Subduction zones and volcanic arcs

In the new drilling program, we envision studying subduction zones and volcanic arcs holistically within the context of the Earth‘s processing zone to answer some fundamental questions of interest to the fields of plate tectonics, geo- and atmospheric chemistry, and geohazards. Four themes concerning the combined subduction-volcanic arc system are: (1) arc growth and growth of continents, (2) forearc structure and evolution, (3) fluxes, fluids, and volatiles, and (4) controls on earthquake processes at subduction zones. The latter is reported on in section 5.1.

Arc growth, mass fluxes, and links to growth of the continents

Volcanic arcs are a direct product of subduction and important for assembly of continents, as recorders of past subduction, and for their hazard potential (Kay, 1985).

One set of key questions about arcs is centered on their formation and growth (Jicha et al., 2006; Nikolaeva et al., 2008). Does steady state or episodic growth dominate arc production? How and over what timescales does arc production vary? What controls the across- and along-arc variation in arc crust, arc magma, and mantle wedge composition and structure? How do subduction inputs control these variations? How does arc growth contribute to the growth of continents? Which processes can transform arcs into parts of continents?

The rate of arc crustal growth is a fundamental parameter towards which arc research should be directed within the new program and in allied programs. This topic requires acquiring samples through upper arc crust, determining crustal thickness, and developing an understanding of middle arc crust composition and formation.

Additionally, the role of serpentine in transferring fluids from the subducting plate to the overriding plate and how it influences arc growth are unknown. To study these processes, we need to drill an early/young arc to understand growth and evolution.

Examining the history of crustal growth from arc initiation through extinction requires transects as crustal growth is not uniform and may vary through time and along arc.

There are clear connections among subduction inputs, magma production, and arc heterogeneity but these processes need to be studied in detail. Subduction inputs and volcanic/arc output therefore need to be researched in concert both along-strike and across-strike. Drilling strategies to address these arc questions should include a combination of shallow drilling transects and a deep hole for crustal sampling targeting

age and chemical composition. Shallow transects should be devised for along- and across-arc variation in crustal composition and volatile emissions. Integration of the geophysical images and models with drilling data should allow for quantitative estimates of the variability. Technological challenges to be overcome for these goals include the ability to drill and log at high temperatures.

Forearc structure and evolution

Beyond studies of the earthquake cycle in subduction zones, there are numerous key questions regarding the forearc structure and evolution of active continental margins (Kukowski and Oncken, 2006; Clift et al., 2009; Scholl and von Huene, 2009). What are the mechanisms and products of tectonic erosion (in contrast to accretion)? What factors control this process, at what rates does it occur, and over what timescales? What occurs when a seamount subducts? How do forearc basins in accretionary or erosive active margins develop? Is their formation related to deformation, rheology, or basal taper changes? How do the mechanisms of stability that produce forearc basins relate to or contribute to growth of continents and arc magmatic processes? What are the links between the forearc, arc, and backarc?

Scientific drilling is the only method that can access and sample the submarine forearc. To help understand the process of tectonic erosion, drilling can provide forearc slope paleobathymetry, lithologies, and rheology of non-accretionary forearcs; however, to examine basal erosion mechanisms deeper drilling is required. To study forearc basins, basin sediments can be used to determine the timing of events and may give clues about the evolution of the forearc (Gulick and Meltzer, 2002; Gulick et al., 2002;

Melnick and Echtler, 2006). Shallow sub-basin sampling or drilling near the flanks of basins may provide tests of varying basin formation mechanisms. For both forearc structure and basin studies, along-strike transects could highlight lateral variations and elucidate how subducting plate characteristics influence arc development. Site characterization is critical for site selection and knowledge of the structural framework will allow results at the borehole to be interpolated between boreholes and correlated regionally.

Fluxes, fluids, and volatiles

Considering the forearc to backarc as a continuous system requires investigating fluxes of mass, fluids, and volatiles through the lithosphere. A number of key questions were highlighted at INVEST. What is the hydrogeology of the subducting plate and the role of faults within this system? What are the rates and distribution of fluid-flow within the subduction system? What is the significance or role of serpentine in different parts of the subduction system and what is its contribution to the hydrogeology of the system and earthquake generation process? What is the role of hydrothermal systems in arc volcanic systems and what are their contributions to the biosphere and mineralization (significant resources)? How do diagenetic and metamorphic processes evolve within the subduction system? What is the effective stress distribution (pore pressure, stress magnitude) in space and time? What is the feedback to other systems, e.g., ocean, atmosphere, biosphere, mantle?

