The Pacific megagash: A future plate boundary?

The oceanic Pacific Plate started forming in Early Jurassic time within the vast Panthalassa Ocean that surrounded the supercontinent Pangea, and contains the oldest lithosphere that can directly constrain the geodynamic history of the circum-Pangean Earth. We show that the geometry of the oldest marine magnetic anomalies of the Pacific Plate attests to a unique plate kinematic event that sparked the plate’s birth at virtually a point location, surrounded by the Izanagi, Farallon, and Phoenix Plates. We reconstruct the unstable triple junction that caused the plate reorganization, which led to the birth of the Pacific Plate, and present a model of the plate tectonic configuration that preconditioned this event. We show that a stable but migrating triple junction involving the gradual cessation of intraoceanic Panthalassa subduction culminated in the formation of an unstable transform-transform-transform triple junction. The consequent plate boundary reorganization resulted in the formation of a stable triangular three-ridge system from which the nascent Pacific Plate expanded. We link the birth of the Pacific Plate to the regional termination of intra-Panthalassa subduction. Remnants thereof have been identified in the deep lower mantle of which the locations may provide paleolongitudinal control on the absolute location of the early Pacific Plate. Our results constitute an essential step in unraveling the plate tectonic evolution of “Thalassa Incognita” that comprises the comprehensive Panthalassa Ocean surrounding Pangea.

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Distribution of the Pacific/North America motion in the Queen Charlotte Islands-S. Alaska plate boundary zone

Geophysical Research Letters ◽

10.1029/2003gl017586 ◽

2003 ◽

Vol 30 (14) ◽

Cited By ~ 22

Author(s):

Stéphane Mazzotti ◽

Roy D. Hyndman ◽

Paul Flück ◽

Alex J. Smith ◽

Michael Schmidt

Keyword(s):

North America ◽

Plate Boundary ◽

Boundary Zone ◽

Queen Charlotte Islands ◽

The Pacific ◽

Plate Boundary Zone

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Crustal structure and seismicity distribution adjacent to the Pacific and North America plate boundary in southern California

Journal of Geophysical Research Atmospheres ◽

10.1029/2000jb900016 ◽

2000 ◽

Vol 105 (B6) ◽

pp. 13875-13903 ◽

Cited By ~ 160

Author(s):

Egill Hauksson

Keyword(s):

North America ◽

Crustal Structure ◽

Southern California ◽

Plate Boundary ◽

Seismicity Distribution ◽

The Pacific

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Characterizing the Foreshock, Main Shock, and Aftershock Sequences of the Recent Major Earthquakes in Southern Alaska, 2016–2018

Frontiers in Earth Science ◽

10.3389/feart.2020.584659 ◽

2020 ◽

Vol 8 ◽

Author(s):

B. G. Bukchin ◽

A. S. Fomochkina ◽

V. G. Kossobokov ◽

A. K. Nekrasova

Keyword(s):

Control Parameter ◽

Empirical Distribution ◽

Scaling Law ◽

Plate Boundary ◽

Scaling Properties ◽

Second Moments ◽

The Pacific ◽

Unified Scaling Law ◽

Aftershock Sequences ◽

Integral Estimation

For each of three major M ≥ 7.0 earthquakes (i.e., the January 24, 2016, M7.1 earthquake 86 km E of Old Iliamna; the January 23, 2018, M7.9 earthquake 280 km SE of Kodiak; and the November 30, 2018, M7.1 earthquake 14 km NNW of Anchorage, Alaska), the study considers characterization of the foreshock and aftershock sequences in terms of their variations and scaling properties, including the behavior of the control parameter η of the unified scaling law for earthquakes (USLE), along with a detailed analysis of the surface wave records for reconstruction of the source in the approximation of the second moments of the stress glut tensor to obtain integral estimation of its length, orientation, and development over time. The three major earthquakes at 600 km around Anchorage are, in fact, very different due to apparent complexity of earthquake flow dynamics in the orogenic corner of the Pacific and North America plate boundary. The USLE generalizes the classic Gutenberg-Richter relationship taking into account the self-similar scaling of the empirical distribution of earthquake epicenters. The study confirms the existence of the long-term periods of regional stability of the USLE control parameter that are interrupted by mid- or even short-term bursts of activity associated with major catastrophic events.

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Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

Solid Earth ◽

10.5194/se-8-1211-2017 ◽

2017 ◽

Vol 8 (6) ◽

pp. 1211-1239 ◽

Cited By ~ 3

Author(s):

Thomas van der Werf ◽

Vasileios Chatzaras ◽

Leo Marcel Kriegsman ◽

Andreas Kronenberg ◽

Basil Tikoff ◽

...

Keyword(s):

Grain Size ◽

Upper Mantle ◽

Lower Crust ◽

Baja California ◽

Plate Boundary ◽

Volcanic Field ◽

Strike Slip ◽

Low Viscosity ◽

The Pacific ◽

Flow Laws

Abstract. The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

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