Search

Highlights • We document marine forearc deformation in the Northern Chile seismic gap. • Upper-plate normal faulting off Northern Chile locally extends close to the trench. • Normal faults indicate that past earthquakes may reached the shallow plate-boundary. Abstract Seismic rupture of the shallow plate-boundary can result in large tsunamis with tragic socio-economic consequences, as exemplified by the 2011 Tohoku-Oki earthquake. To better understand the processes involved in shallow earthquake rupture in seismic gaps (where megathrust earthquakes are expected), and investigate the tsunami hazard, it is important to assess whether the region experienced shallow earthquake rupture in the past. However, there are currently no established methods to elucidate whether a margin segment has repeatedly experienced shallow earthquake rupture, with the exception of mechanical studies on subducted fault-rocks. Here we combine new swath bathymetric data, unpublished seismic reflection images, and inter-seismic seismicity to evaluate if the pattern of permanent deformation in the marine forearc of the Northern Chile seismic gap allows inferences on past earthquake behavior. While the tectonic configuration of the middle and upper slope remains similar over hundreds of kilometers along the North Chilean margin, we document permanent extensional deformation of the lower slope localized to the region 20.8°S–22°S. Critical taper analyses, the comparison of permanent deformation to inter-seismic seismicity and plate-coupling models, as well as recent observations from other subduction-zones, including the area that ruptured during the 2011 Tohoku-Oki earthquake, suggest that the normal faults at the lower slope may have resulted from shallow, possibly near-trench breaking earthquake ruptures in the past. In the adjacent margin segments, the 1995 Antofagasta, 2007 Tocopilla, and 2014 Iquique earthquakes were limited to the middle and upper-slope and the terrestrial forearc, and so a

From north to south, the Southern Chile forearc is affected by the subduction of the aseismic Juan Fernandez Ridge, a number of major oceanic fracture zones on the downgoing Nazca Plate, the active Chile Ridge spreading center, and underthrusting of the Antarctic Plate. The tectonic structure is characterized by intense deformation of the lower continental slope within a variably wide accretionary wedge. In places the middle and upper slope is affected by out-of-sequence overthrusting and by normal faulting. In the area of the Chile Triple Junction at 46°S latitude most of the forearc is destroyed by subduction erosion, to be rebuilt further south by sediment offscraping and accretion from the Antarctic Plate. The Southern Chile forearc has been intensively explored by reflection seismic surveys, and has been drilled by the Ocean Drilling Program during two expeditions (ODP Leg 141 - Behrmann et al., 1992; ODP Leg 202 – Mix et al., 2003). The widespread occurrence of gas hydrates has been known for some time. Regarding the analysis of reflection seismic sections we have used data of R/V SONNE Expeditions 101 and 161, R/V VIDAL GORMAZ Expeditions VG02 and VG06, and R/V ROBERT CONRAD Cruises RC2901 and RC2902. Using bottom water temperature data obtained from the World Ocean Data Base (NOAA) and an acoustic velocity model constrained from the seismic sections, and measurements of temperature, thermal conductivity and acoustic velocity from ODP boreholes, we use the position of the Bottom Simulating Reflector (BSR) in reflection seismic sections to estimate the heat flow through the forearc in an area between 32°S and 47°S latitude. Heat flow in most of the upper and middle continental slope is on the order of 50-80 mWm-2. This is normal for continental basement and overlying slope sediments, and is true also for those parts in the south of the area that are being underthrusted by hot, young oceanic crust. The middle and lower slopes, however, in some places display up

Fluids are suspected to play a major role in the nucleation and rupture propagation of earthquakes. In Chile, seismological data were previously interpreted to indicate that fluids captured in the fault zone are released periodically during large underthrust earthquakes, leading to post-seismic fluid flow. In central Chile, heat flow derived from the presence of a bottom simulating reflector (BSR) show a smooth trend across the margin. BSR-derived data are in excellent agreement with thermal subduction zone models. Over the young accretionary prism, both BSR-derived and measured surface heat flow support a common trend. Landwards of the backstop, however, measured heat flow triples over a distance of 20–30 km, producing a profound discrepancy to the BSR-derived data. We suggest that this disparity is related to transient flow of warm fluids through the gas hydrate stability zone possibly caused by fluids released after large underthrust earthquakes. Such flow events may inherently affect the distribution of solid gas hydrates between the seafloor and the BSR.