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doi:10.2204/iodp.proc.343343T.204.2016

Results and discussion

The linear scanline intersected 2692 structures of all types over 16.11 m of examined core from Cores 10R–20R. Of these, 1102 were categorized as tectonic (Category 1), 1226 were clearly induced (Category 3), and 397 could not be shown to be either tectonic or induced (Category 2). Complete data for structure depths for all categories are included in STRUCTDEPTH in “Supplementary material.” Each core contained structures that fell into all three categories (e.g., see Fig. F3, which shows the categorization method applied to structures in Section 14R-1).

Combining all three categories of structures in Hole C0019E cores, the results show the greatest structure density (measured as the number of structures per 20 cm depth interval) occurs adjacent to the Japan Trench plate boundary décollement (Fig. F4A). The largest structure density occurs near the décollement for all three of the structural categorizations (i.e., the same pattern is observed in calculations from exclusively Category 1 structures, summed Category 1 and 2, and summed Category 1, 2, and 3). In the hanging wall, local maxima for all three category combinations exceed 60 structures/m at ~819 mbsf. In the footwall, local maxima for all three category combinations surpass 80 structures/m at ~824 mbsf. These results show that high structure density correlates with the presence of the shallow Japan Trench décollement and an associated fault damage zone.

Throughout the interval of core spanned by our analysis, distinguishing trends in the spatial variability of structure density is dependent on the identification of induced structures. This is particularly the case in the hanging wall, where the structure density decreases with distance from the décollement for Category 1 structures but not for the cumulative structure density of all three categories. For example, when induced deformation structures are included in the transect results shallower than 790 mbsf, the combined total of Categories 1–3 shows a slight increase in structure density. In contrast, the results for Category 1 structures without induced structures shows a significant decrease over the same interval. Including induced deformation structures therefore conceals the continued decay in tectonic structure density and could result in misinterpretation if high structure densities are used to define tectonic faults. In the footwall, inclusion of induced deformation makes the decay in structure density with distance from the décollement appear less steep than the decay in Category 1 structure density.

Secondary faults have associated minor damage zones that can result in locally high structure density around a major structure (Savage and Brodsky, 2011). Several major secondary faults defined by core observations (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]; Kirkpatrick et al., 2015), radiolarian biostratigraphic age ranges (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]), and/or chemostratigraphic constraints (Rabinowitz et al., 2015) are located in the transect interval (Fig. F4A). Of the major secondary faults, there are two types: (1) those identified within recovered core and (2) those inferred between cores in intervals that were not recovered. We observe slightly elevated tectonic structure density at ~817 and ~832 mbsf, corresponding to the locations of a secondary fault defined from core observations and chemostratigraphic constraints (Rabinowitz et al., 2015), respectively. At both depths, structure density decays both above and below the fault depth (see example from ~832 mbsf in Fig. F4B). Other secondary faults identified in recovered cores (~824.5 and ~825.2 mbsf) do not correlate with locally high tectonic structure density in our data. This is likely because the faults around 825 mbsf have small offsets, indicated by the lack of stratigraphic reversals or gaps associated with them (Rabinowitz et al., 2015). Secondary faults inferred between cores are not resolvable from our structure density data because of large gaps in core recovery and incomplete recovery within coring intervals.

Similar structure density data sets from previous ocean drilling expeditions have been used to investigate deformation in core (e.g., Brown and Behrmann, 1990; Maltman, et al., 1993; Saito et al., 2001). Our results show that the identification of induced structures and the confidence categorization method utilized in this study highlighted trends in tectonic deformation that would have been masked if induced deformation had been included. The induced structures described here are generic to drill core in sedimentary rocks, and the method we used could be applied in many tectonic settings, potentially enhancing the application of structure density characterization for detailed structural analysis.

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