Workshop: "Ocean Mantle Dynamics: from Spreading Center to Subduction Zone"

Workshop Report - posted Nov 2011

Workshop abstracts

Background and Objectives

What have and can we derive from crustal and mantle imaging beneath the ocean together with laboratory and numerical studies? Recent high quality seismic surveys with dense OBS (Ocean Bottom Seismograph) arrays reveal variations in the velocity structure of ocean crust, the uppermost mantle, and the Moho transition zone, even if they were formed at a fast-spreading ridge system. Moreover, recent progress in long-term observational technology on the seafloor enables the imaging of regional mantle structure using both seismological and electromagnetic techniques.  The resulting velocity and resistivity structures, combined with results from laboratory experiments on mantle rocks and numerical simulations, provide important constraints toward understanding ocean mantle dynamics.

In this meeting, we focus on three objectives of crustal and mantle imaging, with an emphasis on mantle dynamics:

1) the structure of oceanic lithosphere (including the crust and sub-crust) and asthenosphere

2) melt migration beneath the spreading axis to form oceanic crust

3) the role of water, especially for subduction and back-arc spreading dynamics.

Each topic is closely inter-related, so that we can seek answers and/or ways to find answers for the questions outlined below. For each topic, we would first like to review all the available structural images of crustal and mantle structure. Second, recent laboratory experiments on crustal and mantle rocks will be presented in order to help interpret the images. Then, petrological and geochemical characteristics of the crust and mantle will be used to address variability of crustal formation, and to provide constraints on melt migration and the role of water. Finally, investigations from numerical simulations are expected to help identify the parameters controlling melt migration, crustal formation, and mantle structure.

Topic 1 - the structure of ocean lithosphere and asthenosphere

# What are the results of imaging the variations in the velocity structure of ocean crust, the uppermost mantle, and the Moho transition zone?
# What are the results of imaging oceanic lithosphere and asthenosphere, and its boundary?
# What is the nature of ocean lithosphere and asthenosphere? Can a thick resistive layer (~60km thickness) be oceanic lithosphere or does it represent some other change in physical or chemical properties?
# How does ocean lithosphere and asthenosphere evolve with crustal age?
# What is the structure and mechanism of anisotropy with depth from the crust to asthenosphere?  Is anisotropy well related to mantle dynamics?

Topic 2 - how melt migrates beneath the spreading axis to form oceanic crust, including possible effects on the formation of oceanic lithosphere

# What do we know about structure beneath the spreading axis?
# How do variations in melt production and transport produce the observed variety in structure of the oceanic crust, the Moho transition zone, and the uppermost mantle even if they are formed at a fast-spreading ridge system?
# How fast does the melt migrate?
# Is the melt connected efficiently?
# Is melt trapped in the asthenosphere and in upper most mantle?
# What are controlling parameters in forming oceanic crust? Can the connection between the different mantle structures as input and the crustal formation as output yield insights to the controlling parameters?

Topic 3 - the role of water especially for subduction and the back-arc spreading dynamics.

# What are the roles of water for mantle dynamics?
# Which part of the slab does the water come from: Crust, or uppermost mantle through plate bending? Does it vary depending on incoming crustal and/or mantle structure?
# What are the major differences between back-arc and mid-ocean ridge spreading?

Organizing Committee (InterRidge Mantle Imaging WG members):
Nobukazu Seama, Kobe University (Chair)
Douglas A. Wiens, Washington University, USA (Co-Chair)
Alan D. Chave, Woods Hole Oceanographic Institution, USA
Douglas R. Toomey, University of Oregon, USA
Pascal Tarits, UBO – IUEM, France
Wayne C. Crawford, Institut de Physique du Globe de Paris, France
John Chen, Peking University, China

Local Organizing Committee
Nobukazu Seama, Kobe University (Chair)
Kyoko Okino, AORI, University of Tokyo (Co-Chair)
Eiichi Takazawa, Niigata University (Chief Field Trip Coordinator)
Tomoaki Morishita, Kanazawa University (Field Trip Coordinator)
Katsuyoshi Michibayashi, Shizuoka University (Field Trip Coordinator)
Takashi Sawaguchi, Toyo University (Field Trip Coordinator)
Takeshi Tsuji, Kyoto University
Kiyoshi Baba, ERI, University of Tokyo
Takehi Isse, ERI, University of Tokyo