THE ROMERAL FAULTS, COLOMBIA: ANATOMY AND KINEMATICS OF A LATE MESOZOIC SUTURE

The Romeral faults define a complex deformation zone in the hanging wall of a Late Mesozoic suture, which separates an accreted, anomalously thick oceanic crust from a Proterozoic (?) Northandean basement. Their significance is brought about by a narrow flexure of the western oceanic plate, an exhumed subduction channel made up of crustal slivers embedded in ultramafic rocks and a crustal flake with a steeply E-dipping Cretaceous cover. This structural setting attests to a progressive tectonic erosion of the continental margin, a consequent subduction of the resulting crustal slivers to intermediate depths, followed by their return to present exposure levels. A regional upper Eocene unconformity marks the final docking of the oceanic crust.

Paleo-stress directions deduced from striated faults point to a WSW-ENE convergence, which was highly oblique to the Mesozoic plate margin south of 5oN, but nearly perpendicular north of this same latitude, where the plate margin defined a large promontory. In the southern segment a corresponding strain partitioning is readily recognized by a dextral reactivation of NNE striking Late Paleozoic lineaments of the Andean basement. In the northern segment a unique dextral displacement of some 25 km accrued within a short distance on the likewise NNE trending Palestina fault near the southeastern corner of the giant Antioquia Batholith, suggesting a significant a crustal weakening by this igneous body.

By its geometry the promontory of the northern segment acted as a buttress for strike-slip movements, ceding as a wholesale block bounded on its western side by the Palestina fault. Internally this block is compartmentalized by NW-striking normal faults, which guided sheet-like intrusions of the now composite Antioquia Batholith. The WSW-ENE convergence was superseded in the Oligocene by a NW-SE convergence, which by its high angle to the Late Paleozoic lineaments put an end to their strike-slip reactivation.