Charles DeMets

Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions

Recent revisions to the geomagnetic time scale indicate that global plate motion model NUVEL-1 should be modified for comparison with other rates of motion including those estimated from space geodetic measurements. The optimal recalibration, which is a compromise among slightly different calibrations appropriate for slow, medium, and fast rates of seafloor spreading, is to multiply NUVEL-1 angular velocities by a constant, α, of 0.9562. We refer to this simply recalibrated plate motion model as NUVEL-1A, and give correspondingly revised tables of angular velocities and uncertainties. Published work indicates that space geodetic rates are slower on average than those calculated from NUVEL-1 by 6±1%. This average discrepancy is reduced to less than 2% when space geodetic rates are instead compared with NUVEL-1A.

A new estimate for present-day Cocos-Caribbean plate motion : Implications for slip along the Central American volcanic arc

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New kinematic models for Pacific‐North America motion from 3 Ma to present, I: Evidence for steady motion and biases in the NUVEL‐1A Model

We use velocities derived from 2–4.5 years of continuous GPS observations at 21 sites on the Pacific and North American plates along with a subset of the NUVEL-1A data to examine the steadiness of Pacific-North America motion since 3.16 Ma, the transfer of Baja California to the Pacific plate, and the magnitude of biases in the NUVEL-1A estimate of Pacific-North America motion. We find that Pacific-North America motion has remained steady since 3.16 Ma, but at rates significantly faster than predicted by NUVEL-1A. In the vicinity of Baja California, our GPS-derived model and recent seafloor spreading rates in the southern Gulf of California both indicate that the NUVEL-1A model underestimates Pacific-North America rates by 4±2 mm yr−1. Steady Pacific-North America motion since 3.16 Myr and increasing seafloor spreading rates since 3.58 Myr in the Gulf of California imply that Pacific-North America motion was partitioned between seafloor spreading in the Gulf of California and decelerating slip along faults in or offshore from the Baja peninsula.

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.

Geologically current motion of 56 plates relative to the no‐net‐rotation reference frame

NNR-MORVEL56, which is a set of angular velocities of 56 plates relative to the unique reference frame in which there is no net rotation of the lithosphere, is determined. The relative angular velocities of 25 plates constitute the MORVEL set of geologically current relative plate angular velocities; the relative angular velocities of the other 31 plates are adapted from Bird (2003). NNR-MORVEL, a set of angular velocities of the 25 MORVEL plates relative to the no-net rotation reference frame, is also determined. Incorporating the 31 plates from Bird (2003), which constitute 2.8% of Earths surface, changes the angular velocities of the MORVEL plates in the no-net-rotation frame only insignificantly, but provides a more complete description of globally distributed deformation and strain rate. NNR-MORVEL56 differs significantly from, and improves upon, NNR-NUVEL1A, our prior set of angular velocities of the plates relative to the no-net-rotation reference frame, partly due to differences in angular velocity at two essential links of the MORVEL plate circuit, Antarctica-Pacific and Nubia-Antarctica, and partly due to differences in the angular velocities of the Philippine Sea, Nazca, and Cocos plates relative to the Pacific plate. For example, the NNR-MORVEL56 Pacific angular velocity differs from the NNR-NUVEL1A angular velocity by a vector of length 0.039 ± 0.011° a−1 (95% confidence limits), resulting in a root-mean-square difference in velocity of 2.8 mm a−1. All 56 plates in NNR-MORVEL56 move significantly relative to the no-net-rotation reference frame with rotation rates ranging from 0.107° a−1 to 51.569° a−1.

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.

Evidence for a post-3.16-Ma change in Nubia–Eurasia–North America plate motions?

