Back-Arc Spreading Systems: Geological, Biological, Chemical, and Physical Interactions

305 Pages·2006·38.7 MB·English

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Geophysical Monograph 166
Back-Arc Spreading Systems:
Geological, Biological, Chemical,
and Physical Interactions
David M. Christie 
Charles R. Fisher 
Sang-Mook Lee 
Sharon Givens 
Editors
Published under the aegis of the AGU Books Board
Jean-Louis Bougeret, Chair; Gray E. Bebout, Cassandra G. Fesen, Carl T.
Friedrichs, Ralf R. Haese, W. Berry Lyons, Kenneth R. Minschwaner, Andrew
Nyblade, Darrell Strobel, and Chunzai Wang, members.
Library of Congress Cataloging-in-Publication Data
Back-arc spreading systems : geological, biological, chemical, and
physical interactions / David M. Christie ... [et al.], editors.
p. cm.
ISBN-13: 978-0-87590-431-3
ISBN-10: 0-87590-431-9
1. Sea-floor spreading. 2. Back-arc basins. 3. Hydrothermal vents.
4. Marine geophysics. I. Christie, David M.
QE511.7.B33 2006
551.1'36--dc22
2006031438
ISBN-13: 978-0-87590-431-3
ISBN-10: 0-87590-431-9
ISSN: 0065-8448
Cover: Saint James' Spires, a chimney complex at 1900 m depth in the Tu'i Malila
hydrothermal field on the East Lau Spreading Center. The snails in the foreground are
Ifremeria nautilei and the mussels are Bathymodiolus brevior. The picture was taken
from the ROV Jason II and is provided compliments of Charles Fisher and the Ridge
2000 Program.
Copyright 2006 by the American Geophysical Union
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CONTENTS
Preface
David M. Christie and Charles R. Fisher ...........................................................................................................vii
Geological, Biological, Chemical, and Physical Interactions in Back-Arc Spreading Systems— 
An Introduction
David M. Christie and Charles R. Fisher .............................................................................................................1
Synthesis Papers
Modes of Crustal Accretion in Back-Arc Basins: Inferences From the Lau Basin
Fernando Martinez and Brian Taylor. ..................................................................................................................5
Present-Day Tectonics in Four Active Island Arcs Based on GPS Observations and Forearc  
Stress Fields
Teruyuki Kato and Atsuki Kubo ........................................................................................................................31
Mantle Structure and Flow Patterns Beneath Active Back-Arc Basins Inferred From Passive  
Seismic and Electromagnetic Methods
Douglas A. Wiens, Nobukazu Seama, and James A. Conder ............................................................................43
Origin of Back-Arc Basin Magmas: Trace Element and Isotope Perspectives
Julian A. Pearce and Robert J. Stern ..................................................................................................................63
Chemical Systematics and Hydrous Melting of the Mantle in Back-Arc Basins
C.H. Langmuir, A. Bézos, S. Escrig, and S.W. Parman .......................................................................................87
Unique Geochemistry of Submarine Hydrothermal Fluids From Arc–Back-Arc Settings  
of the Western Pacific
Toshitaka Gamo, Junichiro Ishibashi, Urumu Tsunogai, Kei Okamura, and Hitoshi Chiba ................................147
Magmatic Fluids as a Source of Metals in Seafloor Hydrothermal Systems
Kaihui Yang and Steven D. Scott ....................................................................................................................163
Microbial Ecology of Mid-Ocean Ridges and Back-Arc Basins
Ken Takai, Satoshi Nakagawa, Anna-Louise Reysenbach, and Joost Hoek .......................................................185
Composition and Biogeography of Hydrothermal Vent Communities in Western Pacific  
Back-Arc Basins
Daniel Desbruyères, Jun Hashimoto, and Marie-Claire Fabri. ..........................................................................215
Physiological Ecology of Back-Arc Basin Fauna: Explorations of the Known and Unknown
James J. Childress, Dijanna Figueroa, and Michael Henry ...............................................................................235

Case Studies
The East Scotia Sea: Mantle to Microbe
Roy Livermore ...............................................................................................................................................243
Tectonic and Magmatic Evolution of the Bismarck Sea, Papua New Guinea: Review  
and New Synthesis
Sang-Mook Lee and Etienne Ruellan ..............................................................................................................263
Origin of Diverse Geochemical Signatures in Igneous Rocks From the West Philippine  
