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.
Research Article| January 01, 2004 Holocene landslides and a 3500-year record of Pacific Northwest earthquakes from sediments in Lake Washington Robert E. Karlin; Robert E. Karlin 1Department of Geological Sciences, University of Nevada, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Mark Holmes; Mark Holmes 2Department of Oceanography, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar S.E.B. Abella; S.E.B. Abella 3Department of Zoology, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Richard Sylwester Richard Sylwester 4Golder and Associates, Redmond, Washington 98052, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Robert E. Karlin 1Department of Geological Sciences, University of Nevada, Reno, Nevada 89557, USA Mark Holmes 2Department of Oceanography, University of Washington, Seattle, Washington 98195, USA S.E.B. Abella 3Department of Zoology, University of Washington, Seattle, Washington 98195, USA Richard Sylwester 4Golder and Associates, Redmond, Washington 98052, USA Publisher: Geological Society of America Received: 25 Mar 2002 Revision Received: 31 Jan 2003 Accepted: 08 May 2003 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2004) 116 (1-2): 94–108. https://doi.org/10.1130/B25158.1 Article history Received: 25 Mar 2002 Revision Received: 31 Jan 2003 Accepted: 08 May 2003 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert E. Karlin, Mark Holmes, S.E.B. Abella, Richard Sylwester; Holocene landslides and a 3500-year record of Pacific Northwest earthquakes from sediments in Lake Washington. GSA Bulletin 2004;; 116 (1-2): 94–108. doi: https://doi.org/10.1130/B25158.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Lake Washington, which forms the eastern boundary of Seattle, is located in a tectonically active area containing several strands of the Seattle fault. High-resolution seismic reflection profiling, sidescan swath imagery, and sediment coring were used to define deformation of Holocene lake sediments and the distribution, geometry, age, and causes of submarine landslides. Numerous large block slides, sediment slumps, and debris flows are present throughout the lake, and large landslides obscure the surficial structure of Seattle fault strands as they pass through the lake. In addition, most bays along the lake margin are the headwalls of large submarine slides. Submerged forests show evidence of deep- seated block failures that have exposed glacial sediments and Tertiary rocks. The massive submarine block slides, and retrogressive submarine slope failures were triggered most likely by large (mb>7) earthquakes on the Seattle fault and/or large to great (mb>8) temblors occurring elsewhere in Cascadia. Buried landslides suggest that submarine slope failures and mass wasting occurred more than once in the last 11,000 yr.Sediments in Lake Washington preserve a record of episodic deposition of turbidites possibly caused by seismically induced submarine landslides. Magnetic susceptibility profiles on 36 gravity cores show a characteristic series of magnetic peaks that can be traced throughout the lake. X- radiography and grain size analyses suggest that the magnetic peaks represent anomalous detrital layers that in some cases are turbidites. The areal extent and magnetic signatures of many of the deposits suggest multiple sources, which is consistent with numerous local landslides caused by large earthquakes. Radiocarbon dating and correlation of the downcore magnetic profiles establish a sediment record in which episodic sedimentary disturbances occurred seven times in the last 3500 yr. If these deposits are seismically induced turbidites (seismites), then large earthquakes have occurred about every 300–500 yr in the Puget Sound region. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
This map summarizes oceanographic data and can be used for geological investigations and as a working chart on shipboard. It contains the following information: ocean contours; major geographic features; offshore earthquake epicenters; bottom sediments and heat flow stations; magnetic lineations; free-air gravity anomaly; seismic reflection profiles; crustal, subcrustal, and free-air gravity profiles; water properties; monthly surface currents and sea surface temperatures; and coastal wave refraction and direction of swells. 12 figures. (RWR)
Maximum heat flow measurements at three locations in the sediment‐filled Escanaba Trough of the Gorda ridge exceed 1200 mW/m². At other ridge crests with thick sediment cover, heat flow values of this magnitude are accompanied by high temperature hydrothermal vent activity and massive sulfide deposition. A dredge haul from the southernmost high heat flow location recovered pyrrhotite, thereby confirming the presence of recent high temperature venting.
