North American Basement Research Articles

The relative roles of ancient and juvenile crust in building accretionary orogens – Minimal ancient crust involved in the magmatic evolution of a North American Cordillera accreted terrane indicated by igneous zircon Hf-O

New igneous zircon HfO data from the Hogem batholith (207 to 125 Ma), emplaced into the Quesnel arc, an accreted oceanic terrane in the northern Cordillera, resolve questions about the magmatic history and source inputs of the terrane and the relationship of the terrane with ancestral North America before, during, and after collision. Juvenile zircon Hf signatures (> + 5 initial ɛHf) with predominantly mantle-like δ18O (+5.3 ± 0.6‰) indicate juvenile, mantle-derived magmas as the major contributors to the Hogem batholith. Minor input from young recycled oceanic crust is indicated by less common heavy zircon δ18O (>6.5‰). Mixing models demonstrate that ancestral North American crust did not underlie or contribute to the construction of north-central Quesnellia during the Triassic to Early Cretaceous - the time of Hogem batholith magmatism. This conclusion contrasts with previous regional tectonic models that suggest Early to Middle Jurassic accretion of Quesnellia onto North American basement. The collision between oceanic arcs and continental margins is generally considered a process that predominates the formation and growth of new continental crust at accretionary orogens. Our combined igneous zircon HfO results track the magmatic evolution of the batholith and indicate the importance of juvenile oceanic arcs in contributing crust to the flanks of some accretionary orogens.

Late Oligocene–Early Miocene Detachment Faulting in Crystalline Basement, Mecca Hills, CA, Documented With Zircon (U–Th)/He Date‐eU‐Visual Radiation Damage Patterns

AbstractZircon (U–Th)/He (zircon He) thermochronometry and visual radiation damage relationships reveal the timing and patterns of Orocopia Schist and North American basement exhumation in the Mecca Hills, CA. Zircon crystals from these units were selected to maximize the spectrum in visual metamictization and effective U (eU) concentrations, and thus He retentivity, in each sample. Zircon He dates (n = 31 analyses) from six of seven samples are ∼24 Ma regardless of eU, despite a large spread in eU (>2,800 ppm) across all grains. One sample (n = 7 analyses) yields a mean ∼65 Ma date. Uniform zircon He dates over a range of eU concentrations and their preserved visual metamictization indicate these grains experienced a thermal history that induced complete He loss, but did not anneal the damage. This is consistent with recent work that indicates zircon radiation damage anneals at higher temperatures than parameterized in current damage annealing models. The zircon He data patterns, together with new zircon U‐Pb results and thermal history modeling, reveal the two basement units in the Mecca Hills exhumed simultaneously at ∼24 Ma, likely by the Orocopia Mountains detachment fault. This requires both basement units lie in the footwall of the detachment, and implies the breakaway zone of the Orocopia Mountains detachment fault was west of the present‐day Mecca Hills basement exposures and has been translated by the San Andreas fault. Combined with prior work, our data suggest large‐magnitude late Oligocene–early Miocene extension in the southwestern Cordillera was more widespread than previously recognized.

Nailed to the craton: Stratigraphic continuity across the southeastern Canadian Cordillera with tectonic implications for ribbon continent models

Abstract Cambrian and Upper Devonian to Mississippian strata can be confidently traced westward, without strike-slip offset, from the autochthonous section above North American basement into the southeastern Canadian Cordillera, and are thus “nailed” to the craton. These strata are in turn stratigraphically pinned to older (Mesoproterozoic Belt-Purcell Supergroup, Neoproterozoic Windermere Supergroup, and Ediacaran), intermediate-aged (Ordovician–Silurian), and younger (Permian to Middle Jurassic) strata found only in the mountains, thus linking them to the adjacent autochthonous craton. The overlapping distribution of linking successions, regionally traceable unique stratigraphic horizons in the Belt-Purcell and Windermere Supergroups, and across-strike stratigraphic features show that the entire Cariboo, northern Selkirk, Purcell, and Rocky Mountains are directly tied to the adjacent North American craton without discernible strike-slip or oblique displacement, or substantial purely convergent plate-scale (>400 km) horizontal displacement. They link the entire width of the Belt-Purcell and Windermere basins in the southeastern Canadian Cordillera to the adjacent craton and show that any proposed Cretaceous ribbon continent suture, with its thousands of kilometers of proposed displacement, cannot run through the southeastern Canadian Cordillera.