In previous drilling efforts these processes have been largely studied in singular settings such as in the context of sediment accretion and dewatering in the forearc or in magma generation within the arc. In the new program we plan to examine these fluxes

and the critical processes that control them through the entire system. To accomplish these goals and answer these questions with drilling is likely to require either new transects across the system, or non-standard drilling implementation allowing these processes to be investigated in differing settings across multiple drilling expeditions.

Arc research has very strong linkages to other programs; thus, focus for the new phase of ocean drilling can build on the US MARGINS program ―The Subduction Factory―, EarthScope, previous ODP/IODP results, seafloor networks and observatories, as well as geophysical data and terrestrial studies. Drilling in forearc environments provides linkages with major existing and planned geophysical datasets, as well as with the numerical modeling community studying margin development and the importance of the subduction erosion process. There is a clear need to combine evidence from land investigations with ocean drilling in arcs. For instance land/sea combined investigations could allow study of the chemistry of volcanism through time including studying lithologies that fingerprint arc initiation, linkages between modern arcs and ophiolite formation, and ash stratigraphy to help establish temporal records and bridge significant stratigraphic gaps in the record of the lavas.

3.6 Science Strategies

3.6.1 Transect concept

For many of the highest priority science objectives regarding the solid Earth (mantle, lithosphere, crust), heterogeneities in composition, structure, material properties, and evolution with time require arrays or transects of drilling sites to capture three-dimensional and four-dimensional variability. These may be modest penetration sites coupled with deep penetration holes, as in the current NanTroSEIZE program, or long transects of modest penetration holes along a crustal flowline to assess aspects of plate aging. Other examples include:

 Arc-parallel transect of sites on the down-going plate to assess variability in slab inputs to the subduction zone ‗factory‘;

 Offset drilling transects of sites to assess variability in ocean crust (particularly that formed at slow-spreading ridges) and ocean plateaus through sampling of composite sections exposed in tectonic windows;

 Cross-hole experiments to determine hydrogeological behavior of the ocean crust;

 Arrays of marine sedimentary sections to examine near- and far-field environmental effects of ocean plateau (LIP) construction; and

 Transects of sites along hotspot lineaments to discover the temporal and spatial scale of mantle source heterogeneities delivered from the deep mantle by plumes.

3.6.2 Land-sea linkages

Processes within the ‗lithospheric membrane‘ by definition cross the boundary between land and sea. Ocean drilling has traditionally focused exclusively on the deep-sea or continental shelf record of these processes and in general on the marine processes themselves; however to fully understand the interactions between the mantle,

crust, ocean, and atmosphere we need to integrate the records of these processes from terrestrial deposits, shallow water and marginal basin sites, continental shelf records, and deep-sea depocenters. These transects from land to sea can be integrated through provenance studies, use of proxies, and thermo-chronology, calibrated with a suite of dating techniques, and mapped using onshore-offshore geophysical methods. Some major research programs now exist that are attempting to cross the shoreline and the new drilling program should integrate with these research endeavors to gain the most complete picture of lithospheric processes and fluxes from mantle to oceanic and continental crust, from crust to atmosphere, and then by erosion from continental crust and basins into the marine environment.

3.6.3 Model − observation integration

Our understanding of crustal growth and cooling is hampered by a near complete lack of direct evidence for the mechanisms of lower crustal accretion. Competing, but untested, conceptual models of crustal accretion have very different predictions for the extent and nature of hydrothermal circulation and for heat and chemical flux (Kelemen et al., 1997; Maclennan et al., 2005). These models provide robust predictions that can be tested by deep drilling of intact lower crust or rare tectonic windows. Sampling of in situ shallow mantle and an associated lower crustal cumulate section would lead to major paradigm shifts in planetary geochemical models. Similarly, we do not yet understand the controls on seawater penetration into the lithosphere and its importance for melt emplacement, plate rheology, and lithospheric architecture. In situ sections of lower crust are needed to determine the penetration depth and flux of seawater in the lower ocean crust and assess its role in crustal cooling.

In terms of crustal evolution, we need tighter constraints on the interrelationships among hydrology, geochemistry, and microbiology. Detailed and comprehensive combined studies involving observatory components are critical for understanding how the transport of heat and solutes are coupled. The development of coupled models of heat and reactive mass transport require a firm knowledge of the rates of geochemical reactions, which is currently missing. Better constraints on the timing of hydrothermal alteration and veining processes and their relation to changes in the hydrological state of the crust are dearly needed.