Basin: Implications for Tectonic Models
Rosemary Hickey-Vargas, Ivan P. Savov, Michael Bizimis, Teruaki Ishii, and Kantaro Fujioka ...........................287
i
Geological, Biological, Chemical, and Physical Interactions
in Back-Arc Spreading Systems—An Introduction
David M. Christie
Global Undersea Research Unit, School of Fisheries and Ocean Sciences, University of Alaska
Fairbanks, Fairbanks, Alaska, USA
Charles R. Fisher
Biology Department, Penn State University, University Park, Pennsylvania, USA
Each chapter of this volume is based on a presentation at the international
Theoretical Institute, “Interactions among Physical, Chemical, Biological, and
Geological Processes in Back-arc Spreading Systems”, which was jointly sponsored
by Ridge 2000 and InterRidge, on Jeju Island, South Korea, in May 2004. The vol-
ume is divided into two sections. Papers in the first section synthesize the state of
knowledge of back-arc basins in general, from the perspectives of a broad range of
disciplines. The papers are arranged according to their subject matter, roughly fol-
lowing the flow of energy and material from their origins in the mantle, through the
magmatic systems that create and modify the oceanic crust, to the hydrothermal and
biological systems that derive their existence from magmatic heat and chemicals.
Papers in the second section are case studies, focused on the geology and geophys-
ics of individual back-arc basins. These include two back-arc systems that are more
than usually complex, as a result of their more complex tectonic settings.
Back-arc basins are elongate oceanic basins formed by the Mid-Atlantic Ridge. Back-arc basin studies repre-
seafloor spreading and related extensional tectonic pro- sent an important emerging area of ocean sciences for a
cesses. They form adjacent to island arcs, on the opposite number of reasons. Because they are closely associated
side from the subducting oceanic plate. Back-arc basins with islands, back-arc basins commonly have considerable
are a characteristic feature of oceanic convergent plate economic, cultural, and political importance, especially
boundaries, and the dominant role of extensional tec- to western Pacific nations that include back-arc regions
tonics in their formation and evolution is a paradox that within their Exclusive Economic Zones. Scientifically,
remains imperfectly understood. Mature back-arc basins back-arc spreading centers are important analogs for, and
are floored by predominantly basaltic lavas erupted along complements to, mid-ocean spreading systems, relative
back-arc spreading centers that are very similar to “nor- to which they are more diverse, shorter-lived, and more
mal” mid-ocean ridges such as the East Pacific Rise or rapidly evolving. Their proximity to islands allows for
real-time geodetic studies using global positioning system
Back-Arc Spreading Systems: Geological, Biological, Chemical, technology that is not possible at most mid-ocean spread-
and Physical Interactions ing centers. Back-arc magma systems are more chemically
Geophysical Monograph Series 66
and physically diverse and variable, providing a window
Copyright 2006 by the American Geophysical Union
0.029/66GM02 to understand their dynamic and complex mantle source

2 INTRODUCTION
regions. Because of their diverse and often extreme chem- arc). This new geophysical evidence for arc-parallel flow
istry, back-arc hydrothermal systems support a diversity of supports earlier hypotheses, based on both geochemistry
microbial fauna with considerable biotechnological poten- and seismology, that mantle material is flowing southward
tial. The chemical diversity of hydrothermal microhabitats behind the Tonga arc from the vicinity of Samoa.
in relatively isolated back-arc basins supports a variety of Patterns and processes of mantle flow may also be inferred
endemic vent macrofauna, each with its own potential to from igneous geochemistry. Pearce et al. use trace element
contribute to our understanding of the physiological limits ratios in back-arc lavas to infer that three types of mantle
to animal life. These shorter-lived, more variable systems material contribute to back-arc magma sources and to esti-
also provide a natural laboratory for studies of larval dis- mate their relative contributions. They identify the three
persal and genetic linkages, both regionally and among the mantle types as ambient mantle from the mantle wedge
major global biogeographic vent faunal provinces. beneath the backarc; material derived from the subducting
slab as a fluid phase; and mantle that has physically migrated
MANTLE STRUCTURE AND DYNAMICS into the region above the slab, through gaps or tears in the
subducting slab or around its end(s).
