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PI-LAB
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2016
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Marcus Langseth
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Project Title: |
Passive Imaging of the Lithosphere-Asthenosphere Boundary |
Project Status: |
Submitted |
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Principal Investigator: |
Catherine A. Rychert, UoS |
Project Institution: |
UoS |
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Project ID: |
104909 |
Version #: |
3 |
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Date Submitted: |
7/16/2015 7:17:00 AM |
Created By: |
Catherine A. Rychert |
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Date Last Modified: |
7/28/2015 4:33:00 PM |
URI Serial #: |
None |
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Funding Agencies: |
OTHER - NE/M003507/1 - Funded |
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Summary of Field Work: |
Plate Tectonics is the foundation of modern earth sciences, and provides basic framework for the origin of continents, ocean basins and mountain ranges. Plate Tectonics describe the division of the surface area of the earth into several plates that move independently over the surface of the planet. Each plate acts as an essentially rigid solid shell, which is called Lithosphere, and floats over the material below that flows slowly, called Asthenosphere (Figure 1). Most of the geological activities occur at plate boundaries as the solid lithosphere moves independently, producing giant earthquakes such as 2004 Sumatra and 2011 Japan, volcanoes, great mountains such as the Himalayas, the Andes.
The base of the lithosphere, Lithosphere Asthenosphere Boundary (LAB), is the lower boundary of the plate. There are many definitions of the LAB, depending upon the method used. In one model, the LAB is defined as an isotherm (a surface of constant temperature), which is ~1300? C, melting point of mantle rocks. The rocks above this isotherm are sufficiently cool to behave rigidly, whereas rocks lying below this isotherm are sufficiently hot so they deform readily and flow. Beneath the oceans under this model, the base of this isotherm, and hence LAB, is controlled by cooling of the lithosphere as it moves away from spreading centres where it could be 2-6 km thick at zero age and thicken to ~100 km by the time it reaches 120 Ma toward continents or subduction zones. The precise depth-age curve of this isotherm depends on the thermal model used. Beneath the continents, the lithosphere is older and hence thicker, 100-250 km depending upon the age of the lithosphere. However, other models suggest that the LAB could be a boundary between dry, depleted mantle above a hydrated and more fertile mantle (Hirth and Kohlstedt, 1996; Karato, 2012). Since continents have gone through a complex geological history, the precise depth of the LAB is rather poorly defined. Therefore, here we focus on the oceanic lithosphere where different models of evolution of the lithosphere can be tested and verified. These results can then be used to understand the nature of the continental lithosphere as well.
The most direct evidence of the base of the lithosphere has come from surface wave studies where the lithosphere is associated with a high S-wave velocity above a low velocity and high attenuation asthenosphere (Priestley and McKenzie, 2006; Eaton et al., 2009) with a gradual decrease in the velocity (Figure 2). One of the limitations of surface wave tomography is that surface wave alone cannot distinguish a change in mantle velocity that occurs instantaneously in depth from a change that occurs over tens of kilometres. For example Eaton et al. (2009) have shown that a typical fundamental mode surface wave data can be fitted by a sharp LAB at 160 km or a transition zone from 125 to 225 km. Body wave tomography can be used to estimate lithosphere thickness, but since the waves travel vertically, there is a trade-off between velocity and thickness, and therefore uncertainty could be more than 20 km (Tan and Helmberger, 2007).
Recently, the mode-converted waves (P to S (Ps) and S to P (Sp)) from a sharp boundary have been used to determine the depth of the boundary and its velocity gradient. All these phases are primarily sensitive to changes in shear-wave velocity structure (either shear-wave velocity or impedance), and hence provide better resolution than surface waves. Rychert et al. (2005) inverted both Ps and Sp waveforms and found that a velocity drop across a LAB of 5-8% in <11 km zone is required. Similar studies have been carried out by several authors such as Kawakatsu et al. (2009) (Figure 3), Rychert and Shearer (2009), Schmerr (2012), but these results do not fit with the conventional understanding of the LAB (Priestley and McKenzie, 2006; Hirth and Kohlstedt, 1996; Faul and Jackson, 2005; Karato, 2012), which have led to heated debates about the plate tectonics and geodynamical processes.
In the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PiLAB) phase 1 our goal is to provide in situ passive seismic and electromagnetic constraints on the structure of the Lithosphere-Asthenosphere system on young seafloor (<40 Ma) as part of a large international multidisciplinary experiment to characterize the oceanic lithosphere in the Atlantic. The goal of the experiment is to use complementary information from different geophysical techniques (active and passive seismic, electromagnetic, shipboard geophysics and heatflow) to constrain at a high resolution the material properties of the lithosphere and asthenosphere. This will allow us to address fundamental questions about the evolution of oceanic lithosphere-is it thermally controlled or is it a compositional boundary, or does its rheological behavior transition with age between the two? For the asthenosphere we will be able to determine whether its relative weakness, high conductivity and low seismic velocity are caused by increased melt, hydration or if it is purely thermally controlled.
