An Examination of Accretionary Prisms at the Nobeoka Thrust

Transcription

1 Evan Tam, University of Connecticut UTRIP 2012 Participant, Kimura Labs An Examination of Accretionary Prisms at the Nobeoka Thrust Introduction Accretionary prisms form at convergent plate boundaries, and are created when sediments of the subducting plate are scrapped off and accreted onto the overlying plate. These formations lie in the seismogenic zones, from where about 90% of seismic moments originate in a subduction zone (Lay and Biley, 2007; Pacheo and Sykes, 1992; Ruff and Kanamori, 1983). The configuration of this seismogenic zone can determine the power of an earthquake and the size of a tsunami if slip occurs within the zone (Park and Tsuru, 2002). This combination of earthquakes and tsunamis, most often caused by movement along the subduction zones, are a grave danger to coastal populations. Formation Accretionary prisms form continuously as long as subduction occurs at a convergent plate boundary. However, as more sediments accumulate on the overlying plate, the accretionary prism begins to deform in order to accommodate the added sediments (Figure B). Pre-lying sediments are pushed away from the subducting plate by newly accreted sediments, creating several different features in the accretionary prism. As pushing continues, the sediments furthest from the subducting plate fold onto each other, by way of a fault-like mechanism. These thrusts are called splay faults, and run along the base of the accretionary prism (Figure B-C). Another deformation mechanism is the continuous deformation of the taper. The taper, or the slope of the frontal accretionary prism, is constantly increasing due to the continual addition of sediments (Figure B). However, as the taper approaches a certain angle, called the critical taper, the accretionary prism will collapse under its weight. The accumulation of sediments causes out-of-sequence thrusts to form towards the back of the prism, shifting the sediments in such a way that the taper is reduced (Figure B-D). These mechanisms continue as more sediments are amassed onto the prism. (Figure A)

2 (Figure B, Gutscher et. al, 1996) Setting In order to better understand accretionary prisms and their formation mechanism, Kimura labs decided to examine the Nobeoka Thrust. The Nobeoka Thrust is located in the Miyazaki Prefecture of Kyushu, Japan (Figure C). It is notable in the study of accretionary prisms, as it the location of a large exhumed accretionary prism, or one that is no longer active and has been pushed on to land. The Nobeoka Thrust is a complex out-o-sequence thrust, with plastically deformed clastic rocks in the hanging wall, and mélange like cataclastic deformation in the footwall (Okamoto et al., 2006; Raimbourg et al., 2009; Kondo et al., 2005). With such complex deformities in both the hanging and the foot wall, it is difficult to decipher how the core of the Nobeoka Thrust was deformed. The deformation of the core is influenced by both hanging and foot wall, and by understanding the deformation styles of both walls we can piece together how the core was deformed. Kimura lab s research in Nobeoka focused on various deformation patterns that we found in the footwall, with a focus area of up to 100 meters from the thrust core. Our goal was to research foot wall deformation in order to better understand core deformation. (Figure C, Kondo et. al, 2005; Figure D, E, Yamaguchi et al, 2011)

3 Research Area My focus area during the research was on an area of rock deformation about 50 meters from the central core. It was located in the foot wall of the accretionary prism, dominated by cataclastic mélange from the Hyuga group (Figure D). The focus area itself was about 8 meters long, and in depth ranged from 1 to 2.5 meters. The maps created from this area were oriented perpendicular to N 52 E. The most important part of the focus area was in the first 5 meters. We discovered a small band of cataclastic deformation, spanning about 2 meters wide (Figure G, 1). The deformation band ran along the bottom of a minor fault line. Additionally, a thin ankyrite vein ran above the minor fault line (Figure F, 1; Figure G, 2). A second minor fault line, perpendicular to the first minor fault line, was located towards the north-west (Figure F, 4). The second minor fault line was noted for also having a thick vein of ankyrite, running for about 2 meters in depth (Figure F, 3). The ankyrite vein running along the second minor fault line was very thick, reaching 5-10 centimeters at times. Moreover, the ankyrite vein was thick enough that there were clasts suspended in it. A third minor fault was noted, although it was not specifically in the mapped focus area. However, it is worth noting, as it seemed to have some influence on the quartz mineral veins above the first minor fault. The quartz veins followed the direction of the third minor fault (Figure G, 3), but more towards the north-west side of the area, began to bend so to line up with the second minor fault (Figure F, 2). (Figure F, by Shimizu Mayuko. Line A connects to Figure G) Significance The reason for the combination of features in this area is not currently known, and thus was the focus of my research. The main interesting feature was the dual existence of both the ankyrite veins and the layer of cataclastic deformation. Cataclastic deformation typically occurs under high levels of pressure, where the localized hanging wall and foot wall press against a layer of rock so powerfully that it is deformed( Yamaguchi et al, 2011). The observed layer of cataclastic

