Change In Absolute Motion Research Articles

The Basement of the Central Andes: The Arequipa and Related Terranes

The basement of the Central Andes provides insights for the dispersal of Rodinia, the reconstruction of Gondwana, and the dynamics of terrane accretion along the Pacific. The Paleoproterozoic Arequipa terrane was trapped during collision between Laurentia and Amazonia in the Mesoproterozoic. Ultrahigh-temperature metamorphism correlates with the collapse of the Sunsás-Grenville orogen after ∼1000 Ma and is related to slab break-off and dispersal of Rodinia. The Antofalla terrane separated in the Neoproterozoic, forming the Puncoviscana basin. Its closure was coeval with the collision of the eastern Sierras Pampeanas. The rift-drift transitions of the early Paleozoic clastic platform showed a gradual younging to the north, in agreement with counterclockwise rotation based on paleomagnetic data of Antofalla. North of Arequipa arc magmatism and high-grade metamorphism are linked to collision of the Paracas terrane in the Ordovician, during the Famatinian orogeny in the Sierras Pampeanas. The early Paleozoic history of the Arequipa massif is explained by a backarc, which further south changed to open oceanic conditions and subsequent collision. The Antofalla terrane reaccreted to the continental margin by the late Ordovician. These accretions and subsequent separations during the Mesoproterozoic, Neoproterozoic–early Cambrian, and late Cambrian–middle Ordovician are explained by changes in absolute motion of the Gondwana supercontinent during plate global reorganization.

Absolute velocity of North America during the Mesozoic from paleomagnetic data

We use paleomagnetic data to map Mesozoic absolute motion of North America, using paleomagnetic Euler poles (PEP). First, we address two important questions: (1) How much clockwise rotation has been experienced by crustal blocks within and adjacent to the Colorado Plateau? (2) Why is there disagreement between the apparent polar wander (APW) path constructed using poles from southwestern North America and the alternative path based on poles from eastern North America? Regarding (1), a 10.5° clockwise rotation of the Colorado Plateau about a pole located near 35°N, 102°W seems to fit the evidence best. Regarding (2), it appears that some rock units from the Appalachian region retain a hard overprint acquired during the mid-Cretaceous, when the geomagnetic field had constant normal polarity and APW was negligible.We found three well-defined small-circle APW tracks: 245–200 Ma (PEP at 39.2°N, 245.2°E, R=81.1°, root mean square error (RMS)=1.82°), 200–160 Ma (38.5°N, 270.1°E, R=80.4°, RMS=1.06°), 160 to ∼125 Ma (45.1°N, 48.5°E, R=60.7°, RMS=1.84°). Intersections of these tracks (the “cusps” of Gordon et al. [Tectonics 3 (1984) 499]) are located at 59.6°N, 69.5°E (the 200 Ma or “J1” cusp) and 48.9°N, 144.0°E (the 160 Ma or “J2” cusp). At these times, the absolute velocity of North America appears to have changed abruptly.North America absolute motion also changed abruptly at the beginning and end of the Cretaceous APW stillstand, currently dated at about 125 and 88 Ma (J. Geophys. Res. 97 (1992b) 19651). During this interval, the APW path degenerates into a single point, implying rotation about an Euler pole coincident with the spin axis.Using our PEP and cusp locations, we calculate the absolute motion of seven points on the North American continent. Our intention is to provide a chronological framework for the analysis of Mesozoic tectonics. Clearly, if APW is caused by plate motion, abrupt changes in absolute motion should correlate with major tectonic events. This follows because large accelerations reflect important changes in the balance of forces acting on the plate, the most important of which are edge effects (subduction, terrane accretion, etc.). Some tectonic interpretations: (1) The J1 cusp may be associated with the inception of rifting of North America away from land masses to the east; the J2 cusp seems to mark the beginning of rapid spreading in the North Atlantic. (2) The J2 cusp signals the beginning of a period of rapid northwestward absolute motion of western North America; motion of tectonostratigraphic terranes in the westernmost Cordillera seems likely to have been directed toward the south during this interval. (3) The interval ∼88 to ∼80 Ma saw a rapid decrease in the paleolatitude of North America; unless this represents a period of true polar wander, terrane motion during this time should have been relatively northward.