Early Mesozoic synrift Eagle Mills Formation and coeval siliciclastic sources, sinks, and sediment routing, northern Gulf of Mexico basin

AbstractThe sedimentary architecture and provenance of the early Mesozoic incipient northern Gulf of Mexico basin remains controversial due to both lack of outcrop exposure and sample scarcity across the southern United States with subcrop depths approaching 6 km. The Eagle Mills Formation and coeval deposition across the northern Gulf of Mexico provides both a stratigraphic foundation for some ∼15-km-thick overlying Mesozoic and Cenozoic deposits, and a coeval pre-salt equivalent for southern synrift deposits, in one of the most economically significant hydrocarbon basins in the world. This study presents more than 3200 new detrital zircon U-Pb analyses from sixteen Late Triassic pre-salt, siliciclastic, subcrop well samples, and combines over 14,000 linear kilometers of 2-D multi-channel seismic reflection data, 1511 geophysical well logs, and biostratigraphic data from 2478 wells to construct basin-scale pre-salt isochore and structure maps spanning the northern Gulf of Mexico margin from Florida to the USA-Mexican border.The data show that incipient Gulf of Mexico paleodrainage pathways held individual distinctions between basement sources and tectonic controls in three primary regions across the northern Gulf of Mexico: (1) The western Gulf of Mexico paleodrainage extended from the Central Texas uplift highlands to the submarine Potosi Fan on the western margin of Laurentia with local tributary sources from the East Mexico Arc, Yucatán/Maya, and Marathon-Ouachita provinces as evidenced by inverse Monte Carlo unmixing of peri-Gondwanan (ca. 700–500 Ma), Appalachian/Ouachita (500–280 Ma), Grenville (1250–950 Ma), and Mid-Continent/Granite-Rhyolite Province (1500–1300 Ma) detrital zircon ages. Isochore and associated geophysical well and seismic data suggest that by Early Jurassic time this depocenter had shifted into the present-day western Gulf of Mexico as East Mexico Arc development continued. (2) Southerly drainage in the north-central Gulf of Mexico region bifurcated around the Sabine and Monroe uplifted terranes with southwestern flow characterized by peri-Gondwanan detrital zircon ages from late Paleozoic accreted basement or discrete flexural successor basins, and southeastern fluvial networks distinguished by traditional North American basement province sources including Grenville, Mid-Continent, and Yavapai-Mazatzal. (3) Eastern Gulf of Mexico regional paleodrainage, with regional southern flow dictated by the brittle extensional tectonics of the South Georgia Rift as well as the regional southern flexure of the South Florida Basin, resulted in almost all pre-salt detrital zircon siliciclastic ages from this region to be dominated by local Gondwanan/peri-Gondwanan aged sources including the proximal Suwannee terrane and Osceola Granite complex. These regional, synrift sediment provenance models provide the first critical allochthonous evidence of Late Triassic–Early Jurassic paleodrainage stemming from the Appalachian-Ouachita hinterlands into the incipient northern Gulf of Mexico basin with critical implications for pre-salt hydrocarbon exploration and carbon sequestration reservoir potential.

Tectonics of the northern Canadian Cordillera imaged using modern magnetotelluric analysis

