10:45 AM - 11:00 AM
[SCG46-07] Structural Characteristics of frontal prism at C0019, recovered by IODP Exp. 405
Keywords:Tohoku Earthquake, Mass Transport Deposit, Splay Faults
Drilling during IODP Exp. 405, at Site C0019, penetrated the Japan Trench frontal prism, plate boundary décollement, and underthrust sediments. Using visual core description to characterize the dominant structures, the major structural findings are followings:
1) Variably-inclined bedding associated with MTDs (Mass-Transport Deposits): In the frontal prism, we identified highly-variable bedding inclinations (0-90°) over/within meter length scales[AL1] , block-in-matrix textures co-located with tight[AL2] folding (<20 cm halfwave length) and mud injections. These textures occurred in both shallow (310-400 mbsf) and deeper (616-803 mbsf) cores within Hole C0019-J, and are indicative of soft-sediment deformation that accompanies MTDs.
2) The presence of a major fault at 610.9 mbsf (Hole C0019-J): [AL3] [YY4] The dominant interval of shearing associated with this major fault is 60 cm thick, and is composed of foliated gouges with scaly clay fabrics. Beneath this dominant interval of shearing, the base of this fault contains a black, 0.5 cm-thick layer, inferred to be fault gouge that marks a localized slip zone. Immediately beneath this fault gouge lies relatively undeformed dark gray mudstone. This major fault likely represents a reverse splay faultthat branched from the décollement zone, as this interval also corresponds to a distinct change in lithology, a major age reversal (Iwai+, this session; Upper Miocene and Pleistocene above and below the fault, respectively), and a fluid chemistry anomaly (Ishikawa+, this session).
3) Plate-boundary décollement: The base of the prism was identified at 826.05 mbsf (C0019J-87K-2, 59 cm) and 846.17 mbsf (C0019K-10K-2, 34 cm) in the Holes C0019-J and C0019-K, respectively. A sharp boundary representing major fabric changes from fractured mudstones, to foliated clay-rich rocks is accompanied by a large age gap as well as a fluid chemistry anomaly. At these depths, we identified numerous indicators of shear, each <1 m thick, and comprised of intercalated units of various clays and mudstones. Detailed observation of the split cores from this interval will be conducted and reported in this presentation.
4) Minor faults, fractures, and deformation bands (network of dark seams) increase in population density around the major faults mentioned in 2) and 3). The distribution is likely to be indicative of the strain accumulation and localization processes. Multi-scale structural [AL5] analyses have been ongoing, as post-cruise researcher work to reveal them. [AL1]I remember what was really striking about the MTDs was how rapidly (over short length scales) the bedding changed dips within the cores [AL2]what is the scale of this folding? (clarify what you mean by ‘tight’) [AL3]I tried to reorganize some of these paragraphs so 1) results came first, and 2) interpretation came second. [YY4]Thanks for your editing making this draft clearer! [AL5]this sentence feels a little out of place, but I do think you need a summary sentence tying the 4 main structural findings together at the end here.
1) Variably-inclined bedding associated with MTDs (Mass-Transport Deposits): In the frontal prism, we identified highly-variable bedding inclinations (0-90°) over/within meter length scales[AL1] , block-in-matrix textures co-located with tight[AL2] folding (<20 cm halfwave length) and mud injections. These textures occurred in both shallow (310-400 mbsf) and deeper (616-803 mbsf) cores within Hole C0019-J, and are indicative of soft-sediment deformation that accompanies MTDs.
2) The presence of a major fault at 610.9 mbsf (Hole C0019-J): [AL3] [YY4] The dominant interval of shearing associated with this major fault is 60 cm thick, and is composed of foliated gouges with scaly clay fabrics. Beneath this dominant interval of shearing, the base of this fault contains a black, 0.5 cm-thick layer, inferred to be fault gouge that marks a localized slip zone. Immediately beneath this fault gouge lies relatively undeformed dark gray mudstone. This major fault likely represents a reverse splay faultthat branched from the décollement zone, as this interval also corresponds to a distinct change in lithology, a major age reversal (Iwai+, this session; Upper Miocene and Pleistocene above and below the fault, respectively), and a fluid chemistry anomaly (Ishikawa+, this session).
3) Plate-boundary décollement: The base of the prism was identified at 826.05 mbsf (C0019J-87K-2, 59 cm) and 846.17 mbsf (C0019K-10K-2, 34 cm) in the Holes C0019-J and C0019-K, respectively. A sharp boundary representing major fabric changes from fractured mudstones, to foliated clay-rich rocks is accompanied by a large age gap as well as a fluid chemistry anomaly. At these depths, we identified numerous indicators of shear, each <1 m thick, and comprised of intercalated units of various clays and mudstones. Detailed observation of the split cores from this interval will be conducted and reported in this presentation.
4) Minor faults, fractures, and deformation bands (network of dark seams) increase in population density around the major faults mentioned in 2) and 3). The distribution is likely to be indicative of the strain accumulation and localization processes. Multi-scale structural [AL5] analyses have been ongoing, as post-cruise researcher work to reveal them. [AL1]I remember what was really striking about the MTDs was how rapidly (over short length scales) the bedding changed dips within the cores [AL2]what is the scale of this folding? (clarify what you mean by ‘tight’) [AL3]I tried to reorganize some of these paragraphs so 1) results came first, and 2) interpretation came second. [YY4]Thanks for your editing making this draft clearer! [AL5]this sentence feels a little out of place, but I do think you need a summary sentence tying the 4 main structural findings together at the end here.