Ranero, Cesar

Vannucchi, Paola

von Huene, Roland

OceanRep

Scientific Drilling, Special Issue No.1, 2007  51Part 3 : Fault Zone Structure, Composition, and Physical PropertiesIntroductionInterplate subduction zone earthquakes occur in the seismogenic zone, downdip of the transition from stable slip to unstable slip along the plate boundary megathrust. Currently, mineral transformations and fluid migration are thought to govern the stable to unstable slip transition. In the seismogenic zone, subducting clastic material in the subduction channel changes structure and physical   properties and releases fluid as it migrates toward the mantle. Where sediment accretion dominates a convergent margin, the subducted clastic input is trench sediment, and the   subducted plate is likened to a conveyor belt traveling from the trench into conditions of elevated temperature and pressure. Trench sediment has been sampled, and the diage-   netic and metamorphic changes can be inferred. Moore and Saffer (2001) describe the decrease in porosity and increase in temperature with depth, which results in a transition from       opal to quartz, the transformation of clay minerals, cemen-tation by carbonate, zeolites, and quartz, all accompanied by   pressure solution. However, where subduction erosion dominates a convergent margin, material input to the seismo-genic zone is unknown because it is derived from the base of the upper plate, beyond the sampling capabilities of past scientific ocean drilling (von Huene et al., 2004). Mineral transformations involved in seismogenesis at erosional margins may be very different from those at accretionary   margins. Zones of upper plate subduction erosion have never been drilled, nor does geophysics resolve their structure,       lithology, and physical properties (Fig. 1). The Japanese riser drill ship Chikyu in the Integrated Ocean Drilling Program (IODP) provides the opportunity to drill the seismogenic zone of an erosional margin and this is the objective of CRISP (Costa RIca Seismogenesis Project). Subduction erosion along the plate boundary of the Middle America convergent margin: During CRISP the seismogenic zone of the Middle America erosional convergent margin will be sampled and instrumented. A key feature is the uncon-formable surface across the top of crystalline basement of the upper plate, a regional unconformity imaged in seismic records. Drilling on DSDP Legs 67 (1979) and 84 (1981) revealed that the inferred accretionary prism of Guatemala is instead an extension of the upper plate igneous basement covered unconformably by Eocene to Miocene shallow water sediment. Drill samples from the lower slope contain micro-fossils that are now more than 3 km deeper than the water depth in which they lived. Microfossil depth indicators in the Figure 1. [A] Seismic depth image (pre-stack depth migration) of the Costa Rica margin near the Osa Peninsula (Ranero and von Huene, 2000). Margin basement is formed by igneous rock; major normal faults are emphasized with lines. Inset on the left shows detail of normal faults displacing the top of basement uncorformity and the slope sediment section. Seafloor topography was acquired with multibeam surveying.4684û 40'W 9û 00'N01234567891011121314Northtrenchaxis2Sonne81-Line43plate boundary reflectionsplate boundary reflections frontal sediment prismDepth [km]QueposPlateauMarginBasementfrontalprismkm123Depth [km]05152025ODP 170 site 1042 (projected)10A)Drilling the Seismogenic Zone of an Erosional Convergent Margin: IODP Costa Rica Seismogenesis Project CRISPby César R. Ranero, Paola Vannucchi, Roland von Huene and the CRISP proponentsdoi:10.0/iodp.sd.s01.9.0075  Scientific Drilling, Special Issue No.1, 2007Part 3 : Fault Zone Structure, Composition, and Physical Propertiesoverlying sediment record the progressive margin subsidence since Miocene times (Vannucchi et al., 2004). Subsidence from the surf zone to trench depths requires crustal thinning by erosion of material along the underside of the upper plate.Drilling on ODP Leg 170 (1996) off Nicoya confirmed that, like Guatemala,   the Costa Rican unconformity surface has subsided from shallow water in the past few Myr (Vannucchi et al., 2003) similarly to the neighbor segment off Nicaragua (Ranero et al., 2000). Additionally, the igneous basement subsequently dredged from the lower continental slope of Nicaragua