Lake Fault Research Articles

Eocene dextral strike‐slip and extensional faulting in the Bridge River Terrane, southwest British Columbia

The early Mesozoic Bridge River Terrane of oceanic affinity lies in the eastern Coast Mountains of southwestern British Columbia and is one of several terranes located near the Intermontane Superterrane‐Insular Superterrane boundary. Eocene strike‐slip faulting and related extensional faulting and plutonism are largely responsible for the present upper crustal configuration of the Bridge River Terrane. This paper presents U‐Pb geochronology and a detailed structural and geochronological analysis of Eocene deformation of the Bridge River Terrane near Lillooet, British Columbia. Evidence for Eocene magmatism and deformation within the Bridge River Terrane includes 48.5–46.5 Ma U‐Pb zircon ages from four deformed granitic rocks, two of which are from the Mission Ridge pluton. All are cut by the Mission Ridge normal fault, which juxtaposes low‐grade Bridge River Group in the hanging wall against medium‐grade Bridge River Schist and associated intrusions in the footwall. The Mission Ridge fault is a low‐angle east‐dipping brittle normal fault with at least 10 km of downdip displacement and a probable dextral strike‐slip component. In the Bridge River Schist, in the footwall of the Mission Ridge fault, penetrative ductile fabrics in northwest‐trending, steeply to shallowly‐dipping mylonites with northwest‐southeast trending subhorizontal stretching lineations record a dextral sense of shear. Development of these fabrics predates normal movement on the Mission Ridge fault and is attributed to dextral movement on the Yalakom fault system within the middle crust. Two Middle Eocene syntectonic foliated intrusions, dated by U‐Pb zircon methods, have the same kinematic signature as the previously described mylonites; these constrain timing of dextral movement on the Yalakom fault system to be earlier than, contemporaneous with, and in part younger than the intrusion of the circa 47 Ma Mission Ridge pluton. The Mission Ridge fault is correlated with the Petch Creek fault, the probable extension of the Ross Lake fault near the United States border. They were broken and offset by about 100 km of dextral movement on the north‐trending Fraser fault. The Petch Creek fault hanging wall consists of the Hozameen Group, which, like the Bridge River Group, is an assemblage of low‐grade oceanic rocks. The footwall of the Petch Creek fault, the Custer Gneiss, is higher grade than the Bridge River Schist in the footwall of the Mission Ridge fault but has the same lithological and kinematic signature. Mylonitic fabrics in both the Bridge River Schist and Custer Gneiss have subhorizontal stretching lineations that were generated prior to normal movement on the Mission Ridge‐Petch Creek fault system and are attributed to earlier dextral motion along the Yalakom fault system. The clear offset of these geologic features constrains about 100 km of movement on the Fraser fault to be younger than 46.5 Ma. The Fraser fault is cut by 34 Ma phases of the Chilliwack batholith.

Hornblende geobarometry of the Nelson Batholith, southeastern British Columbia: tectonic implications

Hornblende geobarometry has been applied to granitic rocks of the Middle Jurassic Nelson Batholith, British Columbia, locally containing magmatic epidote. Geobarometry suggests equilibration pressures of less than 4.5 kbar (1 kbar = 0.1 GPa) in the northern part of the batholith, which lacks magmatic epidote. This part of the pluton shows clear magmatic intrusive relations, and the contact metamorphic rocks contain andalusite, which suggests that the equilibration and emplacement pressures are compatible.In the southern part of the batholith, granitic rocks containing magmatic epidote have equilibration pressures of 4.8–6.4 kbar. South and west of Nelson, there is a distinct contrast in pressure between the pluton and the country rock. Both the contact metamorphic rocks and the low-grade regional metamorphic rocks suggest pressures in the 2–3 kbar range.The pressure difference of about 2 kbar across the southwestern contact and the variation in pressure within the batholith can be explained by a model combining a late postequilibration upsurge (diapiric) of a deeper part of the pluton in the south, with a much later rotation and tilting of the batholith, associated with Eocene motion on the upper listric portion of the Slocan Lake Fault. The late diapiric (?) upsurge may account for the pressure contrast across the southwestern contact.

