Upper‐mantle earthquakes beneath the Arafura Sea and south Aru Trough: Implications for continental rheology

Abstract

The extent and controls of long‐term elastic strength and seismicity in the upper continental lithospheric mantle (UCLM) are controversial topics in continental tectonics. One key issue is the scarcity of UCLM earthquakes, even where the UCLM is likely to be colder than 600°C. The rarity of these earthquakes could be because the UCLM generally relatively hydrous causing it to deform aseismically even when colder than 600°C, unless it is deforming at exceptionally high strain rates. Alternatively, the UCLM could be relatively anhydrous, and potentially seismogenic at temperatures below 600°C; in which case the rarity of UCLM earthquakes may be because areas where the UCLM is colder than 600°C have such a thick seismogenic layer, and such a cool mantle root, that they deform exceptionally slowly. The identification and study of UCLM earthquakes allows us to distinguish between these possibilities. Here we show that two earthquakes occurred in the UCLM beneath the epicontinental Arafura Sea. Both earthquakes occurred where the UCLM is probably cooler than 600°C and one of these earthquakes lies ∼25 km below the Moho in an region where there is no evidence of unusually high strain rates. There at least, it is probable that the UCLM is relatively anhydrous, and seismogenic at temperatures below 600°C. We also find evidence that regions where the UCLM is colder than 600°C also have a seismogenic lower crust. This results in a single, extremely strong layer comprising the entire crust and the UCLM down to the 600°C isotherm.

Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle

“…Our results on the 2013 Wyoming earthquake shed new light on the rupture mechanism of these rare earthquakes ( 12 , 17 , 28 ) and provide important constraints on rheological properties of the lithospheric mantle ( 5 , 6 , 19 , 41 ). …”

The meaning of midlithospheric discontinuities: A case study in the northern U.S. craton

“…Several mechanisms have been proposed as explanations for mantle‐depth earthquakes in continental settings. One hypothesis is that with sufficiently cold (<600°C) and anhydrous mantle, brittle fracture is possible at mantle depths [ McKenzie et al ., ; Boettcher et al ., ; Sloan and Jackson , ]. However, mantle temperatures in the source regions of the Wyoming and Utah mantle earthquakes are likely too high for brittle failure to be relevant, a conclusion also reached for the Utah event by Wong and Chapman [].…”

Continental collisions and the origin of subcrustal continental earthquakes

“…Figure 6 shows that there are a number of places where the uncertainty in the depth of the deepest earthquakes, and also in the crustal thickness, allows, but does not require, the events to be in the uppermost mantle. Regions where earthquakes do occur in the continental upper mantle are where the crust is thin, like the Arafura Sea (Sloan and Jackson 2012), and also where it is being cooled from below by a subducting slab, as it is above the flat subduction zone in Peru (Emmerson 2007).…”