North Pacific Plate Research Articles

The Role of Seismic and Slow Slip Events in Triggering the 2018 M7.1 Anchorage Earthquake in the Southcentral Alaska Subduction Zone

AbstractThe M7.1 2018 Anchorage earthquake occurred in the bending part of the subducting North Pacific plate near the geometrical barrier formed by the underthrusting Yakutat terrane. We calculate the triggering potential related with stress redistribution from deformation sources including the M9.2 1964 earthquake coseismic slip, postseismic deformation, slip from regional M > 5 earthquakes, and the cumulative slip of previously detected slow slip events over the past 55 years. We investigate the deeper shallow depth (20–60 km) seismicity response to these perturbations using an epidemic type aftershock sequence model to describe earthquake‐to‐earthquake interactions. The statistical forecast captures the triggered seismicity during the 1983 M6+ aftershocks in Columbia Bay but performs poorly during the slow slip event period between 1992.0 and 2004.8 that presents a statistically significant rate change (β, Z > 2; M < 4.0). We find that stress effects from the 1964 postseismic source and the 12‐year‐long slow slip event (~M7.8) contribute to the 2018 Anchorage earthquake occurrence and that slow slip events modulate the deeper shallow depth seismicity patterns in the region.

Building and breaking a large igneous province: An example from the High Arctic

AbstractThe genesis of the Amerasia Basin in the Arctic Ocean has been difficult to discern due to overprint of the Cretaceous High‐Arctic Large Igneous Province (HALIP). Based on detailed analysis of bathymetry data, new Arctic magnetic and gravity compilations, and recently published radiometric and seismic data, we present a revised plate kinematic model of the northernmost Amerasia Basin. We show that the smaller Makarov Basin is formed by rifting and seafloor spreading during the latest Cretaceous (to middle Paleocene). The opening progressively migrated into the Alpha Ridge structure, which was the focus of Early‐to‐Middle Cretaceous HALIP formation, causing breakup of the proto‐Alpha Ridge into the present‐day Alpha Ridge and Alpha Ridge West Plateau. We propose that breakup of the Makarov Basin was triggered by extension between the North America and Eurasian plates and possibly North Pacific plate rollback.

Improved Late Cretaceous and early Cenozoic Paleomagnetic apparent polar wander path for the Pacific plate

Understanding of Pacific plate tectonics and geodynamics is aided by refinement of the plate's apparent polar wander path (APWP). We improved the Late Cretaceous and early Cenozoic APWP by analyzing a large, diverse paleomagnetic data set that combines core sample, seamount magnetic anomaly model, and marine magnetic anomaly skewness data. Our preferred APWP has five mean paleomagnetic poles representing the Oligocene (30 Ma), Late (39 Ma) and Early (49 Ma) Eocene, and Paleocene (61 Ma) epochs and the Maastrichtian (68 Ma) stage. Along with a published 80 Ma pole, the APWP shows a stillstand from ∼ 80 to ∼ 49 Ma punctuating the large overall northward drift of the plate. The two youngest poles imply resumption of northward motion during mid-Eocene time with another change of polar motion after ∼ 30 Ma. If unaffected by other phenomena (e.g., true polar wander or change in time-averaged magnetic field geometry), the stillstand implies negligible northward plate motion during the period of Emperor Seamounts formation, contrary to most accepted plate motion models. The stillstand is consistent with paleomagnetic data from the Emperor Seamounts, which imply southward motion of the Hawaiian melting anomaly. It also implies significant westward drift of the hotspot if the Pacific plate was moving west at rates similar to the later Cenozoic. In addition, changes in polar wander after ∼ 49 Ma are consistent with changes of north Pacific plate boundaries.

The genesis of the stable isotope (O, H) record in arc magmas: the Kamtchatka's case