Correction to “Evidence of uppermost Proterozoic to Lower Cambrian Miogeoclinal rocks and the Mojave‐Snow Lake Fault: Snow Lake Pendant, central Sierra Nevada, California”

TectonicsVolume 10, Issue 1 p. 227-228 Free Access Correction to “Evidence of uppermost Proterozoic to Lower Cambrian Miogeoclinal rocks and the Mojave-Snow Lake Fault: Snow Lake Pendant, central Sierra Nevada, California” Mary M. Lahren, Mary M. LahrenSearch for more papers by this authorRichard A. Schweickert, Richard A. SchweickertSearch for more papers by this authorJames M. Mattinson, James M. MattinsonSearch for more papers by this authorJ. Douglas Walker, J. Douglas WalkerSearch for more papers by this author Mary M. Lahren, Mary M. LahrenSearch for more papers by this authorRichard A. Schweickert, Richard A. SchweickertSearch for more papers by this authorJames M. Mattinson, James M. MattinsonSearch for more papers by this authorJ. Douglas Walker, J. Douglas WalkerSearch for more papers by this author First published: February 1991 https://doi.org/10.1029/90TC02778AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume10, Issue1February 1991Pages 227-228 RelatedInformation

Intraplate faults revealed in crystalline bedrock in the 1983 Goodnow and 1985 Ardsley epicentral areas, New York

Bedrock geologic studies in the epicentral areas of the 1983 Goodnow ( m b = 5.1) and 1985 Ardsley ( m L = 4.0) earthquakes in New York State suggest that seismogenic intraplate faults have subtle, but clearly recognizable, expressions in crystalline bedrock along surface extrapolations of these well-defined earthquake ruptures. Both ruptures are small and totally confined to the subsurface. The 1983 Goodnow rupture and the 1985 Ardsley rupture are correlated with the Catlin Lake fault zone and the Dobbs Ferry fault zone, respectively. Although both structures had been recognized by other authors as brittle structures prior to the recent earthquakes, they had not been mapped as faults because of their small accumulated offsets. Our interpretation of these features as the surface expressions of seismogenic structures is based on: 1. (1) the spatial correlation of the rupture plane, as defined by aftershock hypocenters, with a prominent fracture-controlled topographic lineament, and 2. (2) observations of mesoscopic brittle structures along the lineaments that reflect the larger scale structure and, more importantly, are consistent with both the rupture orientation and sense of slip determined from seismic data. In both study areas mesoscopic structures indicate that fault formation at pre-existing joints was important in the overall evolution of the fault zones. The mesoscopic-scale observations are in agreement with studies of fault zone formation and growth in crystalline rock, and provide information on the geometry, segmentation, kinematics, and evolution of the fault zones. Fault zone segmentation seems to be controlled by wavelengths of folds in the pre-existing ductile structure.

Acadian Deformation in the Anticosti Basin, West Newfoundland

It has been traditionally assumed that the limit of the Acadian deformation of the Anticosti basin corresponded with the western limit of the onland exposures in western Newfoundland. Offshore seismic surveys have previously failed to provide evidence to allow any modification of this concept. Recent reprocessing of old marine seismic data reveals that Acadian thrust deformation extends about 80-100 km to the west of Newfoundland. This is entirely compatible with Acadian structures such as the Tenmile Lake and Great Northern faults and their azimuthal projection from their intersection with the shoreline. Both surface analyses and seismic interpretation indicate that these structural features are basement-cored, high-angle reverse faults with minimal foreshortening. Characteristic hanging-wall drape anticlines and footwall synclines can be identified both in outcrop and in the subsurface, indicating structurally focused petroleum potential in the offshore.

Significance of brittle and plastic fabrics within the Massawippi Lake fault zone, southern Canadian Appalachians

Brittle and plastic deformation structures are common features of many well-described brittle-ductile fault zones. These fabrics coexist in the Massawippi Lake fault zone (MLFZ), a NW-SE-oriented fault related to the La Guadeloupe fault in the Dunnage Zone of the Canadian Appalachians. Kinematic analysis of the MLFZ indicates that it is a NW-directed ramp thrust underlain by tectonic slices of mafic-ultramafic and granitic rocks. The association of brittle and plastic deformation structures is well demonstrated within the granitic rocks. Brittle fabrics include cataclasites and quartz-filled fractures and microfractures. Plastic deformation is superimposed on brittle textures. Microstructures and c-axis fabrics suggest plastic and superplastic regimes of deformation. Analysis of coexisting brittle and plastic microstructures in terms of deformation mechanism associations reveals inconsistencies between brittle- and ductile-related conditions of deformation. Combined with the regional structural and metamorphic history, this suggests that brittle fabrics are relict structures preserved within a predominantly ductile shear zone.

