Farallon plate (oceanic) America (cont.)
30 Ma
10 Ma
0 Ma
Rio Grande Rift Basin and range
Rio Grande Rift Basin and range
0 km
1020 km
Rio Grande Rift Basin and range
G eosph ysic al c onstr ain ts Setup
References
"one suture model"
"Two suture model"
No strong effect on the rising of the astenosphere
No strong effect Suture
Sutures
106 108 110
0 250 500 750 1000
x (km)
Tplume=550o Tplume=750o Tplume=950o 106
108 110
0 250 500 750 1000
x (km)
Δx=25km Δx=50km Δx=100km
106 107 108 109
0 250 500 750 1000
x (km)
Δx=25km Δx=50km Δx=100km
?
Enhancing astenosphere rising/lithospheric mantle breaking/Decreasing localisation in the crust/ Enlarging the rift’s size
114 116 118 120
0 250 500 750 1000
x(km)
t=0Myr t=2.2Myr t=3.7Myr t=5.1Myr t=6.7Myr t=9.9Myr
Lithospher e thick ness (Km)
Control of initial heterogeneities and boundary conditions on the deformation partitioning of continental rift: a comparison between Rio Grande Rift and Main Ethiopian Rift.
CédricThieulot1,2, Melody Philippon1, Dimitrios Sokoutis1,3, Jolante van Wijk4, Enrst Willingshofer1 & Sierd A.P.L. Cloetingh1
Introduction Contientntal rifting leads to break up, and is therefore a key process in plate tectonics. Understanding the proces- ses of strain localization and partitioning in such tectonic context appears to be essential. Earth dynamic processes such as subduction, continental collision and continental rifting affect the whole lithosphere. This results in introdu- cing heterogeneities at lithosphere scale in terms of com- position, temperature, structure enhancing regional
strength variations. These heterogeneities strongly
control the deformation pattern as they act either as weak or strong zone within the lithosphere. Rifts development is controlled by such heterogeneities. A deep investiga- tion of the controls of initial heterogeneities and the in- fluence of boundary conditions on rifting is required to understand the mechanics of continental rifting and its evolution toward drifting or its abortion. We propose here to investigates two natural examples by the means of nu- merical modelling, carried out with ELEFANT (http://www.
cedricthieulot.net/elefant.html), an improved version of FANTOM (Thieulot, 2011). We present here a preliminary study of the main controls affecting continental rifting in various regions. The final and non acheived yet goal of this study is to investigate theses problems in 3D.
-10898 0
8271 Main continental rifts
Extensional collapse (after Dewey, 1988)
1. Faculty of Geosciences, Departement of Earth Sciences. Budapestlaan 4. 3584 CD Utrecht (m.m.b.philippon@uu.nl 2. PGP, Sem Selands vei 24 NO-0316 Oslo NORWAY
3. Department of Geosciences, University of Oslo, PO Box 1047 Blindern, N-0316 Oslo, Norway
4. University of Houston. Department of Earth and Atmospheric Sciences312 Science & Research 1 Rm #312Houston, TX 77204-5007
Amante, C. and Eakins, B.W. 2009. ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis. NOAA Technical Memorandum NESDIS NGDC-24, 19 pp. // Baker, B.H., 1987. Outline of the petrology of the Kenya rift alkaline province, in Fitton, J.G., and Upton, B.G.J., eds., 1987, Alkaline igneous rocks: Geological Society London Special Publication 30, p. 293–311.//Bagherbandia M. and Sjöberga, L.E. Modelling the density contrast and depth of the Moho discontinuity by seismic and gravimetric–isostatic methods with an application to Africa//Buehler J.S. and Shearer, P.M. 2010. Pn tomography of the western United States using USArray. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B09315.