Insular Superterrane Research Articles

Orogen-scale inverted metamorphism during Cretaceous−Paleogene terminal suturing along the North American Cordillera, Alaska, USA

The northern North American Cordilleran margin has been active for >200 million years, as recorded by punctuated phases of crustal growth and deformation. Accretion of the exotic Wrangellia Composite Terrane (Insular Belt) is considered the largest addition of juvenile crust to the Cordilleran margin, though margin-parallel translation during the Cenozoic has obscured much of the accretionary history. Three zones of inverted metamorphism spatially correspond to the Insular−North American suture zone from north to south: (1) Clearwater Mountains; (2) Kluane Lake; and (3) Coast Mountains, each preserving kinematics indicative of thrusting of North American−derived rocks over Insular-derived assemblages. We performed in situ monazite petrochronology on samples collected across strike in both the Clearwater and Coast Mountain regions. New and recently published data from these three metamorphic belts indicate that thrust-sense deformation accompanied the formation of inverted metamorphic isograds from 72 to 56 Ma. We leverage recent estimates of Denali fault offset to reconstruct a >1000-km-long zone of inverted metamorphism and interpret it as the Insular−North America terminal suture.

Jura-Cretaceous synorogenic magmatism and relations to supercontinental rifting in the northwestern U.S. Cordillera

Changes in lithospheric composition and strength associated with rifting of the Rodinian supercontinent (ca. 780−485 Ma) served to localize Mesozoic tectonic activity in the McCall region of west-central Idaho, USA. Although discrimination diagrams support slab failure melting, ca. 150−121 Ma calcic magmatism in the Hazard Creek complex and chemically similar intrusive rocks of northeastern Oregon, USA (e.g., Cornucopia stock), is best explained by progressive loading of oceanic crust. Westward-directed thrusting across the Laurentian continental margin and adjacent island-arc terranes (Olds Ferry/Wallowa; Blue Mountains province) led to partial melting of metabasaltic source rocks at pressures and depths sufficient for garnet stability (>10 kbar, ∼35 km). Early pluton emplacement during right-oblique arc−continent collision overlapped in time with subduction zone reorganization, which involved a change in dip direction (westward to eastward) under the accreting Blue Mountains block. Ensuing magmatic activity predated and was concurrent with ca. 116 Ma thrust reactivation of the Wallowa/Olds Ferry boundary, which temporally overlapped with contractional deformation along the terrane−continent accretionary boundary. Later synorogenic magmatism was focused along the accretionary boundary, where rising anatectic melts were caught between the eastward-impinging/northward-migrating (obliquely colliding) terrane block and continental margin backstop (Rodinian rift architecture). Middle Cretaceous dextral transpression viewed in this manner precludes the need for an outboard “hit-and-run” collider (Insular superterrane) and thus, a post-accretion shear zone in western Idaho.

Late Jurassic paleogeography of the U.S. Cordillera from detrital zircon age and hafnium analysis of the Galice Formation, Klamath Mountains, Oregon and California, USA

The Upper Jurassic Galice Formation, a metasedimentary unit in the Western Klamath Mountains, formed within an intra-arc basin prior to and during the Nevadan orogeny. New detrital zircon U-Pb age analyses (N = 11; n = 2792) yield maximum depositional ages (MDA) ranging from ca. 160 Ma to 151 Ma, which span Oxfordian to Kimmeridgian time and overlap Nevadan contractional deformation that began by ca. 157 Ma. Zircon ages indicate a significant North American continental provenance component that is consistent with tectonic models placing the Western Klamath terrane on the continental margin in Late Jurassic time. Hf isotopic analysis of Mesozoic detrital zircon (n = 603) from Galice samples reveals wide-ranging εHf values for Jurassic and Triassic grains, many of which cannot be explained by a proximal source in the Klamath Mountains, thus indicating a complex provenance. New U-Pb ages and Hf data from Jurassic plutons within the Klamath Mountains match some of the Galice Formation detrital zircon, but these data cannot account for the most non-radiogenic Jurassic detrital grains. In fact, the in situ Cordilleran arc record does not provide a clear match for the wide-ranging isotopic signature of Triassic and Jurassic grains. When compiled, Galice samples indicate sources in the Sierra Nevada pre-batholithic framework and retroarc region, older Klamath terranes, and possibly overlap strata from the Blue Mountains and the Insular superterrane. Detrital zircon age spectra from strata of the Upper Jurassic Great Valley Group and Mariposa Formation contain similar age modes, which suggests shared sediment sources. Inferred Galice provenance within the Klamath Mountains and more distal sources suggest that the Galice basin received siliciclastic turbidites fed by rivers that traversed the Klamath-Sierran arc from headwaters in the retroarc region. Thus, the Galice Formation contains a record of active Jurassic magmatism in the continental arc, with significant detrital input from continental sediment sources within and east of the active arc. These westward-flowing river systems remained active throughout the shift in Cordilleran arc tectonics from a transtensional system to the Nevadan contractional system, which is characterized by sediment sourced in uplifts within and east of the arc and the thrusting of older Galice sediments beneath older Klamath terranes to the east.