Silver-lead-zinc veins, metamorphic core complexes, and hydrologic regimes during crustal extension

The Kokanee Range Ag-Pb-Zn-Au vein and replacement deposits formed during Eocene crustal extension and unroofing of the Valhalla metamorphic core complex, in the hanging wall of the transcrustal Slocan Lake fault. The δ34Ssulfide, δ18Oquartz, δ18Osiderite, and galena Pb isotopic ratios for vein minerals display regional zonations revealing fluid-flow paths of a large, fossil hydrothermal system. Sulfur was derived from local country rocks, but carbon was derived from a deep-seated reservoir, possibly the upper mantle. Lead is a mixture of local, upper-crustal Pb with lower-crustal and depleted upper-mantle Pb. Regional isotopic zonations were controlled by deep fracture zones, such as the Slocan Lake fault, which channeled lower-crustal and mantle Pb and mantle CO2 to higher-crustal levels, where mixing occurred with highly evolved meteoric waters that had leached local sulfur and upper-crustal Ph. This is the first Ag-Pb-Zn vein district for which a genetic link between mineralization and metamorphic core complex unroofing is demonstrated.

Speculative reconstruction of the Mojave‐Snow Lake Fault: IMPLICATIONS FOR Paleozoic and Mesozoic orogenesis in the western United States

Available evidence suggests that the Mojave‐Snow Lake fault, a major Early Cretaceous dextral fault with up to 400 km displacement, formerly existed in what is now the axial part of the Sierra Nevada batholith. Regional relations suggest that the fault formerly continued in western Nevada, the Mojave Desert, and in rocks now exposed in the Salinian block. Restoration of 400 km of Early Cretaceous dextral displacement on the Mojave‐Snow Lake fault leads to speculative but testable reconstructions of Mesozoic and Paleozoic tectonic features of California and Nevada. Insights derived from these reconstructions include the following: (1) Two regionally extensive Jurassic thrust belts of opposed vergence appear to have intersected in the northern Sierra Nevada. The eastern, east‐vergent thrust belt, which includes the Fencemaker and Liming thrusts in western Nevada and several east‐vergent thrusts in the northern Sierra, may be a contractional belt along which an Early Jurassic interarc or marginal basin was closed in Middle to Late Jurassic time. The western, west‐vergent thrust belt is part of the classic Nevadan orogen of the western Klamath Mountains and the western Sierra metamorphic belt, in which collisions and/or thrusting of various island arc terranes occurred during Middle and Late Jurassic time. (2) The early Mesozoic continental arc included paired belts of Triassic‐Jurassic blueschist and melange, and igneous rocks including Late Triassic and younger plutons and volcanic rocks. This conclusion is not new, but the reconstruction provides a slightly different geometry than previously thought. (3) Sonomia, the late Paleozoic island‐arc terrane of the western U. S., may have formed a relatively narrow belt extending from northwestern Nevada to the latitude of the western Mojave Desert. The geologic development of the Paleozoic basement complex of Sonomia was very different from that of the Cordilleran miogeocline in Death Valley and the Mojave Desert, requiring that a continuation of the Sonoma suture belt (or Golconda thrust) must have extended southward through the Sierra Nevada batholith to the Mojave region. (4) A heterogeneous belt of Permian to Triassic or Triassic deformation in the Mojave and eastern Sierra Nevada, together with the Golconda allochthon in Nevada, may represent the protracted response to the collision of Sonomia with the western margin of North America. The contractional deformation continued during the evolution of the early Mesozoic continental arc. (5) Several predictions can be made about the nature of concealed basement within the Sierra Nevada batholith and within western Nevada. The reconstruction suggests that elements of the Antler and Sonoma orogenic belts should underlie areas of the batholith and western Nevada, as a result of northward displacement along the Mojave‐Snow Lake fault from the western Mojave and southern Sierra Nevada.

Evidence of Uppermost Proterozoic to Lower Cambrian miogeoclinal rocks and the Mojave‐Snow Lake Fault: Snow Lake Pendant, central Sierra Nevada, California

