Characteristics of Magnetic Fabrics in Mass Transport Deposits in the Nankai Trough Trench Slope, Japan
Submarine landslides are a potential risk to coastal areas all over the world. Studies of mass transport deposit (MTD) contribute to our understanding of the nature and process of submarine landslides. Scientific drilling provides material containing geological records of past landslide events. However, MTDs may not always be uniquely discernible by visual inspection. We applied magnetic fabric analysis to the drilled cores to examine the potential of magnetic fabrics for use in identifying MTDs. Among the sites drilled in the framework of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), in Japan, of the Integrated Ocean Drilling Program (IODP), multiple occurrences of MTDs were observed in the recovered cores. We focused on the slope sediments in the footwall of the megasplay fault at Sites C0008 and C0018. The shape parameter (T) and the orientation of the axes of magnetic ellipsoids are distinctively scattered in MTDs at Site C0018. Downward increments in the lineation parameter (L) near the bottom of the MTDs may result from shear localization near the basal sliding plane. This, in combination with visual observation, suggests cohesive mass flow. By contrast, the results from sediments previously described as mass transport complexes at Site C0008 showed the opposite trend, suggesting a different process during transportation; i.e. the mass transport body evolved to become a complete debris flow. Our results show that magnetic fabric analysis is potent for describing MTDs and their internal structures. This finding may extend the methodology for describing MTDs and add to the discussion of the dynamic formation process.
KeywordsSubmarine landslide D/V Chikyu NanTroSEIZE Accretionary prism Megasplay fault IODP Expeditions 316 and 333
The trench slope along subduction zones is a common location of submarine landslides. Submarine slope failures can be promoted by the intrinsic environment of subduction zones and their depositional conditions such as shaking due to earthquakes or tectonic steepening. The anisotropy of magnetic susceptibility (AMS) is a rapid and nondestructive mean of investigating rock fabrics. The AMS method has been used as a reliable strain indicator in geological settings where conventional strain markers are scarce and it is currently best to investigate subtle tectonic related fabrics in clay sedimentary rocks at the early stages of deformation. This method has successfully provided strain information for various types of accretionary prism material: e.g., present marine sediments (e.g. Owens 1993), ancient accretionary prisms (e.g. Kanamatsu et al. 1996; Yamamoto 2006) and even highly deformed plate boundary rocks (e.g. Kitamura et al. 2005; Kitamura and Kimura 2012).
Here we present magnetic fabric data for the trench slope sediments from Sites C0008 and C0018, from which mass transport deposits (MTDs) were recovered during IODP Expeditions 316 and 333, respectively. Our aim is to examine the potential of magnetic fabrics in identifying MTDs.
58.2 Geological Overview of IODP Sites C0008 and C0018
The Nankai Trough of southwest Japan is among the most extensively studied subduction zones in the world, and has a well documented 1,300-year history of great earthquakes and associated tsunamis (Ando 1975). Here, the Philippine Sea plate is being subducted beneath the Eurasian continental plate at a convergence rate of 4 cm/year (Seno et al. 1993). The expeditions were conducted as a part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a multi-expedition project for characterizing and monitoring the subducting plate boundary process in the Nankai Trough. Multiple drillings have recovered mass transport deposits in various places along a transect across the trough (e.g., Expedition 316 Scientists 2009; Kitamura and Yamamoto 2012; Strasser et al. 2012).
58.2.1 Site C0008
58.2.2 Site C0018
Site C0018 is located ∼7.5 km southwest of Site C0008 and ∼3 km seaward from the megasplay fault tip in a slope basin at 3,112 m water depth (Fig. 58.1). The site was selected to recover MTDs downslope from the megasplay fault zone (Expedition 333 Scientists 2012 and Strasser et al. 2012).
Sediments cored at Site C0018 were subdivided into two lithologic subunits, Units IA and IB (Expedition 333 Scientists 2012). Subunit IA is mainly composed of bioturbated hemipelagic mud intercalated with layers of varying coarse and fine volcanic ash and MTDs. Subunit IB consists of sandy turbidites interbedded with silty clay. The depositional ages of the sediments at Site C0018 are <1.67 Ma at the bottom of the borehole and about ∼1 Ma at the boundary of Subunits IA and IB.
Six intervals of MTDs occur within Subunit IA. They range in thickness from 0.5 to 61 m and have a cumulative thickness of 98 m. For the recognition of MTDs in a single drill site, onboard imaging by X-ray computed tomography (CT) scans of recovered cores provided efficient and clear criteria in addition to the conventional visual core description (Expedition 333 Scientists 2012). Physical properties data show that MTD intervals display an increased compaction gradient compared with the average porosity-depth trend, and slight reversals (porosity increasing with depth) are observed near the base of MTDs 2, 3, 5 and 6 (Expedition 333 Scientists 2012; Strasser et al. 2012).
In this study, AMS measurements were made on a total of 813 samples (7 or 10 cm3 volume) from Sites C0008 (n = 330) and C0018 (n = 483). Clayey or silty samples were collected from every core section, avoiding specific features such as veins, nodules and shear bands, and were measured with a KLY-4S Kappabridge magnetic susceptibility meter at the Japan Agency for Marine-Earth Science and Technology. As much as possible, we selected fine-grained clayey samples. Onboard data suggest that the magnetic minerals may be a mixture of titano-magnetite and clay minerals as reported in this area (e.g. Kitamura et al. 2010).
Magnetic susceptibility is a proportion of the intensity of the induced magnetic field to that of the applied magnetic field. AMS represents the geometrical alignment and intensity of mineral fabrics as a magnetic ellipsoid, which is commonly linked with the strain ellipsoid. The ellipsoids expressed with the principal susceptibility axes (K1 > K2 > K3) and the minimum axis K3, in particular, are widely regarded as the orientation of maximum shortening.
