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doi:10.2204/iodp.proc.344.105.2013

Frontal prism Site U1412¹

R.N. Harris, A. Sakaguchi, K. Petronotis, A.T. Baxter, R. Berg, A. Burkett, D. Charpentier, J. Choi, P. Diz Ferreiro, M. Hamahashi, Y. Hashimoto, K. Heydolph, L. Jovane, M. Kastner, W. Kurz, S.O. Kutterolf, Y. Li, A. Malinverno, K.M. Martin, C. Millan, D.B. Nascimento, S. Saito, M.I. Sandoval Gutierrez, E.J. Screaton, C.E. Smith-Duque, E.A. Solomon, S.M. Straub, W. Tanikawa, M.E. Torres, H. Uchimura, P. Vannucchi, Y. Yamamoto, Q. Yan, and X. Zhao²

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1412 (proposed Site CRIS-9A) targeted the frontal sedimentary prism, the main subduction thrust, the underthrust sedimentary section, and the upper oceanic crust. This site is along seismic Line BGR99-7 at common midpoint 4550 (Figs. F1, F2). The scientific objectives were to

1. Characterize the frontal sedimentary prism by documenting its lithology and age;
2. Sample the décollement at a shallow depth to define the deformation style, stress, fault friction, and behavior that influences coseismic displacement and plays an important in tsunami earthquakes;
3. Characterize any diagenetic processes, microstructures, and potential sealing/healing processes;
4. Characterize the physical properties of the subducting incoming and frontal prism sediments that are thought to mix with eroded upper plate material; and
5. Determine the chemistry of pore water within the wedge, décollement, and underthrust sections and characterize fluid pathways.

Wireline logging this site was a high priority to quantify the changes between underthrust material and input material.

This site was chosen because the frontal prism, subduction thrust, incoming sediment, and upper oceanic crust are well imaged with seismic reflection data (Fig. F1). The décollement is imaged as a high-amplitude negative-polarity reflection that indicates an inversion in seismic impedance. High heat flow measured in this area is interpreted as fluid flow in the upper oceanic crust, and estimating the style of flow is important.

Site U1412 consists of a 780 m thick section of upper plate sediments and 200 m of underthrust sediments (Fig. F1). The bottom-simulating reflector (BSR) is at ~200 meters below seafloor (mbsf). Sediment above the BSR is relatively transparent with few coherent reflectors. Below the BSR, a series of landward-dipping reflectors are interpreted as faults. The décollement is at 780 mbsf and is defined by a high-amplitude negative-polarity reflection. Near the toe of the margin, the décollement appears to cut sharply upward at ~1 km landward of the deformation front. Below the décollement, the incoming sediment shows little internal coherence of reflectors. The top of the oceanic crust is well defined by a large-amplitude reflector. A series of steps in the basement reflector may represent blocks of oceanic crust offset by plate-bending normal faults.

¹ Harris, R.N., Sakaguchi, A., Petronotis, K., Baxter, A.T., Berg, R., Burkett, A., Charpentier, D., Choi, J., Diz Ferreiro, P., Hamahashi, M., Hashimoto, Y., Heydolph, K., Jovane, L., Kastner, M., Kurz, W., Kutterolf, S.O., Li, Y., Malinverno, A., Martin, K.M., Millan, C., Nascimento, D.B., Saito, S., Sandoval Gutierrez, M.I., Screaton, E.J., Smith-Duque, C.E., Solomon, E.A., Straub, S.M., Tanikawa, W., Torres, M.E., Uchimura, H., Vannucchi, P., Yamamoto, Y., Yan, Q., and Zhao, X., 2013. Frontal prism Site U1412. In Harris, R.N., Sakaguchi, A., Petronotis, K., and the Expedition 344 Scientists, Proc IODP, 344: College Station, TX (Integrated Ocean Drilling Program). doi:10.2204/​iodp.proc.344.105.2013

² Expedition 344 Scientists’ addresses.

Publication: 11 December 2013
MS 344-105

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