Displaced uppermost Precambrian to Lower Cambrian miogeoclinal strata occur within Snow Lake pendant in the central Sierra Nevada. These rocks have been correlated with the Stirling Quartzite, the Wood Canyon Formation, the Zabriskie Quartzite, and the Carrara Formation in the western Mojave Desert and the San Bernardino Mountains (Lahren and Schweickert, 1989; Lahren, 1989). This correlation is based on new, updated, and previously reported data including (1) lithologic similarities, (2) overall stratigraphic sequence, (3) vertical sequence within individual formations, (4) approximate stratigraphic thicknesses, (5) Skolithos in the correct stratigraphie position, (6) depositional environments, and (7) petrographic character and provenance of quartz arenites. The correlation is strengthened by the fact that Snow Lake pendant and the western Mojave share many other close similarities including (1) initial 87Sr/86Sr ratios of associated granitic rocks >0.706, (2) passive margin tectonic setting of Precambrian to Cambrian miogeoclinal rocks, (3) dikes of the Independence dike swarm, (4) possible Lower Triassic overlap sequence, the Fairview Valley Formation, (5) petrographically similar gabbroic complexes of the same age, (6) associated eugeoclinal rocks, and (7) identical(?) pre‐Tertiary structural configuration. New U/Pb zircon geochronology unequivocally shows that dikes at Snow Lake pendant are coeval with the Independence dike swarm of the eastern Sierra and the western Mojave desert and that associated gabbroic complexes in both the Mojave and Snow Lake pendant are the same age. Correlation of Snow Lake pendant with the western Mojave requires about 400 km of dextral displacement of the rocks of Snow Lake pendant, together with associated rocks (Snow Lake block), from the western Mojave Desert along the Mojave‐Snow Lake fault. Displacement most likely occurred after 150 Ma, the age of the Independence dike swarm, and before about 110 Ma, the age of major plutons within the Sierra Nevada batholith. This interpretation, if correct, holds major implications for allochthonous terranes west of Snow Lake pendant, which were probably attached to the Snow Lake block before its northward transport. In addition, a number of Paleozoic and Mesozoic tectonic features in western Nevada and eastern California may have been offset dextrally along the proposed Mojave‐Snow Lake fault.

Structure and chronology of the Oval Peak batholith and adjacent rocks: Implications for the Ross Lake fault zone, North Cascades, Washington

The >200-km-long Ross Lake fault zone (RLFZ) forms the northeastern border of the crystalline core of the North Cascades. This Late Cretaceous(?) and Paleogene structure is an ∼15-km-wide system of faults and fault zones, several of which deform the Paleocene (∼65 Ma, U-Pb zircon) Oval Peak batholith. The southern segment of the Gabriel Peak tectonic belt of the RLFZ is defined by strongly deformed rocks of the foliated southwestern margin of the Oval Peak batholith, structurally underlying pre-Late Cretaceous Twisp Valley Schist, and structurally highest part of the subjacent Skagit Gneiss, which here consists of Late Cretaceous (87 Ma) and Paleocene (60 Ma) orthogneiss. Kinematic indicators in moderately steeply northeast-dipping mylonitic orthogneiss record reverse slip. The strong solid-state flattening fabric in the foliated margin of the Oval Peak batholith formed during or shortly after emplacement and overprinted a magmatic fabric observed in the core of the batholith. Foliation and associated stretching lineation within the Gabriel Peak tectonic belt are oblique to the regional structural grain but concordant with the contacts of the batholith. These patterns in part record emplacement of the batholith as an expanding diapir into the tectonic belt. Reverse slip in the tectonic belt occurred before, during(?), and shortly after emplacement but ended by ∼55-58 Ma. The core of the Oval Peak batholith intruded a large tongue of Twisp Valley Schist on the north. Porphyroblast microstructures and Ar-Ar and K-Ar geochronology indicate that amphibolite-facies dynamothermal metamorphism of the schist occurred during intrusion. The Foggy Dew and Twisp River fault zones separate the batholith and its wall rocks on the northeast from weakly metamorphosed strata of the Methow basin. The Foggy Dew fault zone contains mylonitic gneiss derived from the Oval Peak batholith, Skagit Gneiss, and tonalitic rock that displays U-Pb zircon systematics suggestive of an Eocene age. Sillimanite schist and amphibolite are also present. Mylonites record dextral strike slip with a component of normal slip, down to the northeast. Movement in the fault zone continued subsequent to deformation in the Gabriel Peak tectonic belt and ended by 48 Ma. Mylonites in the Twisp River fault zone also reveal dextral strike slip, but apparently with less normal slip. The RLFZ experienced a diachronous history from at least 65 to 45 Ma. This history was dominated by dextral strike slip, but there were significant components of normal slip and reverse slip on some structures. During the latter part of this interval (∼57-45 Ma), the RLFZ probably recorded a transtensional tectonic regime transitional between extension to the east in Omineca metamorphic core complexes and dextral strike slip to the west related to oblique convergence along the continental margin. Early Cenozoic plutonism, metamorphism, and ductile deformation have strongly overprinted any of the major Cretaceous deformation postulated in previous tectonic models for the RLFZ.