indicated an extension of the conti-nental framework to the trench there. In seismic records the regional uncon-formity can be followed continuously from the ODP Leg 170 drill transect off Nicoya to the proposed Osa Peninsula drill transect.Ongoing erosion is indicated by morphology of the slope (Fig. 2). Osa bathymetric high forms the southern flank of a major embayment where erosion and slope retreat are accel-erated by subduction of Cocos Ridge and its associated seafloor ridges and seamounts. The regional erosion surface was ground truth during Leg 170, and the current retreat of the conti-nental slope provides evidence of vigorous tectonic erosion where CRISP drilling is proposed. Results from recent work:  Researchers in the German SFB 574 (Kiel University) have used high-resolution geophysical tools to map the mounds at a series of mid-slope vent sites. In all, about 125 vent sites were located along ~600 km of the margin off Costa Rica and Nicaragua, and they seem to be common from Panama to Mexico. Material and fluids were sampled from several mounds, and probe heat flux     measurements were collected across a number of sites. Cameras just above the seafloor displayed abundant chemo-  synthetic carbonates and fauna at the vent sites. Figure 2. Multibeam bathymetry and topography of the Costa Rica margin between Nicoya and Osa Peninsula. Dashed lines delineate the 90°C and 150°C temperature isotherms along the plate boundary. Large blue circle marks nucleation area of the 2002 Mw6.4 earthquake, and small blue circles are aftershocks occurring during 6 days after the main shock (I. Arroyo, unpublished). Drill site in the upper slope is located on the nucleation area of the 2002 Mw6.4 earthquake (see Fig. 4). Black lines indicate tracks of multichannel seismic reflections profiles collected during SONNE 81 and BGR99 cruises.85 30'W 85 15'W 85 00'W 84 45'W 84 30'W 84 15'W 84 00'W 83 45'W 83 30'W 83 15'W8 00'N8 15'N8 30'N8 45'N9 00'N9 15'N9 30'N9 45'N10 00'NOsaPeninsulaCocos RidgeCrestNicoyaPeninsula150C90CFigure 1. [B] Long-term subsidence history of the Costa Rica margin composed with data from several drill sites offshore the Nicoya Peninsula. Paleodepth  information was obtained from studies of benthic foraminifera in the slope sediment, indicating a non-linear subsidence (two main pulses) of about 4 km for the middle continental slope (Vannucchi et al., 2003).B)0Site 1041S.565Age ofsubsidenceevents:012310 1516.5Present Site 565 water depthPresent Site 1042 water depthDepth of thecarbonate-cementedbreccia at time ofdepositionSite1041Age(Ma)Seafloor Depth (km)Present Site 1041 water depthSite565Oldest sediment drilled at:Age ofcarbonate-cementedbreccia at:Site 1042QUIESCENCEorPOSSIBLEUPLIFTPresent Site 1042 depth of thecarbonate-cemented breccia at top basementBolivina-Bulimina-PseudoparrellaassociationUvigerina-StilostomellaassociationassociationNuttallides Umbonifera5B)Part 3 : Fault Zone Structure, Composition, and Physical Propertiesdrilling technology it also provides the samples to charac-terize upper plate basement rock. Logging and instrumen-tation will provide accurate information on the upper plate thermal structure, interplate earthquake nucleation, and rupture propagation, which are key for a successful planning of deeper riser drilling. In Program B, the riser sites locations are constrained with the results from downhole instruments and conventional geophysical experiments, rock mechanics laboratory experiments, and forward modeling. Program B drilling reaches 3 km below seafloor to sample for the first The mounds are a few hundreds to one thousand meters     across with a very hummocky topography. They occur in the area of normal faulting and locally appear at fault scarps. Heat flux over and around the mounds is high compared to the regional background values, indicating venting of deeply-    sourced fluid (Grevemeyer et al., 2004). Pore fluid chemistry documents low chlorinity fluids, and chemical modeling     indicates that they come from dehydration of clays (Hensen et al., 2004). First-order estimates of fluid flow budget across       the forearc indicate that most fluid released during dehydration of sediment in the subduction channel traverses the upper plate and is released across the mid-slope vent  sites (Ranero et al., 2007). Dehydration of the sediment subducting with the lower plate is proposed as