Allochthonous Jurassic ophiolite in northwest Washington

Whetten, John T.; Zartman, Robert E.; Blakely, Richard J.; Jones, David L.

doi:10.1130/0016-7606(1980)91<359:ajoinw>2.0.co;2

Cited by 36 publications

(16 citation statements)

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“…The Haystack nappe, described more fully later in the text, consists of arc related ophiolite and cover rocks of the Fidalgo Complex (Brown et al 1979) and contiguous mélange belts to the southeast in the Cascades foothills (Whetten et al 1980;Frizzell et al 1987;Tabor 1994). This nappe is of special interest here because the isotopic ages of Tabor (1994) and a new age presented here indicate emplacement of the nappe during or after deposition of parts of the Nanaimo Group.…”

Section: San Juan Islands -Northwest Cascades Thrust Systemmentioning

confidence: 88%

“…The mutual structural relations of the major components have been viewed in different ways. In the original concept of Whetten et al (1980) the eastern and western mélange belts were footwall to the Haystack nappe, whereas in the Tabor (1994) analysis these units lie structurally above the Helena-Haystack mélange. The Tabor model is based on the observations that the Helena-Haystack mélange lies structurally between the underlying Easton terrane to the north and the eastern and western mélange belts to the south and contains blocks of the adjacent units.…”

Section: Details Of the Haystack Nappementioning

confidence: 98%

“…The nappe was originally conceived by Whetten et al (1980) to consist of the Fidalgo ophiolite in the San Juan Islands and the Haystack complex in the Cascade foothills, based on mapped thrust faults putting these units in the hanging wall over other terranes, and a strong aeromagnetic anomaly indicating continuity of ultramafic rocks from the Haystack area to the San Juan Islands (Fig. 2).…”

Section: Details Of the Haystack Nappementioning

confidence: 99%

“…Subsequent studies, including the present data, support and refine this interpretation by further definition of the age and geographic limits of the nappe. In the model of Whetten et al (1980), the Haystack nappe is marked by ophiolitic klippen throughout most of the Cascade foothills, from the Twin Sisters Dunite and gabbros in the Easton Suite in the north to large ultramafic and gabbroic tectonic blocks in the eastern and western mélange belts to the south. However, this broad extent of the Haystack nappe is not supported by other studies that show the structural setting of ophiolitic rocks far north and south of Haystack Mountain to be phacoids structurally interleaved within, not on top of, the regional units with which they are associated (Brown 1987;Gallagher et al 1988;Tabor et al 1993;Tabor 1994).…”

Section: Details Of the Haystack Nappementioning

confidence: 99%

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Obducted nappe sequence in the San Juan Islands – northwest Cascades thrust system, Washington and British Columbia

Brown

2012

Can. J. Earth Sci.

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The San Juan Islands -northwest Cascades thrust system in Washington and British Columbia is composed of previously accreted terranes now assembled as four broadly defined composite nappes stacked on a continental footwall of Wrangellia and the Coast Plutonic Complex. Emplacement ages of the nappe sequence are interpreted from zircon ages, field relations, and lithlogies, to young upward. The basal nappe was emplaced prior to early Turonian time (∼93 Ma), indicated by the occurrence of age-distinctive zircons from this nappe in the Sidney Island Formation of the Nanaimo Group. The emplacement age of the highest nappe in the thrust system postdates 87 Ma detrital zircons within the nappe. The nappes bear high-pressure -low-temperature (HP-LT) mineral assemblages indicative of deep burial in a thrust wedge; however, several features indicate that metamorphism occurred prior to nappe assembly: metamorphic discontinuities at nappe boundaries, absence of HP-LT assemblages in the footwall to the nappe pile, and absence of significant unroofing detritus in the Nanaimo Group. A synorogenic relationship of the thrust system to the Nanaimo Group is evident from mutually overlapping ages and by conglomerates of Nanaimo affinity that lie within the nappe pile. From the foregoing relations, and broader Cordilleran geology, the tectonic history of the nappe terranes is interpreted to involve initial accretion and subduction-zone metamorphism south of the present locality, uplift and exhumation, orogen-parallel northward transport of the nappes as part of a forearc sliver, and finally obduction at the present site over the truncated south end of Wrangellia and the Coast Plutonic Complex.Résumé : Le système de chevauchement formé par l'archipel San Juan et le nord-ouest de la chaîne des Cascades dans l'état de Washington et la province de la Colombie-Britannique est composé de terranes anciennement accrétés et qui sont maintenant assemblés, au sens large, en quatre nappes composites empilées sur l'éponte inférieure continentale de la Wrangellie et du complexe plutonique côtier. Les âges de la mise en place de la séquence des nappes ont été détermi-nés à partir d'âges de zircons, de relations sur le terrain et des lithologies; ils sont plus jeunes vers le haut. La nappe à la base a été mise en place avant le Turonien précoce (∼93 Ma), ce qui est indiqué par la présence de zircons à âge distinctif provenant de cette nappe dans la Formation de Sidney Island du Groupe de Nanaimo. L'âge de la mise en place de la nappe supérieure dans le système de chevauchement est plus tardif que les zircons détritiques âgés de 87 Ma dans cette nappe. Les nappes comportent des assemblages de minéraux de haute pression -basse température (HP-BT) indiquant un enfouissement profond dans un prisme de chevauchement; toutefois, plusieurs caractéristiques indiquent que le métamorphisme est survenu avant l'assemblage de la nappe : des discordances métamorphiques à la limite des nappes, l'absence d'assemblages HP-BT dans l'éponte inférieure de l'empilement d...

