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USGS Completes Second Atlantic Margin Expedition for Law of the Sea/ Submarine-Landslide Studies

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Between April 10, 2015, and May 2, 2015, the U.S. Geological Survey (USGS) utilized the specially-equipped seismic research vessel (R/V) Marcus G. Langseth to collect multichannel seismic (MCS) data in deep water along the U.S. Atlantic Continental Margin between 30° and 40° N (see trackline map, below). The Langseth, owned by the National Science Foundation and operated by Lamont-Doherty Earth Observatory, was specially designed for both 2D and 3D multichannel seismic-reflection data acquisition. This cruise (number MGL1506) was the second of two USGS Atlantic margin surveys using Langseth to address the dual objectives of understanding submarine landslides and identifying the outer limits of the extended continental shelf (ECS). (For more information about the previous survey, see Sound Waves article “USGS Atlantic Margin Expedition Combines Submarine-Landslide Studies with Law of the Sea Mapping.”) Warren Wood of the U.S. Naval Research Laboratory joined this year’s team as a co-chief scientist. The survey lasted 22 days, with approximately 3000 kilometers (km) of MCS tracks surveyed. Approximately 250 km of the survey covered submarine landslide objectives and the rest covered ECS objectives. In addition, gravity, magnetics, continuous multibeam echo-sounder, and chirp sub-bottom data were collected. Frequent deployment of expendable bathy-thermographs (XBTs) enabled the seismic signals in the water column to be calibrated for seismic oceanography studies for the U.S. Navy. Ports of embarkation and disembarkation were Charleston, South Carolina, and Brooklyn, New York, respectively.

Location map showing surveys and features discussed in story
Above: Map showing track line locations of the 2015 Langseth survey (solid black line), locations of 2014 Langseth survey (dotted black line), major submarine landslides (tan), and locations of seismic profiles (red line, yellow line, and dashed white-and-black line) shown in subsequent figures. CFS, Cape Fear Landslide; CLS, Cape Lookout Landslide; MNRS, Munson-Nygren-Retriever slide complex. [larger version]

For understanding submarine landslides, multichannel seismic data were collected across the width of the largest known landslide of the Atlantic margin, the Cape Fear slide (CFS), south of Cape Hatteras, North Carolina. The seismic data will provide information on the structure, stratigraphy, and relative history of failure of the slide. The Cape Fear slide extends approximately 375 km from its headwall failure near the shelf edge to its run-out toe in approximately 5,400 meters of water depth. The 2015 track lines crossed the width of the CFS obliquely twice, making two crossings/tie points (areas where the datasets overlap) with the 2014 data. Most significantly, the high-resolution data along one of the track lines show that the Cape Fear Landslide truncates the Cape Lookout landslide, and is therefore the younger of the two features (see the seismic profile, below). Unfortunately, a late spring storm with winds gusting above 50 knots interrupted data acquisition along the second landslide objective, the Munson-Nygren-Retriever slide complex near the New England seamounts.

Photo of MGL1506 cruise participants
Above: MGL1506 cruise participants. Standing, left to right: Ray Sliter (USGS), Debbie Hutchinson (USGS), Josh Kasinger (Lamont-Doherty Earth Observatory [LDEO]), Roberto Henriquez (LDEO), Mike Martello (LDEO), Wayne Baldwin (USGS), Gilles Guerin (LDEO), Tom Spoto (LDEO), David Martinson (LDEO), Matt Arsenault (USGS), Nathan Miller (USGS), Amy Schmitt (RPS, Inc.), Alan Thompson (LDEO), Amanda Dubuque (RPS, Inc.), Cassadra Frey (RPS, Inc.), Amy Piko (RPS, Inc.). Kneeling, left to right: Carlos Gutierrez (LDEO), Eric Moore (USGS), Warren Wood (U.S. Naval Research Laboratory), Klatyon Curtis (LDEO), Claudia Portocarrero (RPS, Inc.). [larger version]

For identifying the outer limits of the ECS, multichannel seismic data were collected to accurately measure sediment thickness as an input to the sediment thickness formula of Article 76 of the Convention on the Law of the Sea. This Article enables nations to manage the seafloor and sub-seafloor beyond 200 nautical miles where the conditions of Article 76 are met. In 2014, the first survey collected data for ECS objectives that were primarily parallel to the trend of the margin in order to identify fracture zones or valleys where there could be local maxima in sediment thickness. The 2015 lines were primarily dip lines (in a zig-zag pattern) along these mapped or inferred trends. The seafloor and basement (i.e., top and bottom of the sediments) were imaged with great clarity on the new data (see seismic profile, second image below). One of the surprises in the dataset was the identification of faults in the sedimentary section, well above basement, indicating continued tectonic or other activity well after seafloor spreading began (see seismic profile, second image below).

Example of multichannel seismic reflection profile showing a prominent basement ridge
Above: Example of multichannel seismic reflection profile showing a prominent basement ridge with a fault that extends well into the shallow part of the sedimentary section. Location is shown as the yellow line in track line location map above. [larger version]
High-resolution seismic profile showing the Cape Fear Landslide truncating the Cape Lookout Landslide
Above: High-resolution seismic profile showing the Cape Fear Landslide truncating the Cape Lookout Landslide. Location is shown as the red line in track line location map above. [larger version]

Other unexpected features imaged in the data were diapir (dome)-like structures affecting the otherwise flat seafloor. These can be seen at all scales imaged, in the highest resolution chirp data, the multichannel data, and as possible flow-like features in the multibeam echosounding and backscatter swaths (see images of possible mud diapirs, below). These features occurred primarily in the vicinity of the New England seamounts, although there was no indication of the seamounts immediately around or near these features. The origin of these features appears to be shallow in the sedimentary section because deeper horizons are generally not affected.

Images of possible mud diapirs discovered in deep water during the MGL1506 survey
Above: Images of possible mud diapirs discovered in deep water during the MGL1506 survey. A) High resolution seismic profile showing seafloor disturbance by these features; B) Multichannel seismic profile showing disturbance in the upper ~0.5 second two-way travel time, but no deep seated disturbance (location shown by the white dashed line in trackline map above); C) Multibeam (upper) and backscatter (lower) images along the same track as (A) showing the seafloor disturbance. [larger version]

The hard work of processing and interpreting the data is now underway!

Related Sound Waves Stories
USGS Atlantic Margin Expedition Combines Submarine-Landslide Studies with Law of the Sea Mapping
November / December 2014
Department of State Recognizes U.S. Extended Continental Shelf Project Team with Superior Honor Awards
May / June 2013
Submarine Landslides as Potential Triggers of Tsunamis That Could Strike the U.S. East Coast
August 2009
Three-Week Expedition Images Sediments Beneath the Gulf of Alaska
August 2011

Related Websites
Article 76 of the Convention on the Law of the Sea
United Nations
Law of the Sea
United Nations
Law of the Sea—Outer Limits of the US Continental Margins

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in this issue:

cover story:
Sampling Atlantic Margin Methane Seeps and Plumes

USGS Completes Second Expedition for Atlantic Submarine-Landslide Studies

Spotlight on Sandy
“Team Delmarva” Completes Seafloor Mapping off Delmarva Peninsula

Pathways to the Abyss

USGS Residual Oil Research Presented at Two Public Seminars

USGS Hosts USF Oceanography Camp for Girls

USGS Continues Collaboration for Native Youth in Science

USGS Assists in Another Year of Woods Hole Partnership in Education

New Production Team for Sound Waves

Oct. / Nov. Publications

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