Adames-Corraliza, Ángel R.

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    Geomorphic and geophysical characterization of the north Boquerón Bay-Punta Montalva fault zone: A capable fault system in southwestern Puerto Rico
    (2017) Adames-Corraliza, Ángel R.; López-Venegas, Alberto M.; College of Arts and Sciences - Sciences; Santos, Hernán; Martínez, José; Department of Geology; Cancelos, Silvina
    Mapping of near surface deformation of landforms and geologic deposits were characterized and completed on LiDAR and geophysical data along the North Boquerón Bay-Punta Montalva Fault Zone (NBB-PMFZ). A sandy shutter ridge alluvial valley was selected as the site for the geophysical exploration program based upon the geomorphic mapping results. Two geophysical data sets were acquired at the shutter ridge site, a high resolution seismic reflection (SR) profile, a quasi 3-D ground penetrating radar (GPR) survey. Four boreholes spaced along the GPR survey provide groundtruth (lithological) data. Both geophysical data sets imaged the NBB-PMFZ. Radar velocities were determined at the site to be 0.17 m/ns. A 200 MHz GPR antenna provided vertical resolution in the order of 0.10-m. Boreholes along the 200 MHz dataset confirmed the thickness of the surficial deposits at about 0.31-m subsurface depth. On the Quasi 3-D ground penetrating radar survey the shallow stratigraphy is characterized as a series of two alternating positive-negative polarity semi-continuous reflectors. These reflectors are constrained with the borehole data and described as: (a) positive-polarity deformed TOP SOIL LAYER (TSL), (b) a negative-polarity, also deformed and faulted, HIGHLY-WEATHERED LIMESTONE LAYER (HWL), (c) a positive-polarity WEATHERED LIMESTONE LAYER (WLL), and (d) a highly attenuated negative-polarity “SHADOW-ZONE (SZ)” that possibly represents a leached clayey zone or an increased moisture zone. The base of the top soil layer (BTSL) was tracked and mapped. BTSL is deformed and appears displaced at several locations. Structural relations suggests a curvilinear series of tilted and rotated blocks along a narrow zone of deformation that appears to form an apparent ~0.9-m graben. On the GPR data, and at depths greater than 4-m below ground surface, high amplitude-low frequency reflectors are interpreted as weathered Miocene Ponce Limestone. The limestone is block tilted and faulted as a positive flower structure. The block tilted and faulted zone is about 35-m in width. The deformed BTSL structure was interpreted as the NBB-PMFZ. Strata thickening between faulted blocks, was observed on the GPR data suggesting sync-depositional deformation which is also suggested by diagenesis in the thin section analysis of the limestone rock sample. Faulting pattern observed in the GPR data suggest that faulting commenced in Miocene time (Ponce Limestone syn-deposition) and has reactivated in recent times, deforming the base of the top soil layer.From Punta Montalva to Ensenada las Pardas, the total surface rupture length of the NBB-PMFZ, is approximately 6-km and at a shutter ridge alluvial valley (AV #1) is approximately 35-m wide. Evaluation of Wells and Coppermith (1994) empirical regressions between earthquake magnitude and fault Surface Rupture Length provides an estimate of (Mw= 6.03), as the maximum earthquake magnitude capable to sustain NBB-PMFZ. From Punta Montalva to Ensenada las Pardas, the total surface rupture length of the NBBPMFZ, is approximately 6-km and at a shutter ridge alluvial valley (AV #1) is approximately 35-m wide. Evaluation ofWells and Coppermith (1994) empirical regressions between earthquake magnitude and fault Surface Rupture Length provides an estimate of (Mw= 6.03), as the maximum earthquake magnitude capable to sustain NBB-PMFZ. Permission was not granted to trench the site. Therefore, no samples were obtained for radiocarbon dating. But, based on cross-cutting and stratigraphic relationships, there have been at least 3 events that deformed the base of the top-soil unit as observed on the GPR_200 MHz dataset. If this unit is assumed to be Holocene in age, it implies a recurrence rate of about 3,300 years. GPS data (Jansma and Matiolli, 2010) suggests a compressional component of deformation at a rate of about 1.6 ± 0.3 mm/yr between GPS Stations PUR3 and PARG. Geomorphic features mapped on aspect and hillshade maps suggest a left-lateral strike slip fault that appears to bend to the southeast at the shutter ridge alluvial valley forming a complex zone of deformation that is imaged on the geophysical data set, as a positive flower structure. This complex zone of deformation is ~35-m in wide, and is interpreted as a constraining bend of the NBB-PMFZ. This compressional zone of deformation is consistent with Roig et al., (2013) focal mechanism solutions that suggest strain partitioning with predominantly east–west left-lateral displacements and a component of compression.