Mafic High‐Pressure Rocks Are Preferentially Exhumed From Warm Subduction Settings

Abstract

The oceanic crust that enters a subduction zone is generally recycled to great depth. In rare and punctuated episodes, however, blueschists and eclogites derived from subducted oceanic crust are exhumed. Compilations of the maximum pressure‐temperature conditions in exhumed rocks indicate significantly warmer conditions than those predicted by thermal models. This could be due to preferential exhumation of rocks from hotter conditions that promote greater fluid productivity, mobility, and buoyancy. Alternatively, the models might underestimate the forearc temperatures by neglecting certain heat sources. We compare two sets of global subduction zone thermal models to the rock record. We find that the addition of reasonable amounts of shear heating leads to less than 50 °C heating of the oceanic crust compared to models that exclude this heat source. Models for young oceanic lithosphere tend to agree well with the rock record. We test the hypothesis that certain heat sources may be missing in the models by constructing a global set of models that have high arbitrary heat sources in the forearc. Models that satisfy the rock record in this manner, however, fail to satisfy independent geophysical and geochemical observations. These combined tests show that the average exhumed mafic rock record is systematically warmer than the average thermal structure of mature modern subduction zones. We infer that typical blueschists and eclogites were exhumed preferentially under relatively warm conditions that occurred due to the subduction of young oceanic lithosphere or during the warmer initial stages of subduction.

Imaging Subduction Beneath Mount St. Helens: Implications for Slab Dehydration and Magma Transport

“…The persistence of the JdF Moho to depths greater than 60 km is difficult to reconcile with petrologic models that predict the velocity contrast at the JdF Moho should disappear once the subducting crust eclogitizes by 60-km depth (Rondenay et al, 2008;van Keken et al, 2011van Keken et al, , 2018. Similar seismic analyses in Cascadia show a noticeably weaker putative JdF Moho than what we find beneath MSH (Abers et al, 2009;Bostock et al, 2002;Nicholson et al, 2005).…”

Thermal Structure of the Forearc in Subduction Zones: A Comparison of Methodologies

“…To illustrate the impact of this limitation, we parameterize the reduction in the effective strength of the fault past the BDT by assuming a depth‐dependent effective friction coefficient μ′ of the form where and Δ z μ are constants. This leads to a depth‐dependent reduction in μ′ that mimics the shape of the depth‐dependent shear heating from models that use a more complete description for the rheology of the plate interface past the BDT (Wada & Wang, ) as summarized in van Keken et al (, see their Figure 6).…”

“…Red lines show slab surface temperatures for three models from van Keken et al (). Black lines show the slab surface temperature for finite element method models similar to those in this paper with a rigid overriding lithosphere to great depth.…”

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

“…To quantify the extent to which composition explains the lateral velocity variations of the lower crust, we calculate Vs at a range of temperatures consistent with surface heat flow for Siletz gabbro compositions, following the calculation of Till et al (). Forearc heat flow of 34 ± 4 mW/m 2 (see the supporting information) is extrapolated to midcrustal depths for an assumed thermal conductivity of 2.0 ± 0.5 W/m/K (Pollack et al, ; van Keken et al, ). Heat production in gabbros should be negligible and the crust appears to be in thermal steady state so a linear geotherm is appropriate (Till et al, ).…”

“…MSH, the most active Holocene volcano in the Cascades, lies 35–50 km west of this arc front as do abundant vents farther south (Figure ). The melt transport pathway is not understood, since heat flow within a few kilometers of the MSH edifice is markedly low compared to the arc and backarc (Blackwell et al, ; van Keken et al, ). Furthermore, erupted dacites record equilibration temperatures of 925–940 °C at lower crustal pressures of 700–900 MPa (Blatter et al, ), but such high temperatures seem inconsistent with low heat flow and the inferred presence of serpentinized mantle.…”