Magnetotelluric data from LITHOPROBE SNORCLE Corridor 2 in western Canada crossing the Rocky Mountain Trench-Tintina Fault between 56° and 60°N have been analyzed using modern MT methods to provide new insights into the tectonics of the North American Cordillera and Ancestral North America. Resistivity images of the crust were obtained using two independent sets of unconstrained 2-D inversions and constrained 2-D inversions incorporating seismic constraints. Structure to the west of the Tintina Fault includes uniform, moderate, mid to lower crustal resistivity in a seismically-defined westward-tapering wedge of Ancestral North American rocks. To the east, between the Tintina Fault and the Liard line, a transfer zone in asymmetric Neoproterozoic rifting, the wedge includes a major synformal conductor that parallels seismic reflections. The large lateral changes in resistivity can be explained by transition between different blocks of asymmetric rifting. The synformal conductor is attributed to graphitic rocks from the Aida Formation of the Mesoproterozoic Muskwa Assemblage that were re-distributed in a sag basin on the lower plate margin in the Blackwater Block. Spatially-complex upper crustal resistivity in the Cordillera can be interpreted in terms of lithological variations and alteration processes. An east-dipping conductor in Stikinia is attributed to east-dipping Jurassic and relatively high conductivity beneath the Eskay rift may be explained by crustal fertilization. A conductor in the Cache Creek terrane is attributed to listwanite alteration of ultramafic bodies on the King Salmon Fault, and high conductivity in the Slide Mountain terrane within the Sylvester Allochthon to localized carbonaceous alteration. A series of conductors above North American Basement is attributed to deformation and metamorphic processes associated with Mesozoic thrust faulting. To the west of the Tintina Fault the source material for these conductors is likely from the Stikinia terrane, and to the east it is likely from the Muskwa Assemblage.

Geophysical investigation and reconstruction of lithospheric structure and its control on geology, structure, and mineralization in the Cordillera of northern Canada and eastern Alaska

A reconstruction of the Tintina fault is applied to regional geophysical and topographic data, facilitating the definition of west trending lineaments within the lower crust and/or mantle lithosphere, oblique to the NW trending structure of the Cordilleran terranes. The lineaments, which exhibit a range of geophysical and geological signatures, are interpreted to be related to the Liard transfer zone, continuous to the Denali fault, that divided lower and upper plates during late Proterozoic-Cambrian rifting of the Laurentian margin. Three-dimensional gravity models show a density increase in the lower crust and mantle lithosphere to the north. The transfer zone also divides bimodal mantle xenolith suites to the south from unimodal suites to the north. These conclusions suggest that extended North American basement, related to Laurentian margin rifting that would have brought mantle lithosphere rocks to a shallow depth, continuously underlies a thin carapace of accreted terranes in western Yukon and eastern Alaska. The interpreted continuity of North American basement reaffirms that if oroclinal bending of the Intermontane terranes occurred, then it was prior to its emplacement upon the rifted basement. Examination of the spatial relationships between mineral occurrences and postaccretionary, Cretaceous lithospheric lineaments, from their manifestation in geophysical, geological, and topographic data, suggest that the late Proterozoic lineaments influenced Mesozoic mineralization through influence on the development of the shallow crustal structure, intrusion, and exhumation and erosion.

The S-type Ladybird leucogranite suite of southeastern British Columbia: Geochemical and isotopic evidence for a genetic link with migmatite formation in the North American basement gneisses of the Monashee complex

The 62–52 Ma Ladybird granite (LBG) suite is a peraluminous, leucocratic, S-type, quartz monzonitic to granitic suite which occurs as batholiths, stocks, dikes, sills, and pegmatite veins predominantly in the high-grade rocks of the Shuswap complex, in southeastern British Columbia. The emplacement of the LBG was synchronous with the production of abundant migmatites within Thor–Odin dome of the Monashee complex, an exposure of North American basement, exhumed from depths of ca. 26–33 km by Eocene extensional faults. The LBG and the leucosome in migmatites from Thor–Odin dome have similar major and trace element patterns, and are both characterized by zircons which have inherited Precambrian cores. Whole rock Nd isotope compositions show a range of values for the LBG with εNd (55 Ma) values from − 5.0 to − 17.2. The εNd (55 Ma) for the leucosome samples range from − 9.5 to − 23.6, overlapping with those of the granitic suite. These data support the interpretation of a genetic link between formation of the LBG suite and melting of North American basement rocks, such as those exposed in the core of Thor–Odin dome. The leucosome samples have lower high field strength element (HFSE) concentrations and positive Eu anomalies, whereas the LBG samples have higher HFSE concentrations and negative Eu anomalies. The similar trace element characteristics suggest that the leucosome from the migmatites and the LBG are related, whereby most of the leucosome samples are cumulates and the LBG samples represent evolved or residual melts. The initial 87Sr/ 86Sr isotope values for both the LBG and leucosome samples have a large range. However, the initial Sr isotopic ratios for the LBG suite are lower than those of the leucosome samples, with 87Sr/ 86Sr (55 Ma) ranging from 0.70603 to 0.73688 and 0.74256 to 0.76593, respectively. This isotopic discrepancy suggests either: a) isotopic disequilibrium during partial melting in the mid- to lower crust where the leucosome formed, b) the distribution of Sr during partial melting was controlled by different melt-producing reactions, and/or c) isotopic heterogeneity in the source rocks. At least part of the LBG suite likely formed via melting of North American basement rocks that were dominantly of sedimentary origin. Melting of the Proterozoic supracrustal metasedimentary rocks overlying North American basement may also have contributed to the formation of the different phases of the suite found at the regional scale. However, the abundant leucosomes in the basement rocks of Thor–Odin dome may mark the paths along which anatectic melt migrated in the structurally overlying Ladybird granites of the South Fosthall pluton.

