Deformation history recorded in accreted sediments in an evolved portion of the Nankai accretionary prism Kohtaro Ujiie Research Program for Plate Dynamics, Institute for Frontier Research on Earth Evolution Introduction Ocean Drilling Program (ODP) legs and associated geophysical surveys have investigated deformation, fluid flow and physical properties in accretionary prisms. These studies have particularly focused on active processes at the toes of the prisms, improving understanding of initial mountain building and related geophysical and geochemical processes (e.g., Moore and Vrolijk, 1992). In the Nankai accretionary prism off Shikoku Island, southwest Japan, ODP Leg 190 penetrated the prism and the incoming sedimentary section at six sites (Fig. 1a, Moore et al., 2001b). One notable point of this leg was the penetration into an evolved portion of the prism at the upslope site (Site 1178), located ~65km landward of the deformation front (Fig. 1). Site 1178 is halfway between the deformation front of the Nankai prism and the Shimanto accretionary complex on Shikoku Island, which is the ancient analogue of the Nankai accretionary prism (Taira et al., 1988). Thus, cores from Site 1178 provide a rare opportunity to examine not only the structural evolution of the Nankai prism, but also some of the geological characteristics of the proto-shimanto complex. Site 1178 recorded the deformation of shallowly buried (less than 1km) but relatively evolved sediments in the accretionary prism, which are rarely preserved in on-land accretionary complexes due to later overprint and denudation. Therefore, detailed structural and magnetic fabric analyses were performed using samples and data collected from Site 1178. Deformation structures and magnetic fabrics in accreted sediments Fig. 2 shows the variation with depth of dip angles of Site 1178 deformation structures. Increased deformation occurs from 400 to 506 meters below seafloor (mbsf) and from 622 mbsf to the base of Site 1178 at 673 mbsf. These deformed intervals are marked by fractures and brecciated zones in which sediments are broken into mm- to cm-scale polished and slickenlined fragments (Fig. 3a). The brecciated zones are less than 10 m in thickness and their intensity varies within the deformed intervals. Toward the base of the deformed interval at 506 mbsf, the spacing of fractures tends to decrease on the scale of millimeters and downdip slickenlines become common. The density of fractures and bedding dips rapidly decrease below 506 mbsf (Fig. 2). An increase in fracturing and brecciation is also seen at 550 mbsf. Foliations are mostly developed below 400 mbsf (Fig. 2). On the microscopic scale, the foliations are defined by the alignment of phyllosilicates and clastic grains (Figs. 3b and 3c). Many of the foliations are considered to be fissility, because they are parallel to bedding and are present in undeformed intervals. However, a steeply dipping (> 55 ) bedding-oblique foliation is also developed, mostly in the deformed interval between 400 and 506 mbsf (Figs. 2, 3c and 3d). It appears not to be an axial-planar cleavage because there is no evidence of folding. It is seen in intervals that alternate with brecciated zones. No asymmetric fabrics or shear bands are associated with the bedding-oblique foliation (Fig. 3d). The sets of polished and lineated shear surfaces in the brecciated zones, which are marked by a preferred orientation of minerals, crosscut the fissility and/or the bedding-oblique foliation. This indicates that the brecciation is superposed on the planar fabrics. The anisotropy of magnetic susceptibility (AMS) was measured to determine the magnetic fabrics. The AMS is expressed by a symmetrical second order tensor and geometrically represented as an ellipsoid that is commonly coaxial with the strain ellipsoid (e.g., Borradaile, 1988). The AMS ellipsoid has three principal susceptibility axes: Kmax (maximum susceptibility), Kint (intermediate susceptibility) and Kmin (minimum susceptibility). The plane normal to Kmin commonly expresses the magnetic foliation. The AMS ellipsoid reflects a