Abstract
AbstractCalcareous‐pelagic input sediments are present at several subduction zones and deform differently to their siliciclastic counterparts. We investigate deformation in calcareous‐pelagic sediments drilled ∼20 km seaward of the Hikurangi megathrust toe at Site U1520 during International Ocean Discovery Program (IODP) Expeditions 372 and 375. Clusters of normal faults and subhorizontal stylolites in the sediments indicate both brittle faulting and viscous pressure solution operated at <850 m below sea floor. Stylolite frequency and vertical shortening estimated using stylolite mass loss, porosity change, and distribution increase with carbonate content. We then use U1520 borehole data to constrain a P‐T‐t history for the sediments and apply an experimentally derived pressure solution model to compare with strains calculated from stylolites. Modeled strains fail to replicate stylolite‐hosted strain distribution or magnitude, but comparison shows porosity, composition, and grain‐scale effects in diffusivity and mass transfer pathway width likely exert a strong influence on pressure solution localization and strain rate. Stylolite and fault clusters concentrate clay in these sediments, creating weak volumes of clay within carbonates, that may localize slip where the plate interface intersects the carbonates at <5‐km depth. Plate interface slip character and rheology will be influenced by the deformation of intermixed phyllosilicates and calcite, occurring by variably stable frictional slip and pressure solution of calcite. Pressure solution of calcite is therefore important at the shallow plate interface, waning at the base of the slow‐slipping zone because calcite solubility is low at temperatures >150°C where frictional (possibly seismic) slip likely predominates.
Highlights
The mechanical behavior of carbonate‐rich sediments during subduction is not well understood, despite their recognition at several margins around the world (Moore & Mascle, 1990; Morris et al, 2006; Wallace et al, 2019)
Lstyl 1 − εstyl and εrecovered is the shortening strain in recovered core, Lrecovered is length of recovered core, Lo is the undeformed thickness of recovered core, Lstyl is the thickness of stylolites within recovered core, calculated using the aforementioned idealized stylolite and the frequency of stylolites observed within recovered core, and εstyl is the shortening strain on an individual stylolite (0.7 from isocon plots and porosity loss; Figures 8a and 8b)
This description is similar to what is seen at Site U1520, where randomly distributed peaks in stylolite frequency increase in intensity with depth (Figure 6), burrows localize intense stylolite‐rich regions along their margins (Figure 3c), and more fossiliferous regions host more spaced, less well‐developed stylolites (Figure 3c)
Summary
The mechanical behavior of carbonate‐rich sediments during subduction is not well understood, despite their recognition at several margins around the world (Moore & Mascle, 1990; Morris et al, 2006; Wallace et al, 2019). The abundant data gathered during IODP Expedition 375 provide a unique opportunity to constrain the P‐T‐ t history for the pelagic sequence at Site U1520, which we use to apply published models of intergranular pressure solution (Rutter, 1976, 1983) We compare these results to those calculated from stylolite frequency earlier in this study and find agreement requires sustained high fluid pressures or larger length scales of material transport than the average grain size of lithologies hosting stylolites. We extend this model to the future subduction of the sediments and discuss the rheological impact of ongoing pressure solution and pre‐subduction strain weakening on current subduction
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
