ESTIMATION OF THE THERMAL PARAMETER

Transcription

1 GSA Data Repository Baillard et al., 2018, Tracking subducted ridges through intermediate-depth seismicity in the Vanuatu subduction zone: Geology, SUPPLEMENTAL MATERIAL TEXT T1: ESTIMATION OF THE THERMAL PARAMETER The thermal parameter reflects the capacity of a plate to rapidly bring cold material into the mantle and is defined as Φ=A * V C * sin(δ), where A is the age of the downgoing plate, V is the convergence velocity, and δ is the dip of the subduction interface. To estimate the thermal parameter in the central part of the Vanuatu arc we consider: - The age of the Australian plate at the collision front to be A=57 Ma, (Collot et al., 1985). - The long-term convergence rate V C between the Australian plate and the North-Fiji Basin (NFB) micro-plate to vary from 49 to 70 km/myr (Taylor et al., 2005). - The Dip of the subduction interface δ > 60 for depth below 50 km (Baillard et al., 2015). We these parameters the thermal parameter lower values are in between 2400 and 3400 which is higher than the global average of ~1500 over all subduction zones (Heuret et al., 2011). TEXT T2: CALCULATING THE POSITON OF THE SUBDUCTED D ENTRECASTEAUX RIDGE (DER) To estimate the position of the subducted DER under the Vanuatu arc, we perform a first order kinematic reconstruction of the DER by projecting from the point of collision of the DER with the arc 2-3 Ma to its most probable current position on the subducting slab, then connecting this point to the present-day point of collision using a simple curve fit. Kinematic reconstructions based on convergence rates and forearc/trench morphology place the DER collision zone approximately 190 km to the south at 2 to 3 Ma, in front of a morphological feature on the trench known as the Epi reentrant (Collot and Fisher, 1991; Greene et al., 1994; Meffre and Crawford, 2001)(Fig. DR1). The long-term convergence rate, V C, between the Australian plate and the North-Fiji Basin (NFB) micro-plate varies from 49 to 70 km/myr (Taylor et al., 2005) and the direction can be approximated from the interseismic convergence rate direction of ~N80 in the central part of the arc (Calmant et al., 2003;

2 Bergeot et al., 2009). Based on these parameters, we estimate the current position of the subducted ridge as follows: Considering V C between 49 and 70 km/myr and a collision initiated between 2 and 3 Ma, the Australian plate has moved km in the N80 direction since the Epi reentrant episode. We consider the average of these distances: 160 km (Fig. DR1). The Epi reentrant is well south of the back-projected position of the present-day DER, so the section between the present collision point and the Epi reentrant must have been curved. The reference points can be connected by a simple circular curve with radius R=200 km; this radius is similar to the radius of the present-day DER north of New Caledonia (Fig. DR1). We then projected this extrapolated curved ridge onto the surface of the subducting slab (from Baillard et al., 2015) (Fig. DR1).

3 REFERENCES CITED Baillard, C., Crawford, W.C., Ballu, V., Régnier, M., Pelletier, B., and Garaebiti, E., 2015, Seismicity and shallow slab geometry in the central Vanuatu subduction zone: Journal of Geophysical Research: Solid Earth, v. 120, no. 8, p , doi: /2014JB Bergeot, N., Bouin, M.N., Diament, M., Pelletier, B., Régnier, M., Calmant, S., and Ballu, V., 2009, Horizontal and vertical interseismic velocity fields in the Vanuatu subduction zone from GPS measurements: Evidence for a central Vanuatu locked zone: Journal of Geophysical Research, v. 114, no. B6, p. B06405, doi: /2007JB Calmant, S., Pelletier, B., Lebellegard, P., Bevis, M., Taylor, F.W., and Phillips, D.A., 2003, New insights on the tectonics along the New Hebrides subduction zone based on GPS results: Journal of Geophysical Research, v. 108, no. B6, p. 2319, doi: /2001JB Collot, J., Daniel, J., and Burne, R., 1985, Recent tectonics associated with the subduction/collision of the D Entrecasteaux zone in the central New Hebrides: Tectonophysics, v. 112, p Collot, J.-Y., and Fisher, M.A., 1991, The collision zone between the North d Entrecasteaux Ridge and the New Hebrides Island Arc: 1. Sea Beam morphology and shallow structure: Journal of Geophysical Research, v. 96, no. B3, p. 4457, doi: /90JB Greene, H., Collot, J., Fisher, M.A., and Crawford, A., 1994, NEOGENE TECTONIC EVOLUTION OF THE NEW HEBRIDES ISLAND ARC : A REVIEW INCORPORATING ODP DRILLING RESULTS: Proceedings of the Ocean Drilling Program, Scientific Results, v Heuret, A., Lallemand, S., Funiciello, F., Piromallo, C., and Faccenna, C., 2011, Physical characteristics of subduction interface type seismogenic zones revisited: Geochemistry, Geophysics, Geosystems, v. 12, no. 1, p. n/a-n/a, doi: /2010GC Meffre, S., and Crawford, A.J., 2001, Collision tectonics in the New Hebrides arc (Vanuatu): The Island Arc, v. 10, no. 1, p , doi: /j x. Taylor, F.W., Mann, P., Bevis, M.G., Edwards, R.L., Cheng, H., Cutler, K.B., Gray, S.C., Burr, G.S., Beck, J.W., Phillips, D. a., Cabioch, G., and Recy, J., 2005, Rapid forearc uplift and subsidence caused by impinging bathymetric features: Examples from the New Hebrides and Solomon arcs: Tectonics, v. 24, no. 6, p. n/a-n/a, doi: /2004TC