statistical alignment of magnetic minerals, and the most magnetically susceptible minerals can have distributions of shape orientation or lattice orientations influenced by the kinematic history of the fabric. This leads to the relationship between AMS and structural fabrics. A magnetic fabric is parallel to bedding and fissility, showing various magnetic foliation dips with depth (Fig. 4a). Another magnetic fabric shows good correlation with the steeply dipping bedding-oblique foliation in the deformed interval between 400 and 506 mbsf. Magnetic foliation dips decrease rapidly across the base of deformed interval at 506 mbsf. In spite of the various magnetic foliation dips, the Kmax axes were originally subhorizontal. Based on the paleomagnetic directions of the samples, the orientation of AMS axes can be restored to their original positions before drilling. The corrected Kmax are subhorizontal and dominantly oriented NE- SW, while the corrected Kint axes are shallowly to steeply inclined and oriented perpendicular to the Kmax axes directions (Fig. 4b). The corrected Kmin axes are shallowly to steeply inclined and dominantly oriented southeastward. This is consistent with distribution of poles to bedding and foliation below 400 mbsf (Shipboard Scientific Party, 2001). Origin of deformation structures and magnetic fabrics The overall distribution of deformation structures and magnetic fabrics with depth is shown in Fig. 5. Many of the magnetic foliations in the prism are parallel to bedding/fissility (Fig. 4a). Magnetic foliations dip variously, but the Kmax axes are subhorizontal and mainly oriented NE-SW (Fig. 4b). 169
These features indicate that the bedding-parallel magnetic fabrics with the subhorizontal NE-SW trending Kmax axes and fissility were rotated about a horizontal axis. As the deformation front is approached, the principal stresses reorient, resulting in magnetic fabrics characterized by the subhorizontal Kmax axes perpendicular to the plate convergence direction (Byrne et al., 1993). Such tectonic mineral fabric is expected to develop prior to rotation of bedding/fissility due to frontal accretion. Therefore, the bedding-parallel magnetic fabrics with the subhorizontal NE-SW trending Kmax axes was considered to be formed during bedding-parallel compression near the deformation front and then rotated about a horizontal axis during frontal accretion, resulting in landward dipping magnetic fabrics with NE-SW trending subhorizontal Kmax axes. The intensity of fractures and brecciated zones in the deformed intervals and at 550 mbsf suggest that they represent faults in the accretionary prism. Based on calcareous nannofossil biostratigraphy, inversions of biostratigraphic age are apparent in the accretionary prism (Shipboard Scientific Party, 2001). Therefore, some of the brecciated zones in the deformed intervals are presumably imbricate thrusts in the prism. Intense brecciation above 506 mbsf and a decrease in fracture density as well as bedding and magnetic foliation dips below 506 mbsf (Figs. 2 and 4a) imply the presence of a major fault at the base of the deformed interval between 400 and 506 mbsf. In the deformed interval between 400 and 506 mbsf, the bedding-oblique foliation alternates with brecciated zones, suggesting that the foliation is formed under the same stress system that produced the faulting. The most likely interpretation is that bedding-oblique foliation is a flattening plane associated with shearing in a fault zone more than 100 m in thickness. In this case, the magnetic fabric parallel to beddingoblique foliation represent pervasive strain in the fault zone above 506 mbsf. Considering that the maximum principal compressive stress in an accretionary prism is commonly subhorizontal (Davis et al., 1983) and that the Kmax axes are subhorizontal (Fig. 4b), a steeply dipping bedding-oblique foliation may develop perpendicular to the principal shortening direction. No dissolution residues or pressure shadows associated with clastic grains are seen in the deformed interval between 400 and 506 mbsf (Figs. 3b and 3c). This suggests that pressure