#5026 Rift stability in , Alexis A. Martone and Laurent G.J. Montési University of Maryland, Department of Geology [email protected], [email protected]

Introduction

Earth is unique in our solar system because it is the only body with plate Basin and Range province tectonics, which requires an interconnected network of localized shear zones of the western US (taken through the entire thickness of the lithosphere (Regenauer-Lieb et al., 2001). These A USGS diagram of from Parsons, et al. 1998). are areas of concentrated deformation with low strength. Venus’ numerous image of Beta the East African rift. The extension is distributed similarities to Earth begs the question why it lacks plate tectonics. The formation of Regio, clearly The triple junction over a wide region showing the greatly resembles that compared to Venus or East localized shear zones is possibly the cause of the difference. Devana of Devana Chasma. Africa. The Buck (1991) model links the evolution of lithospheric strength with the style Chasma rift. of rifting (wide, narrow, or metamorphic core complex). Narrow rifts like Devana Chasma form as the strength of the lithosphere decreases as the rift develops. However, we show that the Buck (1991) model implies that wide rifts are the most likely outcome of lithospheric extension on Venus. Narrow rifts on Venus may require an efficient strain-weakening process to exhibit localization and resemble narrow terrestrial rifts, such as the East African rift.

Preliminary Results Methods dT ∂2T ∂T dh ∂2h ∂u ∂u Heat equation: = κ 2 −ν + H (1) Lower Crustal flow: = κ f − u − h (2) The crust and mantle are assumed to be dry diabase and dry olivine, respectively (diabase rheological parameters are from dt ∂z ∂z dt ∂x2 ∂x ∂x Mackwell et al. (1998), olivine rheological parameters are from Hirth and Kohlstedt (2003)). The crustal thickness and surface heat flux are varied based on estimated values from Nimmo and McKenzie (1998), Phillips and Hansen (1998), and Buck Eq. 1-2 are solved with finite difference approximations and the MATLAB ode15s solver. (1992). The rift velocity was varied from 0.1 cm/year and 0.01 cm/year, following Smrekar et al. (2010-LPSC), then the ZL ZL h -16 -1 extensional strain rate was defined at 10 s . F = σ dz or F = σ dz Yield Strength: ys ∫ b ys ∫ d (3) Crustal Buoyancy: Fcb = g ∫ (ρm − ρc )dz (4) 0 0 h−δ h ' ! . $1/n * )σ b = gBz σ d = #ε/ A& exp(E / nRT), Blue=Wide, Red=Narrow, Black=Core Complex Blue=Wide, Red=Narrow, Black=Core Complex " % 110 100 ( + Z Figure 1: rift mode 90 L 100 boundaries for F = F + F + F 2 Thermal Buoyancy: F g ( z ) T ( z ) d z (5) Total Force: T ys cb t b (6) 2 = ρ αδ Earth using 80 tb ∫ 0 90 0 ux=1 cm/year 70 80 60 70 50 F ΔF Figure 2: rift Eq. 3-5 are integrated through depth and time to determine the change in total force, Δ T : T > 0 (wide rift), 40 60 mode boundaries Δ F T < 0 (narrow, or core complex if (κ f t) > X e / 2 ) surface heat flux mW/m surface heat flux mW/m for Venus with 30 50 −16 −1 ε! =10 s 20

Surface 740 K Xe width of 40 10 extending region 20 30 40 50 10 20 30 40 50 60 70 temperature Ts crustal thickness km crustal thickness km Thermal Width of initially -1 -1 uniform conductivity, K 3 W m K XL lithosphere Thermal Velocity 0.01 cm/year – expansion 3!10-5 K-1 Blue=Wide, Red=Narrow, Black=Core Complex Blue=Wide, Red=Narrow, Black=Core Complex difference across 0.1 cm/year 100 100 coefficient α Xe 90 Figure 3: rift mode 90 Brittle failure 19.5 MPa/m Strain. rate 10-16 s-1 constant (gB) boundaries for 2 ε = ux/Xe 2 80 80 2 Venus with Acceleration of 8.87 m/s Crustal heat 0-6.4!10-7 W m-3 70 70 ux = 0.01 cm/year gravity production, H 60 60 Surface heat flux Qs Crustal thickness Zc 50 50 modified from Buck, 1991 Figure 4: rift mode 40 boundaries for 40

surface heat flux mW/m 30 surface heat flux mW/m 30 Venus with 20 ux = 0.1 cm/year 20 • A dislocation creep rheology is used for the ductile stress in the Yield strength. This non-Newtonian

