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Upper-Mantle Earthquakes Beneath the Arafura Sea and South Aru Trough: Implications for Continental Rheology R

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Upper-Mantle Earthquakes Beneath the Arafura Sea and South Aru Trough: Implications for Continental Rheology R JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, B05402, doi:10.1029/2011JB008992, 2012 Upper-mantle earthquakes beneath the Arafura Sea and south Aru Trough: Implications for continental rheology R. A. Sloan1 and J. A. Jackson1 Received 4 November 2011; revised 5 March 2012; accepted 8 March 2012; published 1 May 2012. [1] 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 600C. The rarity of these earthquakes could be because the UCLM generally relatively hydrous causing it to deform aseismically even when colder than 600C, unless it is deforming at exceptionally high strain rates. Alternatively, the UCLM could be relatively anhydrous, and potentially seismogenic at temperatures below 600C; in which case the rarity of UCLM earthquakes may be because areas where the UCLM is colder than 600Chave 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 600C 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 600C. We also find evidence that regions where the UCLM is colder than 600C also have a seismogenic lower crust. This results in a single, extremely strong layer comprising the entire crust and the UCLM down to the 600Cisotherm. Citation: Sloan, R. A., and J. A. Jackson (2012), Upper-mantle earthquakes beneath the Arafura Sea and south Aru Trough: Implications for continental rheology, J. Geophys. Res., 117, B05402, doi:10.1029/2011JB008992. 1. Introduction [3] The challenge we face is to make observations that constrain the actual rheological behavior of upper-mantle [2] One of the largest gaps in our knowledge of the factors material beneath the continents. One method is to accurately governing continental tectonics is the rheology of the upper determine the centroid depths of earthquakes. This reveals continental lithospheric mantle (UCLM). Views on the long- the depth to the seismic-aseismic transition and places an term strength and the deformation mechanism of the UCLM important bound on the UCLM conditions that control vary widely. Experimental research on the rheological seismogenic behavior. properties of mantle minerals has shown that the flow laws [4] In oceanic lithosphere the seismogenic and elastic which control deformation in mantle materials are extremely thicknesses increase with plate age, suggesting both are sensitive to minor compositional changes, especially the controlled by temperature [Watts et al., 1980; Chen and presence or absence of small amounts of hydrogen dissolved Molnar, 1983; Wiens and Stein, 1983]. When oceanic geo- in nominally anhydrous minerals such as olivine and therms are calculated, taking into account the temperature pyroxene [Mackwell et al., 1998; Hirth and Kohlstedt, 2003; dependence of thermal conductivity, the seismic-aseismic Boettcher et al., 2007]. If the UCLM is relatively hydrous transition follows the 600C isotherm [Denlinger, 1992; these minerals contain a significant amount of hydrogen and McKenzie et al., 2005]. However, in oceanic lithosphere it is the UCLM will undergo aseismic deformation at tempera- likely that the lithospheric mantle has been dehydrated by tures well below 600 C and will lack significant long-term melting beneath the mid-ocean ridge. It is far from certain strength [Maggi et al., 2000]. Alternatively, relatively that the UCLM should follow the same pattern, especially anhydrous UCLM should remain seismogenic to tempera- beneath ancient shields where the UCLM may have been tures of up to 600C[Boettcher et al., 2007], and could gradually hydrated by metasomatic melts from the astheno- contribute significantly to the long-term strength of the sphere over a long period of time [Harte et al., 1993; Maggi continents in some areas [McKenzie et al., 2005]. et al., 2000]. [5] In the continents, UCLM earthquakes were once 1 thought to be present in many places, leading to the con- Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Cambridge, UK. clusion that the UCLM was generally seismogenic, and contributed greatly to the long-term strength of the con- Copyright 2012 by the American Geophysical Union. tinents [Chen and Molnar, 1983]. Indeed, the UCLM was 0148-0227/12/2011JB008992 B05402 1of13 B05402 SLOAN AND JACKSON: UCLM EARTHQUAKES BENEATH THE ARAFURA SEA B05402 thought to contribute more to the long-term strength of the estimates from