//Ebinger, C. J., 1989. Tectonic development of the western branch of the East African Rift system, Geol. Soc. Am. Bull., 101, 885 – 903.//Ebinger, C.J., Poudjom, Y., Mbede, E., Foster, F., and Dawson, J.B., 1997. Rifting Archean lithosphere: The Eyasi-Manyara-Natron rifts, East Africa: Geological Society [London] Journal, v. 154, p. 947–960.//Golombek, M.P., MCGill, G.E. and Brown, L.1983.Tectonic and geologic evolution of the Espanola basin, Rio Grande Rift: structure, rate of extension and relation to the state of the stress in the western united states. Tectonophysics, 94, 483-507.//Korrnel, T., Acocella, V., and Abebel, B. 2004. The Role of Pre-existing Structures in the Origin, Propagation and Architecture of Faults in the Main Ethiopian Rift. Gondwana Research, 7 ( 2), pp. 467-479.//Liu L. and Stegman D.R. 2011. Segmentation of the Farallon slab. Earth and Planetary Science Letters. 311,(1–2),1–10.//Manley, K.. 1979. Stratigraphy and structure of the Espaiiola basin, Rio Grande rift, New Mexico, In: R.E. Riecker (E!.ditor), Rio Grande Rift: Tectonics and Magmatism. American Geophysical Union. Washington, D.C.. pp. 71-86.//Pérez-Gussinyé, M., Metois, M., Fernández, M., Vergés, J., Fullea, J., Lowry, A.R. 2009. Effective elastic thickness of Africa and its relationship to other proxies for lithospheric struc- ture and surface tectonics. Earth and Planetary Science Letters 287, 152–167.//Priestley K., Tilmann, F..Relationship between the upper mantle high velocity seismic lid and the continental lithosphere. Lithos 109 (2009) 112–124//Pulliam J., S. P. Grand, Y. Xia, C. Rockett, and T. Barrington. Edge-Driven Convection Beneath the Rio Grande Rift .inSights the EarthScope newsletter Summer 2010//Seager, W.R. and Morgan, P.. 1979. Rio Grande rift in southern New Mexico, west Texas, and northern Chihuahua. In: R.E. Riecker (Editor), Rio Grande Rift: Tectonics and Magmatism. Am. Geophysical Union, Washington, D.C., pp. 87-106.//Shen W. and Ritzwoller M. H. . A 3-D Shear Ve- locity Model of the Crust and Uppermost Mantle Beneath the Western US from Bayesian Monte Carlo Inversion of Surface Wave Dispersion and Receiver Functions. 2012 IRIS Workshop//Shen, W., Ritzwoller, M.H., Schulte-Pelkum, V. and Lin, F.C. Joint inversion of surface wave dispersion and receiver functions: A Bayesian Monte-Carlo approach. Submitted.//Tedla, G.E., van der Meijde, M.,Nyblade, A.A. and van der Meer, F. D.A. 2011. Crustal thickness map of Africa derived from a global gravity field model using Euler deconvolution. Geophys. J. Int. Geodynamics and tectonics, 187, 1–9//Thieulot, C., 2011. Two- and three-dimensional numerical modelling of creeping flows for the solution of geological problems. Physics of the Earth and Planetary Interiors, doi:10.1016/j.pepi.2011.06.011.
235˚ 240˚ 245˚ 250˚ 255˚ 260˚
30˚
35˚
40˚
45˚
50˚
3.4 3.5 3.6 3.7 3.8
3.2 3.4 3.6 3.8
Crustal Velocity (km/s)
−8
−6
−4
−2 0 2 4 6 8
Velocity Perturbation (%)
0 20 40 60 80 100 120 140
238 240 242 244 246 248 250 252 254 256 258 260 262
0 2000 4000
0 2000
Great 4000
Valley Sierra Nevada
Basin and Range Colorado
Plateau Great Plains
Rio Grande Rift Cascade
Volcanic Arc
Snake River
Plain Big Horns
Black Hills
Great Plains
0 20 40 60 80 100 120 140
236 238 240 242 244 246 248 250 252 254 256 258 260 262
0 2000 4000
0 2000
A A’ 4000
B B’
High Lava Plains
Vsv, at 17.5 km depth (Shen and Ritzwoller 2012)
Te
(Pérez-Gussinyé et al., 2009) Crustal Thickness
(Telda et al., 2010)
235˚ 240˚ 245˚ 250˚ 255˚ 260˚
30˚
35˚
40˚
45˚
50˚
4.