Nature of the contact zone between the North Cascades crystalline core and the Methow Sequence In the Ross Lake Area, Washington: Implications for Cordilleran tectonics

Previous analyses of the accretionary history of NW Washington‐SW British Columbia have suggested the possibility of large‐scale, dextral‐slip faulting events to explain the present day regional outcrop pattern. The contact zone between the crystalline Skagit Complex and the volcano‐sedimentary Methow sequence has been considered to be a major fault, named the Ross Lake fault by Misch (1966), and has been considered to be a possible large‐scale strike‐slip fault by later workers. Detailed mapping of this contact zone in the Ross Lake area shows that the brittle faults and mylonites thought by Misch to express dextral shearing along the contact are not continuous, throughgoing structures, and that the contact between Skagit orthogneisses and Methow sequence strata is primarily intrusive. Furthermore, the mylonites show both left‐ and right‐lateral shear. The Skagit‐Methow contact is therefore not a major terrane boundary but rather a tectonized intrusive contact that has not accommodated significant motion between the two regions. Based on these observations and regional synthesis of timing relations, deformation in the contact zone is interpreted to express regional ENE‐WSW shortening, not major dextral slip, in the early Tertiary. Since no terrane‐bounding fault exists, the Cretaceous‐Tertiary history of the Ross Lake area can be interpreted as follows: (1) Jurassic(?)‐Early Cretaceous forearc/rift basin sedimentation (deposition of the lower Methow sequence); (2) 110–95 Ma, imbrication of the lower Methow sequence and its basement of oceanic crustal rocks by east‐vergent thrusting, with concomitant development of a west‐derived foredeep in the upper Methow sequence; (3) 95–85 Ma, arc magmatism, with genesis of the Skagit migmatite from Methow and older basement protoliths; and (4) Paleocene‐Early Eocene folding, faulting, uplift, and exposure of a relatively intact 25+ km crustal section, termed the Skagit‐Methow crustal section by Kriens and Wernicke (1987).

Isotopic evidence for the crustal evolution of the Frontenac Arch in the Grenville Province of Ontario, Canada

Nine late-tectonic granitoid bodies from the Central Metasedimentary Belt (CMB) of the Grenville Province were studied by the Sr, Nd and O isotopic systems. They occur on either side of the Rideau Lake Fault (RLF). Four bodies south of the fault were dated by the U-Pb zircon method. Three of the ages are tightly constrained at 1166 ± 3 Ma. This is in contrast to ages reported for late-tectonic plutons north of the fault between 1090 and 1076 Ma. t Nd DM model ages range from 1211 to 1480 Ma, indicating that the plutons were derived from a mixed source involving older crust. Exposed pelitic and psammitic metasediments in the area have t DM ages averaging 1790 Ma. The observed Sr-Nd-O relationships indicate that marble was involved in the evolution of the granitoids. The granites are chemically similar to the Mid-Proterozoic anorogenic plutons that occur elsewhere in North America. Their crystallization ages are not found elsewhere in the CMB in Ontario, but they are similar to ages found to the east in the Adirondacks, New York. This suggests that the Frontenac-Adirondack area represents a separate Penokean-age block assembled with the rest of the CMB ∼ 1070 Ma ago.

Structural cross sections based on a gravity survey of parts of the Quetico and Wawa subprovinces near Thunder Bay, Ontario

In this study, gravity data from 350 new gravity stations are combined with those from 50 previously surveyed stations in a detailed Bouguer anomaly map of a portion of the Quetico and Wawa subprovinces north and west of Thunder Bay, Ontario.In general, high gravity values characterize the southern and southwestern part of the area where metavolcanic rocks of the Wawa subprovince dominate. Much of the Quetico subprovince forms a broad gravitational low, reflecting extensive exposures of gneisses, schists, and migmatites. Well-defined gravity lows are associated with several granitic intrusive bodies.Three- and [Formula: see text]-dimensional gravity models of subsurface configuration of the density contrasts, representative of major rock units, indicate a trough-like structure for the metavolcanic rocks of the Wawa subprovince. This trough-like structure is flanked by a domical feature in the granitoid rocks to the south. North of the metavolcanic rocks, a succession of low-grade greywackes and slates occupies a basinal structure. These structures form the principal subsurface elements of the Wawa subprovince in this area.The gneisses, schists, and migmatites of the Quetico subprovince form a thick, southward-dipping, wedge-shaped structure that may extend under the structures of the Wawa subprovince. This wedge-shaped structure is underlain by a model unit of greater density representative of mafic gneisses and amphibolites. The denser substratum is modelled with local abrupt changes in dip corresponding in position with the Quetico and Hawkeye Lake faults.