Cenozoic dynamics of the African plate with emphasis on the Africa‐Eurasia collision

We investigate the forces acting on the African plate from about 56 Ma to the present‐day. We aim (1) to establish a basis for quantification of the overall dynamics of the Mediterranean/Middle East region and (2) to understand how Africa reacts to temporal variation in the forces associated with the Africa‐Eurasia collision. We use the assumption of dynamical equilibrium which implies that at any given time, the net torque of forces on Africa's northern margin must have been balanced by the other torques acting on the plate. Moreover, temporal variation in the forces on Africa's northern margin must have been balanced by contemporaneous changes in one or more of the other torques. By quantifying those “other” torques we may derive a constraint on the forces at the northern margin. We expect the Africa‐Eurasia collision to have been associated with changes in the associated forces and examine how these changes are balanced. Using alternative parameterizations, we calculate the net torque of ridge push, transform fault resistance, and basal drag. We also quantify changes in the torque of forces associated with the continent‐ocean transition. The results constitute a framework for detailed dynamic models of the Africa‐Eurasia convergence zone. Temporal changes in the ridge push torque prove to be small. The torque of transform resistance does change, but these changes do not relate to forces at the northern margin. Evidence for a relation with collision is found only for the case of basal drag. The results point to a mechanism where the plate maintains its dynamical equilibrium mainly through collision‐induced changes in absolute motion.

Late Neogene motion of the Pacific Plate

Oceanic crust between 38°S and 65°S along the Pacific‐Antarctic ridge records a complete history of Neogene Pacific‐Antarctic motion. We have modeled magnetic stages corresponding to portions of isochrons 1, 2, 2A, 3, 3A, 4, and 5 from 17 high‐quality marine magnetic traverses of the ridge to determine the Neogene finite rotation poles for this plate pair. We chose to model velocities over time intervals of about 0.5 m.y. because this greatly increased the number of estimates of Pacific‐Antarctic plate velocity. Our analysis shows that a change in Pacific‐Antarctic relative motion occurred between chron 2A, 3.40 Ma, and the beginning of chron 3 time, 3.86 Ma. We used 203 estimates of Pacific‐Antarctic spreading velocity to calculate a 0–3.4 Ma and 3.86–10.3 Ma Euler pole. Combined with tracks of seamounts that record the passage of the Pacific plate over mantle plumes our data show that this change in relative motion corresponds to a change in the absolute motion of the Pacific plate with respect to the hotspot frame of reference. Using these Euler poles to calculate Pacific‐Antarctic‐Africa‐North America circuit Euler poles show that this recent change in Pacific plate absolute motion corresponds to a change from strike‐slip to transpressive motion along the coastal California Pacific‐North America plate boundary. Interestingly, this age of increased tectonism agrees in time with deformation in New Zealand and northern Japan.

A revised magnetic polarity time scale for the Paleocene and Early Eocene and implications for Pacific plate motion

Magnetostratigraphic studies of a continental sedimentary sequence in the Clark's Fork Basin, Wyoming and a marine sedimentary sequence at Gubbio, Italy indicate that the Paleocene‐Eocene boundary occurs just stratigraphically above normal polarity zones correlative with magnetic anomaly 25 chron. These data indicate that the older boundary of anomaly 24 chron is 52.5 Ma. This age is younger than the late Paleocene age assigned by LaBrecque et al. [1977] and also younger than the basal Eocene age assigned by Ness et al. [1980]. A revised magnetic polarity time scale for the Paleocene and early Eocene is presented in this paper. Several changes in the relative motion system between the Pacific plate and neighboring plates occurred in the interval between anomaly 24 and anomaly 21. A major change in absolute motion of the Pacific plate is indicated by the bend in the Hawaiian‐Emperor Seamount chain at ∼43 Ma. The revised magnetic polarity time scale indicates that the absolute motion change lags the relative motion changes by only ∼3‐5 m.y. rather than by >10 m.y. as indicated by previous polarity time scales.