4 (Figure G, by Evan Tam. Line A connects to Figure F) deformation was notable because of how limited in size it was. It was about 2 meters wide, and only about 5 centimeters thick. Typically, a large area will show signs of Ankyrite veins tend to form in areas of reduced pressure when an Out of Sequence thrust relaxes, as mineral rich fluid is able to flow into gaps and precipitate. At first glance, the presence of Ankyrite veins and cataclastic deformation is contradictory to each other, as one requires reduced pressure, whereas the other requires intense pressure. Furthermore, the immediate area in which these formations occurred are on or are surrounded by minor fault lines, which imply some movement and pressure have occurred at these points. A possible solution is that the Ankyrite veins and cataclastic deformation formed at different points. We know that the minor faults must have been pressurized, most likely during their formation, so the reduction in pressure may have come from after the creation of the 3 minor fault lines. A reduction of pressure could have allowed for increased fluid flow, allowing minerals deformation, rather than only a small band. This band is also rather confusing when juxtaposed with the layers of Ankyrite that were discovered on the minor fault lines. to seep into cracks that would later form mineral veins, such as ankyrite. Further evidence of fluid flows exist in the quartz veins. Quartz veins, similar to Ankyrite, require fluid flow to form. In the focus area, the Quartz veins bend from being parallel to the 3 rd minor fault to a formation that suggests that the fluid that formed the Quartz vein flowed into the minor fault line running along the think Ankyrite vein. This Ankyrite vein may have developed thicker due to the fluid flow from the quartz. Future Research This research project is still ongoing, and has not been finished due to time constraints. However, there are still many types of experiments that can be performed to discover the nature of the focus area. Information such as grain size and direction, mineral composition, and local stress would be useful finding a conclusion.

5 One technique that is available for examining grain size, mineral composition and stresses is thin sections. Making thin sections involves creating a microscope slide with a rock sample from the focus area. The sample slide must be sanded down to be 30μm thin, so that light is able to pass through it. Since thin sections are observable under a microscope, it is easier to see how the small grains in a rock sample are oriented, and see what minerals are present. Both of these observations aid in finding both the stress of the structure and the presence of fluid flow. An XRD test (X-Ray Diffraction) would also aid in tracking fluid flow, as it can be used to identify minerals within a rock sample. It is able to find the ratio of minerals on the surface of the sample by shooting X-Rays at the sample. The diffraction patterns recorded from that indicate the sample s mineral composition. It is more useful than thin sections for mineral identification, as thin sections depend on one s own observation skills in order to find the different compositions of a sample, making it more difficult to accurately characterize a focus area. Creating a Stereo Net Analysis map would be useful in analyzing the stresses in the given area. A Stereo Net Analysis map is created by gathering strike points of the local rock face. The strike of a rock face indicates of the stress experienced during formation. A collection of strike points in the area, with a Stereo Net Analysis map, would indicate the general trend for the local stresses. This data would especially help in investigating the presence of the small band of cataclastic deformation. As of right now, not enough data has been gathered to come to a final conclusion. More information must be gathered through experiments on rock samples and the focus area. However, once this is done, we will be one step closer in determining how the different modes of deformation work within the Nobeoka Thrust, and other accretionary prism structures around the world. References Gutscher, M., Kukowski, N., Malavieille, J., Lallemand, S. Cyclical behavior of thrust wedges: Insights from high basal friction sandbox experiments. Geology, 24 (1996), pp Lay, T., Bilek, S. Anomalous Earthquake Ruptures at Shallow Depths on Subduction Zone Megathrusts. Dixon T.H., Moore J.C. (Eds.), The Seismogenic Zone of Subduction Thrust Faults, Columbia University Press, New York (2007), pp Kondo, H., Kimura, G., Masago, H., Ohmori-Ikehara, K., Mitamura, Y., Ikesawa, E., Sakaguchi, A., Yamaguchi, A., Okamoto, S. Deformation and Fluid Flow of a Major Out-of-sequence Thrust Located at Seismogenic Depth in an Accretionary Complex: Nobeoka Thrust in the Shimanto Belt, Kyushu, Japan. Tectonics, 24 (2004), TC6008, doi: /2004tc Okamoto, S., Kimura, G., Takizawa, S., Yamaguchi, H. Earthquake Fault Rock Indicating a Coupled Lubrication Mechanism. Earth, 1 (2006), pp Pacheco, J.F., Sykes, L.R. Seismic moment catalog of large shallow earthquakes, 1900 to 1989 Bull. Seismol. Soc. Am., 82 (1992), pp Park, J., Tsuru, T., Kodaira, S., Cummins, P., Kaneda, Y. Splay Fault Branching Along the Nankai Subduction Zone. Science, 297 (2002), pp Raimbourg, H., Shibata, T., Yamaguchi, A., Yamaguchi H., Kimura, G. Horizontal Shortening versus Vertical Loading in Accretionary Prisms. Geochem. Geophys. Geosyst. 10 (2009), Q04007, doi: /2008gc Ruff, L., Kanamori, H. Seismic coupling and uncoupling at subduction zones. Tectonophysics, 99 (1983), pp Yamaguchi, A., Cox, S., Kimura, G., Okamoto, S. Dynamic Changes in Fluid Redox State Assoicated with Episodic Fault Rupture Along a Megaspay Fault in a Subduction Zone. Earth, 302(2011), pp

6 Acknowledgements I would like to thank many people for making my research possible. First off, I am especially grateful to Professor Kimura and all others who accepted me into the program: without them, I would have never had this wonderful opportunity to do such great work. Secondly, I would like to thank all the members of Kimura Labs who took care of me during my stay; Asuka-san, Kameda-san, Koge-kun, Shimizu-san, Hamahashi-san, Hamada-san, Tomonaga-kun, and Fukichi-san. You all taught me so much, both inside the labs and out. Lastly, I want to thank the administrative staff, especially Soeda-san, and all the Utrip members who made this an experience I will never forget.