The Alisitos arc segment forms part of the western zone of Jura-Cretaceous Peninsular Ranges batholith of Baja California. It extends south from the ancestral Agua Blanca Fault to the state boundary between Baja Norte and Sur. The study area is located within a fold and thrust belt intruded by a number of plutons that were emplaced during and after the deformational event. The northern end of the Alisitos arc is characterized by subvertical tight to isoclinal folds and high-angle reverse faults that define a northwest trending, southwest vergent fold and thrust belt. The aABF defines the northern limit to the Alisitos arc segment. In this study we present the results of a petrographic study of igneous rocks in order to determine the relative timing and the distribution of deformation within the northern Alisitos arc segment. The study includes samples of the mylonitic shear zone of the aABF, and plutonic samples from intrusions proximal to the aABF emplaced later during regional deformation. These samples were investigated in order to characterize the distribution of the subsolidus strain in grain scale and the sense of shear during later phases of deformation in the northern Alisitos arc. The results are presented and discussed based on the mineralogical and textural observations from the Balbuena pluton, the Piedra Rodada pluton, and volcaniclastics that were deformed within the aABF. The Balbuena pluton, emplaced at ~ 108 Ma after the surrounding country rocks had already been folded, exhibits little if any evidence for subsolidus deformation. In contrast, the Piedra Rodada pluton, emplaced at ~ 105 Ma just to the southwest of the aABF, exhibits a strong magmatic fabric overprinted by a moderate subsolidus fabric to the southwest that grades into a strong subsolidus fabric with proximity to the fault. Kinematics observed from lineation parallel-foliation normal sections exhibit consistent top-to-the southwest sense of shear.

A ocorrência de uma grande reorganização de placas durante o Neógeno reflete a comlpexa interação entre as placas do Caribe, Farallon, e América do Sul, e o arco intra-oceânico de Panamá. Isto, tem influenciado o registro tectono-estratigráfico dos Andes nor-ocidentais da Colômbia, o qual tem sido escassamente estudado perto da sutura entre os domínios de Panamá o da América do Sul. Novos dados de petrografia de arenitos, análises de minerais densos, geocronologia detrítica em zircão, geoquímica de rocha total, e modelamento termal inverso de múltiplos termocronômetros, realizados em um transepto de 50 km com direção SE-NW, permitem reconstruir a história de deformação e exumação/erosão das rochas do Cretáceo Superior ao Plioceno do flanco ocidental da Cordilheira Ocidental. O início da exumação das rochas do Cretáceo Superior associadas com a margem continental Sul-americana, ocorreu entre 45 e 20 Ma, o qual é sugerido pelo resfriamento inicial desde temperaturas dentro da zona de retenção parcial do hélio em zircão (~180-200°C). Essas rochas, também sofreram um pulso de exumação considerável durante o Mioceno Médio, o qual caracteriza-se por taxas moderadamente rápidas entre 0.3 km/my e 0.7 km/my. Adicionalmente, um período de resfriamento rápido também é documentado com taxas máximas de exumação ~1.3 km/my. As rochas do Eoceno Médio do arco oceânico de Panamá, as quais estão separadas das rochas do Cretáceo Superior da Cordilheira Ocidental por uma faixa de deformação, registram um resfriamento inicial em ~15 Ma desde a zona de retenção parcial de hélio em apatita (APRZ, ~80-60°C, com taxas perto de 0.6 km/my. Termocronologia detrítica em secuências pós-colisionais e sedimentos modernos, também registram estes períodos de exumação. Os novos resultados podem ser relacionados com processos tectônicos de escala regional, que incluem: Convergência rápida e frontal no Oligoceno Superior, entre a recentemente formada placa de Nazca e a placa continental Sul-americana, depois da fragmentação da antiga placa de Farallon, a colisão do Arco de Panamá do Cretáceo Superior-Eoceneo durante o Mioceno Médio, o qual resultou exumação geral ao longo do segmento NW da placa Sul-americana, é o início da subducção da placa de Nazca é a instalação de um segmento plano ao norte durante o Mioceno Superior-Plioceno. Este é provavelmente o mecanismo responsável pelo soerguimento da superfície recente na Colômbia nor-oriental e a contínua deformação-soerguimento das serras costeiras ocidentais e das bacias orientais de foreland ao norte de 5°N. / The occurrence of major plate reorganization during the Neogene, reflects the complex interactions between the Caribbean, Farallon and South-American plates, and the Panama Arc. These, have influenceed the tectono-stratigraphic record of the northwestern Colombian Andes, which has been scarcely studied neart the suture between the Panama and South-American domains. New sandstone petrography, heavy minerals analyses, U-Pb detrital zircon geochronology, whole-rock geochemical data, and thermal inverse modeling of multiple thermochronometers, carried along a 50 km southeast-northwest transect, allow to reconstruct the deformation and exhumation/erosional history of the Late Cretaceous to Pliocene rocks from the western flank of the Western Cordillera. The onset of exhumation of Late Cretaceous sedimentary rocks, associated with the South-American margin, occurred between 45-20 Ma, as suggested by the initial cooling from temperatures within the zircon helium partial retention zone (ZPRZ, ~180-200°C). These rocks also experienced a substantial pulse of exhumation during the Middle Miocene, which is characterized by moderately rapid rates between ~0.3 km/my and 0.7 km/my. Moreover, a Pliocene period of rapid cooling is also documented with maximum exhumation rates of ~1.3 km/my. The Middle Eocene rocks from the intra-oceanic Panama Arc, which are separated by a highly-deformed zone from the Late Cretaceous rocks of the Western Cordillera, record an initial cooling at ca. 15 Ma from the apatite partial retention zone (APRZ, ~80-60°C) at rates around 0.6 km/my. Detrital thermochronology of post-collisional sequences and modern river sediments also record these exhumation events. The we results can be related to more regional-scale tectonic processes including: the Late Oligocene rapid and frontal convergence between the newly formed Nazca plate and the continental South-American plate following the fragmentation of the former Farallon plate, the Middle Miocene collision of the Cretaceous-Eocene Panama Arc that triggered generalized exhumation-deformation widespread in the upper NW South-American plate, and the Middle to Late Miocene initiation of the Nazca subduction and installation of a Late Miocene-Pliocene flat-slab to the north. The latter is likely to represent the mechanism that triggered surface uplift in eastern Colombia and ongoing uplift-deformation of western coastal ranges and eastern foreland basins north of 5°N