Revisiting the Baranof–Leech River hypothesis for early Tertiary coastwise transport of the Chugach–Prince William terrane

According to the Baranof–Leech River hypothesis originally proposed in 1982, (1) schists on southern Baranof Island in southeastern Alaska were contiguous with the Leech River schist on southern Vancouver Island until 40 Ma, and (2) both rock units were part of the 2200 km long Chugach–Prince William terrane, which was displaced northward about 1100 km after 40 Ma. Isotopic data obtained since 1982 show that the syn-magmatic metamorphism that produced the Baranof and Leech River schists occurred at 50 Ma, not at 40 Ma. Large-magnitude coastwise slip of the terrane is therefore post-50 Ma. Igneous rocks in the Baranof and Leech River units are part of the Sanak–Baranof magmatic belt of forearc magmatism, which has been ascribed to the early Tertiary subduction of an oceanic ridge. The slab window also gave rise to early Eocene, near-trench plutonic and volcanic rocks on North American basement in the North Cascades of Washington State, and probably to coeval igneous rocks on the western coast of Vancouver Island. These igneous suites in the forearc fix the location of the intersection of the ridge with the continental margin 50 Myr ago at latitude ca. 48–49°N (present-day coordinates). Paleomagnetic data obtained since 1982 imply that before 50 Ma, the parts of the Chugach–Prince William terrane that were to become the Baranof and Leech River schists were south of 48–49°N. From 61 to 50 Ma, the northward movement of the terrane relative to North America can be reconciled with the southward migration of forearc magmatism in the Chugach–Prince William terrane if the ridge–trench intersection was fixed at 48°N (present-day coordinates). The Border Ranges fault system is the on-land structure that most likely accommodated hundreds of kilometers of post-early Eocene displacement.

Effects of basement composition and age on silicic magmas across an accreted terrane-Precambrian crust boundary, Sierra Madre Occidental, Mexico

