NSF Org:	OCE Division Of Ocean Sciences
Recipient:	THE TRUSTEES OF COLUMBIA UNIVERSITY IN THE CITY OF NEW YORK
Initial Amendment Date:	August 11, 2015
Latest Amendment Date:	August 11, 2015
Award Number:	1536566
Award Instrument:	Standard Grant
Program Manager:	Candace Major OCE  Division Of Ocean Sciences GEO  Directorate for Geosciences
Start Date:	August 15, 2015
End Date:	July 31, 2016 (Estimated)
Total Intended Award Amount:	$80,480.00
Total Awarded Amount to Date:	$80,480.00
Funds Obligated to Date:	FY 2015 = $80,480.00
History of Investigator:	Geoffrey Abers (Principal Investigator) abers@cornell.edu
Recipient Sponsored Research Office:	Columbia University 615 W 131ST ST NEW YORK NY  US  10027-7922 (212)854-6851
Sponsor Congressional District:	13
Primary Place of Performance:	Lamont-Doherty Earth Observatory 61 Route 9W Palisades NY  US  10964-1707
Primary Place of Performance Congressional District:	17
Unique Entity Identifier (UEI):	F4N1QNPB95M4
Parent UEI:
NSF Program(s):	Marine Geology and Geophysics
Primary Program Source:	01001516DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s):
Program Element Code(s):	162000
Award Agency Code:	4900
Fund Agency Code:	4900
Assistance Listing Number(s):	47.050

ABSTRACT

The primary goal of this study is to better understand the mantle beneath tectonic plates that floor the ocean basins. The Cascadia Initiative deployed an Amphibious Array of seismic stations across the complete Juan de Fuca plate and adjacent Cascadia margin, in the Pacific Northwest. This is the first time a seismic array has covered a complete plate from spreading center to trench to volcanic arc. The cooling, rigid structure that characterizes an oceanic plate from its formation, through aging, and its eventual subduction at the continental margin is well understood. In contrast, the nature of the ductile, partially molten mantle underneath is less certain. Does melt occur in the mantle only beneath the magma-producing upwelling zone beneath the oceanic spreading center, or are small amounts of magma beneath the aging plate? As the plate approaches a trench and bends downward to subduct, does fracturing let seawater filter down into the plate, to be absorbed in minerals and carried into the mantle? If so, how and where do these volatiles later emerge to affect the fluidity of the slowly circulating mantle? The seismic analysis carried out in this study aims to answer these questions. A graduate student will carry out most of the analysis, expanding skills in a final PhD training year beyond what has already been gained in earlier thesis research.

Although the thermal structure of oceanic lithosphere is well understood, basic questions regarding asthenosphere still exist. Both partial melt and bound water are proposed to explain low asthenosphere seismic velocities and viscosities but their distribution is not well constrained. It is not known if the base of the asthenosphere is defined by a constant pressure / homologous temperature, or if the asthenosphere is entrained at a subduction zone and carried to depth. Additional questions about temperature, water distribution and melt exist once plates subduct. This study focuses on seismic attenuation across the Cascadia Amphibious Array measured from body waves, since these physical properties can be better elucidated than from seismic velocity studies alone. Because the Amphibious Array provides such comprehensive broadband seismic data, the expected variations in body-wave attenuation can be seen here better than any other oceanic region. Comparisons with geodynamic models and with surface-wave derived images will allow calibration of laboratory-based theories of anelasticity.

PROJECT OUTCOMES REPORT

Disclaimer

This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

Seismic waves as they pass through hot and molten regions of the earth lose energy or “attenuate”.  Conversely, measurement of the amplitudes of seismic waves, and how they vary from place to place, provides insight into the distribution of high temperatures and molten regions.  This project measures for the first time the seismic attenuation of an entire tectonic plate at high resolution, from the formation of the plate at a mid-ocean ridge to its demise at a subduction zone.  The plate studied here is the Juan de Fuca plate offshore the northwest U.S. and southwest Canada. There, the NSF-funded Cascadia Initiative deployed a novel deployment of ocean-bottom seismometers spanning the entire plate and coastal zone of deformation over a four year period, providing some of the first data available for complete studies of this nature.  The primary result is an unexpectedly large amount of attenuation near the mid-ocean ridge, many times larger than standard models predict. The variation in attenuation from the ridge to the subducting end of the plate is in the upper 200 km of the Earth, largely in the mantle, and is some of the largest variation observed anywhere on the planet.  Any plausible explanation appeals to widespread partial melt in a region over 100 km from the ridge crust, where mantle rocks contain small amounts of molten material in pores and cracks.  Although this dataset images melt over a wide region in the mantle, it only erupts in a very narrow region at the mid-ocean ridge, requiring very vigorous mechanisms for focusing melt as it rises to the surface.  This explanation simultaneously allows a better understanding of how magmas form near mid-ocean ridges, the largest volcanic chains on earth, and provides calibration of how seismic observations of this type can identify melt.  These methods now can be used to better understand the deep plumbing and origins of magmas beneath large explosive volcanic centers on land, where they form a significant hazard.

Last Modified: 10/24/2016
Modified by: Geoffrey A Abers

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