Leg 205 Synthesis: Subduction Fluxes and Fluid Flow across the Costa Rica Convergent Margin

Content Provider	Semantic Scholar
Author	Morris, Julie Dianne Villinger, Heinrich W.
Copyright Year	2006
Abstract	Ocean Drilling Program (ODP) Leg 205 was designed to investigate subduction fluxes and fluid flow across the Costa Rica convergent margin. During Leg 205, modified CORK-IIs (new-generation Circulation Obviation Retrofit Kits) were installed within the incoming plate and the décollement zone to monitor pressure and temperature of formation fluids through time and to collect a time series of fluid samples for chemical analysis. One subseafloor observatory is monitoring within a highly fractured region of the incoming igneous section. Vigorous, slightly subhydrostatic flow within high-permeability horizons is observed; fluid compositions appear to be a mixture of bottom seawater and components distinct from seawater. The observatory within the décollement zone reveals pressures that are just slightly overpressured; excursions in the pressure and temperature record correlate temporally with excursions in fluid composition and flow rate variation, all occurring during periods of aseismic strain, recorded geodetically. Igneous and sedimentary core materials collected during Leg 205 have been studied extensively in combination with materials recovered during ODP Leg 170. This synthesis and papers in this volume report on the igneous and alteration history of the incoming oceanic section and its implications for off-axis magmatism and fluid flow, the composition of incoming and underthrust sediments with implications for fluid and element loss from the shallow subduction zone, the structure and permeability characteristics of the forearc sediment wedge, and the consequences of sediment dynamics and fluid flow to the seismogenic zone and subduction factory. 1Morris, J.D., and Villinger, H.W., 2006. Leg 205 synthesis: subduction fluxes and fluid flow across the Costa Rica convergent margin. In Morris, J.D., Villinger, H.W., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 205: College Station, TX (Ocean Drilling Program), 1–54. doi:10.2973/ odp.proc.sr.205.201.2006 2Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, CB 1169, St. Louis MO 63130-4899, USA. Present address: Division of Ocean Sciences, National Science Foundation, 4201 Wilson Boulevard, Arlington VA 22230, USA. jdmorris@nsf.gov 3FB Geowissenschaften, Universität Bremen, Postfach 330 440, 28334 Bremen, Germany. Initial receipt: 14 March 2006 Acceptance: 23 August 2006 Web publication: 13 October 2006 Ms 205SR-201 J.D. MORRIS AND H.W. VILLINGER LEG 205 SYNTHESIS: COSTA RICA CONVERGENT MARGIN 2 INTRODUCTION Fluid flow in convergent margins can have a profound effect on the shallow thermal structure of the downgoing plate, the physical properties of the subduction interface, and the transport of elements to the ocean, the volcanic arc, and the deeper mantle. It is, therefore, a major factor in understanding the impact of subduction zone processes on seismicity and, ultimately, volcanism at convergent margins. Ocean Drilling Program (ODP) Legs 170 (Kimura, Silver, Blum, et al., 1997) and 205 (Morris, Villinger, Klaus, et al., 2003), and their postcruise science, have investigated fluid flow across the Costa Rica margin, in the context of the lithology, structure, tectonics, and thermal history of the incoming plate, the overriding plate, and the décollement zone that separates them. During Leg 205 subseafloor observatories (CORKs: Circulation Obviation Retrofit Kits) were installed within fractured igneous rock of the incoming plate and in the décollement zone to monitor changing pressure and temperature of the formation fluids and to collect fluid samples for shore-based analysis. Data acquired to date represent the first high-resolution time series of combined pressure (P), temperature (T), chemical composition, and rates of fluid flow in the décollement of a subduction zone. Intriguing correlations between excursions in the fluid P, T, and composition records and aseismic strain events recorded geodetically set the stage for a long-term investigation into the role of fluids in seismogenic processes. The theme of fluid flow and long-term monitoring link many of the sections that follow. Another major goal of the two legs was to characterize the mass and element fluxes, including volatiles, into the Middle America Trench, derived from both the altered oceanic basement and the sedimentary veneer. Coring during Leg 205 was restricted to specific target depths to maximize operational time for CORK installation. In conjunction with Leg 170, however, a large amount of cored sedimentary and igneous rock has been studied, with results reported here. During Leg 205, ~170 m of igneous rock was cored in the form of a ~30-m-thick gabbro sill separated from a lower igneous unit (>140 m thick) by intervening sediments. The lower unit also shows gabbroic textures, with chemical composition similar to ocean island basalt (OIB) in general and moderately enriched Galapagos lavas