We present the following parameters derived from the principal susceptibility axes for discussion: The lineation parameter L (= K1/K2) and the flattening parameter F (= K2/K3) commonly used for structural geology, the anisotropy degree (P′) and the shape parameter (T), an amended expression of the L-F diagram (Flinn diagram) in the polar coordinate system proposed by Jelinek (1981), Km, the bulk magnetic susceptibility.
Km reflects the amount of magnetically susceptible components in the specimen and thus displays the lithologic difference. L-F and P′-T are each a pair of factors expressing the shape of magnetic ellipsoids but are different in their main focus. L and F indicate the intensity of the shape components, lineation and flattening, respectively. Given both L and F, we know the shape of the ellipsoid. T itself provides the shape information (oblate if 0 < T < 1 and prolate if −1 < T < 0), where the intensity of distortion compared to the true sphere is presented by P′. Therefore, P′-T are useful for discussing the shape in general while L-F are strong when focusing on the lineation or flattening components.
For normally deposited and compacted marine sediments, it is expected that P′ shows a gradual increase with depth in association with the reduction of porosity and T displays random plots in the shallow part shifting to the oblate field (Kitamura et al. 2010). This compaction trend is seen as a stable L and increasing F with depth. A vertical K3 axis is expected for gravitational compaction (e.g. Kanamatsu et al. 2012).
At Site C0018, the T value shows a scattered distribution and low P′ in the upper 50 mbsf, indicating unconsolidated deposits. Converging T to the positive value in the deeper succession, except within the MTDs, agrees with the trend of compaction against depth inferred from the porosity data. The parameters of T and the axes of magnetic ellipsoids (the inclination of K1 and K3) as well as the bedding dip show remarkable scatter in the MTD intervals. Km at Site C0018 reflects a slight difference in lithology between Subunits IA and IB. The sediments at Site C0018 are characterized by sediment compaction with positive T and vertical K3, although the down-hole progress of compaction is not clear, as well as MTDs with scattered T and K1/K3.
58.5 Discussion and Summary
Average T parameters in the MTDs and coherent intervals in between
Unit IB (MTC)
Unit IB (MTC)
Some parameters bear information on the internal structure and processes of MTD formation. The lineation parameter slightly increases with depth near the base of MTDs 2, 3, 5 and 6 (Fig. 58.2). This increase is associated with a porosity kick back at the base of the MTDs; i.e., the sediments in the lowermost MTD appear to be overconsolidated. We speculate that the loading and shearing near the basal boundary of the transporting mass promoted mechanical dewatering and lineation fabrics. These observations may reflect localized shear flow near the basal sliding plane of transporting debris.
The AMS results from MTD in Subunit IB at Site C0008 contradict the results from MTDs at Site C0018. A high P′ and positive T as well as high F indicate well-compacted sediment (Fig. 58.2). The average T value in MTD is higher than that in coherent interval by contrast (Table 58.1) and vertical K3 axes and horizontal bedding do not demonstrate disturbance, suggesting the promotion of compaction. The MTD was reported as unconsolidated “gravels” in Subunit IB (Expedition 316 Scientists 2009). Preliminary results from coring at Site C0022 located between Sites C0004 and C0008 confirmed similar occurrences of mudclast gravels (Moore et al. 2013). The visual inspection of the cores suggests submarine landslide deposits, although this was accompanied by no particular AMS anomaly. Similarly, the slump deposits in the shallow Subunit IA at Site C0008 (see Sect. 58.2.1) do not show scattering in the AMS parameters. The AMS data from chaotic sediments in Subunits IA and IB imply a different type of MTD, which can be ascribed to a different dynamic process during transportation. The mass transport process to create this AMS pattern would be a debris flow that enforced particles to redeposit, resulting in an isotropic magnetic fabric near the surface and oblate fabrics at greater depths, and such fabrics are commonly observed in sediments under diagenetic compaction. This means that the debris flow here is associated with the re-sedimentation process. This interpretation is consistent with what was previously inferred from sedimentological data (Strasser et al. 2011). Interestingly, evidence for overconsolidation was found in the porosity data in the shallow slump and in the AMS data in the deeper MTD. This apparent overconsolidation is possibly due to uplift associated with the removal of mass at this site suggested by the scars (Expedition 316 Scientists 2009).
In summary, the MTDs that occurred at Site C0018 are formed by ductile flow during transportation, causing a scattered shape parameter and magnetic ellipsoid orientation, while in contrast, the MTDs at Site C0008 are formed by debris flows under conditions where both erosion and deposition take place (Fig. 58.3). These features correspond to the proposed classifications of submarine landslides, i.e. the MTDs in Site C0018 correspond to the “debris avalanche” (Masson et al. 2006) or “discrete cohesive slide” or “blocky flow” (Ogata et al. 2012), whereas the MTDs in Site C0008 match the definition of “debris flow” in both studies cited. Our results showed that the AMS analysis is (1) apt to detect a specific kind of MTD formed with cohesive flow, and (2) in combination with visual description and X-CT analysis, the AMS method can be used to discriminate another kind of MTD that had evolved to a debris flow in which all the transporting particles broke up and lost cohesion, leading to complete re-sedimentation.
This research used samples and data provided by the Integrated Ocean Drilling Program. This work was partially supported by KAKENHI 19GS0211 and 21107001. We thank K. Tsuchida (Marine Works Japan Co. Ltd.) for providing technical support with regard to the AMS measurements. The IODP Expeditions 316 and 333 could not have been completed without the devoted contributions of those involved. Careful review and critical comments by David Völker and Yoshitaka Hashimoto improved this manuscript.
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