Evidence for regionally continuous Early Cretaceous sinistral shear zones along the western flank of the Coast Mountains, coastal British Columbia, Canada

Abstract The plate-boundary conditions of the Mesozoic North American Cordillera remain poorly constrained, but most studies support large (>800 km) southward motion of the Insular and Intermontane superterranes during Jurassic–Cretaceous time. An implicit feature in these models of large coastwise displacements is the presence of one or more continentalscale sinistral strike-slip faults that could have dismembered and displaced terrane fragments southward along the western margin of North America prior to the onset of mid-Cretaceous shortening and dextral strike-slip faulting. In this study, we documented a system of sinistral intra-arc shear zones within the Insular superterrane that may have accommodated large southward motion. Employment of a new large-n igneous zircon U-Pb method more than doubled the precision of measurements obtained by laser ablation–inductively coupled plasma–mass spectrometry (from ~1% to 0.5%) and allowed us to demonstrate the close temporal-spatial relationship between magmatism and deformation by dating comagmatic crosscutting phases. Crystallization ages of pre-, syn-, and postkinematic intrusions show that the intra-arc shear zones record an Early Cretaceous phase of sinistral oblique convergence that terminated between 107 and 101 Ma. Shear zone cessation coincided with: (1) collapse of the Gravina basin, (2) development of a single voluminous arc that stitched the Insular and Intermontane superterranes together, and (3) initiation of eastwest contractional deformation throughout the Coast Mountains. We interpret these concurrent tectono-magmatic events to mark a shift in plate kinematics from a sinistral-oblique system involving separate terranes and intervening ocean basins to a strongly convergent two-plate margin involving a single oceanic plate and the newly assembled western margin of North America.

Improved Provenance Constraints for Nanaimo Group Sediments, British Columbia, Canada, Through Zircon LA‐ICP‐MS Depth‐Profiling

AbstractDetrital zircon from Late Cretaceous rocks of the Insular Superterrane have become an important constraint on Cordilleran paleogeography. These interpretations are largely based on the occurrence of a unique detrital zircon facies characterized by a broad Paleoproterozoic (mode 1.70 Ga) and narrow Mesoproterozoic mode (1.38 Ga) in Cordilleran sedimentary rocks. However, uncertainty about the provenance of this facies and the broad geographic distribution of rocks in which it is found limits its usefulness in constraining paleogeography. Detrital rims found on Mesozoic, Proterozoic, and Archean‐aged zircon cores from the Late Cretaceous Nanaimo Group of Western British Columbia, Canada provide constraints into its provenance. Depth‐profiling of a large population of grains (N = 16; n = 3,000), combined with observations regarding zircon external morphology, internal growth textures, U/Th ratios, and U concentrations, indicate high‐grade metamorphism occurred in the sediment source area between 99 Ma and 72 Ma. Rapid cooling of the metamorphic source area at rates of 35 to over 100°C/m.y. between 84 and 63 Ma was required to supply these metamorphic zircon to the basin. The geological history of the sediment source region defined by the rim populations (age of metamorphism and exhumation rate) is inconsistent with previously proposed sediment source areas for the Nanaimo Group in southern (Mojave region) or northern Laurentia (Lehmi Subbasin). We hypothesize that the sediment source area was the adjacent Coast Mountains Batholith and a metasedimentary rock, similar to the Pelona‐Orocopia‐Rand‐type schists, that was exhumed during the Late Cretaceous.

