Glen S. Mattioli

GPS geodetic constraints on Caribbean-North America plate motion

We describe a model for Caribbean plate motion based on GPS velocities of four sites in the plate interior and two azimuths of the Swan Islands transform fault. The data are well fit by a single angular velocity, with average misfits approximately equal to the 1.5–3.0 mm yr−1 velocity uncertainties. The new model predicts Caribbean-North America motion ∼65% faster than predicted by NUVEL-1A, averaging 18–20±3 mm yr−1 (2σ) at various locations along the plate boundary. The data are best fit by a rotation pole that predicts obliquely convergent motion along the plate boundary east of Cuba, but are fit poorly by a suite of previously published models that predict strike-slip motion in this region. The data suggest an approximate upper bound of 4–6 mm yr−1 for internal deformation of the Caribbean plate, although rigorous estimates await more precise and additional velocities from sites in the plate interior.

Oblique collision in the northeastern Caribbean from GPS measurements and geological observations

Abstract Previous studies along the Andean subduction zones of South America have shown that forearc basins can develop over shallow-dipping the subduction zone dips horizontally or up to 15°, and that these shallow-dipping subduction zones can alternate with more steeply dipping (>30°) subduction zones over distances of 400–1500 km (249–932 mi). This study describes the Cenozoic structural and depositional history of the Lower Magdalena Basin (LMB)—an Oligocene to Recent forearc basin covering an area of 42,000 km2 (16,216 mi2) and overlying a zone of shallow subduction (the depth to the top of the Caribbean slab ranges from 30 km to 90 km [19 to 56 mi] beneath the LMB). Using 7000 km (4350 mi) of two-dimensional (2-D) seismic reflection lines tied to 33 wells, we describe the initial Oligocene subsidence of the forearc basin along a radial array of 70°- to 110°-striking normal faults that remained active until the early Miocene. During this period, the LMB was underfilled by 1–3 seconds two-way-time (TWT) (1500 m [4921 ft]) of shallow-marine and deep-marine facies. During middle Miocene the LMB remained underfilled with marine sediments deposited in water depths of 200–2600 m (656–8530 ft). An angular unconformity spanning the interval of 11–7 Ma marks a shortening and uplift affecting the Sinu accretionary prism west of the LMB that became emergent to form a prominent forearc high along the western edge of the LMB. The regional structure of the LMB is a broad syncline that folds all units older than early Miocene and produces an asymmetrical shape—in profile—with the western edge of the LMB (against the Sinu accretionary prism), steeper than the eastern edge of the LMB. After the late Miocene–Pliocene, the forearc high continued to elevate and separate the LMB from the outer Sinu accretionary prism. During this period, the LMB overfilled with terrigenous sediments of shallow marine facies that spilled offshore into the Caribbean Sea to form the proto-delta of the Magdalena Fan; these spilled sediments led to rapid tectonic accretion and growth of the offshore Sinu accretionary prism from 5 Ma to present. During the period of Oligocene to middle Miocene, different structural styles and subduction-related magmatic intrusions suggest that the Caribbean slab was subducting at an angle greater than 30° with a discontinuous volcanic arc. The decrease in the dip of the Caribbean slab to its modern dip angles of 4–8° occurred during the late Miocene and is interpreted as the entry of thicker Caribbean oceanic plateau crust into the subduction zone. Comparison of the segmented dip of the 400-km-long (249-mi-long) subducting Caribbean slab is consistent with the upper, 220-km-long (137-mi-long) shallow-dipping part subducting at rates of 2 cm/yr (0.78 in/yr) from 11 Ma (late middle Miocene) to Recent. We propose that this change from the steeper to shallower-dipping slab in the middle Miocene led to (1) increasing elevation of the forearc high of the Sinu prism along the eastern edge of the LMB; (2) the regional synclinal structure of the LMB in profile; and (3) the possible elevation of the entire LMB after 11 Ma as it changed from underfilled, deep-water marine environments to overfilled, shallow-water marine and fluvial environments.

GPS estimate of relative motion between the Caribbean and South American plates, and geologic implications for Trinidad and Venezuela

Global Positioning System (GPS) data from eight sites on the Caribbean plate and five sites on the South American plate were inverted to derive an angular velocity vector describing present-day relative plate motion. Both the Caribbean and South American velocity data fit rigid-plate models to within ±1–2 mm/yr, the GPS velocity uncertainty. The Caribbean plate moves approximately due east relative to South America at a rate of ∼20 mm/yr along most of the plate boundary, significantly faster than the NUVEL-1A model prediction, but with similar azimuth. Pure wrenching is concentrated along the approximately east-striking, seismic, El Pilar fault in Venezuela. In contrast, transpression occurs along the 068°-trending Central Range (Warm Springs) fault in Trinidad, which is aseismic, possibly locked, and oblique to local plate motion.

