Constraints on the Rheology of the Lithosphere From Flexure of the Pacific Plate at the Hawaiian Islands

Abstract

The rheology of oceanic lithosphere is important to our understanding of mantle dynamics and to the emergence and manifestations of plate tectonics. Data from experimental rock mechanics suggest rheology is dominated by three different deformation mechanisms including frictional sliding, low‐temperature plasticity, and high‐temperature creep, from shallow depths at relatively cold temperatures to large depths at relatively high temperatures. However, low‐temperature plasticity is poorly understood. This study further constrains low‐temperature plasticity by comparing observations of flexure at the Hawaiian Islands to predictions from 3‐D viscoelastic loading models with a realistic lithospheric rheology of frictional sliding, low‐temperature plasticity, and high‐temperature creep. We find that previously untested flow laws significantly underpredict the amplitude and overpredict the wavelength of flexure at Hawaii. These flow laws can, however, reproduce observations if they are weakened by a modest reduction (25–40%) in the plastic activation energy. Lithospheric rheology is strongly temperature dependent, and so we explore uncertainties in the thermal structure with different conductive cooling models and convection simulations of plume‐lithosphere interactions. Convection simulations show that thermal erosion from a plume only perturbs the lithospheric temperature significantly at large depths so that when it is added to the thermal structure, it produces a small increase in deflection. In addition, defining the temperature profile by the cooling plate model produces only modest weakening relative to the cooling half‐space model. Therefore, variation of the thermal structure does not appear to be a viable means of bringing laboratory‐derived flow laws for low‐temperature plasticity into agreement with geophysical field observations and modeling.

Extensive Magmatic Heating of the Lithosphere Beneath the Hawaiian Islands Inferred From Salt Lake Crater Mantle Xenoliths

“…The first is that the flexural rigidity of the lithosphere along the Hawaiian chain appears significantly weaker than predicted by the relevant, laboratory‐derived rheological laws and assuming a normal 90‐Myr geotherm (Bellas et al., 2020; Pleus et al., 2020; Zhong & Watts, 2013). Given that it is unlikely that solid‐state convection can induce such a shallow temperature anomaly, it was argued that the laboratory‐derived rheologies are too strong, and thus require revision to lower strengths (Bellas et al., 2020; Zhong & Watts, 2013). Another study (Hunter & Watts, 2016), however, found that some of the same rheological laws successfully predicted the flexure of the lithosphere around the Pacific subduction zones, but required substantial weakening to adequately predict the flexure beneath Hawaiʻi.…”

“…Conductive heat transport from the hot mantle plume (Wolfe et al., 2009), alone, is too slow, so some form of advective heat transport is needed. Three‐dimensional geodynamic models of a mantle plume interacting with the lithosphere predict that sublithospheric convection in the solid‐state can advectively erode the base of the lithosphere, but heating at depths of <50 km requires several millions of years longer (Bellas et al., 2020) than allowed by the history of rejuvenated volcanism on Oʻahu.…”

“…If the xenoliths originated in the lithosphere, information about their preextraction temperatures and pressures may provide insight into the thermal structure of the lithosphere. Knowledge of the lithospheric geotherm beneath hotspots such as Hawai‘i is important for addressing questions about the origin of hotspot swells (Detrick & Crough, 1978), the dynamics of plume‐plate interaction (Bellas et al., 2020; Moore et al., 1998), the rheology of the lithosphere as reflected by its flexural response to the weight of the volcanic edifices (Bellas et al., 2020; Pleus et al., 2020; Zhong & Watts, 2013), as well as the interaction between the lithosphere and magma penetrating it (Mittelstaedt et al., 2008).…”

Toward a Self‐Consistent Characterization of Lithospheric Plates Using Full‐Spectrum Viscoelasticity

“…Our description is based on purely material properties and thus does not consider any loading conditions to determine lithospheric thickness (e.g., buoyancy or plate flexure induced by the load). For the real Earth, we do not expect that T e is equivalent to z LAB as additional weakening mechanisms are not included here (such as brittle deformation, dislocation creep and low temperature plasticity; e.g., Burov & Diament, 1995; Bellas et al, 2020). In the following, we have chosen to use the classical and most familiar description to demonstrate the disctinction between our approaches in the simplest manner.…”

Seismic Strain Rate and Flexure at the Hawaiian Islands Constrain the Frictional Coefficient

“…In the following, we present the viscoelastic loading model which computes the surface flexure and internal deformation of the lithosphere in response to time‐dependent loading from volcanic island building. The model has been developed and used in previous studies (Bellas et al., 2020; Zhong et al., 2003; Zhong & Watts, 2013), but we describe all essential aspects in the following and in the supporting information.…”