0 4.
1 4.
2 4.
3 4.
4 4.
5 4.
6 4.
7 4.
8
Vsv, at 120 km depth (Shen and Ritzwoller 2012) A
B
A’
B’
East African rift
Rio Grande Rift
G eosph ysic al c onstr ain ts In order to have an in-depth control, we gathered available geophysical
data from Eastern Africa. The initial crustal thickness is approached by looking at surrounding non-rifted area on a model based on gravity data (Tedla et al. 2010) and is estimated to be at 45km.
Lithospheric thickness is also available and as been obtained from a combination of surface-wave and body-wave tomographies (Priesltey and Tillman 2009). To begin with a classical thickness of 120 km for the whole lithosphere has been chosen.
The rheological behavior of the whole lithosphere can be approached looking at its elastic thickness Te (obtained by the mean of topography and Bouguer anomaly data)(Perez-Gussinyé et al., 2009). The Te of the East african lithosphere shows basically two areas: 1/ north of the rift, in the Afar area, where the rift is itself a zone of low Te compare to the sur- roundings, and 2/ the central part of the rift that show the Tanzania
craton and its surrounding in blue shade showing a high Te whereas the boundaries of the cratons have a low Te.
The setup of our reference model is 2D and consist of a 120km lithosphere containing one suture zone (modeling the North of the East African rift) and two suture zone (for the South of the rift). The suture is implemented in the model thanks to zone of least strength that affects the whole lithosphere (Green line on the srength profile).
The model of the South-Western America crust and upper most mantle has been obtained from receiver function (Shen and
Rotzwoller and Shen et al., in progress) and the Moho depth is deduced from Pn arrival times on USArray (Julliam et al., 2000). The initial state of the lithosphere before rifting can be approach by looking at the northern cross section AA', where the Rio Grande Rift did not developped. The Rio Grande Rift developped at the interface between a Western hot delaminated lithosphere and cold cratonic lithosphere to the East. The setup of our reference model is 2D and consists of a 120km lithosphere. The East-West profile of our lithosphere is closely inspired by the cross section AA'. The first western part of the model consist of a delaminated
"hot" lithosphere (red line on the strength profile) whereas the eastern part of the model consist of a cratonic "cold lithosphere"
(green line on the strength profile).
Moho Depth
(Buehler and Shearer 2010)
Setup
N
NUBIA
SOMALIA
From North to South, the East African rift is composed from north to south: the Main Ethiopian Rift (MER), the Kenya Rift, the eastern and western of the bran- ches that surrounds the Tanzania craton (Map after Krome et al 1983).
In east Africa horn, continental breakup that leads to the individualization of Nubia, Arabia and Somalia plates occurred along the Mozambic Ocean Suture Zone (MOSZ) that trends NNW-SSE (Kazmin et al., 1978, Stern 1994).
During Eocene, the evolution from collision to active subduction of the boun- dary conditions at the northern convergent margin of the African plate
(Bellahsen et al., 2003) , coupled with the presence of the Afar plume, helped strain localization along the MOSZ and lead to the Red Sea sea opening (Gass, 1977).
During Miocene, a second episode of "pre-rift" basaltic flood emplaced (Zanettin et al.,1978; Ebinger et al., 1993) and predates the opening of the Main Etiopian Rift (MER), south of the Afar region, that separates the Nubia and Somalia plates.
The western and eastern branch of the East African Rift starts to open at the same time and (12–10 Ma) (Baker, 1987, Ebinger, 1989, Ebinger et al., 1997),
and is still in its early stage of development contrary to the MER that is close to beakup is its northern part.
ARABIA
East African rift Rio Grande Rift
The Rio Grande Rift is an active zone of extension separating the Colorado plateau from the Great Plains from the Southern Rocky Mountains (North) to the Basin & Range (south) (Map after Golombek et al., 1983, USGS GIS data- base).