We have determined δ 18 O in 120 chemically well-documented samples of lavas characteristic of the major areas and types of volcanism, most of which Quaternary in age, from the subduction zone volcanism in the Kamchatka peninsula, far-eastern Russia. Hydrogen isotopic data have also been obtained on about half of the samples. This volcanism has been highly active from the Cretaceous to the present and results from high convergence rates between the North Pacific plate and the Northeastern part of the Eurasian plate. The peninsula is the northernmost extension of the Japan–Hokkaido–Kurile Island Arc that runs continuously over about 2000 km. Hundreds of Quaternary arc-related volcanic edifices occupy its two main areas: the Eastern Volcanic Front (EVF), and the Central Kamchatka Depression (CKD). In Kamchatka, the contributions of sediments and continental crust are low, which makes easier the specific evaluation of the mantle wedge and the descending slab contributions. δ 18 O range from 5.3 to 8.5‰, δD from −70 to −212‰ and water contents from 0.01 to 0.9% in basic lavas. This high scatter is pristine and inherited from the mantle source of the magmas. Comparable ranges and scatter in evolved lavas (andesites to rhyolites) are mainly due also to parent magma variability. Combined δ 18 O , δD values and H 2O contents are explained in terms of a mixing, differentiation and degassing model. All the lavas result from mixtures of normal mantle magmas with a slab-derived, water-rich fluid/magma which is an essentially equimolecular mixture of water and plagioclase-like silicates, together with Mg and Fe concentrations around 1 and 3%, respectively. The average initial water concentration in the resulting lavas is 5.5%. If that characteristic concentration is a general feature in subduction zones, this would correspond to an upward volcanic and plutonic water flux of around 10 15 g/year. That `leakage' flux equilibrates the flux subducted through sediments and hydrated oceanic crust so that the remaining flux subducted into the mantle is of the order of the ridge flux. The water flux could be a significant part of the water involved at mid crustal levels in anatexis phenomena and explain the convergence of granites' δ 18 O 's towards the 8.5–10‰ range.

The Lithoprobe corridor across the Vancouver Island continental margin: the structural and tectonic consequences of subduction

The deep structure and tectonic history of the continental margin of southwestern Canada have been determined by phase I of the Lithoprobe program across south-central Vancouver Island and associated marine studies across the continental shelf and slope. This article reviews results from the marine portion of the corridor but also presents continuous onshore–offshore data and interpretation. The geophysical data include multichannel seismic reflection, seismic refraction, magnetics, gravity, bathymetry, sea-floor acoustic imagery, heat flow, and seismicity. There has been Ocean Drilling Program (ODP) coring and downhole measurements on the continental slope. The margin structure and Cenozoic tectonic history are dominated by the consequences of subduction. Two narrow terranes, the Eocene volcanic Crescent and the Mesozoic mainly sedimentary Pacific Rim, were emplaced along the coast at the time of north Pacific plate reorganization at 43 Ma. They provide the landward-dipping backstop to a large accretionary sedimentary prism formed from the sediments scraped off the incoming oceanic plate. The prism exhibits active folding, thrusting, and tectonic consolidation, and it provides a model for the formation of ancient fold and thrust belts. An extensive gas hydrate layer was detected beneath the continental slope by a seismic BSR (bottom-simulating reflector) and was penetrated by the ODP drilling. The surface heat flow decreases landward from high values over the young oceanic plate to low values just seaward of the volcanic arc as a consequence of the heat sink provided by the underthrusting oceanic plate. The margin seismicity includes continental crust-events and earthquakes in the downgoing oceanic plate. No earthquakes have been detected on the subduction thrust fault but great thrust events are inferred to occur with an average interval of 600 years. The seismic source zone for such events is restricted to a narrow region beneath the continental shelf because of the high temperatures over the young Juan de Fuca plate.

North Pacific plate convergence: A quantitative record of the past 140 m.y.

A quantitative record of the rate and direction of plate convergence around the North Pacific Ocean can be derived by combining information on Northern Hemisphere plate motion as related to hot spots with the details of North Pacific plate evolution. Such a record permits reconstruction of both north-south and east-west components of convergence and shows that the North and east Pacific margins were characterized by convergence rates in excess of 20 cm/yr during the time period between about 80 and 45 Ma. The west Pacific was characterized by north-northwesterly convergence at 15 to 20 cm/yr prior to 80 Ma and by moderate westerly convergence since then.

