Thermal regimes, mantle diapirs and crustal stresses of continental rifts

Abstract

A coupled, thermomechanical continuum model of a continental rift and platform calculates crustal stresses, free-surface velocities, viscosity profiles, and convective sub- solidus creep of mantle geotherms. Two cases, a Miocene and Plio-Pleistocene model are evaluated. The initial geometry and geotherms for the Plio-Pleistocene case are derived by Bridwell and Anderson from seismic refraction and petrology of mantle xenoliths. Crustal thinning to ~34 km, surface heat flux of ~2.5 HFU, and a mantle with ρ ~ 3.22 g/cm 3 define this case. The Miocene case is an estimate of a prior geometry whose crustal thick- ness exceeds 37 km, whose surface flux is ~2.0 HFU, and whose density is ~3.22 g/cm 3. The isotherms were established at t = ∞. Modeled geotherms of rift and continent are con- sistent with surface flux and xenolith data on crust and mantle temperature and pressure. From these initial values, a series of forward dynamic-flow problems are solved to calcu- late stress, free-surface velocity, viscosity, and mantle flow. Since material properties of the mantle beneath the rift are unknown, a parameter study was performed to deduce physical constants for a Weertman-type non-Newtonian flow law. These values are A = 10 8kbar −ns −1, V ∗ = 17 cm 3/mol , 95< E ∗ < 102 kcal/mol , and n = 3.3. An existing mantle diapir for the Plio-Pleistocene case, whose hot, less-dense material creeps upward and spreads laterally beneath the rift, generates crustal uplift and spreading. Free-surface uplift rates are ~1 km/10 m.y. consistent with geologic constraints. Mantle viscosities for the Plio-Pleistocene case have minima of 10 21 to 10 22 poise for the range in E ∗ . For lower crustal temperatures, surface heat flow, and small mantle diapir of the Miocene case, viscosities range from ⪢10 22 to 10 23 poise. These values are somewhat higher than average values of 10 22 poise for a global model of the asthenosphere proposed by Hager and O'Connell. Velocities of the mantle diapir have the range 1 < v < 5 km/m.y. as tem- perature at the base of the crust increases from 750° to 950°C. Scaling E ∗ by a few per- cent changes uplift times for the Miocene case from 10 to 20 m.y./km. The Plio-Pleisto- cene case achieves uplifts of 1 km in 5–10 m.y. The Miocene mantle diapir can move up- ward 15 km in 15–36 m.y. for a similar range of E ∗ . The hotter Plio-Pleistocene diapir can move upward 25 km in 5–10 m.y. These results support the interpretation that the increase in surface heat flux from Miocene to Present produces an acceleration in the con- tinental rifting process.