upper mantle xenoliths form southern India continents than the lower crust, which is aseismic in many also suggest the Indian Shield has Moho temperatures of deforming regions. This view contributed to the widespread 500C[Priestley et al., 2008]. This observation supports acceptance of a laminated or “jelly sandwich” model of the view that the UCLM is relatively anhydrous and is continental rheology [Chen and Molnar, 1983]. Reassess- capable of seismic deformation at temperatures up to ments of earthquake and Moho depths in active regions have 600C. However, some caution is required due to the tec- since revealed that UCLM earthquakes are in fact rare. It tonic setting of these microearthquakes. India is being was therefore suggested that, in most areas, the UCLM is thrust under Tibet at 20 mm yrÀ1 [Larson et al., 1999] aseismic and contributes little to long-term elastic strength and these UCLM events occur where the Indian plate first [Maggi et al., 2000; Jackson et al., 2008]. Geotherm mod- bends and then unbends in a ramp-and-flat geometry eling, taking into account modern estimates of crustal beneath the Himalaya. The UCLM in this region is thickness, radiogenic heat production and temperature deforming at relatively high strain rates, which may allow it dependent conductivity, revealed that Moho temperatures in to undergo seismic deformation at higher temperatures than actively deforming areas are generally greater than 600C it would under normal conditions [Priestley et al., 2008]. [McKenzie et al., 2005]. The UCLM in those areas would be [9] One, possibly two, UCLM earthquakes have also been expected to deform aseismically, and be relatively weak, reported in the Andean UCLM in an area where a shallow regardless of its composition. and subhorizontally subducting oceanic slab underplates the [6] There are, however, many continental regions, par- Andean continental lithosphere [Emmerson, 2007]. These ticularly in ancient shields, which are underlain by rela- earthquakes occur in the UCLM between the cold subduct- tively thick lithosphere (>150 km), have low-to-moderate ing slab and the surface. This region is therefore being crustal thicknesses (<40–45 km), and lack large amounts of cooled from both above and below, resulting in temperatures radiogenic crustal heat production. In these areas (e.g. the colder than 600C[Emmerson, 2007]. In this unusual setting Canadian and Siberian shields) the Moho temperature is both coupling between the subducting oceanic slab and the significantly cooler than 600C. If the UCLM is relatively UCLM and the release and upward migration of fluid from anhydrous, and UCLM seismicity is controlled by the the subducting slab could result in unusually high strain rates 600C isotherm, then the UCLM in such places should be in the area where the earthquakes occurred. capable of seismic deformation, and will contribute greatly [10] It is therefore important to test the hypothesized to long-term elastic strength. Alternatively, if the UCLM in 600C seismic-aseismic transition temperature in areas such places is relatively hydrous, perhaps due to pervasive where the UCLM is cooler than 600C and is not deforming metasomatism, the UCLM is likely to deform aseismically, at unusually high strain rates. However, most ancient shields and will contribute little to long-term elastic strength. are deforming so slowly that earthquakes occur only very [7] One way of resolving the ambiguity in UCLM rhe- occasionally. The rare earthquakes that do occur in these ology would be to consider the effective elastic thickness of settings are therefore potentially very informative, especially regions where the 600C isotherm is expected to lie well if they occur in the UCLM. Such earthquakes provide the below the Moho. Unfortunately, whilst in principle it is opportunity to test whether the rarity of UCLM seismicity possible to use the relationship between gravity and topog- beneath ancient shields is because the UCLM there is rela- raphy to estimate the effective elastic thickness [Forsyth, tively hydrous and so deforms aseismically, or because the 1985; McKenzie and Fairhead, 1997], in practice differ- UCLM is anhydrous and deforms seismically, but the great ent methods produce estimates from ancient shields which seismogenic thickness and thick cold mantle root result in vary by an order of magnitude (from 20 to 200 km) such slow deformation that this has not yet been recognized. [Pérez-Gussinyé and Watts, 2005; Kirby and Swain, 2009; Pérez-Gussinyé et al., 2009; McKenzie, 2010]. This is
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