Present-day Kinematics of the East African Rift Sarah Stamps, Eric Calais (Purdue University, IN, USA - Elifuraha Saria (UCLAS, University of Dar Es Salaam, Tanzania) Evelyne Mbede (Department of Geology, Univ. of Dar Es Salaam, Tanzania) Cindy Ebinger (University of Rochester, NY, USA) Damien Delvaux and Francois Kervyn (Royal Museum for Central Africa, Belgium) Ludwig Combrinck (Hartebeesthoek Radio Astronomy Observatory, South Africa) Chris Hartnady (UMVOTO (Pty) Ltd, South Africa) Jean-Mathieu Nocquet (CNRS Geosciences Azur, Nice, France) Rui Fernandes (UBI, CGUL, IDL, R. Marques d'Avila e Bolama, Covilha, Portugal) AGU Fall Meeting 2007 San Francisco, CA

2007 AGU Fall Meeting 10 December 2007, San Francisco, CA Global Kinematics

→Boundary conditions: relative motion of surrounding plates? →Spatial distribution of strain: localized or diffused strain within the plate boundary zone? →Temporal distribution of strain: relationship between long-term, far- field plate motions and short-term rifting events? →Dynamics of continental rifting: role of mantle flow? Contribution of buoyancy stresses? →Hazard Assessment: help quantify seismic/volcanic hazards posed by actively deforming areas AGU Fall Meeting 10 December 2007, San Francisco, CA Open Questions

Gordon and Stein, 1991: EAR = a diffuse plate boundary Hartnady, 2002: EAR = 4 rigid plates embedded within Nubia-Somalia plate boundary 2007 AGU Fall Meeting 10 December 2007, San Francisco, CA Early Kinematic Models

2007 AGU Fall Meeting 10 December 2007, San Francisco, CA Recent Kinematic Models Calais et al., 2006: NU-SO pole south of Africa, 2 additional plates (Victoria + Rovuma) Horner-Johnson et al., 2007: 3.2 My NU-SO pole consistent with present-day one, one additional subplate (Lwandle)

→Seismicity in EAR: Focused along narrow rift valleys Consistent with active faults Large aseismic “blocks” (Hartnady, 2000) Focal mechanisms consistently show normal faulting, except… →Calais et al. (2006): GPS + ESV data consistent with CCW rotation of Victoria →Horner-Johnson and Gordon (2007): SWIR 3.2 My average data best fit with additional “Lwandle” plate →Hypothesis: EAR consists of 3 microplates: Victoria, Rovuma, and Lwandle 2007 AGU Fall Meeting 10 December 2007, San Francisco, CA Background

2007 AGU Fall Meeting 10 December 2007, San Francisco, CA Defining Stable Nubia →3 data sets processed independently →Solutions combined 14 parameter transformation into ITRF 2005 →Transformation to Nubia-fixed frame →Used 14 “best-fit” Nubia sites →Reduced chi2 = 1.5 RMS = 0.7 mm/yr

→Model: surface deformation results from rigid plate motions, i.e. 3 parameters per plate. →Data: GPS velocities, assigned to a plate (+12 GPS velocities on Antarctic plate) Earthquake slip vector directions, assigned to a plate boundary 3.2 My average data on the SWIR = transform fault azimuths + spreading rates. →Solve for block angular velocities: joint inversion of GPS, ESV, and SR data. →Use F-test statistics: quantify significance of chi2 difference between various scenarios 2007 AGU Fall Meeting 10 December 2007, San Francisco, CA Method

1. No Lwandle, GPS +ESV only 2. No Lwandle, GPS + ESV My data 3. With Lwandle, GPS + ESV My data GPS/ESV vs. 3.2 My data? F(91,185) = 0.66 => p < 1% Models statistically similar No Lwandle vs. Lwandle? F(3,296) = 5.38 => p > 99% Model significantly improved Results 2007 AGU Fall Meeting 10 December 2007, San Francisco, CA

Model Residuals →Data versus model predictions: weighted RMS →Earthquake slip vector directions = 6.7 o →3.2 My average spreading rates = 4.2 mm/yr →GPS velocities = 0.6 mm/yr

→Present-day and 3.2 My average data →Predicted extension rates: increase from S to N up to ~6.5 mm/yr →Extension directions: ~E-W but vary as a function of the plates involved →Central EAR: 3-6 mm/yr →Extension: At least 30 km cumulative extension 2007 AGU Fall Meeting 10 December 2007, San Francisco, CA Virunga: 2.5 mm/yr Results: Extension Rates

→ Localized strain bounds strong cratonic blocks Rovuma, Victoria → Extension directions across EAR ~ EW Varies spatially as a function of plates involved → Stable plate motion over past ~3 My Consistency in GPS, ESV, and 3.2 My SWIR data → Slower extension rates as rifting initiated → Future work Refine kinematics -- very sparse GPS data set! Investigate dynamics, role of mantle flow 2007 AGU Fall Meeting 10 December 2007, San Francisco, CA Conclusions