Thermal regime around the Chile Triple Junction based on JAMSTEC MR18-06 cruise 'EPIC'

The Chile triple junction (CTJ) is a unique place where a spreading center of mid-ocean ridge is subducting near the Taitao peninsula. Around CTJ, presence of high heat flow on the continental slope and near-trench young granitic rocks on the Taitao peninsula suggests the thermal and petrological impact of subducting ridge on the continental side. The tectonic history of the southeast Pacific since early Cenozoic to the present suggests that ridge subduction continuously occurred along the Chile trench, which migrated northward.

In January 2019, the MR18-06 cruise Leg 2 was conducted at CTJ, as a part of 'EPIC' expedition by using R.V Mirai of JAMSTEC. During the leg, we completed 4 SCS lines, 6 piston coring with heat flow measurements, 2 dredges, and underway geophysics observations, as well as deployment of 13 OBSs. Coring/heatflow sites were located across the ridge axis, HP5 on the seaward plateau of axial graben, HP1/HP2/HP6 on the axis, and HP3/HP7 on the forearc slope near the trench axis. The primary object of heat flow measurement at CTJ is to better constrain the thermal regime around CTJ by adding new data right above CTJ. The key question is whether CTJ is thermally dominated by ridge activity (magmatic, tectonic, and/or hydrothermal) or by subduction initiation (tectonic thickening, accretion, and/or erosion). The ultimate goal is to model the temperature at the plate interface from the heat flow and other data, and to infer how the thermal regime at CTJ contributes the seismogenic behavior at the M~9 megathrust zone.

Onboard and post-cruise measurements include; bulk density, porosity, Vp, resistivity, CT imags, iTracks element scan, age dating, etc. Core saples seaward of ridge axis (HP5) has few turbidites with higher density (~2 g/cc) and low sedimentation rate (SR; 0.2 m/ky), whereas cores on the axis the density are turbidite dominant with lower (1.6~1.8 g/cc) and very high SR (1~3 m/ky). The accretionary prism (landward of trench) cores have the density of 1.6~1.7 g/cc and SR=0.5~1 m/ky. They suggest that the turbidite covers only the axial graben.

Heat flow in the axial graben range 140-210 mW/m^2, which is lower than on the seaward plateau (370 mW/m^2). This apparent controversy may be due to lower magmatic activity and/or high sedimentation rate on the axis. The lower spreading rate (2.6 cm/yr one side) and the rapid convergent rate at the trench (7.2 cm/yr) may suppress sufficient magma supply or hydrothermal circulation. Heat flow on the accretionary prism (230 mW/m^2) is higher than borehole or BSR-derived heat flow (~<100 mW/m^2). It is suggestive of fluid upwelling along the decollement as proposed in the previous study. Some numerical thermal models will be presented to show the effect of ridge subduction.