U‐Pb Zircon Geochronology From the Northern Cordillera, Central Yukon, With Implications for Its Tectonic Assembly

AbstractThe tectonic assembly of the Northern Cordillera is currently disputed and directly impacts Paleozoic‐to‐recent paleogeographic and plate tectonic reconstructions of North America. In this study, we present new U‐Pb zircon geochronology from the allochthonous Yukon‐Tanana terrane and the parautochthonous Cassiar terrane of the Northern Cordillera from south‐central Yukon, Canada. Our data provide new constraints for the assembly of the Northern Cordillera in this region. Metasedimentary samples from the Ingenika Group (Cassiar terrane) and the Snowcap and Finlayson assemblages (Yukon‐Tanana terrane) yielded detrital zircon age spectra that are comparable to known northwest Laurentia age spectra. One of our samples from the Snowcap assemblage yields a detrital zircon age spectrum that is anomalous for northwest Laurentia, but comparable to Early Paleozoic strata deposited in the Nevada‐Idaho‐Utah region. Zircon rim growth and Pb‐loss recorded by detrital zircon in the Snowcap assemblage and Ingenika Group samples record metamorphism at 370 ± 4 Ma and between 171 ± 5 and 135 ± 3 Ma. Late Devonian metamorphism and magmatism possibly corresponds to rifting of the Snowcap assemblage from the Laurentian margin. Middle Jurassic‐Early Cretaceous metamorphic zircon rim ages from the Cassiar terrane record metamorphism during collisions between the Intermontane superterrane (including the Yukon‐Tanana terrane) and Laurentia (Early to Middle Jurassic), and between the Insular superterrane and Laurentia (Middle to Late Jurassic). Our study suggests that collision between the Yukon‐Tanana terrane and the Laurentian margin began no earlier than ∼205 Ma.

Seismic Rupture on an Oceanic-Continental Plate Boundary: Strike-Slip Earthquakes along the Queen Charlotte-Fairweather Fault

Abstract We investigate the 2013 M w 7.5 Craig, Alaska, earthquake and nearby seismicity to understand better how temperature and composition may control the depth of seismic rupture along a strike‐slip fault offsetting contrasting lithosphere types. The Queen Charlotte–Fairweather (QCF) fault lies between the oceanic lithosphere of the Pacific plate and the accreted Insular superterrane of the North American continent. We use point‐source and finite‐fault modeling of teleseismic body waves to characterize the focal mechanism and the depth extent of seismic rupture of five M w 5.9–7.5 earthquakes. Four of the five earthquakes are consistent with rupture on the QCF fault. We find that these four earthquakes have centroid depths between 11 and 18 km (±3 km) and aftershocks with an order of magnitude less than typical continental earthquakes. Finite‐fault modeling of the 2013 Craig earthquake favors bilateral rupture along a 150 km fault with a depth range of slip between 5 and 25 km, with faster rupture (4–5 km/s) to the north than the south (1 km/s). These results suggest that the transition of brittle to ductile deformation along this section of the Pacific–North American plate boundary is thermally controlled by a more mafic rheology than average continental crust, exhibiting behavior consistent with that of an oceanic strike‐slip fault. Online Material: Figures of best‐fitting point‐source focal mechanism and waveform fits, dip sensitivity, directivity tests, aftershock distributions, and finite‐fault results.

Garnet growth during crustal thickening in the Cascades Crystalline Core, Washington, USA

Garnet Sm–Nd and zircon U–Pb ages, and pressure–temperature–time paths elucidate Late Cretaceous crustal thickening which occurred within magmatic arc rocks of the Insular Superterrane. Voluminous tonalitic magma of the Mount Stuart batholith intruded at <3 kbar into upper crustal sedimentary rocks between 96 and 91 Ma, with initial intrusion prior to garnet growth in the metasedimentary rocks. Arc plutonism then shifted northward as crustal thickening commenced. Initial garnet growth, locally with kyanite and staurolite replacing andalusite, at c. 91 Ma was directly associated with intrusion of granodiorite to tonalite sheets at 7 kbar, north of the Mount Stuart batholith, within the Nason Ridge Migmatitic Gneiss. Subsequent heating and garnet growth, which postdates emplacement of large plutons, occurred between 88 and 86 Ma. This late garnet growth occurred at pressures of 6–8 kbar. The history of garnet growth and intrusion indicates that initial garnet zone and higher temperature metamorphism was restricted to contact aureoles. However, later widespread garnet growth at higher pressure probably resulted from heating as the orogenic wedge approached thermal equilibrium after crustal thickening. We conclude that metasedimentary rocks outside narrow contact aureoles remained at temperatures significantly below those of garnet growth and that the growth of garnet lasted <6 Myr. Heating to temperatures that stabilized garnet after pluton emplacement is compatible with intrusion of arc plutons into an accretionary wedge (Chiwaukum Schist) which was tectonically thickened and/or overthrust causing loading and thermal relaxation.

