Constraints on yield strength in the oceanic lithosphere derived from observations of flexure

Abstract

The wavelength and amplitude of outer rises seaward of subduction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing. The data show the clear age dependence in the mechanical thickness of the lithosphere up to 60-100Myr. Saturation of moment at large curvature is interpreted in terms of a depth-dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation. A comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long-term strength below about 40 km depth, with no difference between 100 and 165 Myr old crust. Moderate axial loading forces (+200 MPa) can explain most variations in the moment/curvature observations, except in the case of the Kuril Trench which appears anomalous given the age of the crust. Regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension. The observations point to a lithosphere weaker than the prediction from experimental deformation of rocks. Of the possible weakening mechanisms, elevated porefluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms. Our favoured explanation is that the activation energy, Q, appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures. If recent oceanic geotherms are reliable, Q in the lower lithosphere must be lower than lOOkcal mol-'. The method used here is most appropriate for trench profiles with curvatures greater than lO-'rn-'. For lower curvatures, such as along seamount profdes, small errors in the curvature estimate cause large changes in rheological parameters.

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).…”

“…This is consistent with the scatter observed in pre vious works on other oceanic regions (e.g., Tassara et al, 2007;Kalnins and Watts, 2009). Watts (2001) uncertainties in load, infill and mantle densities, thermal perturba tions due to hot and cold spots (e.g., Tassara et al, 2007), viscoelas tic stress relaxation (Watts and Zhong, 2000), yielding in regions of large loads and high curvature (McNutt and Menard, 1982), or spatial variations in the controlling isotherms that determine Te (Kalnins and Watts, 2009). …”

Crust‐mantle decoupling by flexure of continental lithosphere

“…The effective elastic thickness (he) of oceanic lithosphere has been shown to depend on the thermal age (t) of the lithosphere (equated to the crustal age) at the time of loading; that is, this thickness approximately follows an isotherm [Caldwell and Turcotte, 1979;Watts et al, 1980]. This apparently simple relationship is complicated by the effects of bending curvatures [McNutt and Menard, 1982], thermal rejuvenation [McNutt, 1984], thermal contractional stresses [Wessel, 1992], and in-plane forces [Mueller and Phillips, 1995], but these dependencies are fairly well understood. Some estimates of continental h e also show a first-order relationship to age of the basement at the time of loading ], yet there are significantly greater departures from this trend than is the case for oceanic lithosphere, and the reasons for this scatter are not entirely agreed upon [McNutt et al, 1988;Watts, 1992 called "jelly sandwich" rheological layering, illustrated in Figure 2b, invokes the inherent weakness of crustal rocks compared to mantle rocks as the accommodating mechanism for decoupling.…”

Scientific drilling into the San Andreas fault and site characterization research: Planning and coordination efforts. Final technical report

“…These dynamic ' weakening mechanisms include shear heating during slip, leading to transient high fluid pressures (Lachenbmch, 1980) or melting (Sibson 1975;Spray, 1987); reductions in normal stress accompanying the propagation of dilational waves along the fault (Brune et al, 1993); and the fluidization of fault-zone materials due to the channeling of co-seismic acoustic energy (Melosh, 1979(Melosh, , 1996. While such processes may be operative during an earthquake, they do not relate to the problem of how earthquakes initiate and each requires that very specific fault zone conditions exist to be viable.…”

“…The physics of earthquake rupture propagation has also been the subject of intensive investigation in recent years (e.g., Heaton, 1990;Brune et al, 1993;Melosh, 1996). New data has again drawn into question the standard model of a dynamically expanding crack that heals inward from its outer boundary (Madariaga, 1976).…”