LITHOPROBE: a scientific update-new images of the continental crust

LITHOPROBE, Canada's national earth sciences research program which integrates geophysical, geological and geochemical investigations in a collaborative effort among scientists from universities, government and industry to extend and relate surface geology to structures at depth, is up and running. The Lithoprobe Secretariat is established at the University of British Columbia. The Lithoprobe Seismic Processing Facility (LSPF) for research reprocessing of Lithoprobe and related data is installed at the University of Calgary. Access by the Canadian research community will be available through remote site connections. All scientific aspects of the program are in operation and plans for the future are in hand. Some of the spectacular new images of continental crustal structures that have been obtained and their tectonic significance are highlighted in this summary. Individual contributions of specific studies by Lithoprobe scientists are included in this volume.Along the eastern portion of the Southern Cordillera Transect, 270 km of crustal reflection data acquired in 1985 and subsequent magnetotelluric results define the westward extent of autochthonous North American basement (truncated by the crustal scale east-dipping Slocan Lake fault zone), the thickening of Purcell Proterozoic supracrustal rocks by folding and thrusting and a major compressional shear zone underlying the Valhalla gneiss complex. In 1988, Seismic profiling was extended westward across the allochthonous terranes and plutonic complexes to the west coast. About 900 km of 60-fold, 18 s data were recorded by an industry contract crew; initial processing by contract is in progress.In our Appalachian transect, Lithoprobe East, more than 3000 km of marine crustal reflection data have been acquired around Newfoundland and in the Gulf of St. Lawrence. Three deep crustal blocks, each of which exhibits a characteristic Seismic signature, can be traced through the region. Intracrustal reflectors on the Seismic line run across the continental margin northeast of Newfoundland help determine the nature of extensional tectonics and associated sedimentary basin evolution on the rifted margin.The GLIMPCE Transect has enabled a glimpse of the mid-continent crust by recording marine reflection data in the Great Lakes. The sections from Lake Superior reveal — 10 km of sediments overlying a 20 km thick sequence of strongly reflective volcanics and inter-flow sediments, perhaps representing the greatest vertical extent of intra-continental rift deposits on Earth.The original Archean/Early Proterozoic crust is greatly thinned beneath the rift basin. In the Lake Huron/Georgian Bay profile, the Grenville front is imaged as the westernmost reflection of a spectacular 70 km wide band of southeast-dipping reflections that clearly truncates prominent sub-horizontal crustal reflections of the Superior granite-rhyolite terrane.The Kapuskasing Structure Zone is a sequence of lower crustal rocks that have been thrust upward along the Ivanhoe Lake cataclastic zone and are now exposed at the surface. As part of the Kapuskasing Transect, 340 km of regional deep reflection data and 20 km of “ high resolution” data (20 m station spacing, 20 m source interval and a higher sweep range) over the cataclastic zone were recorded in 1987–1988. The Abitibi sub-province of Ontario and Quebec contains mineralized belts for which it is famous. As a preliminary program for the Abitibi-Grenville Transect, 130 km of regional and 65 km of high-resolution Seismic profiles also were recorded in 1987–1988. These were centred over major fault zones with which mineralization is related. Data quality is excellent and the data are in the process of being interpreted.

The crustal structure of the Wrangellia Terrane along the East Glenn Highway, eastern‐southern Alaska

