The relationship between plate curvature and elastic plate thickness: A study of the Peru‐Chile Trench

Abstract

The age of the Nazca plate where it enters the Peru and northern Chile trenches varies from 30 Ma in the north to 45 Ma in the south as its dip beneath the South American continent steepens from 13' to 300. If the elastic thickness Te of oceanic lithosphere depends only on its age, and therefore thermal state, we would expect that Te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth and geoid anomalies would increase from the Peru Trench in the north to the northern Chile Trench further south. We find that just the opposite is true: the lithosphere appears stiffer outboard of the Peru Trench than it does further south, and the isotherm controlling the elastic/ductile transition must be 600 0 C or greater if the thermal structure of the plate is that predicted by the standard thermal plate model. Because the decrease in plate stiffness to the south is correlated with a decrease in the minimum radius of curvature of the flexed plate over the outer rise and outer trench wall, we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains. The fact that the more highly bent segment of subducting lithosphere also dips at a steeper angle at greater depth beneath the continent might suggest that the amount of inelastic weakening of the lithosphere could be predicted from seismic images of the downgoing slab, but we find little support for this correlation worldwide. The forces and moments controlling the shallow deformation of the plate seaward of the trench do not appear to be linked to upper mantle processes which impose the dip at greater depth. Finally, we consider the possibility that the elastic thickness of the lithosphere would be reduced for trenches that are highly arcuate in map view, again due to inelastic yielding. If such a relationship exists, the effect for oceanic lithosphere is much smaller than what is documented for continental plates where they underthrust highly arcuate fold belts

Earthquake Rupture Stalled by a Subducting Fracture Zone

“…In addition to the presence of large so-called superplumes (8,9), the complex seismic structure of Earth_s lower mantle also reveals lateral heterogeneities that have been explained by compositional and thermal variation, partial melting, and phase transformations (10)(11)(12)(13). The recent discovery of a spin-pairing high-spin (HS) to low-spin (LS) electronic transition of iron in silicate perovskite (14) and magnesiow[stite (15) have also been invoked to explain these anomalies. In (Mg,Fe)O, the HS-LS transition between 50 and 70 GPa strongly influences its bulk elastic properties (16,17) and is expected to blue-shift iron absorption bands in the infrared (IR) (1,18), although the transport properties of low-spin (Mg,Fe)O at high pressure have remained only speculative.…”

“…To determine the cause of the barriers, we have examined the available marine geophysical data (12)(13)(14)(15)(16) in the region of the subducting oceanic plate. The oceanic crust immediately adjacent to the earthquake rupture zone is 45 to 50 million years old and was generated by seafloor spreading at the fossil Pacific/Farallon ridge.…”

Spatial variations of effective elastic thickness of the lithosphere in Central America and surrounding regions

“…3). These Te values decrease sharply under the MAT offshore of Central America, indicating a substantial degree of weakening within the downgoing plate due to the flexure of the lithosphere (see McNutt and Menard, 1982;Judge and McNutt, 1991;Billen and Gurnis, 2005;Contreras-Reyes and Osses, 2010). In fact, the bathymetry of the MAT offshore of Central America shows a complex response of the crust to the subduction process, with widespread outer-rise normal faulting subparallel to the trench axis due to the plate bending, increas ing in number and offset where the bending is more pronounced (Ranero et al 2003(Ranero et al , 2005Harders et al, 2011;Manea et al, 2013).…”

Crustal and Lithosphere Dynamics: An Introduction and Overview

“…As pointed out by Bodine et al (1981) and Lago and Cazenave (1981), a dependence of T e on plate age is compatible with the extrapolation of data from experimental rock mechanics. Not all loads on the oceanic lithosphere, however, are described by a controlling isotherm in the range 300-600 C. Flexure studies at seamounts and oceanic islands in the French Polynesia region, for example, suggest that T e is controlled by a lower isotherm (Calmant and Cazenave, 1987;Maia and Arkani-Hamid, 2002), while studies at some deep-sea trenchouter-rise ( Judge and McNutt, 1991) and fracture zone systems (Wessel and Haxby, 1990) imply a higher isotherm. Moreover, the data imply that since brittle deformation depends on confining pressure and ductile deformation is determined by both pressure and temperature, strength increases and then decreases with depth.…”