Timing and modern character of the western Puerto Rico fault system using carbonate stratigraphy and fluvial terrace chronology for the Río Culebrinas fault valley

Abstract

The Western Puerto Rico Fault System has been the subject of previous geomorphological, geophysical, structural, and paleoseismic studies. However, its modern kinematic style and origin are not well understood. To better understand the modern kinematic style of the initially normal-style fault system samples from fluvial terraces along the Río Culebrinas fault valley were dated using Optically Stimulated Luminescence (OSL). Incision rates derived from the ages of these samples were compared to regional uplift rates to discern if any downward vertical motion is occurring within Río Culebrinas fault valley. The resulting incision rates indicate that downward vertical motion on the fault system, if any, is not detectable within the effects of Pleistocene to Holocene climate cycling. To better understand the origin of the system, fossil Kuphus incrassatus specimens from limestones interpreted to be coeval with initial fault activity were dated via 87Sr/86Sr. The resulting ages suggest initiation of the system began during the Mid Miocene around 15 Ma. This data also highlights a local hiatus in carbonate deposition between 23 and 16 Ma which has also been described by Ortega-Ariza et al. (2015) in Puerto Rico and Hispaniola. This depositional hiatus does not appear in the traditional stratigraphic interpretations of the island emphasizing the need to revise the chronostratigraphy of Oligocene - Miocene limestones in the region. This study dates the onset of regional-scale fault activity in Western Puerto Rico to the Mid Miocene and demonstrates that this period was not a time of tectonic quiescence in the region, as has been traditionally modeled. The presence of seismic activity along the system and the perseverance of the topographic signature of this system in tropical terrain suggest that vertical fault motion was not limited to the Mid Miocene, but that it may have continued to the near present. OSL analyses reveal that at least in the past 65,000 years, there is no detectable fluvial geomorphic signature of dominant dip-slip movement. This suggests that the faults may currently be accommodating dominantly strike-slip motion, similar to the large plate boundary to the north of the island. As potential strike-slip vectors, they pose more of a shaking risk to citizens than if they remained active as normal faults. The timing of the cessation of dip-slip movement is unknown but speculatively may be related to a shift from subsidence to uplift that occurred around 5 Ma. This relatively recent kinematic conversion could account for the lingering half-graben topography.