Recently acquired seismic refraction data from eastern‐southern Alaska provide new information on the structure and composition of the Wrangellia and adjacent terranes. The data comprise a 160‐km‐long refraction profile along the East Glenn (Tok‐Cutoff) Highway that was collected as part of the U.S. Geological Survey's multidisciplinary Trans‐Alaska Crustal Transect program. The upper 3 km of the Wrangellia terrane and associated rocks is characterized by low compressional wave velocities (Vp = 1.9, 3.3, 4.6, 5.6 km s−1) and high‐velocity gradients common to most onshore seismic refraction profiles. There is also clear seismic expression of the West Fork fault system as a steep, down‐to‐the‐southwest fault that separates the Peninsular terrane to the southwest and the metamorphic complex of Gulkana River to the northeast. In contrast, no seismic expression occurs for the Paxson Lake fault, which separates the Wrangellia terrane from the metamorphic complex of Gulkana River. Adjacent to the Denali fault, within the Wrangellia terrane, two high‐velocity bodies (Vp = 6.6 km s−1) occur in the upper crust. One of these extends to ∼10‐km depth and correlates with a late Paleozoic dioritic complex, suggesting that the Wrangellia terrane is at least 10 km thick in this part of Alaska. From 5 to 23 km depth, the crust appears seismically homogeneous, with velocity increasing from Vp = 6.2 to Vp = 6.6 km s−1. Beneath this level, the crust is less well resolved, although evidence exists for a low‐velocity zone between 23 and 26 km and a possible southwest dipping interface at 35 km. No identifiable mantle refraction or reflection is observed, possibly indicating a crust as thick as 55 km. The relatively low seismic velocities in the upper 23 km of the crust compare favorably with laboratory‐measured velocities on pelitic schists and intermediate‐composition plutonic rocks (granites and granodiorites), both of which are recognized in Wrangellia. We interpret the seismic velocities to indicate that silicic‐to‐intermediate‐composition rocks are important constituents of the basement of this part of Wrangellia. Geologic evidence indicates that the Alaskan part of the Wrangellia terrane is a Paleozoic and Mesozoic island arc: our seismic evidence indicates it may have been built mostly on continental crust as opposed to the fragment of Wrangellia from Vancouver Island which was probably built on oceanic crust.

Fission-track evidence for Cenozoic uplift of the Nelson batholith, southeastern British Columbia

Apatite and zircon fission-track data from the Nelson batholith in southeastern British Columbia reveal that a significant amount of uplift has occurred since Paleocene time, including an episode of rapid uplift during Eocene time. Age versus elevation curves for apatite and zircon, combined with a calculated present depth to the 105 °C apatite-annealing isotherm, suggest that some 6 km of apparent uplift has occurred in the vicinity of the Nelson batholith since Paleocene time. A period of rapid cooling and uplift occurred from 59 to 45 Ma, when the bounding faults of the adjacent Valhalla gneiss dome, the Valkyr shear zone, and the Slocan Lake fault zone were active. The rapid uplift is interpreted as being related to Eocene extension and the rise of the adjacent Valhalla gneiss dome during Eocene time.

Paleomagnetism of the 1.1 Ga Lackner Lake Complex and tectonics of the Kapuskasing Structural Zone

The Middle Proterozoic Lackner Lake Complex is a circular alkalic syenite–carbonatite stock with a diameter of about 5.5 km. It intrudes granulite-rank Archean gneisses in the Kapuskasing Structural Zone of the Wawa Subprovince in the Superior Province of the Canadian Shield. It adjoins the Ivanhoe Lake fault zone, which forms the boundary with the Abitibi Subprovince and is the probable locus of maximum motion between the subprovinces. Specimens from 18 sites in the complex were analyzed paleomagnetically by alternating-field and thermal demagnetization and by saturation isothermal remanence tests. Large, recent viscous remanence components required removal before a stable remanence with a mean direction of 305.4°, 64.1 °(α95 = 5.2°) was isolated. Its pole of 53.7°N, 156.5°W (dp = 6.7°, dm = 8.3°) indicates emplacement at 1108 ± 10 Ma during a brief normal interval in a predominantly reversed-polarity time. This study indicates that there has been no postintrusion tilting of the Kapuskasing Structural Zone and that postintrusion uplift by unroofing did not exceed about 8 km.

Paleomagnetism of Eocene rocks of the Kelowna and Castlegar areas, British Columbia: studies in determining paleohorizontal

