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Mark L. Holmes

Publications and source records attributed to Mark L. Holmes.

7 recordsLinked to original sources
USGS2012

Upper crustal densities derived from sea floor gravity measurements: Northern Juan De Fuca Ridge

A transect of sea floor gravity stations has been analyzed to determine upper crustal densities on the Endeavour segment of the northern Juan de Fuca Ridge. Data were obtained using ALVIN along a corridor perpendicular to the axis of spreading, over crustal ages from 0 to 800,000 years. Calculated elevation factors from the gravity data show an abrupt increase in density with age (distance) for the upper 200 m of crust. This density change is interpreted as a systematic reduction in bulk porosity of the upper crustal section, from 23% for the axial ridge to 10% for the off-axis flanking ridges. The porosity decrease is attributed to the collapse and filling of large-scale voids as the abyssal hills move out of the crustal formation zone. Forward modeling of a plausible density structure for the near-axis region agrees with the observed anomaly data only if the model includes narrow, along-strike, low-density regions adjacent to both inner and outer flanks of the abyssal hills. The required low density zones could be regions of systematic upper crustal fracturing and faulting that were mapped by submersible observers and side-scan sonar images, and whose presence was suggested by the distribution of heat flow data in the same area.

Geophysical Research Letters↗
USGS2012

Volcanism and massive sulfide formation at a sedimented spreading center, Escanaba Trough, Gorda Ridge, northeast Pacific Ocean

Seismic‐reflection profiles over the sediment‐filled Escanaba Trough at the southern Gorda Ridge reveal a series of volcanic centers that pierce the sediment. The volcanic edifices are 3 to 6 km in diameter and are spaced at 15 to 20 km intervals along the axis of the trough. The volcanic intrusions have uplifted the sediment as much as 100 m, and in places fresh basaltic flows are exposed at the sea floor. Unoxidized pyrrhotite‐rich massive‐sulfide samples were obtained from the flanks of one of these volcanic centers. The composition and form of the sulfide samples suggest significant interaction between hydrothermal fluids and sediment at depth, and deposition of sulfide within the sediment pile.

Geophysical Research Letters↗
USGS2012

Fluid expulsion sites on the Cascadia accretionary prism: mapping diagenetic deposits with processed GLORIA imagery

Point-discharge fluid expulsion on accretionary prisms is commonly indicated by diagenetic deposition of calcium carbonate cements and gas hydrates in near-surface (<10 m below seafloor; mbsf) hemipelagic sediment. The contrasting clastic and diagenetic lithologies should be apparent in side scan images. However, sonar also responds to variations in bottom slope, so unprocessed images mix topographic and lithologic information. We have processed GLORIA imagery from the Oregon continental margin to remove topographic effects. A synthetic side scan image was created initially from Sea Beam bathymetric data and then was subtracted iteratively from the original GLORIA data until topographic features disappeared. The residual image contains high-amplitude backscattering that we attribute to diagenetic deposits associated with fluid discharge, based on submersible mapping, Ocean Drilling Program drilling, and collected samples. Diagenetic deposits are concentrated (1) near an out-of-sequence thrust fault on the second ridge landward of the base of the continental slope, (2) along zones characterized by deep-seated strikeslip faults that cut transversely across the margin, and (3) in undeformed Cascadia Basin deposits which overlie incipient thrust faults seaward of the toe of the prism. There is no evidence of diagenetic deposition associated with the frontal thrust that rises from the dècollement. If the dècollement is an important aquifer, apparently the fluids are passed either to the strike-slip faults which intersect the dècollement or to the incipient faults in Cascadia Basin for expulsion. Diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates.

Cascadia Basin↗
USGS1978

Sonobuoy refraction data near Kodiak, Alaska

A total of 88 unreversed sonobuoy refraction lines were shot over the continental shelf and slope near Kodiak, Alaska, in 1976 and 1977. Useful results were obtained from 59 of these attempts. The field measurements were carried out aboard the USGS research vessel SAMUEL P. LEE. The refraction lines were concentrated in three major areas: (1) the outer continental shelf from Portlock Bank to Chirikof Island; (2) Shelikof Strait; and (3) the continental slope between Kodiak Seamount and Portlock Bank. Sound sources used during the refraction survey included 1326 and 2501 cubic inch air gun arrays, and a 160 kilojoule arcer. Most of the sonobuoys employed were U. S. Navy SSQ-41A and SSQ-41B models; a few commercial buoys (Refraction Technology and Fairfield Industries) were also used. Line lengths ranged from 5 to 30 km, and maximum subbottom penegration was 5.1 km. The field data were recorded on magnetic tape as well as a conventional facsimile recorder. Data reduction was accomplished using the slope-intercept method with the assumption of plane layers. A computer program was used to calculate interval velocities, thicknesses, reflection times, and critical distances for each of the refracting layers; the program also generated a travel time plot showing refracted arrivals as well as wide angle reflections associated with each of the interfaces.

Alaska↗
USGS1975

Geologic framework of the Alaskan Continental Terrace in the Chukchi and Beaufort seas

Seismic, magnetic and gravity data indicate that the Chukchi and Beaufort epicontinental seas off northern Alaska overlie three sedimentary basins, or provinces, separated by structural highs of regional extent. The basins trend west to northwest and become increasingly marine from south to north. The Chukchi-Beaufort continental margin is similar to those of Atlantic type. Hope basin, in the southern Chukchi Sea, overlies strongly deformed Paleozoic to mid-Cretaceous rocks of the Brooks Range orogen. The basin is inferred to contain nonmarine and marine clastic sedimentary rocks in a 1-km-thick Upper Cretaceous(?), a 1 1/2-km-thick Paleogene(?), and a 3/4-km-thick Neogene(?) sequence. A large anticline and many faults and smaller folds disrupt mainly the older sequences. The Hope basin sedimentary units onlap Herald arch, which trends northwest from Cape Lisburne in the central Chukchi Sea. At the Herald fault zone Brooks Range rocks in the arch are thrust east or northeast over Mississippian to Jurassic shelf carbonate and clastic rocks of the Arctic Alaska (Ellesmerian) basin and overlying Cretaceous flysch and molasse of the Colville geosyncline. These Mississippian to Cretaceous rocks underlie the northeast Chukchi Sea and reportedly are about 10 km thick near the Herald fault zone on the Lisburne Peninsula. The great Chukchi syntaxis in western Brooks Range rocks and structures is thought to result from intersection of the west-trending Brooks Range orogen by the northwest-trending Herald fault zone. The Mississippian to Jurassic shelf sequence thins northward, and onlaps the Barrow arch, which trends northwest from Point Barrow to 161° W.long., thence west-southwest to the Herald fault zone. The Colville geosyncline sequence oversteps both the pre-Cretaceous rocks and. the Barrow arch to form the North Chukchi basin west of 161°W. long. and the progradational Beaufort continental terrace east of it. The North Chukchi basin may contain about 6 km of section, probably Cretaceous and Tertiary and possibly deltaic. Diapirs (of Cretaceous shale?) pierce the gently northward-dipping strata of this basin, in places reaching the sea floor. Thick Tertiary marine and nonmarine clastic rocks of the Camden basin overlie the Cretaceous rocks of the North Slope and inner Beaufort continental terrace east of the Colville River delta. These rocks dip gently seaward west of 146°W. long. but are thrown into long, high-amplitude, east-northeast-striking folds to the east.

Alaska↗