Neotectonics of Puerto Rico and the Virgin Islands, northeastern Caribbean, from GPS geodesy

The boundary between the North American and Caribbean plates is characterized primarily by left-lateral motion along predominantly east-west striking faults. Seismicity and marine geophysical survey data are consistent with at least two, and possibly three, microplates in the diffuse boundary zone in the northeastern Caribbean: (1) the Gonave, (2) the Hispaniola, and (3) the Puerto Rico-northern Virgin Islands (PRVI). We discuss results from GPS geodetic measurements acquired since 1994 to test the microplate hypothesis, define PRVI translation and rotation within the boundary zone, and constrain PRVI neotectonics. GPS-derived velocities are analyzed with respect to both North American and Caribbean plate reference frames. Integrated displacements across PRVI are limited to a few millimeters per year, consistent with a rigid PRVI and permitting calculation of an average velocity for PRVI. The motions of PRVI relative to North America and the Caribbean are 16.9±1.1 mm/yr toward N68°E±3° (1σ) and 2.4±1.4 mm/yr toward S79°W±26° (1σ), respectively. In contrast with some recent models, ongoing rotation of PRVI about a nearby (< 25° distant) vertical axis is not supported by the geodetic data. In addition, we argue against eastward tectonic escape of PRVI and favor a simple, progressive increase in velocity across the plate boundary zone, requiring that the summed magnitude of strike-slip fault slip rates will equal the total plate motion rate between the Caribbean and North America. GPS data are consistent with components of left-lateral strike-slip faulting along the Muertos trough south of Puerto Rico and shortening across the Puerto Rico trench. Comparison of GPS velocities for PRVI with respect to North America with total North America-Caribbean relative motion suggests up to 85% of North American-Caribbean plate motion is accommodated by the Puerto Rico trench and offshore faults north of Puerto Rico. Differences in GPS-derived velocities from Hispaniola and PRVI yield east-west extension across the N-S trending Mona rift of a few millimeters per year when estimated elastic strain accumulation effects along the north Hispaniola deformed belt and the Septentrional fault zone are considered. The opening rate implies an age of the Mona rift of 2–3 million years, agreeing with marine geophysical data that support a young age for the structure.

Implications of magma transfer between multiple reservoirs on eruption cycling

Volcanic eruptions are episodic despite being supplied by melt at a nearly constant rate. We used histories of magma efflux and surface deformation to geodetically image magma transfer within the deep crustal plumbing of the Soufrière Hills volcano on Montserrat, West Indies. For three cycles of effusion followed by discrete pauses, supply of the system from the deep crust and mantle was continuous. During periods of reinitiated high surface efflux, magma rose quickly and synchronously from a deflating mid-crustal reservoir (at about 12 kilometers) augmented from depth. During repose, the lower reservoir refilled from the deep supply, with only minor discharge transiting the upper chamber to surface. These observations are consistent with a model involving the continuous supply of magma from the deep crust and mantle into a voluminous and compliant mid-crustal reservoir, episodically valved below a shallow reservoir (at about 6 kilometers).

GPS measurement of surface deformation around Soufriere Hills volcano, Montserrat from October 1995 to July 1996

Global Positioning System geodesy was used to measure surface deformation on Soufriere Hills volcano, Montserrat from October 6, 1995 to July 1, 1997 during initial dome growth and gravitational collapse. Our data from this period show non-axially symmetric horizontal displacements, and decreasing subsidence as a function of radial distance from the former topographic high of the volcanic edifice. Forward modeling suggests that surface deformation is caused by a shallow vertical dike (< 3 km), which expanded approximately 1 m, coupled with a deflating Mogi source at about 6 km depth. These inferred source parameters are in good agreement with independent observations of regional dike widths and preemption magma storage depth.

Global Positioning System detection and energy estimation of the ionospheric wave caused by the 13 July 2003 explosion of the Soufrière Hills Volcano, Montserrat

[1] Volcanic explosions or shallow earthquakes are known to trigger acoustic and gravity waves that propagate in the atmosphere at infrasonic speeds. At ionospheric heights, coupling between neutral particles and free electrons induces variations of electron density detectable with dual-frequency Global Positioning System (GPS) measurements. Using GPS data collected in the Caribbean, we identified an ionospheric perturbation after a major volcanic explosion at the Soufriere Hills Volcano (Montserrat, Lesser Antilles) on 13 July 2003. Spectral analysis reveals peaks centered at 1 and 4 mHz, similar to those in previous observations and consistent with theory, suggesting both gravity and acoustic wave components. We retrieve a horizontal velocity of ∼624 m/s for the acoustic component, which implies upward propagation at ∼33°, consistent with ray-tracing results. We model the acoustic wave using an N-wave pressure source at ground level combined with ray tracing to propagate the neutral pressure wave; this accounts for the dispersive characteristics of the atmosphere while conserving total acoustic energy. Plasma velocity is derived from neutral velocity using a finite difference solution of the magnetohydrodynamic momentum equation. The continuity equation for charge densities is used to compute corresponding electron density variations, which are then numerically integrated along satellite-to-receiver line of sights, simultaneously accounting for the satellite displacements. We minimize the misfit between observed and model waveforms to estimate a total acoustic energy release of 1.53 x 10 10 J for the primary explosion event at Soufriere Hills Volcano associated with the peak dome collapse. This method can be applied to any explosion of sufficient magnitude, provided GPS data are available at near to medium range from the source.

