TSK 11 Göttingen 2006 Mora et al. The role of penetrative defor- mation in orogenic processes. An example from the Eastern Cordillera of Colombia Vortrag Andrés Mora1 Mauricio Parra1 Man- fred Strecker1 The Lower Cretaceous units of the East- ern Cordillera of Colombia have been deposited during a stage when the area where nowadays lies a contractional orogen was an actively deforming rift. Therefore, they are syn-rift sedimen- tary units. These rocks have a unique feature in the Cretaceous sedimentary column of this mountain chain. This is a widespread planar fabric expressed mostly as a penetrative slaty cleavage. This planar fabric is only macroscopi- cally evident in Cretaceous units older than Barremian. Planar fabric (e.g. slaty cleavage) is one of the products of internal or penetrative deformation (i.e. contractional deformation at mi- croscopical scale in rock units). Pres- sure solution and even cleavage have been recently reported in areas under- going only subtle burials (Engelder & Marschak 1985). However, in the East- ern Cordillera, the units where pla- nar fabric is evident are the base of a sequence of at least 5 km and they have vitrinite reflectance values up to 2. These data and the formation of clorithoid contemporary with the slaty cleavage, allow us to propose that this planar fabric was developed when the Lower Cretaceous units had an amount of overburden close to the thickness of the entire Cretaceous sequence. We carried out finite strain measure- 1 Institut für Geowissenschaften, Universität Potsdam, 14415 Potsdam, Germany ments using the enhanced normalized Fry method (Erslev 1988) in order to quantify the shortening by internal de- formation in various localities in the Eastern Cordillera. For this purpose we used thin sections of sandstones con- taining macroscopically evident planar fabrics as recommended by Dittmar et al. (1994). In some cases we de- duced 3D-strain ellipsoids from individ- ual Fry-method measurements following von Winterfeld & Oncken (1995). In ad- dition we took strike and dip measure- ments of all the observed structural fea- tures observed in the outcrop to carry out a structural and orientation analy- sis mostly in the same localities where fi- nite deformation was measured. Finally we also produced stratigraphic profiles in some of the locations in order to check relationships between lithology and de- formation features. First finite strain measurements allowed us to quantify an additional amount of shortening by penetrative deforma- tion (i.e. contractional deformation at the microscopical scale that mostly pro- duces slaty cleavage) in the Eastern Cordillera that is locally near 20%. Shortening by internal deformation is localized in the steeply dipping limbs of anticlines, the hinge zones of some synclines and the contractionally re- activated boundaries of former nor- mal faults. As the observed planar fabric (e.g. slaty cleavage) is mostly continuous throughout different litholo- gies rather than discontinuous cleavage within spaced deformation zones; we assumed that shortening by penetra- tive deformation is not discrete. Thus, we propose that it represents a ho- mogeneous flattening of the outcrop- ping units in those localities where it is present. We calculated that shortening 1 Mora et al. TSK 11 Göttingen 2006 by folding in the Eastern Cordillera is close to 20%. This means that short- ening by penetrative deformation com- pared to shortening by folding is not mi- nor. Orientation analysis of fractures and SiO2-filled or CaCO3-filled veins was done when these features were spa- tially associated with the planar fabric. With the collected data we propose that planar fabric and fractures have been formed mostly at the early stages of folding under a dominant WNW–ESE to E–W compressional stress field. For instance, fractures present in tilted beds only give a consistent pattern of com- pressional stress if they are restored to a prefolding position. In such cases the obtained direction is coincident with the direction of maximum finite deforma- tion (This direction is assumed to be roughly perpendicular to the plane of slaty cleavage when finite strain mea- surements are not available at a given location). After restoring the frac- tures to a prefolding position their dip is vertical and perpendicular to bed- ding. In the folded state fracture ori- entation strongly depends on their posi- tion within the folded structures. How- ever fracture intensity appears to de- pend both on structural position and lithology. In addition, we found the presence of similar sets of fractures with similar ori- entations in different Lower-Cretaceous lithologies along the stratigraphic sec- tions. However, fractures found in ter- rigenous pellites always lack any kind of filling. In contrast when found in terrigenous sandstones they have SiO2 filling and in carbonates they are filled with CaCO3. When evaporites are the host rocks of mineralization the veins have the paragenesis of albite, pyrite, calcite and emeralds. This paragenesis is a guide for emerald exploration and therefore represents the only mineraliza- tion of economic interest. In such evap- oritic facies, fracture density reaches a maximum. Thus, evidence of miner- alization strongly dependent on lithol- ogy, allow us to we hypothesize about a closed mineralization system, at least partially. However, by the field evi- dence the possibility of the influence of fluids coming from outside the forma- tions cannot be discarded completely. In addition, in some of the analyzed localities we have found specific types of deformation, only observed when the sequence is composed of interbedded medium to thick layers of terrigenous mudstones with marls: A close associa- tion of slaty cleavage restricted to cal- careous beds with CaCO3-filled veins restricted to adjacent terrigenous mud- stones. In such localities both features have been formed under the same stress field. The exposed relationships allow us to propose the following situation. Fluids ascending from terrigenous mud- stones to adjacent marls. They ascend due to them being expelled because of a reduction in porosity caused by pres- sure solution processes. The exposed mineralization scenario would be then a highly autochthonous one. The paragenesis of the veins associ- ated with evaporites provides the entire Colombian emerald production, which in fact is one of the largest in the world. In such context those veins in the emerald-bearing areas have been dated (Cheilletz et al. 1996) as old as 37 ± 0.1My in the western flank of the Eastern Cordillera and 65 ± 3My in its eastern flank. We propose that the mentioned ages are giving a proxy of the age of the kinematic indicators 2 TSK 11 Göttingen 2006 Mora et al. deduced from fracture and vein orien- tations. Fluid inclusion studies demon- strate that these mineralizations oc- curred at more than 250°C (Cheilletz et al 1996). Thus fluid inclusions and vit- rinite reflectance data illustrate that at this time exhumation of the Cretaceous rocks was minor or absent. Mineralization is also present as hy- drothermal breccia frequently localized in the hinge zones of folds. In some cases those areas are also the location of intenser finite deformation values. Therefore we propose that mineraliza- tion associated with Lower Cretaceous units were formed in presence of high fluid pressure during folding and inter- nal deformation under epithermal con- ditions. With all the collected evidence it appears that penetrative deformation (Slaty cleavage) is instrumental for min- eralization. Therefore, our contribution shows pieces of evidence supporting that the role of penetrative deformation has been underestimated in the Eastern Cordillera. Penetrative deformation does not only account for important additional crustal shortening in this orogen, but also plays a role in the formation of prolific economic mineral deposits. References Cheilletz A & Giuliani G (1996) The genesis of Colombian emeralds: a restatement. 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