John Weber, Ph.D.
Pablo Llerandi-Roman, Ph.D.
Vertical tectonic motions were studied in the Caribbean using data from continuously operating Global Positioning System stations (cGPS). The island of Puerto Rico (PR) is located in the plate boundary zone between the Caribbean and North American plates. Today, this zone is dominated by east-to-west strike-slip motion with additional minor convergence, which created the PR trench north of the island. Uri ten Brink (2005) developed a series of models that show possible sub-surface trench geometries and possible causes of vertical motion in PR. Differences in long-term (1955- 2012) tide-gauge sea-level rise rates from PR suggested that differential vertical tectonic motion might be resolvable. Our preliminary results from seven cGPS sites in PR show that the northernmost sites with sufficiently long time series (2008- 2013) may be sinking at rates (2-sigma uncertainties) as follows: MOPR (-1.33 mm/yr ±2.76), MAYZ (-1.47 mm/yr ±2.77), AOPR (-5.33 mm/yr ±2.42), BYSP (-1.26 mm/yr ±1.73) and CUPR (-2.52 mm/yr ±1.76). This contrasts with the vertical motions of the suite of southern cGPS sites, which appear to be more vertically static: P780 (-0.5 mm/yr ±1.94) and MIPR (-0.6 mm/yr ±1.70).
Trinidad is clearly tilting to the west into the Gulf of Paria’s pull-apart basin based on macroscopic geomorphic features (Ritter & Weber, 2007), and sits in nearly a mirror image plate tectonic setting to PR; Trinidad is located in the southeast corner of the Caribbean plate, in the Caribbean-South American plate boundary zone. In addition, causes of vertical motion are better understood in Trinidad than they are in PR (Weber et al., 2011). Therefore, we also determined preliminary rates of vertical tectonic motion for Trinidad using a similar approach, and we used this island as an analogue to better understand vertical motions in PR. We analyzed data from five Trinidadian cGPS sites (ALBI, CALD, FORT, GALE, GRAN), and from one episodic GPS site (POST) located on the sinking northwest coast. Our preliminary vertical GPS rates (2-sigma uncertainties) are as follows: ALBI (-2.20 mm/yr ±1.52), CALD (-0.97 mm/yr ±1.36), FORT (0.49 mm/ yr ±1.67), GALE (-0.90 mm/yr ±1.78), GRAN (-1.36 mm/yr ±1.62), and POST (-4.21 mm/yr ±1.96). Results from the northwestern Trinidadian sites (ALBI, POST) are consistent with subsidence. FORT on the southwestern coast has apparently moved down and then up, perhaps due to volcano mud inflation/ deflation. The other central and eastern Trinidad sites (CALD, GALE, GRAN) appear to be more vertically static. Our study now aims to define local vertical reference frames in PR and Trinidad and to use these to quantitatively solve for the differential vertical tectonic motions on each island. This approach should reduce formal uncertainties and bring the qualitative differential signals described above out of the noise.
*This scholar and faculty mentor have requested that only an abstract be published.