Relationship between curved thrust belts, rift inversion, oblique convergence and strike-slip faulting : an example of eastern cordillera in Colombia

Abstract

The northern Andes mountain belt in Colombia is a complex tectonic region close to the triple junction of the continental plate of South America with the Caribbean and the Nazca oceanic plates. In this complex region, the South Caribbean tectonic evolution reflects the interaction between the Caribbean plate and the Northwestern border of South America since Late Cretaceous times (Cortes et al, 2005). The Mesozoic tectonic evolution of the NW Andes has been related to the accretion of several tectonostratigraphic terranes of both oceanic and continental origin (Barrero, D; 1979, Etayo, F; 1983, Toussaint, F y Restrepo, J; 1989). The Eastern Cordillera (EC) is a Mesozoic rift inverted since Miocene; as well as the EC is a regional curved thrust belt with regional change in the strike of local structures (faults and folds). By considering the regional strike of EC, this thrust belt can be defined by three domains. (1) In the southern zone the NS strike domain, (2) the central zone the NE strike domain and (3) to the north the NW strike domain. Those changes in strike could be related to rotations of blocks in the younger phases of shortening or to accommodation of the deformation in the shortening phases, controlled by faults in the Jurassic and early Cretaceous basement, likely related to the rift phase. At least two hypotheses can explain the curved shape of the EC: (1) orocline hypothesis or (2) nonrotational chain with rift structures controlling the subsurface to surface geometry. In order to gain insights in some of these problems, a detail structural, paleomagnetism, anisotropy of magnetic susceptibility (AMS) and analog modeling study will be carried out. Paleomagnetism and AMS technics were focus on the EC, Bucaramanga Fan, Santa Marta Massif. Analog modeling technic was used to explore the tectonic inversion into the brittle and ductile regimens under oblique convergence. The paleomagnetism is the main technique allowing a quantification of rotation and translation of blocks. Ferromagnetic minerals record the ancient direction of the earth’s magnetic field. The anisotropy of magnetic susceptibility (AMS) is considered an important proxy for the strain characterizing a rock. Sediments have a magnetic foliation sub-parallel to bedding. Deformation phases modify primary fabrics developing magnetic lineation, and this lineation can be related with strain ellipsoids and major tectonic structures. AMS is very sensitive to the pristine tectonic phases occurring early after diagenesis. Therefore the AMS study of sediments of different age may provide the evidence for different tectonic episodes developing synchronous with the geologic history of an orogen. A model - either analog or numerical - is a simplified scaled representation of nature, though on a more convenient geometric and temporal scale (smaller and faster). The purpose of models is not simply to reproduce natural observation, but primary to test by controlled experiments hypotheses as to the driving mechanisms of tectonic processes. The main advantage of analogue modeling lies in the fact that a correctly constructed model passes through an evolution, which is the physical response of the system to the applied experimental conditions. These advantages of laboratory models make this methodology a valid tool for the advancement of our understanding of structural and geodynamic processes. This work was focus in four topics: the spatial evolution of the EC, the Bucaramanga Fault, Santa Marta Fault and rift inversion. Each topic is presented in the next chapters. Chapter 2 shows the paleomagnetic and AMS results of Cretaceous to Cenozoic units of the EC and Cucuta zone. 23 sites reveal no rotation of the EC range with respect to stable South America. Magnetic lineations from AMS analysis do not trend parallel to the chain, but are oblique to the main orogenic trend. By also considering GPS evidence of a ~1 cm/yr ENE displacement of central-western Colombia accommodated by the EC, we suggest that the Miocene-recent deformation event of this belt arises from ENE oblique convergence reactivating a NNE rift zone. Oblique shortening was likely partitioned in pure dip-slip shear characterizing thick-skinned frontal thrust sheets. Finally, the 35°±9° clockwise rotation observed in four post-Miocene magnetically overprinted sites from the Cucuta zone reflects late Cenozoic and ongoing right-lateral strike-slip displacement occurring along buried faults parallel to the Boconó fault system. Chapter 3 shows a paleomagnetic investigation of a continental alluvial, Bucaramanga fan juxtaposed to the Bucaramanga Fault, and horizontally displaced by 2.5 km with respect to its feeding river. Nine reliable paleomagnetic directions define a succession of six different magnetic polarity zones that, lacking additional age constraints, can be correlated with several tracks of the Plio-Pleistocene magnetic polarity time scale. The Bucaramanga Fan can be reasonably dated at 0.8 Ma (Brunhes-Matuyama chron transition), translating into a maximum 3 mm/yr slip rate for the Bucaramanga Fault. Older age models would obviously yield smaller slip rates. Paleomagnetic sites, located at 4-10 km from the fault, do not show significant rotations, implying weak fault coupling and/or ductile upper crust behavior adjacent to the Bucaramanga Fault. Chapter 4 shows structural and AMS results of Cretaceous metamorphic and Paleogene intrusive rocks. Foliations, lineations, faults and AMS reveal ductile and brittle regimens. Foliations and lineations in Metamorphic and granitic rocks shows ductile deformation related to NE extension during the Paleogene. Brittle deformation is related to regional wrench faults as the Santa Marta and Oca faults. AMS and faults kinematic solutions show a NW-SE (aprox) compression vector. Finally, chapter 5 shows the results for experiment in sand box for brittle and ductile-brittle regimens. Rift inversion mechanism was evaluated in 3D for brittle and ductile regimen in oblique convergence for 0º, 15º, 30º and 45º. The brittle experiments show a rift inversion located in the principal normal fault plane. During the 45º experiment, rift inversion reveals strike-slip component. On the other hand the ductile-brittle experiments reveal a positive flower for all experiments, as well as, in 30º and 45º convergence develop a strike-slip component. Overall the results of the four chapters highlight the importance of the basin geometry, structures related and dextral strike-slip component to explain the non-rotational curved thrust belt. Deformation related to the strike-slip Bucaramanga Fault occurs with low slip rate during the last 0.8 ma. The Cretaceous metamorphic rocks in the Santa Marta Massif, records NE-SW extensional structures related to Paleogene intrusions. The strike-slip Santa Marta Fault deforms the intrusive rocks in brittle regimens with a normal component. Finally ductile layers in normal to oblique convergence develop a positive flower during rift inversion.