solution was not operative during the development of bedding-oblique foliation. The bedding-oblique foliation occurs in shallowly buried and poorly lithified sediments of porosity ranging from 30% to 40% (Shipboard Scientific Party, 2001). Presumably, reoriented detrital and/or diagenetic phyllosilicates define the bedding-oblique foliation. On the other hand, the fault at 550 mbsf and brecciated zones in the deformed interval between 622 and 673 mbsf are not accompanied by a fault-related planar fabric. This indicates the localized nature of deformation within the prism. Timing of deformations recorded in accreted sediments in an evolved portion of the Nankai accretionary prism In contrast to the Large Thrust Slice Zone (LTSZ), where active out-of-sequence thrusts develop, no faults appear to crosscut slope sediments in a seismic profile across the Landward Dipping Reflector Zone (LDRZ; Fig. 1b). This suggests that faulting at Site 1178 occurred before deposition of slope sediments, prior to 5.54 Ma. One of the main results from ODP Leg 190 is the recognition of the rapid seaward growth of the Nankai accretionary prism; the outer 40 km of the prism (from LTSZ to prism toe) accreted within the past 2 Ma (Moore et al., 2001a). Therefore, faulting at Site 1178 occurred when Site 1178 was located at the frontal part of the accretionary prism. The repetitions in biostratigraphy suggest that the faults at Site 1178 could have formed in association with the development of in-sequence thrusts at the frontal part of the prism. At Sites 1174 and 808 (Fig. 1), deformation bands appear to reflect pervasive strain related to faulting at the toe of the Nankai accretionary prism (Byrne et al., 1993). Although the deformation style is different, the bedding-oblique foliation in the deformed interval (400-506 mbsf) may record pervasive strain associated with faulting at the frontal part of the prism. Timing of faulting and magnetic fabric development at Site 1178 implies that tectonic fabrics formed at the frontal part of the accretionary prism have been preserved ~65 km landward of the deformation front. Studies of ancient accretionary complexes have showed that offscraped sequences buried more than a few kilometers document the progressive development of tectonic fabrics during frontal accretion and the subsequent penetrative deformations associated with intra-prism compression (e.g., Ujiie, 1997). In contrast, the accreted sediments at Site 1178 do not record the modification of pre-existing tectonic fabrics and/or overprinting of deformations during subsequent rapid seaward growth of the Nankai accretionary prism. This suggests that penetrative deformations associated with seaward growth of the prism were not significant at shallow burial depths in the prism. Conclusions Leg 190 penetrated the accreted sediments in an evolved portion of the Nankai accretionary prism. Structural and magnetic fabrics formed at the frontal part of the prism have been preserved even ~65 km landward of the deformation front. As the sediments approached the deformation front, the bedding/fissility-parallel magnetic fabric took on a NE-SW trending subhorizontal maximum AMS axis, due to the NWdirected bedding-parallel compression. Subsequent faulting represented by brecciated zones took place in the frontal part of accretionary prism, presumably equivalent to the imbricate thrusts in the prism. During this process, a steeply dipping bedding-oblique foliation composed of reoriented detrital and/or diagenetic phyllosilicates was developed in a fault zone more than 100 m in thickness. It was formed as a flattening plane associated with shearing deformation and alternates with brecciated zones. The magnetic fabric oriented parallel to bedding-oblique foliation records pervasive strain related to faulting. Bedding and its parallel magnetic fabric together with fissility were rotated about a horizontal axis during frontal accretion, resulting in a seaward-verging magnetic fabric with NE- SW trending subhorizontal maximum AMS axes. Overprinting of deformations and/or modification of pre-existing fabrics during subsequent rapid seaward growth of the Nankai accretionary prism has not been recorded in accreted sediments at Site 1178, suggesting that later penetrative deformations were not significant at shallow burial depths in the prism. 170
