The Ecuador transform-fracture system and its role in the evolution of the Costa Rica and Ecuador spreading systems

Abstract

Fracture zones are intraplate continuations of transform faults that, according to plate tectonic theory, are tectonically inactive. These systems, and their relationship with spreading centres, are poorly understood. The Ecuador Fracture Zone (EFZ) is an example of a complex tri-strike-slip fault system. In this study, multiple geophysical datasets were used to model and investigate the structure of the EFZ, using ODP borehole 540B as a geological ground-truth. Previous modelling along the Costa Rica Rift was also used to inform modelling in this study. A 25-30 km-wide region of low density and ∼3 km thick crust is modelled beneath the EFZ. This anomalous structure contrasts with the surrounding crust, which exhibits characteristics of ‘normal’ oceanic crust extending right to the edge of the transform-fracture zone, suggesting a magma supply to the Ecuador Rift ridge tip. The anomalous crustal structure is thought to represent highly fractured basaltic and gabbroic crust, while the anomalous abrupt thinning is proposed to be due to a transtensional stress regime, resulting from past plate readjustment of the Panama Basin. In addition, the thermal edge effect caused by the ∼4-5 Myr age contrast across the ridge-transform boundary is also thought to contribute. An anomalously low density block is also observed in the upper mantle, consistent with the presence of a 40-50% serpentinised mantle peridotite region. This is suggested to be caused by the ingress of seawater through a highly fractured crust, and its flow along low-angle, basement-cutting normal faults to the mantle. This is thought to be the primary control on the observed vertical tectonism, in addition to thermal conduction across the fracture zone. However, the transverse ridge is thought to be a flexural response of the lithosphere to normal faulting, caused by the aforementioned changes in spreading direction. However, the transverse ridge located adjacent to the EFZ, is thought to be a flexural response of the lithosphere to normal faulting, as a consequence of changes in the spreading direction. This study also finds evidence of potential recent uplift at the EFZ, opposing the idea of fracture zones being tectonically inactive locked systems. Supported by heat flow measurements taken along the EFZ, residual serpentinisation by the continued hydrothermal circulation of seawater is proposed to potentially control this present activity.