Along‐strike variations in underthrust sediment dewatering on the Nicoya margin, Costa Rica related to the updip limit of seismicity

Abstract

Along the Costa Rican subduction zone offshore the Nicoya peninsula, an offset in the updip limit of seismicity coincides with a transition between subduction of warm crust generated at the Cocos‐Nazca Spreading Center and cool crust formed at the East Pacific Rise. We evaluate whether the observed difference in thermal state of incoming crust would result in significant differences in sediment dehydration reaction progress along strike, thought to be a control on the updip limit of seismicity. We combine thermal models with models of dehydration reaction kinetics for opal and smectite to estimate the distribution of diagenetic fluid sources. The modeled distribution of diagenetic fluid sources mimics the pattern of the updip limit of seismicity; seismicity begins ∼15–20 km landward of most of the smectite‐to‐illite transition. This suggests that the location of the updip limit of seismicity may be influenced by the dissipation of fluid overpressures landward of smectite‐to‐illite dehydration.

Fluid transport properties in sediments and their role in large slip near the surface of the plate boundary fault in the Japan Trench

“…They assumed a continuous influx of fluid from the deep crust into a low-permeability fault zone to cause the high pore pressure in the fault zone. Dehydration reactions of smectite transformation and opal-to-quartz transformation also can generate pore pressure at depth on the plate boundary (Spinelli and Saffer, 2004;Saffer et al, 2008). The hypothesis of Kato and Yoshida (2011) is consistent with the very low shear stress of 21 to 22 MPa at the plate interface of the Tohoku earthquake rupture zone estimated from stress tensor inversion by Hasegawa et al (2011).…”

Organic matter cracking: A source of fluid overpressure in subducting sediments

“…We assume as an upper bound that it contains a volumetric proportion of ~20% of water (Saffer et al, 2008;Spinelli and Saffer, 2004) , corresponding to two layers of water in interlayer space.…”

Expedition 334 summary

“…Collectively, the geochemical data in the décollement offshore Nicoya Peninsula indicate that this flow system is active and that a fraction of the fluid is derived from depths within the subduction zone where temperatures are ~80°-150°C (Chan and Kastner, 2000;Silver et al, 2000;Kastner et al, 2006;Solomon et al, 2009). The sharpness of the geochemical anomalies in the décollement and the estimated temperature of the fluid suggest updip flow from a source region ~38-55 km landward of the trench at 9-14 km depth, near the updip limit of the seismogenic zone (Harris and Wang, 2002;Spinelli and Saffer, 2004;Kastner et al, 2006;Ranero et al, 2008).…”