Studies of the dynamics and deep structure of Island By combining a broad array of geochemical data with new
Arc–Back-Arc Systems are primarily centered in the dis- geochemical models for melting in the presence of water,
ciplines of Geodesy, Geodynamics, and Seismology, which Langmuir et al. have developed a new view of the shape of
depend heavily on instrumental observations, and in Igneous the region within which melting occurs beneath the backarc,
Geochemistry, which uses high-precision trace element and and of variations in the melting process throughout this
isotopic data to infer conditions of melt formation and details relatively complex region. Key features of this new melting
of mantle composition. model are the spatial asymmetry that is imposed on the melt
For more than a decade, the Global Positioning System region by the proximity of the active island arc and a funda-
(GPS) has allowed accurate measurement of plate motions mental contrast between shallow, hydrous, iron-poor melts
in real time. Arrays of GPS instruments are especially on the arc side of the spreading center and deeper, dryer,
suited to arc and back-arc geodynamic studies, because more iron-rich melts on the farther side.
GPS stations, which must be stable and above water, can be
placed on islands within and adjacent to areas of interest. HYDROTHERMAL VENTS AND
Arrays of GPS stations allow for measurement of actual HYDROTHERMAL DEPOSITS
earth motions with high precision and accuracy. Such
measurements are rarely possible for normal mid-oceanic The inherent geological variability of backarc basins pro-
spreading centers. Kato and Kubo summarize four major vides a greater variety of substrates for active hydrothermal
GPS studies of convergent margins that include back-arc systems relative to mid-oceanic spreading centers. This
systems. Although each of the areas has a different inter- variety, in turn, provides a broader range of opportunities for
nal structure, these studies show that back-arc extension evaluating the interactions between substrate (volcanic rock)
is measurable on timescales of just a few years and that compositions and those of hydrothermal fluids and deposits.
arc-parallel extension is common, though not universal, in Of particular interest is the significantly greater abundance
island arc and forearc terrains. of volatile species (ultimately derived from devolatiliza-
The subduction of a cold slab beneath arc–back-arc sys- tion of the subducting slab) in arc and back-arc magmatic
tems generates frequent, deep (~00 to ~500 km) earth- systems. Known back-arc hydrothermal systems include a
quakes that vary in intensity and are widely distributed in range of styles and temperatures of hydrothermal venting,
both arc-parallel and cross-arc directions. In some cases, and a number of valuable ancient ore deposits have been
such as the Lau Basin, these high-frequency, powerful interpreted as having formed in back-arc settings.
seismic sources are distributed beneath the entire back Gamo et al. have reviewed current knowledge of back-
arc region, providing an almost ideal “illumination” of arc hydrothermal chemistry. As expected from the vari-
the back-arc spreading system. Wiens et al. have com- able underlying geology, back-arc hydrothermal fluids are
bined passive seismic data that exploit this natural seismic much more variable than their mid-ocean ridge counterparts.
source beneath the Lau Basin with electromagnetic data Backarc hydrothermal fluids are often quite distinct in their
to evaluate three-dimensional mantle flow patterns. Key very high volatile contents, relative to those of mid-ocean
observations include contrasting mantle anisotropies that spreading centers. Back-arc hydrothermal volatile enrich-
are consistent with arc-parallel flow beneath the Tonga ment reflects the volatile-rich character of the host lavas,
arc and with cross-arc flow beneath the Lau Basin (back- and perhaps of still-molten magmas. In some back-arc hydro-
CHRISTIE AND FISHER 
thermal systems, high SO contents give rise to high acidity, the well-studied symbiont-containing vent fauna from mid-
2
with significantly lower pH values (as low as 2) than those ocean ridges to provide a background for understanding the
of mid-ocean ridge systems. back-arc fauna, to constrain their physiological capabilities
The pivotal role of volatiles in hydrothermal metal trans- and the habitat conditions they are likely to tolerate, and to
port and ore deposition is discussed by Yang and Scott. They suggest productive areas for future research on back-arc
conclude that volatiles derived directly from molten magma, vent fauna.