Geochemical evidence of a near-surface history for source rocks of the central Coast Mountains Batholith, British Columbia

Major and trace elemental concentrations as well as Sr and Pb isotopic data, obtained for 41 plutonic samples from the Coast Mountains Batholith ranging in age from ∼108 to ∼50 Ma, indicate that the source regions for these rocks were relatively uniform and typical of Cordilleran arcs. The studied rocks are mineralogically and chemically metaluminous to weakly peraluminous and are mainly calc-alkaline. Initial whole-rock 87Sr/86Sr ratios range from 0.7035 up to 0.7053, whereas lead isotopic data range from 18.586 to 19.078 for 206Pb/204Pb, 15.545 to 15.634 for 207Pb/204Pb, and 37.115 to 38.661 for 208Pb/204Pb. In contrast to these relatively primitive isotopic data, δ 18O values for quartz separates determined for 19 of the samples range from 6.8 up to 10.0‰. These δ 18O values preclude the possibility that these melts were exclusively generated from the Mesozoic mantle wedge of this continental arc, just as the Sr and Pb data preclude significant involvement of an old (Precambrian) crustal/mantle lithospheric source. We interpret the high δ 18O component to represent materials that had a multi-stage crustal evolution. They were originally mafic rocks derived from a circum-Pacific juvenile mantle wedge that experienced a period of near-surface residence after initial crystallization. During this interval, these primitive rocks interacted with meteoric waters at low temperatures, as indicated by the high δ 18O values. Subsequently, these materials were buried to lower crustal depths where they remelted to form the high δ 18O component of the Coast Mountains Batholith. This component makes up at least 40% (mass) of the Cretaceous through Eocene batholith in the studied area. The remainder of the source materials comprising the Coast Mountains Batholith had to be new additions from the mantle wedge. A prolonged period of contractional deformation beginning with the Early Cretaceous collisional accretion of the Insular superterrane is inferred to have been responsible for underthrusting the high δ 18O component into the lower crust. We suggest that mafic rocks of the Insular superterrane (e.g. Alexander–Wrangellia) are of appropriate composition, and were accreted to and overthrust by what would become the Coast Mountains Batholith just prior to initiation of magmatism in the region.

Further taxonomic differentiation in the quadrispinosa complex of Dicraspeda Chaudoir in the Papuan Region and additional records of the genus (Coleoptera: Carabidae: Odacanthinae).

The peculiar odacanthine species Dicraspeda quadrispinosa Chaudoir, 1869 is composed of several quite distinct populations which provisionally were described as subspecies (Baehr 2006a). The species ranges from Halmahera in the north-west, through New Guinea and New Britain, to the Solomon Islands in the south-east. From the latter island group, however, the species was so far reliably recorded only from Bougainville Island. The species belongs to the quite diverse Indo-Australian genus Dicraspeda Chaudoir, 1862 which occupies a large range that extends from southern mainland Asia through the Indonesian and Philippine insular belts, the Papuan subregion, into northern and eastern Australia. Within the genus, D. quadrispinosa is distinguished by large size, glossy black colour, and quadrispinose elytra whose spines are remarkably elongate when compared with other species of the genus. Baehr (2006a) examined a large number of specimens throughout the species’ range and described three subspecies: moluccensis from Halmahera, novabritannica from New Britain, and brevipennis from Bougainville Island in the Solomon Islands, while the specimens from New Guinea belong to the nominate subspecies. During a recent visit at National Museum of Natural History Naturalis in Leiden and while examining a collection of New Guinean carabid beetles from the Zoological Museum of the University of Amsterdam, I sorted out a number of specimens of D. quadrispinosa from various localities throughout its range which turned out to be quite different in their external and genitalic morphology and thus herein are described as additional new subspecies. The material includes in particular specimens from a number of islands of the Solomon Archipelago which exhibit strikingly different morphological character states Further taxonomic differentiation in the quadrispinosa complex of Dicraspeda Chaudoir in the Papuan Region and additional records of the genus (Coleoptera: Carabidae: Odacanthinae)

Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho

Research Article| September 01, 2008 Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho Scott Giorgis; Scott Giorgis † 1Department of Geological Sciences, State University of New York–Geneseo, 1 College Circle, Geneseo, New York 14454, USA †E-mail: giorgis@geneseo.edu Search for other works by this author on: GSW Google Scholar William McClelland; William McClelland 2Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA Search for other works by this author on: GSW Google Scholar Annia Fayon; Annia Fayon 3Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455-0219, USA Search for other works by this author on: GSW Google Scholar Brad S. Singer; Brad S. Singer 4Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Basil Tikoff Basil Tikoff 4Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2008) 120 (9-10): 1119–1133. https://doi.org/10.1130/B26291.1 Article history received: 25 May 2007 rev-recd: 21 Feb 2008 accepted: 14 Mar 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Scott Giorgis, William McClelland, Annia Fayon, Brad S. Singer, Basil Tikoff; Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho. GSA Bulletin 2008;; 120 (9-10): 1119–1133. doi: https://doi.org/10.1130/B26291.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The western Idaho shear zone is one of several Cretaceous high-strain zones in the Cordillera that are thought to have been associated with the northward translation and/or docking of terranes presently in British Columbia. Located in west-central Idaho, this zone of intense deformation consists of a mid-crustal exposure of a lithospheric-scale, intra-arc dextral shear zone that overprints the Salmon River suture zone along the western edge of the Idaho batholith. U/Pb zircon geochronology constrains the main phase of deformation to between ca. 105 and 90 Ma. Cessation of movement on the shear zone occurred by 90 Ma, as determined by dating of the syntectonic Payette River tonalite and a crosscutting pegmatite dike in the Little Goose Creek complex. The 40Ar/39Ar thermochronology indicates that the shear zone passed through both the hornblende (~550 °C) and biotite (~325 °C) closure temperatures between 85 and 70 Ma. The 40Ar/39Ar biotite dates from an outcrop-scale, crosscutting shear zone are indistinguishable from that of the host rock, indicating that deformation occurred above the closure temperature of biotite. Apatite fission-track analysis suggests that exhumation to shallow crustal levels occurred ca. 40 Ma during mid-Tertiary regional exhumation or renewed tectonic activity along the Salmon River suture zone. Taken together, the 40Ar/39Ar results and apatite fission-track analyses indicate a two-stage uplift history for the western Idaho shear zone. Overall, the geochronology indicates that dextral transpressional movement on the western Idaho shear zone was temporally distinct from the Early Cretaceous suturing event. Additionally, the first stage of exhumation recorded by the western Idaho shear zone immediately followed transpressional deformation. Cessation of displacement on the western Idaho shear zone by ca. 90 Ma indicates that the exhumation did not solely occur as a result of ductile deformation on the shear zone itself. Moreover, dextral strike-slip movement on the western Idaho shear zone had also ceased by 90 Ma, indicating that terrane translation models for the Cordillera can only use the western Idaho shear zone to accommodate northward translation up to ca. 90 Ma. Lastly, the timing of movement on the western Idaho shear zone and contractional deformation recorded in the Insular terrane suggests a correlation between these events. This hypothesis implies that the deformation recorded in the western Idaho shear zone may have been linked to the oblique collision of the Insular superterrane with North America. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

New taxa of Dolichoctis Schmidt-Goebel and Paradolichoctis Baehr from the Australian-Papuan Region (Coleoptera: Carabidae: Lebiinae)

