic studies, and the location of forearc basin gravity lows. Global Positioning System (GPS) data indicate that some areas of the eastern Aleutian-Alaska megathrust, such as that beneath Sanak Island, are weakly coupled. We suggest that although these areas will have reduced slip during a giant earthquake, they are not really large enough to form a barrier to rupture. A key aspect in defining an earthquake source for tsunami generation is determining the possibility of significant slip on the updip end of the megathrust near the trench. Large slip on the updip part of the eastern Aleutian-Alaska megathrust is a viable possibility owing to the small frontal accretionary prism and the presence of arc basement relatively close to the trench along most of the megathrust.

Alaska↗
USGS2011

Discussions of ODP Leg 205 and drilling of Middle America Seismogenic Zone

Erosional convergent margins, where material is removed from the base of the upper plate and subducted on the lower plate, are fundamental features of the Circum Pacific. The erosional Middle America Trench convergent Pacific margin, remarkable for its broad diversity of dynamic environments, is a natural laboratory for studying convergent margin processes and seismogenesis. These environments include a shallow and deep trench axis, shallow-to-steep-dipping plate interfaces, abnormally hot-to-cold subducting plate temperatures, and a subducting plate with smooth morphology bordering basement ridges and seamounts. The subducting topography accelerates erosion and localizes seismicity.

Middle America Trench, Pacific Ocean↗
USGS2010

Large-scale deformation related to the collision of the Aleutian Arc with Kamchatka

The far western Aleutian Island Arc is actively colliding with Kamchatka. Westward motion of the Aleutian Arc is brought about by the tangential relative motion of the Pacific plate transferred to major, right-lateral shear zones north and south of the arc. Early geologic mapping of Cape Kamchatka (a promontory of Kamchatka along strike with the Aleutian Arc) revealed many similarities to the geology of the Aleutian Islands. Later studies support the notion that Cape Kamchatka is the farthest west Aleutian “island” and that it has been accreted to Kamchatka by the process of arc-continent collision. Deformation associated with the collision onshore Kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rocks directly west of Cape Kamchatka. The trend of the thrust faults is concave toward the collision zone, indicating a radial distribution of maximum horizontal compressive stress. Offshore, major crustal faults trend either oblique to the Kamchatka margin or parallel to major Aleutian shear zones. These offshore faults are complex, accommodating both strike-slip and thrust displacements as documented by focal mechanisms and seismic reflection data. Earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost Aleutian shear zone and to the west by an apparent aseismic front. Analysis of focal mechanisms in the region indicate that the present-day arc-continent “contact zone” is located directly east of Cape Kamchatka. In modeling the dynamics of the collision zone using thin viscous sheet theory, the rheological parameters are only partially constrained to values of n (the effective power law exponent) ≥ 3 and Ar (the Argand number) ≤ 30. These values are consistent with a forearc thermal profile of Kamchatka, previously determined from heat flow modeling. The thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence, not only of compressive stresses resulting from the west directed collision, but also of sediment-induced coupling of the subducting Pacific plate.

Kamchatka Peninsula↗
USGS2006

Leg 197 synthesis: Southward motion and geochemical variability of the Hawaiian hotspot

The bend in the Hawaiian-Emperor volcanic chain is an often-cited example of a change in plate motion with respect to a stationary hotspot. Growing evidence, however, suggests that the bend might instead record variable drift of the Hawaiian hotspot within a convecting mantle. Paleomagnetic and radiometric age data from samples recovered during Ocean Drilling Program (ODP) Leg 197 define an age-progressive paleolatitude history, indicating that the Emperor Seamounts volcanic trend was formed principally by rapid (4–5 cm/yr) southward motion of the Hawaiian hotspot during Late Cretaceous to early Tertiary time (81–47 Ma). Paleointensity data derived from Leg 197 suggest an inverse relationship between field strength and reversal frequency, consistent with an active lower mantle that controls the efficiency of the geodynamo. Petrochemical data and observations of volcanic products (lava flows and volcaniclastic sediments) from Detroit, Nintoku, and Koko Seamounts provide records of the evolution of these volcanic systems for comparison with recent activity in the Hawaiian Islands. We find that the Emperor Seamounts formed from similar mantle sources for melting (plume components and lithosphere) and in much the same stages of volcanic activity and time span as the Hawaiian volcanoes. Changes in major and trace element and Sr isotopic compositions of shield lavas along the lineament can be related to variations in thickness of the lithosphere overlying the hotspot that control the depth and extent of partial melting. Other geochemical tracers, such as He, Pb, and Hf isotopic compositions, indicate persistent contributions to melting from the plume throughout the volcanic chain.

