MARGINS-Related Sessions at AGU Joint Assembly
Acapulco, Mexico, May 22-25, 2007

Every year the MARGINS Office assembles a list of sessions to be held at the AGU Joint Assembly that we think may be of special interest to the MARGINS community. The summaries included with our subjective choices are edited excerpts from the original AGU session abstracts (http://www.agu.org/meetings/ja07/?content=home).

• G02: Steady and Transient Strain Along the Middle America Subduction Zone

The Middle America subduction zone has long been a superb target for studies of shallow subduction. Fast subduction rates, coupled with relatively small trench-to-coast distances, and shallow subduction angles along much of the coast, makes this area a unique location for land-based GPS and seismic observations to study both the locked, seismogenic area of a subduction interface and region of deeper transitional slip, where episodic tremor and slip may originate.

• G06: Aseismic Slow Slip: Where, When and Why?

Aseismic slow slip events (SSE) have been observed with a variety of geodetic techniques (GPS, strainmeters, tiltmeters, tide gauges etc) during the last decade. SSE has been noted on the San Andreas fault, in Cascadia as well as other subduction zones (Japan, Costa Rica, New Zealand). Indeed, the 2007 Joint Assembly is being held in Acapulco, Guerrero, a region with a rich history of SSE. Recent seismological studies have revealed that non-volcanic tremors (NVT) are in general correlated with SSE. The origin of slow slip events and tremors is not yet clear but it is hypothesized that both phenomena are related with dehydration of the subducted slab. New studies at different subduction zones and large crustal faults are important in order to compare the physical conditions of SSE and NVT occurrence.

• G08: Earthquake cycle and fault mechanics

The earthquake cycle is poorly understood. Earthquakes continue to occur on previously unrecognized faults. Earthquake prediction seems impossible. Nowadays, space geodetic techniques, especially InSAR, provide crucial data on earthquakes and the seismic cycle. Measuring the response of the Earth to the known shock of an earthquake can provide vital clues to the properties of the Earth’s crust and upper mantle. As well as upper mantle flow, earthquakes can also induce changes in pore-water pressure and in some cases the deeper sections of faults are subject to so-called afterslip. Additional outstanding problem in crustal deformation research is the role of the lower crust and mantle in the earthquake cycle. It’s important to know the strength of lower crust-mantle flow and the behaviors during the interseismic period and the postseismic period. This session aims at discussing on these issues.

• OS16: Sedimentation Associated with Wet-Tropical Rivers: Interdisciplinary Linkages

This session will address issues of sedimentation associated with the discharge of wet-tropical rivers entering the ocean in low-latitude settings. These systems dominate the supply of particulate and dissolved components to the global ocean, and processes in the adjacent ocean control the fate of this material. Contributions are encouraged across the spectrum of oceanographic disciplines, especially those that highlight interdisciplinary linkages (e.g., how sedimentary processes impact carbon burial). Emphasis will be placed on the coastal ocean, but studies/observations are also encouraged: from adjacent environments (e.g., river drainage basins, continental slope), that provide a broad perspective (e.g., remote sensing), address forcing mechanisms (e.g., sea-level change), and contrast with sedimentation in other latitudinal settings (e.g., temperate locations).

• S02: Upper Mantle Structure and Geodynamics of the South American Plate

South America is unique in having the longest continuously-active subduction of oceanic lithosphere beneath continental lithosphere. The long history of subduction along western South America implies its subcontinental mantle lithosphere and mantle wedge have been affected by widely varying convergence conditions and slab morphology. Thus, observations of crustal and upper mantle structure of South America, and geodynamic modeling of this subduction history and attendant effects are important avenues to better understanding the dynamics of the plate. Recent field experiments and new geodynamic analysis tools are refining our perception of South America's upper mantle structure and composition and their relationships to its plate motion history and structural evolution.

• S09: New Results and Future Directions for Understanding the Gulf of California Rift

The Gulf of California stands as one of the world’s primer examples of a young rift that shows different stages of the rifting process from the rupturing of continental lithosphere to the formation of oceanic lithosphere along a new ridge-transform plate margin. Through numerous multidisciplinary projects, the international earth science community has shown an intense interest in understanding the geodynamic evolution of the Gulf of California. This session will provide a forum for presenting new data to characterize the lithospheric structure, kinematics and volcanic evolution, and regional patterns of faulting and seismicity. These new results have important implications for understanding several key issues including 1) how distinct modes of continental extension affect the future style and magnitude of seafloor spreading, 2) role of the middle Miocene plate tectonic reorganization that included the mutual annihilation of a ridge and a trench along the entire length of the Baja California Peninsula, and 3) significance of the asymmetry of the rifted continental margins on either side of the gulf. Additionally this session will be used as a forum for discussing the directions of future studies including the identification of key sites for IODP drilling.