even if they constitute only a small fraction of the total vola-
tile budget, can supply most of the metals carried by seafloor CASE STUDIES OF INDIVIDUAL BACK-ARC BASINS
hydrothermal systems and incorporated into sub-seafloor
mineral deposits. Back-arc magmas are generally more vola- The Lau Basin has an overall south-pointing V-shape,
tile-rich, and felsic magmas (which concentrate many metals indicating that the back-arc spreading center is extending
as they evolve) are much more common in back-arc settings (propagating) to the south. One consequence of this south-
than along normal mid-ocean ridges. As a result, back-arc ward growth is that the tectonic environment, and therefore
hydrothermal systems are often metal-rich, and large ore the geological character, of the back-arc spreading center
deposits are much more likely to occur in backarc settings changes systematically from north to south. Martinez and
than on most mid-ocean ridges. Taylor describe this variation in some detail and use it to
examine the interplay of processes and conditions that influ-
HYDROTHERMAL VENT BIOTA AND ECOSYSTEMS ence the spreading process in this basin and, by inference, in
other back-arc spreading environments.
Takai et al. extend the discussion of geological and hydro- Livermore summarizes current knowledge of the east
thermal diversity in back-arc systems to the field of micro- Scotia Sea “from mantle to microbe”. This unique, isolated
bial ecology, reviewing the available information on the backarc basin lies behind (to the west of) the South Sandwich
microbial inhabitants of these geologically diverse spreading Arc in the southwest Atlantic Ocean. This arc–back-arc
centers. The greater variability in back-arc fluids, lavas, and system provides a particularly good case study because it
mineral deposits (relative to mid-ocean spreading centers) has a relatively short geological history and is built entirely
is paralleled by a wide diversity among microbiological on “oceanic” crust with no underlying older arc material. It
communities. This variability facilitates detailed studies of is of great geobiological interest because it is both isolated
individual groups and of the relationships between microbial from other known (or potential) hydrothermal vent systems
populations and their microhabitat parameters. In addition, and situated at an important gateway between the Pacific
the diversity of microbial life associated with back-arc basin and Atlantic Oceans.
hydrothermal systems provides new insights into the rela- Some back-arc basins, especially those to the west of the
tionships among hydrothermal microbial populations from western Pacific rim, occupy more complex tectonic settings
around the world. than the “simple” island arc–back-arc systems, such as the
Desbruyères et al. have summarized the distribution and East Scotia, Lau, and Mariana back-arc basins described
composition of hydrothermal animal communities from in earlier chapters. These more complex back-arcs undergo
five back-arc spreading centers. They provide a compre- changes in local (micro) plate motion in response to exter-
hensive compilation of the occurrence of species endemic nal forces and are therefore more tectonically and magmati-
to the western Pacific back-arc basins. On the basis of cur- cally complex. Lee and Ruellan review the history of the
rent knowledge, they identify a global-scale biogeographic Bismarck Sea, a complex back-arc basin to the north of New
provincialism. At the generic level, they recognize three Guinea. The history of this basin has been complicated both
population clusters: the Mid-Atlantic Ridge; the East Pacific by its location between two opposing subduction zones
Rise, subdivided into northern and southern subgroups; and and by its ongoing collision with the northward migrating
a third, widespread group consisting of the western Pacific Australian Plate. These external forces have induced local
back-arc spreading centers, the Central Indian Ridge, and plate motion changes within the basin, resulting in micro-
perhaps including the Gorda and Juan de Fuca ridges in the plate rotation and highly asymmetric back-arc spreading.
NE Pacific. Hickey-Vargas et al. focus on the magmatic evolution of
Childress et al. focus on the physiological ecology of vent the West Philippine Basin, which was actively spreading
fauna, in particular, the role of microbial symbionts in the from 60 to 5 Ma and which remained magmatically active
animals that dominate the biomass at vents around the world. until 26 Ma, when the Izu–Bonin–Mariana arc to the east
Because no back-arc species has been the focus of intensive began to dominate the tectonics of the region. An interest-
physiological study, Childress et al. use what is known about ing and unexplained feature of both of these complex basins
4 INTRODUCTION
is the occurrence of relatively voluminous, late-stage mag-
David M. Christie, Global Undersea Research Unit, School of
matism that is not directly related to spreading processes.
Fisheries and Ocean Sciences, University of Alaska Fairbanks,
These magmas are described as “enriched” in character,
Fairbanks, Alaska 99752, USA. ([email protected])
meaning that their trace element contents are more similar Charles R. Fisher, Biology Department, 208 Mueller Lab., Penn
to those of many ocean island volcanoes, commonly attrib- State University, University Park, Pennsylvania 6802, USA.
uted to deep mantle sources. ([email protected])