Paleomagnetic test of the Early Tertiary Plate circuit between the Pacific Basin Plates and the Indian Plate

The internal consistency of early Tertiary reconstructions of the Pacific, Antarctic, and Indian plates has been tested by combining seafloor spreading data and paleomagnetic data. These tests show that substantial motion has taken place during the Tertiary across a fossil plate boundary located within one of these plates. We consider two alternative models to reconcile the available paleomagnetic and plate tectonic information. In the first model, East and West Antarctica were separate plates, the early Tertiary position of West Antarctica being one in which the Antarctic Peninsula is adjacent to the southwest border of South America. This model is consistent with the paleomagnetic data from Australia and Antarctica. In the second model the northern and southern parts of the Pacific plate have undergone left‐lateral strike slip motion across a fossil plate boundary located northeast of Chatham Rise. This second model is supported by paleomagnetic data from the northern and southern segments of the Pacific plate. The joint assumptions (1) that the hot spots are fixed and (2) that during the early Tertiary the present Pacific plate was two separate plates (the Chatham Rise plate and the north Pacific plate) are consistent with all of the paleomagnetic data and all of the geometrical constraints of early Tertiary plate reconstructions. The joint assumptions (1) that the hot spots are fixed and (2) that the present Antarctic plate was two early Tertiary plates produce unacceptable overlap and are not consistent with Pacific plate paleomagnetic data unless the Chatham Rise has moved independently of adjacent seafloor.

A look at the Bouguer anomaly map in central Alaska with reference to plate tectonics

The Bouguer anomaly map in interior Alaska is examined in relation to the underthrusting north Pacific plate. The relatively large negative anomalies are observed closely above the flexure of the underthrust slab and there is a systematic increase in the values proceeding along the true dip direction. A large percentage of the minima may be accounted for by the low-density sediment in the forearc basin. It is further ascertained that the strike of the slab under Alaska is in a N17° E direction, while it dips in a N73° W bearing with an average dip value of 35°–45°. Downdip extension is predominant within the Benioff zone and a composite stress axes diagram indicates that the average plunge for the tension axes is about 65°.

Implications of north pacific plate tectonics in central Alaska: Focal mechanisms of earthquakes

The focal mechanisms for 86 selected earthquakes ( 3.0⩽ m b⩽ 5.5 ) located in central Alaska have been investigated from P-wave first motions; the data were gathered by local seismic networks. The results show a depth-dependent characteristic to the fault-plane solutions. For earthquakes having focal depths shallower than 60–70 km, the focal mechanisms indicate either strike-slip or normal faults, while for earthquakes with foci at intermediate depths the focal mechanisms correspond to thrust faults. The nature of the seismicity indicates the hinge line of the Pacific lithospheric plate under the study area to be striking N17°E from Cook Inlet towards interior Alaska. The comparison of the focal mechanisms with the seismicity shows that the strike-slip and normal faults are the predominant processes of stress release along the shallow section of the plate. The earthquakes with intermediate foci systematically occur along the inclined section of the plate. If the gently dipping nodal planes for these earthquakes are chosen as the fault planes, the focal mechanisms correspond to underthrust motions at the foci. In these, the slip vectors are oriented either to the west or north with the resultant being in the N30°W direction. The tension axes for the underthrust solutions are also found to be parallel to the local dip of the plate, indicating that the subducted plate in interior Alaska is undergoing gravitational sinking.

Equatorial Acceleration and Continental Paths: ABSTRACT

A mantle weak enough for convection is also weak enough to End_Page 1902------------------------------ show zonal effects due to the earth's rotation. Zonal rotation--or equatorial acceleration--in a plastic mantle, underneath rigid crustal blocks, should have certain specific results. For instance, major continental blocks in the Northern Hemisphere should, over the long term, describe counterclockwise loops. North America has had such a path, as has Europe. Present motions along the rim of the Pacific Ocean suggest that the North Pacific plate also has been following a counterclockwise loop. Major continental blocks in the Southern Hemisphere should describe clockwise loops. This has been the pattern for Antarctica, Australia, and South America. Africa, located primarily in the Southern Hemisphere during Paleozoic time, traced a clockwise loop during that era; in Cenozoic time, largely in the Northern Hemisphere, it has been moving with counterclockwise curvature. Further, major blocks close to the equator should have a greater west-to-east acceleration than major blocks located far from the equator. Hence, South America should be moving eastward relative to North America, and Africa should be moving eastward relative to Europe, North America, and South America. The northward paths of Europe and North America, relative to Africa and South America, also indicate major tensional effects in the Mediterranean and Gulf of Mexico-Caribbean areas. The Northern Pacific block should be moving eastward faster than the North American block as it is closer to the equator, and should be turning in a counterclockwise sense. This movement provides a north-south zone of east-west compression along the western edge of North America and down the East Pacific Rise. Reconstruction of block paths indicates that Gondwanaland, a hypothetical Paleozoic supercontinent, was more likely a convergence of individual blocks which had separate previous histories. End_of_Article - Last_Page 1903------------

Tectonic stresses in Southern Alaska in relationship to regional seismicity and the new global tectonics

Abstract Fault plane solutions and a strain release map imply that Southern Alaska performs in the role of a sink area unlike that of more classic trench-arc combinations. A broad, apparently interrelated zone of seismic activity extends from the Aleutian Trench to the base of the Alaska Range. While hypocentral distribution and relative seismicity imply that the entire area is performing as a sink for the North Pacific plate, fault plane solutions at the base of the Alaska Range are inconsistent with this premise. It is suggested that Cook Inlet and the lowlands north of Anchorage may represent an ancient trench.