• S14: Characteristics of the Subducted Cocos Plate beneath Mexico and Central America and related seismic hazards

In the last few years, several field experiments, deploying large numbers of portable broadband seismographs, have been carried out in Mexico and Central America. Some of these experiments are still in progress. The main goal of these ambitious projects is to map the characteristics of the subducted Cocos plate beneath the North American and Caribbean plates. In a parallel development, strong-motion networks in the region have been strengthened. Thus, it seems timely to bring together scientists involved in mapping of the subducted plates and those working on related seismic hazard to share their knowledge with each other.

• T02: Subduction Erosion and Tectonic Underplating Along the Central America Trench and Beyond

The modern Acapulco trench is a classic eroding convergent boundary - a site where accretionary wedge sediments and parts of the forearc are clearly missing. How much has been recycled, when, at what rates and the physical processes responsible are unclear. Quantifying long term crustal recycling into the mantle by subduction vs. lateral slicing and tectonic underplating at this margin and numerous other circum-Pacific examples is a first order task for continental tectonics. We propose a forum that focuses primarily on geologic, geochemical and geophysical studies aimed at deciphering the crustal structure of the over-riding plate in southern Mexico, its Cenozoic geologic evolution, and implications for subduction erosion. Studies from similar settings worldwide are welcome.

• T03: Untangling Subduction Zone Plate Boundary Coupling and Trench Migration Using Multi-Scale Observations and Modeling

The deformation state of the overriding plate and thrust interface in convergent margins has been studied for more than forty years. Yet, it is still not entirely clear what controls the transition from uncoupled (Marianas) to coupled (Chilean) margins. This question is intimately related to large scale dynamic processes such as trench migration and to regional scale issues, such as the variation of seismic coupling along strike. Particularly, the question of what controls seismicity is of paramount importance for society, including our host country, as unexpected, large earthquakes have recently shown. Numerous attempts to predict plate boundary behavior, such as expected rupture lengths, focusing on a range of processes, from global plate motions to local, structural features, have only been partially successful. Recent advances including the use of geopotential fields to estimate seismic coupling, mantle dynamics modeling of forces, temperatures, and volatiles, as well as seismologic, geodetic and geological constraints on all parts of the seismic cycle, provide an outstanding opportunity to make progress.

• T06: Mexican and Central America Subduction Zones: Bringing Together Seismology, Petrology, Geology and Geodynamics

Subduction zones host a broad diversity of fundamental and important geological processes. Fluids expelled from subducted sediments and metamorphosed oceanic crust change the rocks rheology, which in turn play an essential role on the evolution of subduction systems. The amount of water and volatiles can vary through geological time, and also the downdip extension of the dehydration front. Also they play a key role in arc magma genesis and the maintenance of compositional heterogeneity in the Earth's mantle. The Mexican and Central America subduction zones offer good examples of these complex and interrelated processes, whose understanding requires a an interdisciplinary approach.

• U08: Processes Controlling Earthquake Potential of Subduction Zones

The shallow subduction zone environment is responsible for ninety percent of all great earthquakes and can generate massive tsunamis. A surge of interest in large megathrust earthquakes followed the disastrous 2004 Sumatra event. Megathrust events raise new questions, such as: What are the similarities and differences between them? Are all subduction zones potential scenarios of megathrust earthquakes? How does interseismic strain accumulation evolve before the next major event? How does this relate to episodic and continual creep, and interface rheology? With advances in understanding of interface processes, are we gaining new insight about the unique character and potential of megathrust earthquakes?

• U12: Disaster Mitigation and Capacity Building Through Geophysical Monitoring

Geophysics is an especially effective science for facilitating international collaboration and technology transfer because research generally requires both technical expertise and “in-place” observational campaigns. Geophysical monitoring contributes directly to natural disaster mitigation such as earthquakes, tsunamis and volcanic eruptions by providing better data for hazard analysis, warning systems, emergency response, and rapid loss estimation. Operation of monitoring networks can contribute to broader capacity building, especially if network technology and operations are frequently improved and coupled with technical training, open data exchange, and research collaboration. Recent developments suggesting that recognition of these societal benefits is broadening include initiatives at the UN Development Program and the World Bank to include natural disaster risk and mitigation in plans for development projects, national commitments to seek societal benefits through a Global Earth Observation System of Systems, and widely-recognized examples of success that include AfricaArray.