Kinematics of the Nicaraguan Forearc from GPS Geodesy

[1] Campaign GPS data from a network in the Nicaraguan forearc show a strong component of arc-parallel motion indicating northwest translation of a nearly rigid forearc sliver. Our measured mean velocity for forearc sites of 15.1 mm yr 1 agrees well with the arc-parallel sliver motion predicted previously by DeMets (2001) derived from closure constraints on oblique convergence between the Cocos and Caribbean plates. The lack of a northeasterly oriented arc-normal component of motion in forearc velocities indicates that there are complexities involved beyond a simple interpretation of sliver motion being driven by oblique convergence. The forearc is reasonably well-fit by rigid rotation about an Euler pole with a rms misfit of residual velocities of 4.9 ± 2.6 mm yr 1 . Current motion of the forearc sliver relative to the stable Caribbean plate yields predominantly boundary parallel NW motion with boundary normal extension in the northwestern region averaging 5m m yr 1 . Citation: Turner, H. L., III, P. LaFemina, A. Saballos, G. S. Mattioli, P. E. Jansma, and T. Dixon (2007), Kinematics of the Nicaraguan forearc from GPS geodesy,Geophys. Res. Lett., 34, L02302, doi:10.1029/2006GL027586.

Pyroclastic flow and explosive activity at Soufrière Hills Volcano, Montserrat, during a period of virtually no magma extrusion (March 1998 to November 1999)

Abstract Dome growth at Soufrière Hills Volcano halted in early March 1998. After dome growth ceased, seismicity reduced significantly, but activity related to dome disintegration and degassing of magma at depth continued. A sustained episode of pyroclastic flows on 3 July 1998 marked the single largest collapse from March 1998 to November 1999. This led to a remarkable episode of dome collapses, low-energy explosions and ash-venting that resulted in the regular production of ash plumes, commonly reaching 1.5-6 km above sea level (a.s.l), but sometimes up to 11 km a.s.l., and the development of a small block-and-ash cone around the explosion crater. During the period of this residual activity, higher levels of activity occurred approximately every five to six weeks. This periodicity was similar to the cycles observed during active dome growth during 1995 to 1998, and probably had a similar cause. The relatively high level of observed activity caused continued concern regarding volcanic hazards and their potential to impact upon the resident population. Vigorous magma extrusion resumed in November 1999. The activity of the intervening period is attributed to the continued cooling and degassing of the dome, conduit and deep magma body, the impact of rising volcanic gases in the volcanic edifice, and limited magma flow in the conduit.

Tectonic relationships between forearc-basin strata and the accretionary complex at Bath, Barbados

The island of Barbados exposes the crestal zone of the accretionary prism of the Lesser Antilles forearc. Tertiary rocks at Bath, Barbados, include four lithic suites: (1) inter-bedded volcanogenic and mainly calcareous pelagic rocks (the Oceanic beds), (2) radiolarite, (3) siliceous hemipelagic rocks, and (4) melange. Suites 1–3 are in structurally discrete groups of fault packets; melange occurs in apparently diapiric crosscutting bodies. The Oceanic beds occupy a pre-Pleistocene nappe complex that underlies much of Barbados. Radiolarite and hemipelagic rocks belong to a thick accretionary basal complex and constitute the relative autochthon to the Oceanic nappes. Fault rocks of the sub-Oceanic fault zone developed during emplacement of the Oceanic nappes. The lower portion of the tectonic stack is deformed in a major south-verging fold couple with wavelength of ∼250 m. Lower, early emplaced Oceanic nappes are restricted to the core of the major synform and are increasingly deformed with depth. Structures suggest easterly transport of Oceanic nappes and upward younging of nappe emplacement, indicating that the deformation of Oceanic beds propagated westward. The Oceanic nappes provided a seal to rising fluids; transport of nappes may have been facilitated by elevated fluid pressure beneath their sole thrust. Pre- and post-nappe folds record north-south shortening about shallow, west-plunging axes, consistent with folded accretionary strata elsewhere on Barbados. Deformation occurred under shallow nonmetamorphic conditions. The Oceanic beds are interpreted as being trapped outer forearc-basin strata that have been tectonically shouldered by the arcward migrating prism flank or crest. The hemipelagic and radiolarite suites may be offscraped abyssal-plain and trench-wedge accumulations. Paleogene-lower Miocene Oceanic strata were deposited at depths between 2.0 and 4.8 km. Middle Miocene Oceanic beds accumulated at shallower depths (1–1.5 km); thus, early Miocene was the probable time of inception of major uplift and arcward shouldering in the crestal zone of the accretionary prism. Accretion of the basal complex of Barbados probably occurred in late Eocene time.