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Section: San Juan Islands -Northwest Cascades Thrust Systemmentioning

confidence: 88%

Section: Details Of the Haystack Nappementioning

confidence: 98%

Section: Details Of the Haystack Nappementioning

confidence: 99%

Section: Details Of the Haystack Nappementioning

confidence: 99%

See 2 more Smart Citations

Obducted nappe sequence in the San Juan Islands – northwest Cascades thrust system, Washington and British Columbia

Brown

2012

Can. J. Earth Sci.

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“…These papers postulated five major, single-plane thrust sheets in the islands that constituted the San Juan Island Thrust (Nappe) System. This hypothesis has remained unchallenged for 40 years and forms the basic structural framework for many published papers [9][10][11][12][13][14][15][16][17][18][19][20][21].…”

Section: David Publishingmentioning

confidence: 99%

The San Juan Islands Thrust System: New Perspectives from LIDAR and Sonar Imagery

Easterbrook¹

2015

JEASE

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New LIDAR (Light Detection and Ranging) and sonar imagery have revealed remarkable geomorphic details never seen before and not visible by any other means. Numerous faults and other geologic structures are plainly visible on LIDAR and sonar images. Many previously unknown faults criss-cross the islands and large fault scarps are visible on sonar imagery along the margins of the larger islands. Sonar images of sea floor morphology show many submerged faults as long linear scarps with relief up to 300m (1,000 ft), some of which visibly truncate geologic structures. The San Juan Lopez fault, the largest fault in the islands, extends for at least 65 km (40 mi) from Stuart Island to Rosario strait with a scarp up to 330m (1,000 ft) high. Since 1975, the basic structural framework of the San Juan Islands has been considered to consist of five stacked thrust faults, the Rosario, Orcas, Haro, Lopez, and Buck Bay faults, constituting the San Juan Thrust (Nappe) System that has shuffled together far distant terranes. However, the new LIDAR and sonar imagery shows that most of the mapped extent of these postulated faults are actually segments of high angle, dipslip faults and are not thrust faults at all. Thus, the San Juan Thrust (Nappe) System does not exist. The age of these faults is not accurately known and more than one period of high angle faulting may have occurred. Faults shown on LIDAR images of the surface of the islands appear as visible gashes, etched out by erosion of fault zones with few fault scarps. However, the sea floor faults have bold relief and high scarps. A late Pleistocene moraine lies undisturbed across the San Juan Lopez fault.

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The Interplay of Accretionary and Attritionary Tectonics along the California Margin

Saleeby

Gehrels

1988

The Ocean Basins and Margins

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Allochthonous Jurassic ophiolite in northwest Washington

Cited by 36 publications

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Obducted nappe sequence in the San Juan Islands – northwest Cascades thrust system, Washington and British Columbia

Obducted nappe sequence in the San Juan Islands – northwest Cascades thrust system, Washington and British Columbia

The San Juan Islands Thrust System: New Perspectives from LIDAR and Sonar Imagery

The Interplay of Accretionary and Attritionary Tectonics along the California Margin

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