We combine updated GPS velocities from the Nubian (NU),Eurasian (EU),and North American (NA) plates with 500 new 3.16-Myr-average seafloor spreading rates and nine transform fault azimuths from the northern Atlantic and Arctic basin seafloor spreading centers to estimate and test for changes in the relative motion between these plates. The numerous new seafloor spreading rates and GPS velocities improve our ability to detect recent changes in the relative motions of these plates. The angular velocity vector that best fits the EU^NA GPS velocities lies significantly north of the 3-Ma-average pole,in accord with previously published geologic evidence that the EU^NA pole has migrated northward since V3 Ma. Although we also find evidence for a significant post-3-Ma change in NU^NA motion,it is less compelling because the Nubian plate GPS velocity field is sparse and NU^NA seafloor spreading rates appear to have remained steady within the 1 mm yr 31 uncertainties if we systematically decrease the seafloor spreading rates to correct for outward displacement of seafloor spreading magnetic lineations. The NU^EU pole derived from GPS site velocities lies more than 30 angular degrees south of the tightly constrained 3-Ma-average estimate and predicts significantly slower and more oblique present-day NU^EU convergence in the Mediterranean. Both models for NU^EU motion pass a key test for their accuracy,namely,they correctly predict strike-slip motion along the well-mapped Gloria fault east of the Azores. The change to more oblique NU^EU motion may reflect increasing difficulty in maintaining margin-normal convergence within this continent^continent collision zone. 8 2003 Published by Elsevier B.V.

Relative motion between the Caribbean and North American plates and related boundary zone deformation from a decade of GPS observations

Global Positioning System (GPS) measurements in 1986, 1994, and 1995 at sites in Dominican Republic, Puerto Rico, Cuba, and Grand Turk define the velocity of the Caribbean plate relative to North America. The data show eastward motion of the Caribbean plate at a rate of 21 ± 1 mm/yr (1 standard error ) in the vicinity of southern Dominican Republic, a factor of 2 higher than the NUVEL-1A plate motion model prediction of 11 ± 3 mm/yr. Independent measurements on San Andres Island, and an Euler vector derived from these data, also suggest a rate that is much higher than the NUVEL-1A model. Available data, combined with simple elastic strain models, give the following slip rate estimates for major left-lateral faults in Hispaniola: (1) the North Hispaniola fault offshore the north coast of Hispaniola, 4 ± 3 mm/yr; (2) the Septentrional fault in northern Dominican Republic, 8 ± 3 mm/yr; and (3) the Enriquillo fault in southern Dominican Republic and Haiti, 8 ± 4 mm yr. The relatively high plate motion rate and fault slip rates suggested by our study, combined with evidence for strain accumulation and historical seismicity, imply that seismic risk in the region may be higher than previous estimates based on low plate rate/low fault slip rate models and the relatively low rate of seismicity over the last century.

Thirty-Five-Year Creep Rates for the Creeping Segment of the San Andreas Fault and the Effects of the 2004 Parkfield Earthquake: Constraints from Alignment Arrays, Continuous Global Positioning System, and Creepmeters

We present results from differential Global Positioning System (gps) surveys of seven alignment arrays and four continuous gps sites along the creeping segment of the San Andreas fault. Surveys of four alignment arrays from the central creeping segment yield 33- to 36-year average minimum slip rates of 21–26 mm/yr. These rates are consistent with previous alignment array surveys spanning a 10-year period and with rates determined by creepmeters, indicating approximate steady- state creep along the central creeping segment for at least 35 years. Motion between continuous gps sites that span the central creeping segment is 28.2 ± 0.5 mm/yr for two sites that are 1 km apart and 33.6 ± 1 mm/yr for two sites that are 70 km apart. Slip rates therefore increase with distance from the creeping segment of the San Andreas fault. All rates reported here are significantly slower than the 39 ± 2 mm/yr rate predicted for motion between the Sierra Nevada–Great Valley block and the Pacific plate. Repeat surveys of three alignment arrays following the 2004 Parkfield earthquake demonstrate that its coseismic and short-term postseismic offsets decrease rapidly with distance from the epicenter, from 150 mm to 15 mm to <5 mm at respective distances of 9, 36, and 54 km to the northwest. Continuous gps data confirm that little coseismic and postseismic slip occurred along the central creeping segment. Geodetic and geologic slip rates are compared and different models for the accommodation of transcurrent deformation across the creeping segment are discussed.