The Queen Charlotte Islands refraction project. Part II. Structural model for transition from Pacific plate to North American plate: Discussion

1990 ◽  
Vol 27 (6) ◽  
pp. 879-880
Author(s):  
H. G. Miller
Keyword(s):  
North American ◽  
Structural Model ◽  
Pacific Plate ◽  
North American Plate ◽  
Queen Charlotte Islands

The Queen Charlotte Islands refraction project. Part II. Structural model for transition from Pacific plate to North American plate

1989 ◽  
Vol 26 (9) ◽  
pp. 1713-1725 ◽  
Author(s):  
D. J. Mackie ◽  
R. M. Clowes ◽  
S. A. Dehler ◽  
R. M. Ellis ◽  
P. Morel-À-l'Huissier
Keyword(s):  
Fault Zone ◽  
North American ◽  
Velocity Structure ◽  
Pacific Plate ◽  
North American Plate ◽  
Lithospheric Structure ◽  
Gravity Modelling ◽  
Oblique Subduction ◽  
Queen Charlotte Islands ◽  
Thin Crust

The oceanic-continental boundary west of the Queen Charlotte Islands is marked by the active Queen Charlotte Fault Zone. Motion along the fault is predominantly dextral strike slip, but relative plate motion and other studies indicate that a component of convergence between the oceanic Pacific plate and the continental North American plate presently exists. This convergence could be manifest through different types of deformation: oblique subduction, crustal thickening, or lateral distortion of the plates. In 1983, a 330 km offshore–onshore seismic refraction profile extending from the deep ocean across the islands to the mainland of British Columbia was recorded to investigate (i) structure of the fault zone and associated oceanic–continental boundary and (ii) lithospheric structure beneath the islands and Hecate Strait to define the regional transition from Pacific plate to North American plate and thus the nature of the convergence. Two-dimensional ray tracing and synthetic seismogram modelling of many record sections enabled the derivation of a composite velocity structural section along the profile. The structural section also was tested with two-dimensional gravity modelling. Part I of the study addressed the structure of the fault zone; part II addresses lithospheric structure extending eastward to the mainland.The derived velocity structure has some important and well-constrained features: (i) anomalously low crustal velocities (5.3 km/s with a 0.2 km/s per km gradient) underlain by a steep, 19 °eastward-dipping boundary above the mantle in the terrace region west of the main fault; (ii) a thin crust of 21–27 km beneath the Queen Charlotte Islands; and (iii) a gentle 4 °eastward dip of the Moho below Hecate Strait as crustal thickness increases from 27 km to 32 km. The gravity modelling requires that mantle material extend upwards to a depth of about 30 km below the mainland and indicates that an underlying subducted slab, if it exists, extends eastward no farther than the mainland.Unfortunately, the velocity structure delineated by this study could not unambiguously determine the mode of deformation, because the lowermost crustal block beneath Queen Charlotte Islands and Hecate Strait can be interpreted as subducted oceanic crust or middle to lower continental crust. Thus, two different tectonic models for the transition from Pacific plate to North American plate are discussed: in one, oblique subduction is the principal characteristic; in the other, oceanic lithosphere juxtaposed against continental lithosphere across a narrow boundary zone along which only transcurrent motion occurs is the dominant feature. Based on the thin crust beneath the Queen Charlotte Islands, the lack of a wide zone of deformation along the plate boundary region, and other geological and geophysical characteristics, oblique subduction is the more plausible model.

scholarly journals Possibility of recovery of slip deficit rate between the North American plate and the Pacific plate off Sanriku, northeast Japan

2007 ◽  
Vol 34 (20) ◽  
Author(s):  
Shinzaburo Ozawa ◽  
Hisashi Suito ◽  
Takuya Nishimura ◽  
Mikio Tobita ◽  
Hiroshi Munekane
Keyword(s):  
North American ◽  
Pacific Plate ◽  
North American Plate ◽  
The North ◽  
Slip Deficit ◽  
The Pacific ◽  
Off Sanriku