The area consist of the upper plate of the Farallon subduction zone (cross sections modilied after Liu and Stegman 2011). Dynamic of subduction and deformation of the upper plate are closely linked. Since Eocene, the subduc- tion started to retreat and the upper plate is first affected by wide type rif- ting: the basin and range developped till Mid Oligocene (30 Ma).
Then, extension switch from wide to narrow rift in the Rio Grande Area. The early rifting stage started in mid-Oligocene (30 Ma). Opening continued during late Miocene (10 Ma). The rift is still active these days (Seager and Morgan, 1979, Manley 1979).
Cross sections (Shen and Ritzwoller 2012)
UPPER CRUST MOHO DEPTH LITHOSPHERIC MANTLE
Utrecht University
500 km
TANZANIA
M AD AGASC AR
0 5.107 108 1,5.108
Strength (Pa)
brittle upper crust
upper mantle ductile Lower crust
lower mantle
0
40 20
60
80
100
120
"Normal lithosphere"
"Suture zone"
D epth (k m)
Plate boundaries extension compression
T53F-2732
∆x
One suture zone/North of the EAR
Two suture zones/South of the EAR
1000 km
120 k m
1000 km 120 k m w et Qz dr y O livine
∆x
T °C plume
Reference
Reference
550
T Ma
0 5 10
T Ma
0 5 10
T Ma
0 5 10
Eastern Branch
Western Branch MER
Kenya Rift
Craton
Neo-Proterozoic Suture zones Cenozoic lava flows
Rift valley
Basement+Cenoizoic sediments strike slip (Inherited strucures) Normal fault
Afar
High : 4385 Low : 421 Cretaceous
Trias to Jurassic
Pr ot er oz oic Paleo zoic M eso zoic Paleogene Neogene
Sediments Volcanic rocks Granite
Sediments Volcanic rocks Granite
Sediments Volcanic rocks Granite
Sediments Volcanic rocks Sediments Volcanic rocks Sediments Volcanic rocks
Granite Q
200 km
Colorado plateau
Great plains Basin &
range
N
0 km
1020 km
0 km
1020 km
v=6mm.y-1 v=6mm.y-1
v=6mm.y-1
25
950 750
50 100
∆x
25 50 100
∆x=50km,
T = 750°C plume
116 118 120
0 250 500 750 1000
t=0Myr t=5Myr t=10Myr
Eastward migration
x (km)
Lithospher e thick ness (Km)
117 118 119 120
0 200 400 600 800 1000
t=0Myr t=5Myr t=10Myr
x (km)
Lithospher e thick ness (Km)
At the initiation of the experiment, the strain is strongly located in the brittle crust at the place of the suture zone. The lower crust and lithospheric mante are affected by large-scale localised shear zones. At 5 Ma, the thinning concentrates mainly in the lithospheric mantle and at 10 Ma the astenosphere reaches the base of the crust. The time evolution of the topography shows that the rift is asymetric with an eastern margin is
smoother and displaced toward the East. The western margin topography stays steep and steady during the experiment.
At the initiation of the experiment, the strain is strongly located in the brittle crust at the place of the suture zone. This situation is comparable to the previous experiment containing one suture. However, he following of the experiment differs greatly. As there are two zones of weaknesses in the lithosphere, deformation is equaly accommodated by both of these zones. The final result is different in terms of astenosphere exhuma- tion, that is in this case simply not happening. The thinning affect the whole lithosphere and the litho-
spheric mantle is least thinned than in the one suture zone experiment.
20 40 60 80 100 120
0 2e+08 4e+08
depth K m
strength (Pa)
Western part Eastern part
Lithospher e thick ness (Km)
2e+08 4e+08
Depth (Km)
Strength (Pa)
steep initial T. gradient
102 104 106 108 110 112 114
0 250 500 750 1000
x (km)
steep initial T. gradient medium initial T. gradient low initial T gradient
Lithospher e thick ness (Km)
final
0 20 40
60 80 100 120
2e+08 4e+08
Depth (Km)
Strength (Pa)
medium initial T. gradient
0 20 40
60 80 100 120
2e+08 4e+08
Depth (Km)
Strength (Pa)
low initial T. gradient
0 20 40
60 80 100 120