According to the catalogue of Australian Carabid beetles (Moore et al.1987) three species of the large Indo-Australian genus Dolichoctis Schmidt-Goebel, 1846 occur in Australia: the widespread D. striata Schmidt-Goebel, 1846 of the nominate subgenus that is mainly characterized by unarmed and usually spotted elytra, and D. aculeata Chaudoir, 1869 and D. subrotunda Darlington, 1968, subgenus Spinidolichoctis Baehr, 1999 which occurs only in the Papuan Subregion and is characterized by the dentate or spinose apex of the elytra and usually uniformly black colour. Whereas the first two species were recorded from Iron Range in mid Cape York Peninsula, northern Queensland, Moore did not give any Australian locality record for D. subrotunda. Both, D. aculeata and D. subrotunda are common in New Guinea, and D. aculeata occurs also on Sulawesi, the Moluccas, New Britain, and New Ireland (Baehr 1999). D. striata certainly is the most widespread species of the genus and is said to occur throughout the whole range of the genus, which extends from India and Nepal in the northwest, through the whole of tropical Asia to Japan in the east, and in the south through the Indonesian and Philippine insular belts to New Guinea, the Bismarck Archipelago, Solomon Islands, and northeastern Australia. However, the taxonomic status of many populations of D. striata is not yet settled and so a comparative examination of the whole complex is currently carried out. It seems that the Australian “D. striata” are different from those from mainland Asia, and that the whole complex consists of several different taxa. Moreover, most populations which usually are referred as “D. striata” probably do not belong to this species, because in most previous comprehensive papers the real D. striata seems to have been confounded with other species. The species of the genus Dolichoctis occurring in the Papuan Subregion have been revised first by Darlington (1968) in his monumental treatment of the Carabidae of New Guinea, then by the author (Baehr 1999, 2003a, b, 2006). At present 44 taxa are recorded from this area, most of these occurring in New Guinea (see checklist in Baehr 2006). In the course of my work on the Papuan species of New taxa of Dolichoctis Schmidt-Goebel and Paradolichoctis Baehr from the Australian-Papuan Region (Coleoptera: Carabidae: Lebiinae)

U–Th–Pb geochronologic constraints on the structural evolution of the Selkirk fan, northern Selkirk Mountains, southern Canadian Cordillera

In the southern Canadian Cordillera a zone of structural divergence marks the eastward transition from penetrative ductile deformation and metamorphism in the Omineca belt to the more brittle ‘thin-skinned’ style of deformation typical of the Foreland belt. In the Selkirk Mountains of southern British Columbia, this zone includes a regional-scale structure termed the Selkirk fan. The fan trends northwest, consists primarily of medium- to high-grade metamorphic rocks, and comprises at least three generations of superposed structures. IDTIMS and SHRIMP analyses provide new U–Th–Pb age constraints for the structural evolution of the Selkirk fan. The data demonstrate that the thermo-structural development of the fan's west flank occurred principally in the Middle Jurassic (ca. 172–167 Ma), whereas in the east flank significant Cretaceous (ca. 104–84 Ma) deformation was superimposed on an early transposition fabric. These data require revision of previous models that concluded fan formation occurred primarily during Middle Jurassic time. Rather, the Selkirk fan is a composite structure comprising Middle Jurassic and Cretaceous deformation. Development of the fan during the Early–Middle Jurassic accretion of the Intermontane Superterrane was followed by extensive reworking and tightening of structures in the fan's east flank during the Cretaceous accretion of the Insular Superterrane.

Structural correction of paleomagnetic vectors dispersed about two fold axes and application to the Duke Island (Alaska) ultramafic complex

A new analysis of paleomagnetic data from the mid‐Cretaceous (∼110 Ma) ultramafic complex at Duke Island (southeast Alaska) supports large poleward transport of the Insular superterrane relative to North America consistent with the Baja British Columbia hypothesis. Previous paleomagnetic work has shown that the characteristic remanence of the ultramafic complex predates kilometer‐scale deformation of the very well developed cumulate layering but that the layering was not horizontal everywhere before the folding. It is possible, however, to estimate paleohorizontal for the Duke Island ultramafic complex because the postremanence deformation of the intrusion occurred about two well‐defined and spatially separate fold axes. In such a case the tectonically rotated paleomagnetic directions should be distributed along small circles centered on each of the two fold axes. The ancient field direction will lie on both small circles and therefore will be identifiable as one of their two intersection points. Interpreted this way, the tectonically rotated remanence of the Duke Island ultramafic complex defines a mid‐Cretaceous (i.e., ancient) field direction that is within 2° of the paleomagnetic direction found by assuming the cumulate layering was initially horizontal (despite the paleomagnetic evidence to the contrary) and performing the standard structure correction. The inferred mid‐Cretaceous paleolatitude of Duke Island is 21.2° (2350 km) anomalous with respect to cratonic North America. This result is concordant with southerly paleolatitudes determined by many other workers from bedded rocks of terranes farther inboard in the Insular and Intermontane superterranes.