The middle Eocene Marron volcanics (mean age 52 ± 2 Ma) of the Kelowna outlier form the upper part of the hanging wall of the westerly dipping Okanagan Valley fault (OVF) in south-central British Columbia. They overlie Quesnellia. The OVF is currently interpreted as the westernmost member of a network of low-angle extension faults in the southern Omenica belt. The OVF was active in the middle Eocene at much the same time that the Marron volcanics were cooling. Relative to present horizontal, the magnetizations are widely scattered (Fisher's precision parameter k = 8) and after correction for bedding attitudes, there is no significant improvement (k = 9). Evidently, some magnetizations were acquired before (referred to as category 1) and others after (category 2) tilting; that is, the horizontal plane at the time of magnetization sometimes did and sometimes did not coincide with the bedding plane. Partial unfolding experiments, carried out on the two categories separately, yield a precision comparable to that expected for paleosecular variation, and a mean direction (D, I) of 352°, 70° (24 sites spanning 2000 m, 275 specimens, k = 23, α95 = 6°, paleopole 86°N, 230°E, A95 = 10°). The Marron is predominantly normally magnetized. Rock units slightly older and others slightly younger are reversely magnetized. The transition from reversed to normal polarity occurs in basal beds of the Marron Formation. The overall mean direction of the Marron and stratigraphically adjacent units is 352°, 69 °(28 sites, 300 specimens, spaning 4000 m, k = 21, α95 = 6°), yielding a paleopole at 85°N, 197°E (A95 = 10°), which is in excellent agreement with that for middle Eocene rocks of cratonic North America. Hence this part of Quesnellia had reached its present position relative to North America by middle Eocene time, and there has been no significant rotation of it. In contrast, the mean direction (020°, 72°, k = 9, α95 = 11°) after correction for bedding (calculated assuming the magnetization to be entirely pretilting) implies a clockwise rotation of 28°. We believe that this is incorrect; the apparent rotation, we argue, is caused by wrongly assuming that the bedding plane always coincides with the paleohorizontal at the time magnetism is acquired.Further tests have been carried out on intrusive and metamorphic core-complex rocks in the region of Eocene crustal extension 100 km to the east of Kelowna. These rocks are coeval with the Marron, and are located in both the hanging walls and footwalls of the Slocan Lake normal extension fault, which dips 30° eastward. Paleodirections are very different from those at Kelowna (four bodies, mean direction (D, I) 60°, 52°, k = 66, α95 = 6°), and we argue that this divergence is caused by tilting 37 ± 10° to the west antithetical to the Slocan Lake fault. We suggest that paleomagnetism provides a means by which tilts in such plutonic and metamorphic terrains can be determined. We suggest further that such tilts may have been responsible for some of the aberrant magnetizations observed in plutonic rocks of the Coast Plutonic Complex being much more widespread in the cordillera than previously envisioned.

Geology of the East Bull Lake gabbro–anorthosite pluton, Algoma District, Ontario

The East Bull Lake Pluton, a layered gabbro–anorthosite intrusion 90 km west of Sudbury, Ontario, is in an inward-dipping lopolith and is 780 m thick in the centre and elliptical in plan view (13.5 km × 3.5 km). It intruded Archean metavolcanic and metaplutonic rocks of the Superior Province during the Early Proterozoic (2480 Ma).The intrusion consistes of a basal anorthositic unit, succeeded by rhythmic-layered gabbro, troctolite, layered gabbro, and massive and dendritic gabbro units. It is offset by the west-northwest-striking, subvertical Folson Lake fault. South of this fault, only anorthosite and massive and dendritic gabbro are exposed. North of the fault, subsurface lithologies intersected by me boreholes correlate with surface lithologies. Troctolite, the most distinctive marker that can be correlated between boreholes and surface exposures, confirms the general attitude and shape of the layers and lopolith.Chemical composition of the intrusion ranges from high-Mg tholeiite to calc-alkaline high-alumina basalts.Fractures occur in several preferred orientations, the most common being northwesterly, subparallel to the Folson Lake fault, numerous mafic dykes, and topographic lineaments. Complex fracture-filling and alteration mineralogies formed under a wide range of P–T conditions representative of epidote-amphibolite – greenschist facies, pumpellyite–prehnite facies, zeolite facies, and low-temperature rock–water interaction processes.The last movement on the Folson Lake fault was a dextral strike slip of up to 3.0 km that postdates most mafic dykes.

Proterozoic and Lower Cambrian miogeoclinal rocks of Snow Lake pendant, Yosemite-Emigrant Wilderness, Sierra Nevada, California: Evidence for major Early Cretaceous dextral translation

Detailed stratigraphic and structural studies at Snow Lake pendant have established new evidence for the presence of uppermost Precambrian and Lower Cambrian miogeoclinal strata within the axial region of the Sierra Nevada batholith. Lithologic and stratigraphic data along with trace fossils, a distinctive Triassic overlap sequence (Fairview Valley Formation?), dikes probably related to the Jurassic Independence dike swarm, and structural history suggest that the older rocks of Snow Lake pendant correlate with the Stirling Quartzite, Wood Canyon Formation, Zabriskie Quartzite, and Carrara Formation in the western Mojave Desert and San Bernardino Mountains. This correlation implies approximately 500 km of dextral transport of the rocks of Snow Lake pendant along the proposed Mojave-Snow Lake fault. Movement on the fault probably took place after 148 Ma, the age of the Independence dike swarm, and prior to 110 Ma, the age of plutons in the central part of the Sierra Nevada batholith.