one of the significant causes for the transition to stick-slip behavior and seismogenesis. The massive fluid release across the upper plate of erosional margins is one of the unique processes        investigated during CRISP.Also unique to CRISP is the possibility to drill the nucle-ation area of the 20 November 2002, Mw6.4 interplate thrust earthquake. The earthquake was recorded on an array of ocean bottom seismometers as well as on the Costa Rican National Seismological Network on land (Fig. 3). The full        array provides a database for accurate relocation of the earth-       quake and its aftershocks (I. Arroyo, in preparation). The     main shock is essentially located in the position of the       proposed 5-km-deep riser drill hole to the seismogenic zone.        Downhole instrumentation, including seismometers, will provide a 3-D seismic image around the drill hole with a radius of several kilometers including the area of the Mw6.4       nucleation. Attribute analysis of the seismic data will permit an estimation of the general physical properties of the upper         plate and plate boundary.CRISP: The shallow dip of the subduction zone off southern Costa Rica and the relatively high   temperature of the subducting ocean crust bring materials and processes involved in seismo-genesis to depths reachable by drilling with the riser ship Chikyu. CRISP is structured in non-riser (Program A) and riser (Program B) stages that systematically lead from shallow non-riser to deep riser drilling (Fig. 4). Program A provides background information for Program B, which reaches the    unstable slip of the seismogenic zone. Program A drilling allows the characterization of lower plate oceanic igneous rock and hydrology before subduction, as  well as under the front of the margin. With standard ODP Figure 4. [A] Depth image across the location of proposed drill sites (see Fig. 2). Shallow non-riser sites are shown in orange, and deeper riser holes in red. Note that drilling of the deepest hole will penetrate the nucleation area of the 2002 Mw6.4 earthquake (approximate location shown by blue arrow). [B] Diagram of main geological units, based on seismic image of [a], showing the plan to drill on either side of the updip limit of the seismogenic zone.0123456789Depth [km]80016002400320040005600kmcmp05101520253035Pre-Stack Depth Migration BGR99 Line 7frontal   prism60ºC90ºC120ºC150ºCPlate BoundaryPlate BoundaryNon-Riser drilling to characterize material and fluid input, and output from seismogenic zone Riser drilling sampling and logging, deploy down-hole observatory, monitor dynamics, in aseismic and seismic areas05igneousocean crustslope aprontrench   axisfrontal      prismcontinental basement5 kmDepth [km]A)B)Mw 6.44800Figure 3. Location of the main shock and aftershocks of the 2002 Mw6.4 earthquake (red star). Triangles indicate the array of ocean bottom seismometers that recorded the quake; black squares are seismographs of the local network. Previous locations without the offshore seismograph array are shown in yellow and blue stars.Costa Rica National Seismological Network SFB offshore-onshore seismological networkI. Arroyo (pers. com.) S. Bilek (pers. com.) S. Husen (pers. com.)Scientific Drilling, Special Issue No.1, 2007  55  Scientific Drilling, Special Issue No.1, 2007Part 3 : Fault Zone Structure, Composition, and Physical Propertiestime the subducting eroded debris along the plate interface, in the transition area from stable to stick slip, where temper-atures of ~100°C occur. Thus, it defines conditions in the zone before the unstable slip and provides the observations to later isolate material transformations, fluid abundance and flow, and dynamic changes causing seismogenesis. Program B riser drilling is engineered based on results of Program A cores and down hole geophysics. The most recent relocation of the 2002 Mw6.4 earthquake epicenter is at the proposed location of the drill site into the seismogenic zone. Thus, the  nucleation area of a large earthquake may be sampled. Riser hole results input to laboratory experiments and modeling can indicate conditions deeper in the seismogenic zone. If found attractive for further work, the results from modeling can be verified in the future with ~7 km drilling under the auspices of the International Continental Drilling Project at sites on the Osa Peninsula.With a low sediment supply, fast convergence rate, abundant seismicity, subduction erosion, a shallow plate boundary, and optimal mid-latitude weather conditions, the Middle America Trench offers excellent opportunities to learn causes of earthquake genesis along erosional margins.  