Tertiary extension and tilting in the Queen Charlotte Islands, evidence from dyke swarms and their paleomagnetism

1992 ◽  
Vol 29 (9) ◽  
pp. 1878-1898 ◽  
Author(s):  
E. Irving ◽  
J. G. Souther ◽  
J. Baker
Keyword(s):  
North American ◽  
Late Eocene ◽  
North American Plate ◽  
Western Margin ◽  
Queen Charlotte Islands ◽  
The North ◽  
Early Oligocene ◽  
Magnetic Stability ◽  
Transform Margin ◽  
Dyke Swarms

The Queen Charlotte Islands form the western margin of the Tertiary Queen Charlotte Basin, which is situated on the western margin of the North American Plate. They contain seven major dyke swarms of Late Eocene to Miocene age, a period when the relative motions of the Pacific and the North American plates in this region were dominantly dextral strike slip (transform margin), with intervals of highly oblique divergence and convergence. Within each swarm, dykes have a systematic trend. However, trends vary from swarm to swarm, indicating that the stress field varied. A total of 678 cores (1352 specimens) were collected from 129 dykes in six swarms over a distance of about 200 km. Magnetic stability is variable. One hundred and one dykes yielded records of the paleofield. Data are also reported from an Oligocene pluton (5 sites, 27 cores, 52 specimens) and Miocene lavas (8 sites, 52 cores, 101 specimens). Both normal and reversed magnetizations occur, but irrespective of sign, the mean directions of remanent magnetization of each swarm and of the pluton and the lavas have systematically steeper inclinations than the value calculated from coeval rocks in North America. To explain this it is proposed that, after dyke emplacement, the sampling areas were tilted to the north or northwest by amounts that vary between 9 and 16°. Apparently, crustal tilting, similar in magnitude and direction, extended over distances of approximately 200 km. This cannot reflect tilting of a single block. Instead, it is argued that at least the southern Queen Charlotte Islands underwent considerable northerly or north-northwesterly directed extension and normal block faulting, which followed and in part was concurrent with the formation of widespread mid-Tertiary dyke swarms, plutons and lava flows. Making use of the fact that dykes propagate perpendicular to the direction of extension, and combining previously measured dyke orientations with paleomagnetic data, three stages of extension are proposed: east–west extension sometime during the Late Eocene to Early Oligocene; north–south extension sometime in the interval Late Oligocene to Early Miocene; and northwest–southeast extension sometime during Late Miocene or later time.

Geotectonics of Cretaceous and Eocene plutons in British Columbia: a paleomagnetic fold test

1977 ◽  
Vol 14 (6) ◽  
pp. 1246-1262 ◽  
Author(s):  
D. T. A. Symons
Keyword(s):  
British Columbia ◽  
North American ◽  
Large Scale ◽  
Structural Model ◽  
Pole Position ◽  
North American Plate ◽  
The North ◽  
Field Evidence ◽  
Coast Plutonic Complex ◽  
Skeena River