References Borradaile, G. J., Magnetic susceptibility, petrofabrics and strain, Tectonophysics, 156, 1-20, 1988. Byrne, T., W. Brückmann, W. Owens, S. Lallemant, and A. Maltman, Structural synthesis: Correlation of structural fabrics, velocity anisotropy, and magnetic susceptibility data, in Proc. ODP, Sci. Results, 131, I. A. Hill, A. Taira, J. V. Firth, eds., College Station, TX (Ocean Drilling Program), 365-378, 1993. Davis, D., J. Suppe, and F. A. Dahlen, Mechanics of fold-and-thrust belts and accretionary wedges, J. Geophys. Res., 88, 1153-1172, 1983. Moore, G. F., A. Taira, A. Klaus, L. Becker, B. Boeckel, A. Cragg, A. Dean, C. L. Fergusson, P. Henry, S. Hirano, T. Hisamitsu, S. Hunze, M. Kastner, A. J. Maltman, J. K. Morgan, Y. Murakami, D. M. Saffer, M. Sánchez-Gómez, E. J. Screaton, D. C. Smith, A. J. Spivack, J. Steurer, H. J. Tobin, K. Ujiie, M. B. Underwood, and M. Wilson, New insights into deformation and fluid flow processes in the Nankai Trough Accretionary Prism: results of Ocean Drilling Program Leg 190, Geochem. Geophys. Geosyst., 2, 10.129/2001GC000166, 2001a. Moore, G. F., A. Taira, N. L. Bangs, S. Kuramoto, T. H. Shipley, C. M. Alex, S. S. Gulick, S, D. J. Hills, T. Ike, S. Ito, S. C. Leslie, A. J. McCutcheon, K. Mochizuki, S. Morita, Y. Nakamura, J. -O. Park, B. L. Taylor, G. Toyama, H. Yagi, and Z. Zhao, Data report: structural setting of the Leg 190 Muroto transect, in G. F. Moore, A. Taira, A. Klaus, et al., Proc. ODP, Init. Repts., 190, 1-14 (CD-ROM). Available from: Ocean Drilling Program, Texas A&M University, College Station TX 77845-9547, USA, 2001b. Moore, J. C., and P. Vrolijk, Fluids in accretionary prisms, Rev. Geophys., 30, 113-135, 1992. Shipboard Scientific Party, Site 1178, in Proc. ODP, Init. Repts., 190, G. F. Moore, A. Taira, A. Klaus, et al., 1-108 (CD-ROM). Available from: Ocean Drilling Program, Texas A&M University, College Station TX 77845-9547, USA, 2001 Taira, A., J. Katto, M. Tashiro, M. Okamura, and K. Kodama, The Shimanto belt in Shikoku, Japan: evolution of cretaceous to miocene accretionary prism, Modern Geol., 12, 5-46, 1988. Ujiie, K., Off-scraping accretionary process under the subduction of young oceanic crust, the Shimanto belt of Okinawa island, Ryukyu Arc, Tectonics, 16, 305-322, 1997. 171
FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1 Figure 2. Dip angle of bedding, foliation, and fractures. Shaded areas show deformed intervals. Note that there is an increase in fracturing at 550 mbsf but the orientation of the structures was not possible to measure. Figure 1. (a) Location map showing the Ocean Drilling Program Leg 190 (solid circles), previous drilling sites (solid squares), and a 3-D seismic survey (shaded outline) off Shikoku Island, southwest Japan. (b) Generalized depth section from seismic reflection data in the Muroto Transect showing tectonic domains and the location of ODP Legs 131 and 190 drilling sites (after Moore et al., 2001a). Figure 3. Deformation structures of accreted sediments at Site 1178. (a) Core photograph of brecciated zone in the deformed interval between 400 and 506 mbsf. (b) Photomicrograph of foliation in the deformed interval between 400 and 506 mbsf. Phyllosilicates are parallel to sand lamina (dash lines) defining bedding-parallel fissility. (c) Secondary mode SEM image of foliation in the deformed interval between 400 and 506 mbsf. Although bedding cannot be discerned in this sample, the dip of foliation attains 79, which is too steep for bedding-parallel foliation (see also Fig. 2). (d) Core photograph of bedding-oblique foliation. 172
Figure 5. Summary of deformation structures and magnetic fabric at Site 1178. Figure 4. (a) Dip angle of poles to magnetic foliation, bedding, and foliation with depth. Shaded areas indicate deformed interval. (b) Distribution of principal susceptibility axes after paleomagnetic correction. 173