in particular. These results, together with studies in the literature, point to significant and extensive overprinting of the incoming oceanic plate by off-axis magmatism, likely related directly or indirectly to hotspot volcanism. Alteration is low overall but strongly concentrated along fractured horizons, including one that is the target of long-term monitoring. This volume also reports results from the incoming and underthrust sediment sections. Comparison of equivalent sediment horizons cored on the incoming plate and from below the décollement sets the stage to investigate directly changes in sediment and pore fluid composition resulting from the initial compaction dewatering associated with the earliest stages of subduction. This is possible because Leg 170 showed no sediment accretion in the forearc and documented the sediment compaction associated with dewatering between the reference site on the incoming plate and the equivalent underthrust sediments. Several contributions discussed below address this topic. The goals and accomplishments of Legs 170 and 205 nicely address objectives of those parts of the Integrated Ocean Drilling Program J.D. MORRIS AND H.W. VILLINGER LEG 205 SYNTHESIS: COSTA RICA CONVERGENT MARGIN 3 (IODP) and MARGINS science plans that emphasize the seismogenic zone and the subduction factory. The seismogenic zone is that part of the convergent margin plate boundary where earthquakes occur on the interface between the two plates. The subduction factory refers to the contributions from the downgoing slab to the overlying plate (aqueous fluids, sediments, and hydrous melts) and their impact on fluid flow, hydrate formation, microbial activity, continent-building (and frequently explosive) volcanism, ore formation, and chemical modification of the deep mantle. Central America has been a high-priority region for both seismogenic zone and subduction factory studies. Correlated variations in sediment dynamics, seismicity, and volcanism along strike allow the linkages among them to be investigated. It is also one of the few active margins where a significant carbonate sedimentary section is being subducted, permitting investigation of CO2 recycling in subduction zones. For the seismogenic zone, central questions involve the role of thermal structure, mineralogy, physical properties, and dehydration behavior of the subducting plate in controlling the onset of seismogenesis, all addressed by the two legs. The fate of underthrusting sediments and pore fluids, the composition and alteration of the igneous section of the incoming plate, and the advection of fluids from depth along the décollement, addressed here, all speak to issues important to the subduction factory as well as the seismogenic zone. GEOLOGICAL AND GEOPHYSICAL BACKGROUND FOR ODP LEGS 170 AND 205 The general tectonic setting for Legs 170 and 205 is shown in Figure F1, and the bathymetry and topography of the margin are shown in Figure F2. Sites for the two legs were drilled across the deformation front outboard of the Nicoya Peninsula. Sites 1039 (Leg 170) and 1253 (Leg 205) penetrate the sedimentary and upper igneous sections of the incoming plate. They are located ~1 and 0.2 km, respectively, outboard of the deformation front. Holes 1040C and 1254A are located within 50 m of each other, ~1.5 km inboard of the deformation front. Both penetrate the forearc sediment prism and the décollement zone; coring in Hole 1040C recovered the entire underthrust sediment section and the top of a gabbro sill on the subducting plate. Sites 1043 and 1255 are again within 50 m of each other, ~0.4 km inboard of the deformation front. Both penetrated the sediment prism and the décollement zone. Leg 205 CORKs were installed at Sites 1253 and 1255. The lithology and structure of incoming and underthrust sediments are shown in Figure F3. Incoming Oceanic Plate The convergence rate of the Cocos plate relative to the Caribbean plate increases only slightly from 83 mm/yr offshore Guatemala to 85 mm/yr offshore Nicaragua, reaching 88 mm/yr offshore southernmost Costa Rica (DeMets et al., 1990). The convergence direction offshore the Nicoya Peninsula is almost perpendicular to the trench with the subducting plate dipping to the northeast (25°N–30°E). The maximum depth of seismicity gradually becomes shallower from Nicaragua (~200 km) to southern Costa Rica (~45 km). The dip angle of the slab in the upper 100 km is similar from Nicaragua to Central Costa Rica (~30°), F1. Leg 205 bathymetry, p. 38. 5 . 8 7 6 5 4 3 2 1 0 1 2 3 4 E st P aific R se (E P R ) s o c o C e t a l p e t a l p a c z a N s o g a p a l a G s d n a l s I e g d i R s o c o C e g d i R e i g e n r a C h t u o S a c i r e m A a u g a r a c i N a c i R a t s o C iddle merica rench MAT)
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DOI	10.2973/odp.proc.sr.205.201.2006
Alternate Webpage(s)	http://www-odp.tamu.edu/publications/205_SR/VOLUME/SYNTH/SYNTH.PDF
Alternate Webpage(s)	https://doi.org/10.2973/odp.proc.sr.205.201.2006
Language	English
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