For the first phase of the experiment, we will deploy 30 broadband ocean bottom seismometers to image the crust and upper mantle down to 300 km using passive seismic techniques and 3 Magnetotelluric instruments to measure ressitivity. We will use surface and body waves to determine the isotropic and anisotropic structure of the crust and upper mantle, we will use converted phases to determine the character and depth of the LAB and other upper mantle discontinuities. We will also use shear wave splitting to determine azimuthal anisotropy across the region. We will construct 1-D resistvity profiles at 0, 25 and 40 Ma seafloor.
Our colleagues in France (IPGP) are already funded to perform an active seismic experiment designed to image the LAB and upper mantle structure from 0-100 Ma seafloor, as well as performing compliance measurements to determine the shear velocity structure of the crust and upper mantle. Our colleagues in the US (Scripps Instituiton of Oceanography) have applied for funding to perform active and passive electromagnetic studies along the same transects. Finally, our colleagues in Germany (GEOMAR) have applied for funding to do complementary high resolution electromagnetic, heat flow and active source experiments.
We are requesting shiptime to deploy 30 broadband instruments from the UK (15), French IPGP (9) and German DEPAS (6) ocean bottom seismometer pools. Figure 1 shows the array geometry for our 2 phase deployment (green triangles show locations of stations for phase 1, red phase 2, and black circles EM). After the success of phase 1 we will request funds for an additional passive deployment to cover 30-80 Ma seafloor. |
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Summary of Facility Requirements: |
We will be deploying 30 broadband ocean bottom seismometers provided by the OBSIP pool, and 30-40 OBEM from PI Constable at Scripps Oceanography. We will need Crane or A frame for deployment of equipment.
Ideally would would also run underway geophysical equipment, gravity, swath bathymetry, magnetometer, and sidescan sonar and technical support.
We will need lab space for prepping equipment and/or deck space to facilitate shipping containers for the OBS and OBEM equipment. |
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Summary of other requirements and comments: |
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Type of Request: |
Primary Ship Use |
Request Status: |
Submitted |
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Request ID: |
1007866 |
Created By: |
Catherine A. Rychert |
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Date Last Modified: |
7/28/2015 4:33:00 PM |
Date Submitted: |
7/16/2015 7:17:00 AM |
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Year: |
2016
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Ship/Facility: |
Marcus Langseth
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Optimum Start Date:
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1/1/2016 |
Dates to Avoid: |
No dates to avoid. |
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Earliest Start Date: |
1/1/2016 |
Multi-Ship Op: |
No |
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Latest Start Date: |
3/31/2016 |
Other Ship(s): |
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Operating Days Needed: |
Science Days |
Mob Days |
De-Mob Days |
Estimated Transit Days |
Total Days |
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17 |
2 |
2 |
8 |
29 |
Repeating Cruise?
(within same year) |
No |
Interval: |
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# of Cruises: |
1 |
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Description of Repeating cruise requirements: |
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Justification/Explanation for ship choice, dates,
conflicts, number of days & multi-ship operations: |
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Short Description of Op Area
for use in schedules: |
OBS deployment |
| Description of Op Area: |
Mid Atlantic Ridge to 40 Ma seafloor. |
| Op Area Size/Dia.: |
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Lat/Long |
Marsden Grid |
Navy Op Area |
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Beginning
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Ending
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Show Degrees Minutes |
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Foreign Clearance Required? |
Yes |
Coastal States:
United Kingdom |
 Important Info on Foreign Research Clearances
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Are you or any member in your science party bringing in any science equipment items which are regulated for export by the International Traffic in Arms Regulations (ITAR) and/or the Export Administration Regulations (EAR)?
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No |
If yes, have you applied for the necessary permits through your export control office?
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No |
| Questions about ITAR/EAR regulations?
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Comments about foreign clearance requirements or
description of any other special permitting requirements
(e.g., MMPA, ESA, IHA, Marine Sanctuaries, etc.) |
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Requested Start Port |
Intermediate Port(s) |
Requested End Port |
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Praia, Sao Tiago, Cape Verde Islands |
None |
Praia, Sao Tiago, Cape Verde Islands |
Explanation/justification for requested
ports and dates of intermediate stops
or to list additional port stops |
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Important Info on Working in Foreign Ports
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Chief Scientist: |
Catherine A. Rychert, UoS |
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# in Science Party |
12 |
# of different science teams |
1 |
# Marine Technicians to be
provided by ship operator:
(include in science party total)
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1 |
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Explanation of Science Party Requirements and Technician Requirements |
The berths are for watchstanders/PI for underway geophysics and for technicians for the OBS and OBEM deployments. |
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 Dynamic Positioning | ADCP | Multibeam | Seismic |
| Dredging/Coring/Large Dia. Trawl Wire | Stern A-frame | Fiber Optic (.681) | 0.680 Coax Wire |
| SCUBA Diving | Radioisotope use - briefly describe | NO Radioisotope use/Natural level work | Other Operator Provided Inst. - Describe |
| 0 PI-Provided Vans - briefly describe | MOCNESS | | |
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Explain Instrumentation or Capability
requirements that could affect choice
of ship in scheduling. |
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Explain Major Ancillary Facilities
Requirements and list description
and provider for "other" systems. |
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