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

Abstract

Samples with vein structures were taken from Sites 787 and 793 in the forearc basin of the Izu-Bonin island arc off Aoga Shima and Sumisu Jima, respectively, between the present volcanic front and the outer arc high. The samples were studied by thin section, X-ray radiograph, and magnetometer; they are discussed with respect to the tectonic implication of the vein structures to the island-arc development.Vein structures are developed in finer, more clayey, preferentially radiolarian-bearing mudstone, subvertical to the bedding plane, which is mostly horizontal. The veins are restricted to certain horizons: in the upper Oligocene at Site 787 and in the lower Miocene at Site 793. The veins are filled with a dominant clay mineral (montmorillonite), which flowed into the vein when the fracture and concomitant stress drop occurred. Some clay mineral was deposited from the fluid that invaded the vein. Some veins might have occurred as hydraulic fractures. The shape, mode of occurrence, and other structural features indicate that the veins originated either as extension fractures or shear cleavages, and then were rotated by the following shearing parallel to the bedding. Sometimes the bedding-parallel slip planes are dislocated by the veins, and sometimes vice versa. This suggests that the vein formation and bedding parallel slip alternately occurred within the same stress environment.Vein attitude was measured by a magnetometer, after alternating field demagnetization; we interpret that they originally formed as subvertical planes, the trends of which average to N45W. The quantity of samples studied was small, but the trends suggest that the stress field for veining might have had a relative extensional stress axis that lay subhorizontally and trended generally northeast. This stress orientation might be attributed to either bending or normal faulting in the forearc basin, at a time when the arc trended northwest.

Vein structures, like ripple marks, are formed by short-wavelength shear waves

“…Similar structures have been recognized in DSDP-ODP cores, mostly from trench slopes, for example, from the Mid-America trench off Guatemala (Cowan, 1982;Ogawa and Miyata, 1985;Helm and Vollbrecht, 1985), the Japan trench (Arthur et al, 1980;Leggett et al, 1987;Knipe, 1986), the Izu-Bonin trench (Ogawa et al, 1992), and the Chile trench ; Kemp, 1990;Lindsley-Griffin et al, 1990), as well as in many on-land examples from the Monterey and Santa Cruz formations, California (Brothers et al, 1996).…”

Layer‐parallel faults, duplexes, imbricate thrusts and vein structures of the Miura Group: Keys to understanding the Izu fore‐arc sediment accretion to the Honshu fore arc

“…Formerly one of the present authors, YO, interpreted the vein structure as an extentional fracture or tension gash (Ogawa 1980;Ogawa & Miyata 1985;Ogawa et al 1992). It is still possible as mentioned above, but from a consideration of the consistent nature of dislocation in the sense of both the synthetic and antithetic Riedel shears in a shearing regime, we now favour a shearing origin by means of horizontal shaking.…”

“…15). As in the former case, it corresponds to the examples from sites 787 and 793 of the ODP Leg 126, where abundant vein structures are reported (Ogawa et al 1992) from the sediments whose bedding at present is almost horizontal avoiding any gravity sliding. In this case, vein structures might form due to horizontal shearing by means of earthquake shaking.…”

“…One more possibility is that the vein could first originate as an extension fracture, then dislocation occurs when the principal stress axes rotate during shearing strain proceeds (Ogawa & Miyata 1985;Charvet et al 1992). Some veins may be so; however, some non-sigmoidal veins have a considerable dislocation as shown in the examples of Ogawa et al (1992) We consider that the best candidate in nature for a source of the two-way horizontal shaking is an earthquake, particularly the s-or Rayleigh-wave, which shakes within a horizontal plane and has the largest inertia within the earthquake waves. In the case of natural vein structures, the sediment must be very wet, but they often cut sharply pre-existing structures such as sedimentary structures, bioturbation or early faults (Figs 4,11).…”

Site 948

“…Mudfilled veins have been interpreted as early extensional features not necessarily related to accretion (Ogawa et al, 1992), whereas scaly fabric and stratal disruption are typical features of shear zones in clay-rich sedimentary rocks. We use the term scaly fabric in its usual mesoscopic sense, based on visual observation of the cores, noting that scaly fabric may refer to any of several distinct microstructural textures (e.g., Agar et al, 1989).…”