Low heat flow from young oceanic lithosphere at the Middle America Trench off Mexico

Abstract

Seismic reflection profiles across the Middle America Trench at 208N show a high amplitude bottom simulating reflector interpreted as marking a phase transition between methane hydrate and free gas in the pore space of both accreted and trench sediments. We determine the depth of the hydrate-gas phase boundary in order to estimate the geothermal gradient and hence the heat flow beneath the trench and the frontal part of the accretionary wedge which overlies the downgoing plate. After correction for sedimentation, heat flow values in the trench and through the accretionary wedge are only about half of the values predicted by plate cooling models for the 10 Ma subducting lithosphere. There is no systematic correlation between heat flow in the accretionary wedge and distance from the trench. A comparison with heat flow predicted by a simple analytical model suggests that there is little shear heating from within or beneath the wedge, despite the high basal friction suggested by the large taper angle of the wedge. The geothermal gradient varies systematically along the margin and is negatively correlated with the frontal slope of the wedge. Some local peaks may be attributed to channelised fluid expulsion. D

Thermal regime of the Costa Rican convergent margin: 1. Along‐strike variations in heat flow from probe measurements and estimated from bottom‐simulating reflectors

“…The transition between hydrate and water to free gas and water produces an acoustic impedance contrast that often generates a well‐developed high‐amplitude, reversed polarity reflection that is relatively straightforward to identify. Because the base of the hydrate stability field is primarily sensitive to temperature, with only moderate sensitivity to pressure and composition, BSRs correspond to an isotherm and provide a way to map regional estimates of average geothermal gradients on continental margins [e.g., Hyndman et al , 1992; Brown and Bangs , 1995; Kaul et al , 2000; Grevemeyer and Villinger , 2001; Minshull et al , 2005]. BSR depths are converted to estimates of heat flow based on the approach developed by Yamano et al [1982] and reviewed in detail by Grevemeyer and Villinger [2001].…”

Imaging the Seismic Crustal Structure of the Western Mexican Margin between 19°N and 21°N

“…The upper and Vol. 168, (2011) Imaging the Seismic Crustal Structure 1377 western part of profile 205 was already published in a study of the heat flow through the margin (MINSHULL et al, 2005). From offshore to onshore (NE orientation) we have identified the following morphological features: deformation front (CMP 1,500), trench (CMP 2,500), accretionary prism (CMP 2,500-3,600), a welldeveloped forearc basin (CMP 3,600-5,600), backstop limit at (CDP 4800), and continental slope and mid slope terrace (CMP 5,600-6,400).…”

“…. The BSR identified in seismic data off Puerto Vallarta have a reversed polarity with respect to the seafloor(MINSHULL et al, 2005), consequently related with the presence of natural gas hydrates. The formation of natural gas hydrates is well-known and widely described as a global phenomenon that has been recognized all along the Eastern Pacific margin [e.g.…”

Subducted oceanic asthenosphere and upper mantle flow beneath the Juan de Fuca slab

“…Thus, although this fl uid hydrates the mantle wedge and likely pervasively modifi es LPO (Jung and Karato, 2001;Karato, 2003;Jung et al, 2006;Bostock et al, 2002), there is no evidence that mantle beneath the slab is affected at all. Furthermore, given their cooling histories, a melt fraction at the base of subducted lithosphere or below-in the absence of any breach in the slab, or fl ow around a slab edge-is highly unlikely (e.g., Currie et al, 2004;Minshull et al, 2005;Peacock et al, 2005). Available tomographic studies have not been interpreted to show partial melt beneath the Juan de Fuca slab (e.g., Xue and Allen, 2007;Sigloch et al, 2008).…”