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Kopf, Achim J (2013): (Table 1) Results from XRD analysis as well as sediment type and selected results from ring shear experiments. PANGAEA,, Supplement to: Kopf, AJ (2013): Effective strength of incoming sediments and its implications for plate boundary propagation: Nankai and Costa Rica as type examples of accreting vs. erosive convergent margins. Tectonophysics, 26, 958-969,

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The location of the seaward tip of a subduction thrust controls material transfer at convergent plate margins, and hence global mass balances. At approximately half of those margins, the material of the subducting plate is completely underthrust so that no accretion or even subduction erosion takes place. Along the remaining margins, material is scraped off the subducting plate and added to the upper plate by frontal accretion. We here examine the physical properties of subducting sediments off Costa Rica and Nankai, type examples for an erosional and an accretionary margin, to investigate which parameters control the level where the frontal thrust cuts into the incoming sediment pile.
A series of rotary-shear experiments to measure the frictional strength of the various lithologies entering the two subduction zones were carried out. Results include the following findings: (1) At Costa Rica, clay-rich strata at the top of the incoming succession have the lowest strength (µres = 0.19) while underlying calcareous ooze, chalk and diatomite are strong (up to µres = 0.43; µpeak = 0.56). Hence the entire sediment package is underthrust. (2) Off Japan, clay-rich deposits within the lower Shikoku Basin inventory are weakest (µres = 0.13–0.19) and favour the frontal proto-thrust to migrate into one particular horizon between sandy, competent turbidites below and ash-bearing mud above. (3) Taking in situ data and earlier geotechnical testing into account, it is suggested that mineralogical composition rather than pore-pressure defines the position of the frontal thrust, which locates in the weakest, clay mineral-rich (up to 85 wt.%) materials. (4) Smectite, the dominant clay mineral phase at either margin, shows rate strengthening and stable sliding in the frontal 50 km of the subduction thrust (0.0001–0.1 mm/s, 0.5–25 MPa effective normal stress). (5) Progressive illitization of smectite cannot explain seismogenesis, because illite-rich samples also show velocity strengthening at the conditions tested.
Median Latitude: 13.078206 * Median Longitude: -112.275867 * South-bound Latitude: 7.921317 * West-bound Longitude: 134.011883 * North-bound Latitude: 31.652517 * East-bound Longitude: -86.179117
Date/Time Start: 1973-07-15T00:00:00 * Date/Time End: 2002-08-22T19:26:00
Minimum DEPTH, sediment/rock: 1.00 m * Maximum DEPTH, sediment/rock: 783.30 m
138-844 * Latitude: 7.921317 * Longitude: -90.480767 * Date/Time Start: 1991-05-08T00:00:00 * Date/Time End: 1991-05-12T00:00:00 * Elevation: -3414.5 m * Penetration: 514 m * Recovery: 499.3 m * Location: North Pacific Ocean * Campaign: Leg138 * Basis: Joides Resolution * Method/Device: Composite Core (COMPCORE) * Comment: 52 cores; 490 m cored; 0 m drilled; 101.9% recovery
170-1039 * Latitude: 9.639767 * Longitude: -86.200033 * Date/Time Start: 1996-11-01T00:00:00 * Date/Time End: 1996-11-20T00:00:00 * Elevation: -4352.0 m * Penetration: 1268 m * Recovery: 449 m * Location: Costa Rica subduction complex, North Pacific Ocean * Campaign: Leg170 * Basis: Joides Resolution * Method/Device: Composite Core (COMPCORE) * Comment: 56 cores; 497.9 m cored; 407 m drilled; 90.2% recovery
170-1040 * Latitude: 9.661800 * Longitude: -86.179117 * Date/Time Start: 1996-11-05T00:00:00 * Date/Time End: 1996-11-24T00:00:00 * Elevation: -4177.7 m * Penetration: 1520.6 m * Recovery: 514.8 m * Location: Costa Rica subduction complex, North Pacific Ocean * Campaign: Leg170 * Basis: Joides Resolution * Method/Device: Composite Core (COMPCORE) * Comment: 76 cores; 705.4 m cored; 655.9 m drilled; 73% recovery
VS = velocity strengthening, VW = velocity weakening
#NameShort NameUnitPrincipal InvestigatorMethod/DeviceComment
1Event labelEvent
2Sample code/labelSample labelKopf, Achim J
3DEPTH, sediment/rockDepthmGeocode
4Depth commentDepth commKopf, Achim JDepth below decollement (mbsf)
5Opal, biogenic silicabSiO2%Kopf, Achim J
6Clay mineralsClay min%Kopf, Achim JX-ray diffraction (XRD)Total clay, relative abundance in bulk
7CommentCommentKopf, Achim JX-ray diffraction (XRD)
8QuartzQz%Kopf, Achim JX-ray diffraction (XRD)relative abundance in bulk
9CommentCommentKopf, Achim JX-ray diffraction (XRD)
10PlagioclasePl%Kopf, Achim JX-ray diffraction (XRD)relative abundance in bulk
11CalciteCal%Kopf, Achim J
12CommentCommentKopf, Achim JX-ray diffraction (XRD)
13SmectiteSme%Kopf, Achim JX-ray diffraction (XRD)Abundance of individual clay in bulk
14IlliteIll%Kopf, Achim JX-ray diffraction (XRD)Abundance of individual clay in bulk
15ChloriteChl%Kopf, Achim JX-ray diffraction (XRD)Abundance of individual clay in bulk, Chlorite (NT) Kaolinite (CR)
16Sediment typeSedimentKopf, Achim J
17Shear strengthµpeakKopf, Achim JRing shear experiments
18Residual friction coefficientµresKopf, Achim JRing shear experiments
19CommentCommentKopf, Achim JRate-dependent behaviour
449 data points

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