The effects of ridge subduction on the thermal structure of accretionary prisms: A Tertiary example from the Shimanto Belt of Japan

Abstract

The Shimanto accretionary complex on the Muroto Peninsula of Shikoku comprises two major units of Tertiary strata: the Murotohanto subbelt (Eocene-Oligocene) and the Nabae subbelt (Oligocene-Miocene). Field-based structural analyses and laboratory measurements of thermal maturity show that both subbelts were affected by thermal overprints long after the initial stages of accretion-related deformation. Paleotemperatures for the entire Tertiary section range from about 140 to 315°C, based upon mean vitrinite reflectance values of 0.9 to 5.0%R m . In general, older rocks of the Murotohanto subbelt display higher levels of thermal maturity and better cleavage than rocks of the Nabae subbelt. The Murotohanto subbelt also displays a spatial decrease in thermal maturity from south to north, and this pattern probably was caused by regionalscale differential uplift following peak heating. Conversely, the paleothermal structure exposed within the Nabae subbelt is fairly uniform, except for the local effects of mafic intrusions at the tip of Cape Muroto. The most important structural discontinuity to display a large component of post-metamorphic vertical offset is the Shiina-Narashi fault; this out-of-sequence thrust forms the boundary between hanging-wall rocks of the Murotohanto subbelt and the younger Nabae subbelt. Rapid heating of the Shimanto Belt evidently occurred immediately after a middleMiocene reorganization of the subduction boundary of southwest Japan. Whether or •Present addresses: DiTullio, 326 12th St., Apt. 2L, Downloaded from 152 M. B. Underwood and Others not all portions of the inherited (Eocene-Oligocene) paleothermal structure were overprinted during the middle Miocene remains controversial. Subduction prior to middle Miocene time probably involved the Kula, or fused Kula/Pacific, Plate. Hot oceanic lithosphere from the Shikoku Basin back-arc spreading ridge first entered the subduction zone at approximately 15 Ma; this event also coincided with the opening of the Sea of Japan and the rapid clockwise rotation of southwest Japan. Middle Miocene thermal overprints elsewhere in the Shimanto Belt are spatially related to and, therefore, probably synchronous with widespread acidic magmatism, local mafic intrusions, and formation of anthracite coals in the forearc. The Kii Peninsula of Honshu appears to be the hottest spot along the strike of the subduction zone; this locality may well correspond to the collision point between the Shimanto accretionary complex and the central ridge of the Shikoku Basin. However, the spreading ridge probably was unstable and disorganized at the time of the collision, which allowed widespread, strike-parallel flux of mantle heat and magma into the accretionary prism. Apatite fission-track data show that most of the Eocene-Oligocene Shimanto strata experienced differential uplift and cooling through the 100°C isotherm shortly after the ridge-trench collision (11 to 8 Ma).The Shimanto Belt provides an excellent example of an ancient accretionary complex that does not follow the...

Deep seismic imaging of the eastern Nankai trough, Japan, from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration

“…This gradient zone could be proposed to be associated with a diagenetic front in the Shimanto unit, with possibly a slight metamorphism below it. This would be consistent with heating of the old prism [Underwood et al, 1993] at the onset of the subduction of the young and hot crust of the Philippine Sea plate at 15 Ma [Aoike, 1999]. The compressive tectonics offsetting this front would be posterior, possibly corresponding to the strong reactivation of the Philippine/Eurasia convergence at 8 Ma [Taira, 2001;Le Pichon, 1997] with the accretion of the Tanzawa massif of the Izu-Bonin arc at the collision front [Niitsuma, 1989].…”

The Mediterranean Ridge: A mass balance across the fastest growing accretionary complex on Earth

“…If the overall width of accreted strata of SW Japan is traced to the onshore outcrops of the Tertiary Shimanto Belt accretionary complex on Shikoku island, a total width of ∼170 km (from the deformation front to the onshore suture between the Tertiary and Cretaceous part of the Shimanto Belt) corresponds to 65 Ma period of subduction‐accretion. The average outward growth at Nankai is hence less than 3 km Myr −1 (which admittedly does not account for the complex tectonic framework and collisional history in the area [ Underwood et al , 1993], which makes Nankai one of the slowly growing margins on Earth [see also Kastens , 1991]. Similarly, Barnard [1978] suggested some 30–50 km for the Quaternary (i.e., ∼2 Ma) outward growth of the Cascadia wedge off Washington.…”

The late Paleozoic paired metamorphic belt of southern Central Chile: Consequence of a near-trench thermal anomaly?

“…Although most of the processes forming slab gaps induce more or less similar magmatic and thermal effects in the upper-plate, mid-ocean ridge subduction is characterized by a substantial impact on the forearc area. In that region, ridge subduction may cause uplift and unconformity development, local ophiolites emplacement, near-trench MORB (Mid-Ocean-Ridge-Basalt) intrusions, felsic magmatism by crustal anatexis of the accretionary prism (blow-torch effect, DeLong et al, 1979), and low-pressure/high-temperature metamorphism (Marshak and Karig, 1977;DeLong et al, 1979;Nelson et al, 1993;Underwood et al, 1993;see Sisson et al, 2003, for a synthesis). In this context, heating and melting of the accretionary prism can take place with or without cessation of subduction (Underwood et al, 1993;Figure 5.…”