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Early Structure influence on the geometry of the San Cornelifault propagation fold, Pyrenees: 3D restoration map
Aubiès-Trouilh Alexandre, Kirkwood Donna and Pouliot Jacynthe
Contact: [email protected]
The Pyrenees is a natural boundary between Spain and France and its general orientation is WNW-ESE (Casas et al., 2003). This mountain range is an intercontinental orogen resulting from the collision and subduction of the Eurasian and Iberian plates. Its formation started with two Mesozoic extensive phases (Muñoz et al. 1986; Vergés and Muñoz, 1990; Soto et al. 2002; Capote et al. 2003), the first occuring duringthe Triassic-Jurassic and the second during the Early Cretaceous (Albian-Aptian). During the LateCretaceous, a compressive phase gave rise to the Pyrenees, a doubly vergent collisional orogen flanked by two foreland basins (fig.1).The opening of the Atlantic Ocean during Early Jurassic (Sussman. et al., 2004) influenced the uplift of the Pyrenees. Initially, an extension phase was accompanied by basaltic volcanism, suggesting an early rift stage without ophiolitic rocks. At the end of the Aptian, the Iberian plate moved south along the NorthPyrenean Fault (NPF). This fault is an inherited structure of the Hercynian phase which affected all Europe during the Carboniferous. NPF reactivation allowed the creation of several pull-apart basins between the Iberian and the European plates. Several normal faults are associated to these basins. During the LateCretaceous, the Iberian plate moved north and collided with the European plate; this episode is the beginningof the Alpine compressive phase. The folding then started in the oriental Pyrenees and it progressively closedall the pull-apart basins.The South Pyrenean Zone can be divided in three units (fig. 2) located between the Axial Zone to the northand the Ebro foreland basin to the south. These units are the south-vergent Boixòls-Cotiella, Montsec and Sierras Marginales thrust sheets. They overthrust the Eocene foreland basin. The sedimentary rocks withinthe thrust sheets are Meso-Cenozoic in age. The Triassic evaporites (Keuper) correspond to the detachmentsurface. The Jurassic deposits are made up of platformal limestones that are affected by several normal faults. The Early Cretaceous deposits are syn-rift sediments that are relatively thick in the Boixòls-Cotiellathrust sheet and thin progressively to the south in the Sierras Marginales thrust sheet (Fig. 2). The LateCretaceous deposits consist of post-rift deposits that are present in the entire zone and unconformablyoverlie syn-folding deposits in the Boixòls-Cotiella thrust sheet. In the Montsec and Sierras Marginales thrustsheets, the Late Cretaceous coarse-grained clastics (Garrunmian) derived from the Axial Zone thin towardsthe south.
Geological setting
The study area covers an area of 8 km by 10 km (fig. 3). San Corneli anticline is a fault propagation foldlocated in the Boixòls thrust sheet (fig. 2). The axial surface of the anticline is oriented N100°, which iscoherent with the Pyrenean compression directions. The folding affects the Meso-Cenozoic sedimentaryrocks. In this region, only the Late Cretaceous platform deposits outcrop. The Early Cretaceous sequence isvisible in the core of the Boixòls anticline, the eastern extension of the San Corneli fold (fig. 3). The sedimentary sequence can be separated in to three sedimentary facies that rest unconformably on top of each other. The Albian-Aptian syn-rift limestones deposed in the Organyà basin, the overlying the Cenomanian-Coniacian limestones that represent a carbonate platform, and the Santonian-Maastrichianclastic wedge corresponding to the syn-folding sedimentation.The Triassic-Jurassic is marked by a rift, with appearance of normal faults and creation of a pull-apart basin. The formation of the Organyà basin continued during the Early Cretaceous. During the Cenomanian-Coniacian, a carbonate plateform developed indicating a period of tectonic quiescence. During the Alpine compression normal faults of which the Organyà Fault were inverted in the Santonian-Maastrichian times (fig.4). The consequence of this inversion is the creation to San Corneli anticline. On this anticline, numerousfractures and normal faults have been described. There are 5 fracture sets, 2 of which are related to foldingand 3 related to normal faults in the northern limbs; these were probably reactivated during the folding event(Kirkwood et al., 2003).
