Transient formation fluid pressures and temperatures in the Costa Rica forearc prism and subducting oceanic basement: CORK monitoring at ODP Sites 1253 and 1255

Abstract

Seafloor and formation-fluid pressure data from two Ocean Drilling Program (ODP) borehole hydrologic observatories installed at the toe of the subduction-zone prism off Costa Rica provide new information about the average and transient state of this non-accretionary prism. Data collected to date span a 16-month period from the time of installation during ODP Leg 205 in late 2002 to the most recent submersible site visit in March 2004. Pressure monitoring is part of a larger coordinated effort involving temperature monitoring and continuous fluid sampling within the formation and at the seafloor. The holes are positioned 800 m apart and monitoring points include two in igneous basement just seaward of the prism toe, one in the decollement that separates the underthrust sediments of the incoming Cocos plate from the Costa Rica prism, and one in the overthrust-prism sediments. Response of formation-fluid pressure to oceanographic loading at the seafloor constrains the framework compressibility of basement (ca . 1.1–1.3 × 10 − 10 Pa − 1) and the prism and decollement sediments (ca . 4–7 × 10 − 9 Pa − 1). Values are equivalent to ones determined elsewhere in similar sections. Once effects of seafloor loading are removed, pressures at both basement levels are seen to be steady, nearly identical, and less than but very close to hydrostatic (− 6 kPa). This state probably reflects the local hydrothermal regime of the oceanic crust, not the hydrologic regime of the consolidating subduction complex, and is consistent with basement being highly permeable and hydrologically well connected to distant igneous outcrops where free exchange of water between the crust and the ocean can occur. To what depth in the subduction zone high basement permeability persists is not known, but until permeability is reduced by alteration or mechanical fracture closure, basement must serve to provide a drainage path for water expelled from the consolidating underthrust sedimentary section. The decollement and overlying prism are observed to be superhydrostatic, although not highly so during this phase of observation. Pressures (expressed as the pore pressure ratio) range from λ ⁎ ≈ 0.25 at the decollement early in the monitoring period to ≈ 0.1 in the overlying prism at the end of the monitoring period. The cause of the initially elevated pressures is not known. If generated by contractional strain, elevated pressures appear not to be maintained for long periods of time at these lithologic/structural levels. The cause of the decline in pressure is also not known; it may be the consequence of strain relaxation or hydrologic drainage. No observations were made in the underthrust sediments, where greater hydrologic isolation may allow higher average pressures and transient pressures of greater amplitude and persistence. Two minor transients were observed at the decollement- and prism-monitoring levels that correlate with deformational events that occurred during a GPS monitoring experiment on the Nicoya Peninsula. One of these is inferred by Protti et al. [M. Protti, T. Gonzalez, T. Kato, T. Iinuma, S. Miyazaki, K. Obana, Y. Kaneda, P. LaFemina, T. Dixon, S. Schwartz, A creep event on the shallow interface of the Nicoya Peninsula, Costa Rica seismogenic zone, EOS, Trans. Am. Geophys. Union, Fall Meeting Program with Abstracts, 85 (2004) F1378; M. Protti, P. LaFemina, V. Gonzalez, T.H. Dixon, S.Y. Schwartz, T. Kato, T. Iinuma, S. Miyazaki, K. Obana, Y. Kaneda, A possible slow slip event within the seismogenic zone, Nicoya peninsula, Costa Rica, Geophys. Res. Lett. (submitted for publication)] to have propagated some 60 km to the northeast across the peninsula over the course of 2–3 weeks. The pressure transients at the ODP drill sites, located roughly 60 km offshore, began on May 24 and October 12, 2003, also 2–3 weeks after the initiation of the GPS-recorded Nicoya strain events at the coast. Propagation of dislocations updip (offshore) as well as downdip along the subduction thrust may be the cause of these transients.