• V08: Diversity of the Subarc Mantle- Insights from Studies of Peridotite Xenoliths, Ophiolites and Metamorphic Rocks from Subduction Zones

Constraining the nature and extent of mantle heterogeneities in the sub-arc region is essential to understand and model island arc magmagenesis. Recent advances in trace element and isotope geochemistry, as well as in experimental petrology, have brought a plethora of new findings unraveling a much more complex picture of the mantle in subduction zones as was previously thought. This session aims to bring together scientists studying the chemical, mineralogical and geophysical diversity of the mantle regions in currently operating, as well as ancient, subduction zones. Reports from any aspects of exposed sections of subduction zone successions (including mapping results) will be particularly welcome. We encourage submissions linking the geochemical signatures of subduction-related mafic and ultramafic rock assemblages to those of island arc volcanic rocks.

• V09: Towards Integrated Studies on Volatiles in Magmas: from the Mantle to the Atmosphere

Magmas transfer volatiles from the Earth’s interior to the oceans and atmosphere. On their way to the surface, volatiles can be separated from magma and contribute to hydrothermal systems and/or produce fumarolic emissions. Exsolution and loss of volatiles, particularly H2O, also leads to major changes in magma crystal content, density and viscosity. Finally, gas emissions during volcanic eruptions can have important effects on the atmosphere and climate. These factors make volatiles an essential concern of volcanology, and understanding their behavior and influence on a wide variety of geological phenomena is crucial. Processes of deep or shallow degassing are important to identify processes and monitor/forecast changes in eruption style (explosive to dome building), which in turn is essential for hazard evaluation and risk assessment. The goal of this session is to integrate information on volatiles exsolution, movement, their role in fragmentation processes, and injection into the atmosphere or oceans. It is also hoped that this session will stimulate collaboration between the different fields of study concerning volatiles in magmas.

• V11: Deep subduction zone metamorphism and rheology: role of fluids

Fluid-driven processes in subduction zones are of great importance because on the large scale they can trigger earthquakes and generate arc magmatism. On the smaller scale, fluid speeds mineral reactions and can dramatically change the rheology of the minerals making up the lithospheric plates, including metamorphosed sediments. High-pressure (HP) and ultrahigh-pressure metamorphic (UHPM) rocks created during deep subduction of oceanic or/and continental lithosphere are an ideal natural laboratory for observation of the microstructures produced by recrystallization and deformation acting together. Combined with high-pressure experiments, these microstructures are critical for quantitative interpretation of the conditions and kinetics of metamorphic reactions and rheology. Unfortunately, these two important parts of the problem, field observations and laboratory measurement of reaction kinetics and/or rheology, are traditionally studied by two separate groups of scientists: one having a strong expertise in the geological processes recorded in the rocks, the other having a strong understanding of mineral physics. The target of this session is to bring together mineralogists and petrologists who work on UHPM rocks with experimentalists involved in quantitative measurement of the role of H2O on mineral reaction kinetics and rheology.

• V18: High Mg-Andesites, Slab Melts and Wedge Melts: Significance for Crustal Genesis?

Subduction zones are key sites of crustal growth and the chemical composition of magmas erupted provide important insights into subduction zone processes. While most models for arc magma petrogenesis involve hydrous melting of peridotite in the mantle wedge, several studies have suggested that the role of high Mg-andesites is critical for the genesis of arc magmas and continental crust in general. We invite abstracts on this subject in which integrated petrologic observations combined with geochemical and numerical studies are used to evaluate the various roles of high-Mg andesites, slab melting and hydrous wedge melting in continental crust genesis. The range of subjects to address may include: physical and chemical interactions at the base of the arc crust; the role of water; models of arc growth through time; the fate of slab melts and related topics that include an interdisciplinary approach to understanding continental crust growth in arc settings.

• V21: Volcanoes, Plutons and Ore Deposits: What's the Connection?

The magmatic plumbing system above subduction zones is a mystery. Those who study the extrusive products see processes that differ substantially from those that study the high pressure metamorphic rocks. The plutons and arc-related ores provide a third differing view, such that a combined holistic view of arc magmatic systems is lacking. The arc-related porphyry and epithermal type ore deposits supply the majority of our silver, copper, gold and molybdenum and, thus, are of significant interest. When viewed in space and time, these geologic entities are parts of a magmatic continuum. Thus, any processes affecting one entity clearly affect the others. That is, the chemical signature of a volcano is a function of fractionation and assimilation in the plutonic magma chamber and likewise the chemistry of ore deposits in volcanic arc settings is controlled by magmatic processes in the pluton and volcanic edifice. Extant geochemical data (i.e., trace elements, isotopes, fluid and melt inclusions) strongly support this chemical link. Yet we still have but a cursory understanding of the physicochemical mechanisms which result in ore production. This session aims to bring together volcanologists, plutonists and ore geologists who are pursuing the study of both natural and experimental assemblages in an effort to describe and understand the mobility of elements in arc environments.