Remarkably consistent thermal state of the south central Chile subduction zone from 36°S to 45°S

Abstract

Delineating rupture areas of subduction zone earthquakes is necessary for understanding the controls on seismic and aseismic slip. For the largest recorded earthquake, the 1960 Chile event with moment magnitude 9.5, the rupture area is only loosely defined due to limitations in the global seismic network at the time. The rupture extends~900 km along strike. Coastal deformation is consistent with either a constant rupture width of~180-200 km along the entire length or a narrower (~115 km) rupture in the southern half. A southward narrowing of the seismogenic zone has been hypothesized to result from warming of the subduction zone to the south, where the subducting plate is younger. We present results of thermal models at 36°S, 38°S, 43°S, and 45°S to examine potential along-strike changes in thermal state. Models most consistent with observed surface heat flux include fluid circulation in the oceanic crust that advects heat to the ocean. This ventilated hydrothermal circulation preferentially cools transects with young subducting lithosphere; frictional heating preferentially warms transects with older subducting lithosphere. The combined effects of frictional heating and hydrothermal circulation increase décollement temperatures in the 36°S and 38°S transects by up to~155°C and decrease temperatures in the 45°S transect by up to~150°C. In our preferred models, décollement temperatures 200 km landward of the trench in all four transects arẽ 350-400°C. This is consistent with a constant~200 km wide seismogenic zone for the 1960 M w 9.5 rupture, with decreasing slip magnitude in the southern half of the rupture.

Diagenetic, metamorphic, and hydrogeologic consequences of hydrothermal circulation in subducting crust

“…In this volume, Harris et al (2017) summarize the need to consider the competing effects of frictional heating and hydrothermal circulation in estimating subduction-zone temperatures; frictional heating increases the plate-boundary temperature (i.e., the "subduction geotherm"), while hydrothermal circulation decreases the subduction geotherm. Several studies have suggested that hy-drothermal circulation transports a significant portion of the heat generated by frictional sliding toward the deformation front, thereby confounding attempts to use surface heat-flux observations to constrain the amount of frictional heating (Rotman and Spinelli, 2014;Wang et al, 2015;Hass and Harris, 2016). Harris et al (2017Harris et al ( , p. 1441 note: "in order to use heat flow observations above a subduction thrust to constrain the amount of frictional heating on the fault, independent knowledge of the vigor of hydrothermal circulation or the lack of hydrothermal circulation is required. "…”

“…By cooling the hottest portions of the margin (i.e., those closest to the triple junction) the most, hydrothermal circulation reduces the along-arc variability in subduction-zone temperatures relative to what is expected in the absence of hydrothermal circulation. This can help to explain a lack of variation in arc lava composition (Shinjoe et al, 2013) over a region with a large along-margin variation in subducting plate age (<2-14 Ma), which, in the absence of hydrothermal circulation, is expected to result in a substantial variation in the subduction-zone thermal structure (Rotman and Spinelli, 2014).…”

Thermal environment of the Southern Washington region of the Cascadia subduction zone

“…The relatively shallow depth of these strongly temperature‐dependent reactions will result in the generation of unbound fluid within the accretionary sediments of the offshore section of the accretionary wedge. This is in contrast to models of fluid generation that occur farther landward beneath the continental plate for subduction zones with lower accretionary wedge temperatures [ Hyndman et al ., ; Spinelli and Wang , ; Rotman and Spinelli , ]. Supplemental fluid injection into the accretionary wedge could serve as a potential source of fluid, in addition to sediment compaction, that assists in the observed elevated BSR heat flow values when compared to modeled surface heat flow.…”

Invited review paper: Some outstanding issues in the study of great megathrust earthquakes—The Cascadia example

“…Spinelli and Wang (2008) showed that heat flow measurements in one area of the Nankai subduction zone cannot be explained unless very vigorous hydrothermal circulation is assumed to occur in the subducted crust. The same concept has been applied to other subduction zones (e.g., Rotman and Spinelli, 2014;Wang et al, 2015), including Cascadia (Cozzens and Spinelli, 2012). In these models, the fluid circulation itself is not simulated, but the thermal effect of the circulation is parameterized by using a very high thermal conductivity value representing high Nusselt number following the recipe of Davis et al (1997).…”