Proceedings of the Ocean Drilling Program: Scienti↗
USGS2006

Possible deep-water gas hydrate accumulations in the Bering Sea

Seismic reflection images from the deep-water Aleutian and Bowers Basins of the Bering Sea contain many hundreds of acoustic Velocity-AMPlitude (VAMP) anomalies, each of which may represent a large accumulation of natural gas hydrate. Against a backdrop of essentially horizontal sedimentary reflections, the VAMP anomalies stand out as both high-amplitude bright spots and zones of vertically aligned horizon distortions. The VAMPs are interpreted as natural gas chimneys overlain by concentrated hydrate caps.

Fire in the Ice: NETL Methane Hydrate Newsletter↗
USGS2001

Dynamic computer model for the metallogenesis and tectonics of the Circum-North Pacific

The digital files on this report consist of a dynamic computer model of the metallogenesis and tectonics of the Circum-North Pacific, and background articles, figures, and maps. The tectonic part of the dynamic computer model is derived from a major analysis of the tectonic evolution of the Circum-North Pacific which is also contained in directory tectevol. The dynamic computer model and associated materials on this CD-ROM are part of a project on the major mineral deposits, metallogenesis, and tectonics of the Russian Far East, Alaska, and the Canadian Cordillera. The project provides critical information on bedrock geology and geophysics, tectonics, major metalliferous mineral resources, metallogenic patterns, and crustal origin and evolution of mineralizing systems for this region. The major scientific goals and benefits of the project are to: (1) provide a comprehensive international data base on the mineral resources of the region that is the first, extensive knowledge available in English; (2) provide major new interpretations of the origin and crustal evolution of mineralizing systems and their host rocks, thereby enabling enhanced, broad-scale tectonic reconstructions and interpretations; and (3) promote trade and scientific and technical exchanges between North America and Eastern Asia.

Open-File Report↗
USGS1998

Summary terrane, mineral deposit, and metallogenic belt maps of the Russian Far East, Alaska, and the Canadian Cordillera

This report is part of a project on the major mineral deposits, metallogenesis, and tectonics of the Russian Far East, Alaska, and the Canadian Cordillera. The project is to provide critical information for collaborators and customers on bedrock geology and geophysics, tectonics, major metalliferous mineral resources, metallogenic patterns, and crustal origin and evolution of mineralizing systems for the Russian Far East, Alaska, and the Canadian Cordillera.

Open-File Report↗
USGS1994

Origin, transport, and emplacement of an exotic island-arc terrane exposed in eastern Kamchatka, Russia

The regional stratigraphy of eastern Kamchatka includes an exotic, Early-Late Cretaceous ophiolite and Late Cretaceous island-arc volcanic sequence. Integrating the existing geologic and geophysical data, we examine the origin, transport, emplacement, and postemplacement deformation of the island-arc terrane, which is named the Olyutorsky island arc. Results from several paleomagnetic studies consistently indicate that the island-arc terrane originated >1000 km to the south of where it is presently exposed. Although the formative paleolatitudes of the island-arc rocks approximately correspond to the location of the Izanagi-Farallon subduction zone, the age of the volcanic rocks postdates the cessation of Izanagi-Farallon convergence, thus indicating that an unnamed plate or back-arc basin existed in the northwest Pacific during Late Cretaceous time. We examine two possible models for northward transport of the island-arc terrane to Kamchatka: (1) infra-oceanic transport with the Pacific or Kula plates and (2) coastwise translation of the island-arc terrane after accretion to the Eurasian margin far to the south of Kamchatka. For both models, the dominant Eocene and Miocene deformation ages observed in eastern Kamchatka are used as two possible age limits for the cessation of northward transport. Although the observed paleolatitudes from paleomagnetic data correspond best with the infra-oceanic transport model, the provenance of the Paleogene "transport" stratigraphy indicates a near-shore sediment supply. Our preferred interpretation is that the island-arc terrane (1) accreted onto the Eurasian margin concurrent with cessation of island-arc volcanism (Maastrichtian-Danian) and (2) underwent northward coastwise translation along a major strike-slip fault zone ending by middle-late Eocene time (43-50 Ma). It is unclear whether the ophiolite was exposed during arc-continent collision or whether the ophiolite was obducted onto the island arc prior to collision. A consequence of either infra-oceanic transport or coastwise translation is that an open corridor between the western terminus of the Aleutian Arc and Kamchatka must have existed until middle to late Eocene time. Spreading within the Komandorsky Basin, subduction of sea-mounts, and collision of the Aleutian Arc with Kamchatka are proposed to have instigated the second Miocene phase of deformation, which uplifted and reexposed the island-arc terrane.

Kamchatka Peninsula↗