Origin of σ3 re-orientation during oblique magma-assisted rifting. Example of the Main Ethiopian Rift T33F- 2726 Melody Philippon, Giacomo Corti , Federico Sani, Ernst Willingshofer, Dimitrios Sokoutis, Cedric Thieulot Tec Lab 1. Faculty of Geosciences, Departement of Earth Sciences. Budapestlaan 4. 3584 CD Utrecht ([email protected]) 2. Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, Firenze, Italy 3. Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Firenze, Italy, Universiteit Consiglio Universita’ 4. Department of Geosciences, University of Oslo, PO Box 1047 Blindern, N-0316 Oslo, Norway Utrecht Nazionale degli Studi 5. PGP, Sem Selands vei 24 NO-0316 Oslo NORWAY delle Ricerche di Firenze Faulted volcano Boundary faults Internal faults Teza At regional scale SE 64 At local scale Introduction Volcano SEET2011-18 65 In East Africa horn, continental breakup leads to the individualiza- Arabia Oblique rifting in the central and northern Main Ethiopian Rift N Strain partitioning is observed at regional scale as a result of oblique rifting. At tion of Nubia, Arabia and Somalia plates that occurred along the Northern MER (MER) has resulted in a complex structural pattern characterized local scale, perturbation of the stress eld induced by volcanic activity may also Nubia ET2011-06 MER Mozambic Ocean Suture Zone (MOSZ) that trends NNW-SSE by two differently oriented fault systems (e.g., Corti, 2009): a set ET2011-07 lead to strain re orientation along faults. Therefore we investigate one of the tecto- (Kazmin et al., 1978, Stern 1994). Northern of NE-SW-trending boundary faults and a system of roughly no-magmatic segments along the MER., in order to constrain the kinematics and Somalia 34 data 43 data Horst During Eocene, the evolution from collision to active sub- NNE-SSW-oriented fault swarms affecting the rift floor (Wonji timing of the deformation as well as the timing of volcanic eruptions. Graben SE 39 & 40 duction of the boundary conditions at the northern convergent σ N110° σ N92° faults). Boundary faults formed oblique to the regional extension SEET2011-08 48 & 49 ET2011-09 The western SMER margin is a magmatic margin that shows a well-developed SE 41 to 43 ARABIA margin of the African plate (Bellahsen et al., 2003) , coupled with vector, likely as a result of the oblique reactivation of a pre- ET2011-05 faults network that trend NNE-SSW. Numerous volcanic edices are observed A SEET2011-04 46 SE 44 & 45 Central MER the presence of the Afar plume, helped strain localization along N existing deep-seated rheological anisotropy, whereas internal SE 52 along the margin. SE 51 the MOSZ and lead to the Red Sea sea opening (Gass, 1977) and Wonji faults developed sub-orthogonal to the stretching direc- SE 53 Volcanoes show three dierent kind of relationship with faults: ET2011-03 SE 54 Nubia and Arabia's continental break up. Contemporaneously, up tion. Previous works have successfully reconciled this rift archi- 1/ cut by fault Aligned volcanoes ET2010-22 MER SE 50 to 3km of basaltic oods have been emplaced above the plume in Central tecture and fault distribution with the long-term plate kinemat- 2/ Aligned above a blind fault Afar Duguna NUBIA N the Afar region (Mohr and Zanettin 1988). Somalia kinematics ics (e.g., Corti, 2008); however, at a more local scale, fault-slip Volcano 3/ at the tip of the fault 220 data 31 data Damota During Miocene, a second episode of "pre-rift" basaltic GPS data reveal significant variations in the orientation the minimum Volcano ET2011-14 The geological map is a compilation eldwork, google earth images analyses, σ N103° σ N96° A' MER ood were emplaced and predates the Main Etiopian Rift opening principal stress and related fault-slip direction across the rift ET2011-13 satellite images and SRTM 90, Kazmin 1979, Chernet 2011and google earth images Paleostress B ET2011-17 ET2011-16 ET2011-15 Graben (MER)(Zanettin et al.,1978; Ebinger et al., 1993). The MER develops Southern MER valley (Agostini et al., 2011). Whereas fault measurements indi- ET2010-10 SE 62 analyses (volcanoes occurrences), satellite images and SRTM 90 (fault pattern & Global SEET2010-07 11 & 12 ET2010-09 ET_05 south of the Afar region, and separates the Nubia and Somalia Western approach cate a roughly N95°E extension on the axial Wonji faults, a SE 61 lava ows). Our preliminary geochemical analyses conrm the nature of the Turkana plates. Dierent scenarii are exposed concerning the timing and SMER N105°E to N110°E directed minimum principal stress and slip di- Hobitcha mapped lithologies. SOMALIA Caldeira basin development of the MER: rection is observed along boundary faults, pointing to a stress Horst MER ET2010-11 ACID LAVAS BASIC LAVAS 1/ A northward propagation starting at 18 Ma in the south and Southern (and slip) reorientation supported by the available focal mecha- ET2010-12 Volcano at the tip of a fault SE 13 Rhyolite dome Basement Mozambic reaching the Afar at 11Ma (Wolfenden et al., 2004). 