A total of 295 cores (590 specimens) were collected at 59 sites in the Coast plutonic complex along an E–W section southwest of Kitimat, British Columbia. The sites represent the Ponder, Alastair Lake, and Quottoon plutons in the 40–50 Ma eastern K–Ar age zone and the Ecstall and Butedale plutons in the 64–80 Ma central age zone. After af demagnetization a stable remanent magnetization was isolated at 32 sites and these data were combined with available data from the Skeena River section about 100 km to the north. The remanence directions in sites from the NNW-trending north and south limbs of the Hawkesbury Warp provide a positive fold test when compared to the WNW-trending centre limb directions.In the Eocene eastern age zone the NNW limbs give a concordant pole position relative to the cratonic North American pole whereas the centre limb has undergone ≈ 50° of the counter-clockwise rotation and ≈ 10° of upward tilt of its western end to give a discordant pole. In the late Upper Cretaceous central age zone, the Ecstall–Butedale pluton was tilted 15° to the west on all limbs before the Eocene intrusion and Hawkesbury Warp deformation events to give a NNW-trend pole and WNW-trend pole diverging in opposite directions from the cratonic reference pole.The geologic field evidence from structural trends, from fault, fold, contact, and foliation attitudes, and from distribution of plutonic phases is consistent with the structural model. The regional geotectonic events are related to possible Cenozoic plate interactions on the western margin of the North American plate. This combination of concordant and discordant poles cannot be explained in terms of an excursion of the geomagnetic paleopole during intrusion, a large scale northward translation of the western Cordillera during the Cenozoic, or a combination of clockwise rotations and northward translations on the margin of the advancing North American plate. The fold test and polarity reversal pattern indicate that all plutons acquired a primary thermoremanent magnetization (TRM) during cooling and probably within ≈ 1 Ma after emplacement.

The Queen Charlotte Islands refraction project. Part I. The Queen Charlotte Fault Zone

1988 ◽  
Vol 25 (11) ◽  
pp. 1857-1870 ◽  
Author(s):  
Sonya A. Dehler ◽  
Ron M. Clowes
Keyword(s):  
Fault Zone ◽  
Structural Model ◽  
Seismic Refraction ◽  
Oceanic Lithosphere ◽  
North American Plate ◽  
Ocean Bottom ◽  
Queen Charlotte Islands ◽  
Lower Terrace ◽  
Vertical Fault ◽  
Ocean Bottom Seismographs

The active margin between the continental North American plate and oceanic Pacific plate west of the Queen Charlotte Islands was the site of an extensive onshore–offshore seismic refraction project in 1983. An airgun line shot over two ocean-bottom seismographs (OBS's) and a 32-charge explosion line recorded on the two OBS's and eight land-based seismographs (LBS's) deployed across northern Moresby Island were selected to study the structure of the predominantly transform Queen Charlotte Fault Zone and the associated offshore terrace. Two-dimensional ray tracing and synthetic seismogram modelling produced a pronounced laterally varying velocity structural model showing three major crustal components (oceanic, terrace, and continental) separated by an outer, crustally pervasive fault and active Queen Charlotte Fault, respectively. The 3 km thick block-faulted upper terrace unit, overlain by deformed sediments, is indistinguishable from adjacent oceanic sediments and upper crustal basalts located to the west. The upper part of the 10–17 km thick lower terrace unit has anomalously low velocities relative to the adjacent oceanic and continental crustal units. A high gradient increases terrace velocity rapidly with depth until the contrast becomes negligible at approximately 17 km depth. Changes in depth to Moho beneath the terrace suggest an increase in eastward Moho dip from 2–5 °observed west of the terrace to 19 °below it. Tectonic mechanisms proposed to explain the anomalous terrace structure involve sediment accretion during subduction of oceanic lithosphere, alternating or combined with compressive upthrusting of material along near-vertical fault planes during periods of active transform motion.

scholarly journals Global Eocene tectonic unrest: Possible causes and effects around the North American plate

2019 ◽  
Vol 760 ◽  
pp. 136-151 ◽  
Author(s):  
Carmen Gaina ◽  
Johannes Jakob
Keyword(s):  
North American ◽  
North American Plate ◽  
The North

North American Plate

2019 ◽  
pp. 441-441
Keyword(s):  
North American ◽  
North American Plate

Quaternary Faulting Along the Caribbean–North American Plate Boundary in Central America

1979 ◽  
pp. 431-445 ◽  
Author(s):  
DAVID P. SCHWARTZ ◽  
LLOYD S. CLUFF ◽  
THOMAS W. DONNELLY
Keyword(s):  
Central America ◽  
North American ◽  
Plate Boundary ◽  
North American Plate ◽  
The Caribbean
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