10 10 rheology requires an approximation in Eq. 2 (to κ ), which is derived for Newtonian rheology. 10 20 30 40 50 60 70 10 20 30 40 50 60 70 f crustal thickness km crustal thickness km • Lower crustal flow produces lateral pressure gradients due to crustal thickness variation. When this force • In figures 1-4 each colored square is the result of one model run, resulting in either a wide rift (blue), narrow rift (red), or becomes significant a core complex would form; Venus’ strong lower crust does not favor this type of rift. core complex (black). • Crustal buoyancy arises from crustal thickness variation between the rift center and X /2. L • • Figure 1 shows the results for the initial model for Earth, under the condition of a rift velocity of 1 cm/year. These are Thermal buoyancy effects are from the temperature change through the center of the rift. comparable to the results under the same conditions in Buck (1991), which gives us confidence in the validity of our code and our application to Venusian tectonics. Discussion and Conclusions • This model has been applied to Venus in two previous cases (Smrekar et al. 2005-LPSC and Smrekar et al. 2010-LPSC). Smrekar et al. (2005) used it to understand the corona/rift relationship (which this work avoids by focusing on Devana • The model for Earth is within good agreement to the rift mode boundaries in Buck (1991), and provides Chasma). Smrekar et al. (2010) applied it to Hectate Chasma (an extensional zone with no evidence of an underlying assurance that Venus results are within reason.

plume). -16 -1 • The majority of runs for ux = 0.01 cm/year, ux = 0.1 cm/year, and ux = 10 s produce wide rifts. • Figure 2 shows results for a defined rift strain rate of 10-16 s-1. The strain rate should be less on Venus since tectonics are • While core complexes are predicted, we do not expect these to be significant due to the lack of a weak driven by mantle convection which imposes smaller stresses on the lithosphere (the value is taken from Nimmo and lower crust, which would inhibit the lateral pressure gradients associated with lower crustal flow. McKenzie, 1998). • Only the 10 km thick crust allows for a large range of surface heat fluxes to produce narrow rifts; all other crustal thickness values have narrow rifts over a smaller range of heat fluxes. James et al. (2013) determined • The crustal heat production can be assumed to be the same as Earth, however, these model runs assume H=0 because crustal thickness between 8-25 km, for which our results could lead to narrow rifting. However, Anderson its inclusion steepened the temperature gradient too much for some values of Qs and Zc. and Smrekar (2006) predict crustal thickening beneath of at least 50 km.

• The geologic parameters of crustal thickness and heat flux are not well constrained on Venus: crustal thickness values for • It appears necessary to include rheological weakening mechanisms to produce narrow rifts for the range of Beta Regio are on average 30-40 km (Leftwich et al., 1999; Kiefer and Peterson, 2003), and heat flux values on Venus also conditions speculated at Devana Chasma. The next step in this work is to correct this problem by including range from about 25 mW m-2 (Nimmo and McKenzie, 1996) to 75 mW m-2 (Solomon and Head, 1982). potential weakening mechanisms (following Montési, 2013), which would serve to concentrate the deformation, likely favoring narrow rifts. • The core complex mode is only predicted for the thickest and hottest crust and for very fast stretching rate; the • As the model was derived for Earth, edge-driven boundary conditions are used, which would not be temperatures at the base of the crust in these predicted modes are always above the melting point of basalt (≈1100°C). appropriate for Venus since it experiences bottom-driven tectonics; this will be another addition to generate

more accurate results.