2000m1000m
0m-1000m-2000m-3000m-4000m
2000m1000m0m
-1000m-2000m-3000m-4000m
Late Eocene
Early EocenePaleocene
Late Cretaceous
Early Creataceous
Jurassic
Triassic
Paleozoic
S N
Central South Pyrenean Zone Axial Zone
Boixols thrust sheetMontsec thrust sheetSierras Marginales thrust sheet
Ebro Basin
Fault
5000 m
Burgos
Saragosse
Pau
Tarbes
Toulouse
Perpignan
Llieda
Pampelune
Tremp
Girone
Barcelonne
Tarragon
Santender
Jaca
Huesca
BilbaoSaint Sebastien
CarcassonneNarbonne
Andorre
Ripoll
Ebro Basin
Aquitanian Basin
Duero Basin
Axial Zone
CSPZ
NPZ
Massif CentralBiscay
Gulf
Atlantic ocean
Mediterranean
Sea
QuaternaryEocene-Miocene
Eocene-Oligocene
Late Cretaceous
Paleocene
Early Cretaceous
Palsozoic
normal fault
thrust0 50 100
N
4 3 2 1 0 1 2 344
43
42
41
Fig. 1: Geological map of the Pyrenees
Fig. 2: ECORS seismic profile across the Pyrenees
Fig. 3: Geological map of San Corneli anticline (modified from J. Mencos)
3D Geological model
The Pyrenees
The San Corneli Anticline
Fig. 4: Palinspatic evolution map of the Pyrenees (modified from Mattauer)
3D restoration
Problem statementThe main objective of the project is to determine the influence of the early structures, like normal faults and fractures, on the final geometry of the San Corneli fault propagation fold. Indeed, whena basin is included in an imbricate fault zone, the structures inside this basin can replay. It ispossible to understand that faults and fractures formed in this basin would have influence the final fold geometry.Short term objectives are to:
# Restore the folded sedimentary horizons to their initial sedimentation configuration.# Determine the initial orientation of the early structures.# Use the forward modeling technique, in order to understand the folding mechanism
responsible for the development of the San Corneli fold.# Determine which structures were reactivated during inversion of the rift basin due to the
Alpine compression.# Evaluate the influence of these early structures on the development of the fold.
Fig. 5: 3D model of San Corneli anticline
N
Top of the Jurassic
Top of the Aptian-Albian
Top of the Cenomanian
Normals Faults
Boixols Faults
1 km
Fig. 6 : 3D restoration of San Corneli anticline, with and without normal faults
Figures 6 and 8 show the first results of our study. With gOcad modeling software and the 3D restoration plug-in (J. Massot, 2003 and P. Muron, 2005), we have unfolded 2 surfaces in our model. These surfaces correspond to the major discontinuities of the Boixol thrust sheet. The first surface, in green, is the interface between the syn-rift deposits and the platform deposits (top of the Aptian Albian). The second surface, in blue, is the interface between the platform deposits and the syn-folding deposits (top of the Cenomanian). Each surface is constructed twice: once with the early structures and the other one without these normal faults. The goal of the restoration is to appreciate the variation of the geometry within the model.With the 3D restored model, we can re-build the normal fault surfaces (fig. 9). We observe in this figure a little rotation of the faults (10° clockwise).The next step of our project is to restore the top of the Jurassic to have the initial rift basin geometry. With the 3D forward modeling, we are studying many hypothesis for the formation of the fold and for the early structures influences.
R_Vector: 3D restoration vector
Fig. 7: Restoration by 3D r_vector
Fig. 8: Restored platform without normal faults and with normal faults
Fig. 9: Reconstruction of the different faults showing the initial geometry of the Organya basin
With faults Without faults
Top of the Aptian-Albian
With faults Without faults
Top of the Cenomanien
References
•Casas A.M. Oliva B., Romàn-Berdiel T., Pueyo E., 2003, Basement deformation: tertiary folding and fracturing of the Variscan Bielsa granite (axial zone, central Pyrenees), Geodinamica acta 16, p. 99-117.•Capote R., Muños J.A., Simon J.L., Liesa C.L. et Arlegui L.E., 2003, Alpine tectonics I: the Alpine system north of the Betic Cordillera, p. 367-386.•Kirkwood, D., Muñoz, J., J. Bausà, J. Mencos, Berastegui, X. and Arbuès, P. 2003. Detailed fracture studies of a fault-propagation fold in the Spanish Pyrenees fold-and-thrust belt. SpecialSession on Field-Based Geological Models: Spanish Outcrops as Reservoir Geology Analogues, AAPG International conference and exhibition, Barcelona, Spain, September 21-24.•Muñoz, Martinez et Vergés, 1986, Thrust sequences in the eastern Spanish Pyrénées Journal of Structural Geology, 8, p. 399-405•Soto R., Casas A.M., Storti F. et Faccenna C., 2002, Role of lateral thickness variations on the development of oblique structures at the Western end of the South Pyrenean central unit Tectonophysics 350, p. 215-235.•Vergés J., Muñoz J. A. 1990, Thrust sequences in the southern central Pyrenees, Bull. Soc. Géol. Fr. 8 pp: 265-271.•Sussman A.J., Butler R.F., Dinarès-Turell J. et Vergès J, 2004, Vertical-axis rotation of a foreland fold and implications for orogenic curvature: an example from the Southern Pyrenees, Spain, Earth and planetary sciences letter 218, p. 435-449.
