Elastic moduli and physical properties of fault rock and protolith associated with SSEs at the Northern Hikurangi margin, NZ
Abstract
International Ocean Discovery Program (IODP) Expeditions 372 and 375 focused on investigating recurring shallow (<2-15 km) slow slip events (SSE) at the N. Hikurangi subduction margin through ocean drilling. The two expeditions conducted a coordinated program of logging while drilling, coring, and borehole observatory installation along a transect of drill sites. These include a borehole that crosses an active thrust splay fault that has accommodated km of displacement and lies in the region where SSEs may propagate near to the seafloor (Site U1518); and two sites that sampled the incoming sedimentary succession. The "inputs" sites include a borehole (Site U1520) in the Hikurangi Trough that sampled a Cretaceous to Pliocene sequence of volcaniclastic sediments and pelagic carbonates overlain by a Quaternary trench wedge; and a site (U1526) on Turanganui Knoll seamount that sampled a volcaniclastic sequence overlain by a thin pelagic sediment cover. Here, we report on the rock physical properties measured by logging and on core samples, focusing on trends spanning the fault and within the pelagic and volcaniclastic portions of the sedimentary sequence, which comprise the protolith for the fault zone in the deeper SSE source region
We find that the thrust splay fault zone is more compliant than the surrounding sediment, and characterized by a tens of m-wide zone of decreased elastic moduli and density. The low fault stiffness, in tandem with fault frictional properties, may play a role in driving an otherwise creeping fault toward transitional slip behavior and conditional stability that could promote SSE. We also find that the incoming sediments exhibit a wide range of shear and bulk moduli, density, and porosity. In particular, the volcaniclastic sequence exhibits large variations in properties (P-wave velocities ranging from 2 to 6 km/s and porosity from <20% to >40%) over length scales less than 1 m, likely related to variations in cementation. This variation is consistent with the observed high-amplitude seismic reflectivity of incoming and subducted sediments. The extreme heterogeneity in physical properties, particularly over such short length scales, should lead to a plate interface and surrounding deformation zone characterized by highly variable rheologies that would promote emergent slow slip.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T54C..04S
- Keywords:
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- 1207 Transient deformation;
- GEODESY AND GRAVITYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8170 Subduction zone processes;
- TECTONOPHYSICSDE: 8185 Volcanic arcs;
- TECTONOPHYSICS