A compaction correction for the paleomagnetism of the Nanaimo Group sedimentary rocks: Implications for the Baja British Columbia hypothesis

A paleomagnetic and magnetic anisotropy study of the Late Cretaceous Northumberland formation on Hornby Island, British Columbia, was conducted to determine if burial compaction could have caused its anomalously shallow inclinations. The shallow Nanaimo Group inclinations have been used to support the Baja British Columbia (Baja BC) model of continental dynamics in which superterranes were transported thousands of kilometers northward along the active North American continental margin in the Cretaceous. A mean of the site means from the Northumberland formation is D = 5.8°, I = 50.9°, α95 = 7.9° (N = 8). Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence (AAR) were measured to identify and correct for any inclination shallowing caused by depositional/compactional processes. Both concretion and fine‐grained rock samples had typical depositional/compactional AMS and AAR fabrics with bedding perpendicular minimum principal axes. A revised correction equation was used to account for the effects of either triaxial magnetic fabrics or small angular deviations of principal axes from the bedding plane. The average inclination was increased by 9.6° after the compaction correction (D = 6.1°, I = 60.5°, α95 = 7.1°, N = 8), and the mean inclination of concretion and fine‐grained rock sites was significantly steeper than Ward et al.'s [1997] result (I = 42.5°) placing Hornby Island and the Insular superterrane at a paleolatitude of 41°N in the Late Cretaceous. This paleolatitude indicates that Baja BC did translate northward since the Late Cretaceous by about 1600 km but not by the 3500 km previously indicated.

Neogene tilting of crustal panels near Wrangell, Alaska

A late Oligocene–Miocene igneous complex south and west of Wrangell, Alaska, contains mafic dikes that yield a discordant paleomagnetic direction (inclination, I = 70.4°; declination, D = 39.3°; α 95 = 4.8°; N = 72 sites). Combined with local and regional geobarometric, metamorphic, and structural observations, the discordant paleomagnetic direction indicates east-side-up tilt by 16° about a tilt axis with azimuth = 8°. Neogene tilt of crustal blocks in the Insular superterrane accounts for much of the paleomagnetic discordance in Cretaceous plutons without the coastwise translation of >1000 km, as suggested by the Baja British Columbia hypothesis.

Deciphering shallow paleomagnetic inclinations: 1. Implications from correlation of Albian volcanic rocks along the Insular/Intermontane Superterrane boundary in the southern Canadian Cordillera

Geologic and paleomagnetic data lead to two contradictory hypotheses regarding the paleoposition of the Insular and Intermontane Superterranes that presently constitute the western Canadian Cordillera. Paleomagnetic data from the Insular and Intermontane superterranes suggest a southerly origin coinciding with the latitude of Mexico and the northwest United States, respectively, during the mid‐Cretaceous. Geologic evidence points to a northerly origin for these same tectonic entities during this period; both models cannot be correct. Geologic and paleomagnetic data from the Empire Valley–Churn Creek area in south central British Columbia (51.5°N, 122.5°W) are critical to resolving these contradictory hypotheses. Late Cretaceous rocks correlated to the Insular Superterrane with large paleomagnetic displacements unconformably overlie mid‐Cretaceous rocks correlative to the Spences Bridge Group of the Intermontane Superterrane. We provide paleomagnetic evidence of this correlation based on similar magnetic properties, opaque mineral assemblages, demagnetization behavior, fold test results, mean inclinations, clockwise vertical axes rotations, and statistically indistinguishable paleomagnetic poles and displacement estimates. This correlation and the observed geologic relationships in the Empire Valley–Churn Creek area indicate that the Insular and Intermontane Superterranes were linked by the mid‐Cretaceous. Sites from the two previous Spences Bridge Group studies are combined with their correlatives in the Empire Valley–Churn Creek area to give 81 sites that yield a paleomagnetic pole of 60.5°N, 304.5°E, dp = 3.7°, dm = 5.5° which corresponds to 1050 ± 450 km of displacement from the south. This new displacement estimate suggests that the Spences Bridge arc formed at the latitude of southern Oregon during the mid‐Cretaceous.