CRISP is complementary to other seismogenic zone drilling projects (e.g., San Andreas Fault Observatory –SAFOD- and        NantroSeize) because it includes studies of earthquake generation in a different setting, one common to about 50% of      all convergent margins worldwide. AcknowledgementsWe thank Ivonne Arroyo (IfM-GEOMAR) for providing the relocation of the 2002 Mw 6.4 thrust event shown in figures 2 and 3 and aftershocks in figure 2, and for producing figure 3.ReferencesGrevemeyer, I., Kopf, A.J., Fekete, N., Kaul, N., Villinger, H.W., Heesemann, M., Wallmann, K., Spieß, V., Gennerich, H.-H., Müller, M., and Weinrebe, W., 2004. Fluid flow through active mud dome Mound Culebra offshore Nicoya Peninsula, Costa Rica: evidence from heat flow surveying. Marine Geology 207, 145–157, doi:10.1016/j.margeo.2004.04.002.Hensen, C., Wallmann, K., Schmidt, M., Ranero, C.R., and Suess, E., 2004. Fluid expulsion related to mud extrusion off Costa Rica –a window to the subducting slab. Geology, 32:201–204, doi:10.1130/G20119.1.Moore, J.C. and Saffer, D.M., 2001. Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low-grade metamorphic processes and increasing effective stress, Geology, 29:183–186, doi:10.1130/0091-7613(2001)029<0183;ULOTSZ>2.0CO;2.Ranero, C.R. and von Huene, R., 2000. Subduction erosion along the Middle America convergent margin. Nature, 404:748–752, doi:10.1038/35008046.Ranero, C.R., von Huene, R., Flueh, E., Duarte, M., and Baca, D., 2000. A cross section of the forearc Sandino Basin, Pacific Margin of Nicaragua. Tectonics, 19:335–357, doi:10.1029/1999TC900045.Ranero, C., Grevemeyer, I., Sahling, H., Barckhausen, U., Hensen, C., Wallmann, K., Weinrebe, W., Vannucchi, P., von Huene, R., and McIntosh, K., 2007. The hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis Geosystems, Geophysics, Geochemistry (in press).Vannucchi, P., Ranero, C.R., Galeotti, S., Straub, S.M., Scholl, D.W., and McDougall-Ried, K., 2003. Fast rates of subduction erosion along the Costa Rica Pacific margin: implications for non-steady rates of crustal recycling at subduction zones. J. Geophys. Res., 108:2511, doi:10.1029/2002JB002207.Vannucchi, P., Galeotti, S., Clift, P., Ranero, C.R., and von Huene, R., 2004. Long term subduction erosion along the Middle America Trench offshore Guatemala. Geology, 32:617–620.von Huene, R., Ranero, C.R., and Vannucchi, P., 2004. A model for subduction erosion. Geology, 32:913–916; doi: 10.1130/G20563.1.AuthorsCésar R. Ranero, ICREA Research Professor, Instituto de Ciencias del Mar, CMIMA, CSIC, Pg. Maritim de la Barceloneta 37-49, 08003 Barcelona, Spain, e-mail: cranero@icm.csic.es.Paola Vannucchi, Department of Scienze della Terra, Università di Firenze, Via La Pira, 4, Italy. Roland von Huene, Department of Geology, University of California, Davis and Geomar, Kiel, 2910 North Canyon Rd., Camino, Calif. 95709, U.S.A.and the CRISP proponents: Guillermo E. Alvarado, Udo Barckhausen, Peter Baumgartner, Susan Bilek, Peter Clift, Philippe Charvis, Jean-Yves Collot, Heather DeShon, Tim Dixon, Giulio Di Toro, LeRoy Dorman, Donald M. Fisher, Simone Galeotti, Ingo Grevemeyer, Robert Harris, Kaj Hoernle, Stephan Husen, Miriam Kastner, Gaku Kimura, Masataka Kinoshita, Serge Lallemand, Alex J. Maltman, Chris Marone, Takeshi Matsumoto, Kirk McIntosh, Christian Mueller, Soenke Neben, Lisa Patino, Adina Paytan, Jason Phipps Morgan, Marino J. Protti, Christian Reichert, Sanny Saito, Dave Scholl, Susan Schwartz, Glen Snyder, Eli Silver, Volkhard Spiess, Erwin Suess, Heiner Villinger, Sergio Vinciguerra, Klaus Wallmann.

Drilling the seismogenic zone of an Erosional convergent margin: IODP Costa Rica Seismogenesis Project CRISP

Ranero, C.

vonHuene, R.

Florence Research

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Drilling the Seismogenic Zone of an Erosional Convergent Margin: IODP Costa Rica Seismogenesis Project CRISP

https://flore.unifi.it/bitstream/2158/348437/1/sd-SpecialIssue-51-2007.pdf

Drilling the seismogenic zone of an Erosional convergent margin: IODP Costa Rica Seismogenesis Project CRISP

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