4 SUPPLEMENTAL MATERIAL FIGURES Figure DR1: Kinematic reconstruction of the current and past geometry of the DER. The dashed green curve indicates the DER projected back 2.5 Ma using the average plate convergence rate and the position of the Epi reentrant (red star). The green solid curve indicates the current position of the ridge and its extrapolation at the surface. The red curve indicates the projection of the green curve onto the subducting slab. Black isopachs are from Baillard et al. (2015).

5 Figure DR2: Intermediate-depth earthquake distributions in the vicinity of the subducted DER. (Left) Map view showing only intermediate-depth earthquakes (50 km < z < 300 km). Red line shows the extrapolated part of the DER projected onto the subduction interface. (Center) Cross-section 1 of intermediate-depth seismicity, containing area N. Dashed black line shows the subduction interface. (Right) Cross-section 2, containing Area C. The red line showing the subduction interface is dashed where the DER is outside of the cross-section.

6 Figure DR3: Ternary and polar diagrams showing the type of focal mechanisms and the strike and dip angles of the nodal planes for intermediate-depth events in Areas N, C and S (Fig. 2A). Events were extracted from the GCMT catalog (Dziewonski et al., 1981; Ekström et al., 2012).

7 Figure DR4: South-American seismicity near to the Carnegie Ridge from 1976 to 2018 (ISC catalog). The solid black box designates the area used for projection, 0 indicates the cross-section s origin. Dotted line overlies the Grijalva Fracture Zone and its projection into subduction zone (gray area indicates range of possible projections).

8 Figure DR5: Seismicity in the vicinity of the Nazca Ridge from 1976 to 2018 (ISC catalog). Dotted line represents the Nazca Fracture Zone and its projection into subduction zone (gray area indicates range of possible projections).

9 Figure DR6: Seismicity in the vicinity of the Louisville Ridge from 1976 to 2018 (ISC catalog). Dotted line represents the average trace of the Louisville Ridge and its projection into subduction zone (gray area indicates range of possible projections).

10 Figure DR7: Along volcanic arc variations of isotopic ratios (from Peate et al. (1997)) and intermediate-depth earthquakes in the Vanuatu subduction zone. Blue rectangles highlight areas associated with the inferred subducted ridges and plateau. Despite non-negligible scattering of isotopic ratios we observe high Ba/La and Pb/Nd for volcanoes above the subducted features indicating potentially large amounts of water released by the later. This correlates with an increase of intermediate-depth earthquakes. Ana: Anatom; Fut: Futuna; Tan: Tanna; Err: Erromango; Efa: Efate; Ngu: Nguna; Ton: Tongoa; Epi: Epi Paa:Paama; Amb: Ambrym; Aob: Aoba; Mer: Mere Lava; Gau: Gaua: Mot: Mota; Van: Vanua Lava; Mot: Moto Lava; Ure: Ureparapara: Vot: Vot Tande.

11 REFERENCES CITED Dziewonski, a. M., Chou, T.-A.T. a., and Woodhouse, J.H., 1981, Determination of earthquake source parameters from waveform data for studies of global and regional seismicity: Journal of Geophysical Research: Solid Earth, v. 86, no. B4, p , doi: /JB086iB04p Ekström, G., Nettles, M., and Dziewonski, A.M., 2012, The global CMT project : Centroid-moment tensors for 13,017 earthquakes: Physics of the Earth and Planetary Interiors, v , p. 1 9, doi: /j.pepi Peate, D.W., Pearce, J. a., Hawkesworth, C.J., Colley, H., Edwards, C.M.H., and Hirose, K., 1997, Geochemical Variations in Vanuatu Arc Lavas: the Role of Subducted Material and a Variable Mantle Wedge Composition: Journal of Petrology, v. 38, no. 10, p , doi: /petroj/