10:45 AM - 12:15 PM
[SCG61-P05] Spatial distribution of mineralization developed around the plate boundary fault in the Nankai Trough from X-ray CT data
Keywords:Calcite, Barite, X-ray Computed Tomography, Ocean Drilling Program, International Ocean Discovery Program, Nankai Subduction Zone
The spatial distribution of mineralization in the crust may modify crustal-properties such as brittleness or permeability. Thus, it is important information to understand crustal deformation. Hydrothermal mineralization were reported from International Ocean Discovery Program (IODP) Expedition 370 Site C0023, located at the toe of the Nankai accretionary complex off Muroto. In this study, we investigated the spatial distribution of mineralization assemblages under the seafloor at the Nankai Trough off Muroto using X-ray computed tomography (XCT) data of geological cores collected by Ocean Drilling Program (ODP) at sites 808, 1173 and 1174. Site 1173 is located on the ocean side of the Nankai Trough and the other sites are located at the toe of the Nankai Accretionary complex. The main lithology of these sites are post-middle Miocene hemipelagic sediments and trench-fill sediments on a basaltic basement and are divided into the Lower Shikoku Basin facies, Upper Shikoku Basin facies, and Trench-fill facies.
The profiles of mean CT-numbers generated for each of the three sites were analyzed at various spatial scales. First, the mean CT-number of the sediments gradually increases from about 1100 to 1800 with depth. Against this background positive spikes with mean CT-numbers ranging from 2000 to 9000 are observed. At Site 1173, these positive spikes concentration area is developing in three intervals: 1) the interval from the trench-to-basin to top of the upper Shikoku Basin facies, 2) an interval from the top of the lower Shikoku Basin facies to the proto-decollement, and 3) an interval within the lower sections of the lower Shikoku Basin facies. In the areas where high mean CT-numbers are concentrated, as thick as several centimeters, are distributed at intervals of several meters to several tens meters. The three intervals where the high mean CT-numbers are concentrated are identical to the calcite-rich intervals reported by XRD data and naked-eye observations. So, the high CT numbers are caused by calcite concentrations. The presence of high CT numbers at the same stratigraphic intervals at all three sites attests to similar mineralization processes occurring across the seaward part of the Nankai Accretionary complex off Muroto. In addition, the mineralization probably occurred prior to accretionary action. To examine the detailed distribution of the high CT numbers, the high CT numbers were divided into three categories: 1) 3000-4000 corresponding mainly to calcite, 2) 4000-10000 corresponding mainly to pyrite, and 3) more than 10000 corresponding to barite. Their spatial distributions show that CT numbers higher than 10000 are only present in the lower two intervals of the three high mean CT-numbers area.
The profiles of mean CT-numbers generated for each of the three sites were analyzed at various spatial scales. First, the mean CT-number of the sediments gradually increases from about 1100 to 1800 with depth. Against this background positive spikes with mean CT-numbers ranging from 2000 to 9000 are observed. At Site 1173, these positive spikes concentration area is developing in three intervals: 1) the interval from the trench-to-basin to top of the upper Shikoku Basin facies, 2) an interval from the top of the lower Shikoku Basin facies to the proto-decollement, and 3) an interval within the lower sections of the lower Shikoku Basin facies. In the areas where high mean CT-numbers are concentrated, as thick as several centimeters, are distributed at intervals of several meters to several tens meters. The three intervals where the high mean CT-numbers are concentrated are identical to the calcite-rich intervals reported by XRD data and naked-eye observations. So, the high CT numbers are caused by calcite concentrations. The presence of high CT numbers at the same stratigraphic intervals at all three sites attests to similar mineralization processes occurring across the seaward part of the Nankai Accretionary complex off Muroto. In addition, the mineralization probably occurred prior to accretionary action. To examine the detailed distribution of the high CT numbers, the high CT numbers were divided into three categories: 1) 3000-4000 corresponding mainly to calcite, 2) 4000-10000 corresponding mainly to pyrite, and 3) more than 10000 corresponding to barite. Their spatial distributions show that CT numbers higher than 10000 are only present in the lower two intervals of the three high mean CT-numbers area.