Initial Tectonic Deformation of Hemipelagic Sediment at the Leading Edge of the Japan Convergent Margin

Abstract

Diatomaceous mudstones at depth under the deep sea terrace and the trench inner slope off Japan have been variably affected by tectonic stress. Veins, healed fractures, and microfaults occur at all sites except the shallow Site 435 on the upper trench inner slope and Site 436 on the Pacific Plate. Veins, fractures, and faults occur in cores from below 620 meters (lower to middle Miocene) in the landward sites (438 and 439) on the deep sea terrace, and are probably related to normal faulting seen in seismic records. The depth to "consolidated" sediment and to the first occurrence of veins and healed fractures shallows progressively toward the trench. The intensity of deformation also appears to increase seaward. However, no sediments younger than upper Pliocene are deformed. Open fractures may exist in situ at Sites 434 and 441 at levels between about 150 and 500 meters sub-bottom. The Japan Transect sediments-in contrast to deposits in the zone of initial deformation at other convergent margins-though highly deformed, are not highly overconsolidated. However, sediment at depth in the trench inner slope sites is overconsolidated relative to that at the same depth in the landward reference site. None of the deformed Japan margin sediments recovered at Legs 56 and 57 sites originated by accretion of oceanic plate material-also in contrast to sediments at some of the margins previously studied. We suggest that tectonic stress related to convergence has been communicated to the slope sediments on the trench inner slope, either continuously or periodically, causing rapid tectonic dewatering and inducing fracturing and faulting. If episodic, the latest of these deformational periods may have occurred during the late Pliocene. The faults and fractures are either rehealed by continued overburden pressure (sediment loading) or may remain open at shallower levels. Fracturing and dewatering of semiconsolidated sediment beneath an unconsolidated but impermeable mud veneer may cause overpressured zones at depths of 200 to 500 meters. These overpressured zones possibly locally reduce shear strength and cause downslope mass movement of sediment, even on low-angle slopes on the trench inner slope.

Site 792

“…Similar features occur in Unit III and in the Eocene and Oligocene sections at Sites 787 and 793 (see "Site 787" and "Site 793" chapters, this volume). The veinlets are identical to those described from the Middle America Trench slope by Ogawa and Miyata (1985) and from the Japan Trench by Arthur et al (1980) and Lundberg and Leggett (1986), and are interpreted as extensional fracture cleavages formed in rapidly consolidating (wet) sediments. The microfaults in Units HI and IV also formed in an extensional regime.…”

The damage is done: Low fault friction recorded in the damage zone of the shallow Japan Trench décollement

“…Under optical microscope, thin zones of subangular clasts in a fine-grained cataclastic matrix separate larger clasts in breccia zones. Arthur et al [1980], Carson et al [1982], Lundberg and Moore [1986], Maltman et al [1993], and Maltman [1998] recognized breccia zones in drill core from other convergent margins.…”

Vein Structures and Their Tectonic Implications for the Development of the Izu-Bonin Forearc, Leg 126

“…Many examples have been cited by Deep Sea Drilling Project (DSDP) drilling in the area, particularly on Legs 57 and 87 in the Japan Trench landward slope (Arthur et al, 1980;Leggett et al, 1987), and on Legs 67 and 84 in the Middle America Trench landward slope off Guatemala (Cowan, 1982;Ogawa and Miyata, 1985). Most of those examples are known from the trench-landward slopes of nonaccretion-type convergent margins, that is, the veins favor the trench-slope covers.…”