Structural characterization of the Costa Rica décollement: Evidence for seismically-induced fluid pulsing

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Abstract

Ocean Drilling Program Legs 170 and 205 offshore Costa Rica provide structural observations which support a new model for the geometry and deformation response to the seismic cycle of the frontal sedimentary prism and décollement. The model is based on drillcore, thin section, and electron microscope observations. The décollement damage zone is a few tens of meters in width, it develops mainly within the frontal prism. A clear cm-thick fault core is observed 1.6 km from the trench. The lower boundary of the fault core is coincident with the lithological boundary between the frontal prism and the hemipelagic and pelagic sediment of the Cocos plate. Breccia clast distributions in the upper portion of the décollement damage zone were studied through fractal analysis. This analysis shows that the fractal dimension changes with brecciated fragment size, implying that deformation was not accommodated by self-similar fracturing. A higher fractal dimensionality correlates with smaller particle size, which indicates that different or additional grain-size reduction processes operated during shearing. The co-existence of two distinct fracturing processes is also confirmed by microscopic analysis in which extension fracturing in the upper part of the damage zone farthest from the fault core is frequent, while both extension and shear fracturing occur approaching the fault core.

The coexistence of extensional and shear fracturing seems to be best explained by fluid pressure variations in response to variations of the compressional regime during the seismic cycle. During the co-seismic event, sub-horizontal compression and fluid pressure increase, triggering shear fracturing and fluid expulsion. Fractures migrate upward with fluids, contributing to the asymmetric shape of the décollement, while slip propagates. In the inter-seismic interval the frontal prism relaxes and fluid pressure drops. The frontal prism goes into diffuse extension during the interval when plate convergence is accommodated by creep along the ductile fault core. The fault core is typically a barrier to deformation, which is explained by its weak, but impermeable, nature. The localized development of a damage zone beneath the fault core is characterized by shear fracturing that appears as the result of local strengthening of the detachment.

Introduction

Theoretical models of faults are based on geometrical, mechanical and mathematical assumptions; their success at predicting fault behavior is a direct consequence of their ability to match observations (Ben-Zion and Sammis, 2003). Field geologists, however, often find that faults are complex features with laterally varying characteristics, and that fault zone mechanisms and structures appear to be strongly influenced by the interaction with fluids (Sibson et al., 1988, Sibson, 1992, Segall and Rice, 1995). Fluids shape fault zones by influencing their hydrogeological and mechanical properties. However we still do not know how episodes of fluid flow relate to episodes of fault slip and the mechanisms of fault deformation. Perhaps the most striking and most important examples of fault-fluid interaction come from modern convergent plate margins which have been explored during the Ocean Drilling Program (ODP) activity (Maltman and Vannucchi, 2004, and references therein).

Subduction zones are characterized by the massive fluid release that occurs when the fluid-rich sediments of the incoming plate are either trapped during underthrusting or incorporated into an accretionary prism. When frontal accretion is absent, subduction margins build up a frontal prism formed by fluid-rich sediments reworked from the slope and deposited by gravitational mass movements (Aubouin and von Huene, 1985, Vannucchi and Tobin, 2000, von Huene et al., 2004). The interaction between deformation and fluid flow in subduction zones is not only particularly important along the plate boundary (Bangs et al., 1999, Bangs et al., 2004, Ranero et al., submitted for publication) but also within the upper plate. In the last few years both modeling (Wang and Hu, 2006) and fluid flow measurements on the seafloor and along fault zones within the frontal prism of subduction zones (Morris et al., 2003, Brown et al., 2005) have revealed a dynamic deformation response of the system to the seismic cycle. Here we report on the structural analysis of the Costa Rica décollement drilled near its deformation front by ODP Legs 170 and 205. Here shallow drilling through the décollement has helped to constrain its architecture and the relationship to fluid flow. We will concentrate on the geometrical properties of the décollement to infer the long term – slow – and short term – fast – evolution of the fault and the mechanisms of particle size reduction within the fault zone. We also discuss the deformation pattern of the Costa Rica convergent margin, which has a well developed upper portion of the damage zone and an absent or poorly developed lower damage zone. Asymmetric development of the décollement has important consequences for the gravity driven deformation of the frontal prism and suggests the occurrence of subduction erosion at much shallower levels than previously thought to be feasible (von Huene et al., 2004).

Finally, we investigate the occurrence of pressure transients or fluid discharge associated with slip. Several physical models have been proposed to explain these processes, both seismic and aseismic, as the seismic fault suction-pumping mechanism (Sibson, 2000), and stress changes models in subduction prism (Wang and Hu, 2006). We hypothesize that deformation mechanisms, fluid pressure and composition in the upper plate and in the décollement vary as a response to slip, and also during quiescent period due to fluid–rock interaction and general upper plate relaxation.

Section snippets

Costa Rica décollement

The frontal part of the Costa Rica décollement has been drilled four times during ODP Legs 170 and 205 (Kimura et al., 1997, Morris et al., 2003) (Fig. 1). Leg 170 and 205 data constrain a detailed cross section across the décollement. Regionally, the décollement is slipping of 88 mm/yr top to S30°W (DeMets, 2001). The local trend of the décollement is N40°W, and in the shallow portion of interest, geophysical imaging shows a subhorizontal dip (Fig. 1). At ODP Site 170-1043, located ~ 0.6 km

Analysis of fault zone deformation

The geometrical description of the décollement largely depends upon the scale of observation, since the thickness of the damage zone is two orders of magnitude larger than the ZLS, i.e. 10's of m vs. a few cm. Most of the fault slip is concentrated in the narrow ZLS, so that the décollement could be treated as a sequence of planar slip-discontinuities within a deforming continuum solid. Considering the fault breccia, instead we will describe the deformation using a fractal description for

Fractal analysis

Analysis of the clast size distribution within the brecciated damage zone reveals a not self-similar geometry. Although the fractal dimensions are consistent for each size-class, the overall analysis indicates that brecciation was not a self-similar process. This lack of self-similarity has also been observed in other fault zones, for example in the strike-slip Mattinata Fault (Storti et al., 2003, Billi and Storti, 2004). In the Costa Rica décollement the fractal dimension D of the 10–3 cm

Conclusions

The Costa Rica décollement deforms in a brittle deformational regime where extensional and shear fracture co-evolve with the fluid system. Yet the distributions of particle size reductions imply an increasing amount of shear toward the zone of localized shear, in agreement with increasing displacement, and extensional fractures as precursor evidence of fault damage. The abrupt change in texture between the breccia of the damage zone and the gouge of the zone of localized shear suggests an

Acknowledgements

We are grateful to Francesca Remitti for reading and Jason Phipps Morgan for helping in improving an early version of the ms. All shipboard participants to ODP Leg 205 were precious companions, in particular PV thanks Demian Saffer for sharing the duty of drillcore description. Harold Tobin was a great help from onshore. This research has been funded by PRIN 2005 Cyclic distributed vs. localized deformation and seismic signature in subduction-related fault zones to G. Molli and PRIN 2005

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