Emergence Model for Formation of Aleutian Arc: ABSTRACT

It is generally believed that the Aleutian arc has been formed as a result of a North Pacific crustal plate being thrust into the Aleutian Trench and beneath the Aleutian Ridge. Several bathymetric and magnetic features along and south of the Aleutian arc conflict with that model of convergence between crustal plates. A new model is proposed which calls for the development of the Aleutian arc by the emergence of material from the underlying mantle as the North Pacific plate is pulled away. The pattern of magnetic anomalies suggests that an ancestral Aleutian Ridge may have been the source for Late Cretaceous and early Tertiary southward sea-floor spreading. The present Aleutian Ridge was built during the Cenozoic by uplift and the accumulation of volcanogenic material. The Aleutian Terrace and Trench are half-grabens which have been collecting sediments since their probable late Tertiary formation. Tensional first motions of earthquakes beneath the trench may reflect its continuing formation, although there is no evidence of appreciable horizontal motion associated with its development. Basins at t e bottoms of submarine canyons that cut into the Aleutian Ridge suggest that the Aleutian Terrace is still opening. The Aleutian and Bowers basins have been collecting thick sedimentary deposits since the formation of the ancestral Aleutian Ridge. Bowers Ridge probably was pushed up from the Aleutian basin during late Tertiary deformation of the Aleutian Ridge. End_of_Article - Last_Page 2501------------

Unified basis for tectonic theory

Five important principles are adopted as guides in developing a tectonic theory. These are: 1. (1) strike-slip motion is more important than any other kind. 2. (2) the crust is essentially a passive layer, riding on top of an active deeper zone. 3. (3) the degree of crustal deformation is, commonly, a greatly diminished measure of actual displacement at depth. 4. (4) the orientation of structural features such as faults and fold axes is more important than their relative sizes. 5. (5) present deformational patterns are typical, rather than unique, and therefore can be used as a key to tectonic history. These five principles are combined to produce a zonal rotation hypothesis for earth deformation. According to this hypothesis, the hierarchy of deformational units is as follows: 1. I. Equatorial belt, consisting of a variety of features, but specifically including a number of axisymmetric systems such as the Caribbean system which extends (north-south) from Cuba into Venezuela. The motion in each system is west-east, hence the entire belt is assumed to have a west-east acceleration. 2. II. The north Pacific plate, rotating in counter-clockwise fashion, as a result of being driven from the south by the equatorial belt. 3. III. Individual continents or fragments of continents, such as North America, which is circling the north Pacific plate — also in a counterclockwise direction, but at a retarded rate — as a result of drag along the Alaska-California coast. 4. IV. Smaller features, such as the Ouachita Mountains of Arkansas and Oklahoma, and the Alps. The two truly unique parts of the surface of the planet are the two junctions of the equatorial belt and the Pacific rim: 1. (1) a convergence zone, in the vicinity of the up-thrust Himalayan plateau. 2. (2) a divergence zone, in the vicinity of the subsiding, primarily tensional, Gulf of Mexico. The rate of displacement, through much of this four-level system, is on the order of 1 cm/year. Mountain systems of North America can be analyzed in terms of the theory. West Coast ranges are permanently active right-lateral structures, although perhaps they have experienced crescendoes at intermittent intervals. The Appalachians were more-or-less continuously active left-lateral structures only during late Precambrian and Paleozoic time, when they were the northern limb of an axisymmetric system in the equatorial belt. The Gulf Coast “geosyncline” has a long depositional history still ahead of it. The theory also predicts that the Pleistocene, if extended into the future to cover all immediately succeeding glaciations, has only begun. The force problem remains unsolved, but past attempts to resolve this difficulty have ignored several important facets of the problem. Regardless of the order of the forces involved, however, the present theory is no more handicapped than any other.