Mathematics – Logic
Scientific paper
Apr 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006e%26psl.244..270y&link_type=abstract
Earth and Planetary Science Letters, Volume 244, Issue 1-2, p. 270-284.
Mathematics
Logic
1
Scientific paper
The interrelations between deformation and variations in the physical properties and fabrics of a shallow-level accretionary prism have been clarified using measurements of porosity, pore-size distribution, and the anisotropy of magnetic susceptibility (AMS) within the Miura Boso accretionary prism, central Japan. Based on the mode of deformation, this onland example of an off-scraped body can be divided into three parts from bottom to top: imbricate thrusts corresponding to a décollement zone; a thrust unit in the lower part of the off-scraped body; and an upper coherent unit in the top part of the off-scraped body. Physical properties and fabrics are quite variable in each of the three parts, even within a particular lithology. These observations reflect each structural unit's individual strain history during shallow off-scraping and accretion. Prior to the accretion, the sediments underwent vertical compaction. When horizontal compressional stresses subsequently began to deform the sediments, precursory shear disturbed the previous compaction fabric accompanied by a pervasive porosity decrease over a zone several tens to hundreds of meters in thickness. The porosity decrease occurred by large pore collapse, which led to the development of an impermeable over-consolidated horizon and inevitable high pore fluid pressures below that controlled the position of the subsequent décollement zone. Displacement along imbricate thrusts within the décollement zone took place just below the over-consolidated horizon where high fluid pressure had been generated. Here, a flattening fabric parallel to the imbricate thrust and associated further porosity and pore size reductions occurred in association with the fault movement. These findings imply unrecoverable pre-fractured strain information of sediments before being included in the décollement zone and suggest that the precursory formation of an over-consolidated zone via grain rearrangement and pore size reduction is an essential control on subsequent large deformation (i.e., within a décollement zone) in the shallow part of an accretionary prism, especially within lithologically homogeneous material.
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