259 data 34 data nism solutions of earthquakes. Both fault-slip data and analysis SE 14 Holocene Scoria cone Major normal fault SE 10 Pyroclatic flow σ N°98 σ N° SE 8 & 9 Ophiolites Ocean 2/ A southward migration from Miocene to present day (Bonini et Boundary faults of seismicity indicate a roughly pure dip-slip motion on the ET2010-05 SE 7 Fold & trust belt SE 21 B' Rhyolite dome Basaltic / yaloclastites Volcanic edifice suture al., 2005, Keranen & Klemperer 2007). Internal faults boundary faults, despite their orientation (oblique to the re- Pleistocene Eo-Oligocene traps Ignimbrites Basalt The rift presents a curved shape at angle with the direc- Measurement points gional extension vector) should result in an oblique displace- SE 15 & 16 SE 22 Minor normal fault Miocene to Quaternary traps tion of Somalia plate's motion, making it a prefect example to ment. To shed light on the process driving the variability of data ET2010-17ET2010-18 Pliocene Trachite Agostini et al 2011 ET2010-14 SE 60 SE 19 ET2010-16 SE 37 Associated rifts WRZ SE 17 & 18 ET2010-15 Rhyolites, Ignimbrites study how the fault develops and accommodates oblique exten- this study derived from fault-slip (and seismicity) analysis we present SE 55 ET2011-10 SE 20 SE 38 Mio-Pliocene Quaternary volcao-sediments Horst ET2011-12 & trachytes SE ET2011-1156 sion. 4500 m crustal-scale analogue models of obliquity rifting. SE 58 Miocene Basalts 0 500 km 50 km Quaternary aluvions 85 data 8 Km Oligo-Miocene Rhyolite & Trachite Graben 700 m σ N°93 SE 57 Abaya lake 0° Analogue modeling of strain partitioning during oblique rifting Fieldwork along the Southwestern volcanic margin of the Main Ehtiopian Rift Volcanoes Caldeira Duguna volcano Decreasing of the water column The models reproduced extension of a conti- At the end of the experiment, after 15 mm of extension, the surface of all the models A A' Maar Maars and tu rings results nental crust containing a weak zone oblique Top view show two sets of faults: large boundary faults that develop above the weak/strong duc- from phreatomagmatic to the stretching direction (0° to 45° of 5 10 5 10Stress tile discontinuity and internal fault that develops in the mean time but above the 15° explosions.Maar and tu rings Stress (kPa) (kPa) obliquity by increment of 15°, in order to 10 10 homogenous/weak ductile crust present a crater oor below and Volcanoes are organized 20 20 30 30 along lines that trend compare the analogue modeling results mm mm 1 km above original ground level, respectively parallel to the NNE-SSW with the northern, central and southern hangingwall trending faults. MER). The top to bottom vertical layering Duguna volcano is located on the rift oor and is a relatively young feature as it is Tu ring Tu Ring Explosion of the experiments consisted of brittle not aected by faults. Its western boundary is controlled by an east-dipping fault. /Maar ashes+ bombs Suggesting that (feldspar powder), analogue to the volcano 14 cm footwall Hobitcha Caldeira upper crust and ductile material Hydrofracturation emplacement Weak zone 30° 30° B B' (plasticine mixture) analogue to the Aquifer is triggered particle 0 mm v hangingwall A by faults lower crust. 3 mm of symmetric ex- position 15 mm Randomly spread particles on top of the Ejections of 18 cm v footwall develo- tension was performed removing 1,5 vector footwall model, were tracked trough time. The ob- Scoria cone (in the middle of a maar) scories eld hangingwall slip on the fault pment. mm spacers on both side of the tained displacement eld serves as a proxi A' model each time steps. to recover the motion across each fault. 1 km Cross section This, coupled with the dip of the faults, obtained via the cross section allow us to 45° Hobitcha caldera is located on the western rift shoulder and the rift oor is aected Centrifuge force eld compute the paleostress tensor on each by a well developed fault pattern and volcanic system comprising numerous scoria brittle/upper crust fault plane of the model using WinTensor cones. Normal faulting = feldpar sand + A A' (Delvaux et al., 2003). The western SMER margin shows a well-developed NNE-SSW trending, SE dipping fault pattern. Paleostress ductile/lower crust Strong reconstruction of the least principal stress axis σ3 from eld data range between N30° and N105° *, displaying a wild Crust Weak zone Crust Weak = silicone puty Analogue and numerical modeling “dispersion” (white arrows), whereas the faults trend remain NNE-SSW (See rose diagram). Regional mean trend of the fault Dispersion of σ3 orientation Equal Area N20 ± 5° Moving walls The models reproduced the extension of a brittle upper crust (quartz sand) containing a magma chamber (light = weak zone normal fault transfert faults sense of shear bloc rotation N Spacers PDMS putty), lying on a decollement layer (heavy pink putty) (analogue either to a lithological interface or to the brittle ductile transition).