Geology of Denman and Hornby islands, British Columbia: implications for Nanaimo Basin evolution and formal definition of the Geoffrey and Spray formations, Upper Cretaceous Nanaimo Group

The Upper Cretaceous Nanaimo Group of southwest British Columbia is a &gt;4 km-thick succession consisting mostly of deep marine siliciclastics deposited directly on the Insular Superterrane. As such, this succession has been the focus of several paleomagnetic, isotope geochemistry, paleontology, and sedimentology studies in attempts to elucidate the tectonic history and paleolatitude of the Insular Superterrane and associated entities during the critical time of Nanaimo Group deposition (ca. 90–65 Ma). However, disagreement as to whether deposition occurred into a single or multiple basins has led to confusion concerning the formal stratigraphy and formation names for the succession, and has resulted in problems with both local and regional correlations. The upper two-thirds of the succession is continuously and well exposed on Denman and Hornby islands and represents the best example of this part of the succession in the northern half of what we consider the single Nanaimo Basin. This area includes the previously only informally defined type areas for the Geoffrey and Spray formations, defined here formally for the first time with type sections and detailed descriptions. New interpretations of the geology of these islands demonstrate that previously interpreted major faults do not exist, resulting in stratigraphic and age controls that are both different and simpler than previously interpreted. The redefined stratigraphy of the northern part of the basin is remarkably similar to that of southern areas in both type and age, affirming both a single basin evolution and a single stratigraphic nomenclature.

Syntectonic remagnetization in the southern Methow block: Resolving large displacements in the southern Canadian Cordillera

The Upper Cretaceous Ventura Member of the Goat Wall unit in the southern Methow block of southern British Columbia and northern Washington State holds a syntectonic magnetization. Eight new sites from Manning Park in British Columbia give a mean direction of D = 27.5°, I = 60.1°, k = 304.7, α95 = 3.2° after optimal partial tilt correction. Of five groups of bedded sites from farther south in the basin reported by Bazard et al. [1990], four have a syntectonic remanence with a direction similar to what we observe. The exception is one group which has optimal concentration of remanence directions on &gt;100% untilting and an abherent direction which must be rejected. Combining the accepted sites, the optimal differential syntilting direction is D = 11.8°, I = 61.5°, k = 39.3, α95 = 3.4° (N = 47), giving a mean pole of 79.8°N, 359.2°E, K = 19.5, and A95 = 4.8°. The age of the remagnetization is constrained to be between 88 and 80 Ma. Compared to cratonic North America, this result indicates that the southern Methow block was displaced from the south by 1800 ± 500 km, meaning it lay south of the Sierra Nevada subduction zone but well north of other paleomagnetically constrained Cretaceous rock units from the Insular superterrane, including correlative strata of the Mount Tatlow area in the northern Methow block. Among several possibilities to reconcile this discrepancy, the most plausible has the whole Methow block translated coherently but with the southern Methow block strata remagnetized during transit.

Paleomagnetism of the Upper Cretaceous Nanaimo Group, southwestern Canadian Cordillera

The Baja B.C. model has the Insular Superterrane and related entities of the Canadian Cordillera subject to &gt;3000 km of northward displacement with respect to cratonic North America from ~90 to ~50 Ma. The Upper Cretaceous Nanaimo Group (on and about Vancouver Island, British Columbia) is a prime target to test the model paleomagnetically because of its locality and age. We have widely sampled the basin (67 sites from seven islands spread over 150 km, Santonian to Maastrichtian age). Most samples have low unblocking temperatures (&lt;450°C) and coercivities (~10 mT) and strong present-field contamination, forcing us to reject three quarters of the collection. Beds are insufficiently tilted to provide a conclusive fold test, and we see evidence of relative vertical axis rotations. However, inclination-only analysis indicates pretilting remanence is preserved for many samples. Both polarities are observed, and reversals correlate well to paleontological data, proving that primary remanence is observed. The mean inclination, 55 ± 3°, is 13 ± 4° steeper than previously published results. Our new paleolatitude, 35.7 ± 2.6° is identical to that determined from the slightly older Silverquick and Powell Creek formations at Mount Tatlow, yet the inferred displacement is smaller (2300 ± 400 km versus 3000 ± 500 km) because North America was drifting southward starting around 90 Ma. The interpreted paleolatitude conflicts with sedimentologic and paleontologic evidence that the Nanaimo Basin was deposited near its present northern position.