Trace element and isotopic compositions of mid-Tertiary siliceous magma sequences from two localities of the Sierra Madre Occidental, northern Mexico, display differences that reflect the composition and age of the basement through which they erupted. The crust beneath the section at San Buenaventura is thicker and more evolved and forms part of the North American basement, while that under El Divisadero consists of allochthonous terranes of island arc/oceanic? crust accreted during the Mesozoic. The volcanics are highly differentiated and range in composition from basalt to rhyolite (SiO 2=50–76%). Those erupted through the accreted terranes display a small range of isotope ratios and have lowest initial (age-corrected) Sr isotope ratios (>0.7044) and the highest Nd (<0.5126) and Pb isotope ratios ( 206Pb/ 204Pb ∼18.9). Isotope ratios of the continental suite are more variable and form an array which trends away from that of the accreted terrane suite toward compositions more typical of old crust (to 87Sr/ 86Sr ∼0.710 and 143Nd/ 144Nd ∼0.5123). The volcanics in the continental zone are relatively more enriched in moderately incompatible elements compared with those within the accreted terranes (Ce/Yb=25–45 vs. 13–33, respectively), but are depleted in some highly incompatible elements such as U and Rb (e.g., Th/U=3.8–7.5 vs. 2.5–4.0, respectively). Those higher in the stratigraphic sections have higher 87Sr/ 86Sr, 208Pb/ 204Pb, and Th/U ratios, and lower 143Nd/ 144Nd ratios than those lower in the sections. The data have implications for the nature of the sources and the petrogenesis of these volcanics. The isotope ratios of both suites fall between those of mafic magma compositions from the Sierra Madre Occidental, and intermediate and felsic lower crustal xenoliths in northern Mexico and the southwestern USA. The relationship between the isotope ratios of the sequences and the age of the basement, combined with the fact that the overall data set forms well-defined isotopic arrays, demonstrates the strong effects of the crust on the chemistry of the silicic magmas. In the continental suite, isotope ratios covary with Th/Pb and U/Pb ratios, approaching the compositions found in the intermediate and felsic granulite facies xenoliths, strongly indicating that they are not anatectic melts of the lower crust but rather reflect interaction between mantle-derived basaltic parental magmas and the crust. Crustal contributions appear to be large, on the order of 20–70%. The small range of isotope ratios in the accreted terrane suite appears to reflect interaction of the basaltic parent with relatively juvenile crust whose isotopic composition is similar to the mantle-derived magmas. High Th/U and Th/Rb ratios indicate that the crustal contamination occurs in the lower crust. Moreover, the less radiogenic 206Pb/ 204Pb and 207Pb/ 204Pb ratios in the continental suite indicate that the depletion in highly incompatible elements in the continental lower crust is an old feature. The secular changes in the isotope ratios within the stratigraphic sections indicate increasingly shallow crustal contributions with time, initially by predominantly mafic deep lower crust and later by more felsic middle crust. Using lavas from outside of the two heavily sampled stratigraphic sections, the differences in the isotopic compositions between volcanics erupted through the accreted terranes and the continental basement help to delineate the location of the boundary.

Northeastward extrusion and extensional exhumation of crystalline rocks of the Monashee complex, southeastern Canadian Cordillera

North American basement and cover rocks of the Monashee complex (MC) are exposed through a tectonic window within the hinterland of the southeastern Canadian Cordillera. The complex records a history of Late Cretaceous to early Tertiary crustal thickening ( F 1/ F 2) related to emplacement of the Selkirk allochthon (SA). Part of the MC and overlying SA then formed a mid-crustal wedge that was extruded towards the foreland from the overthickened hinterland ( F 3, F 4). Finally, the complex was exhumed by Eocene ductile and brittle extensional deformation ( D 5). Rocks along the studied west flank of the MC are thoroughly transposed by F 1/ F 2 (into S 2). The extent of the transposition, and a well-developed northeast-trending L 2 lineation, indicate intense strain during F 1/ F 2 throughout the studied portion of the MC (4–5 km thickness exposed) and overlying SA. Ductile flow continued, resulting in northeast-verging F 3 folds in the MC, west-southwest-verging F 3 folds in the SA and broad F 4 warps in the MC and SA. A shear zone records significant reactivated slip on S 2 during D 5. Rapid exhumation of the MC is attributed to ductile flow during extrusion and extensional deformation; this ductile flow is correlated with foreland thrusting, which ended in the Early Eocene.

Nd Isotopes and the Source of Sediments in the Miogeocline of the Canadian Cordillera

Nd isotopes in clastic 600-75 Ma sedimentary rocks from the miogeocline of Alberta and British Columbia are used to (1) constrain the importance of proximal and distal North-American basement and juvenile Cordilleran sedimentary sources; (2) document large-scale changes in provenance; and (3) develop a North American miogeoclinal reference for Nd isotopes as a baseline for continental input to Cordilleran terranes. Initial $\epsilon_{Nd}$ for Neoproterozoic to Upper Ordovician samples range from -14 to -22 and can be explained by derivation from proximal Precambrian basement. A positive shift of six $\epsilon_{Nd}$ units occurs between Late Ordovician and Late Devonian time and persists until foreland basin formation in the Late Jurassic. $\epsilon_{Nd}$ for this 370-170 Ma period ranges from - 6 to - 9 and requires involvement of a more juvenile source. Cretaceous foreland basin sediments show extreme heterogeneity, with $\epsilon_{Nd}$ from zero to -12. High values can be explained by incorporation of v...

Nd–Sr isotopic constraints on the interactions of the Intermontane Superterrane with the western edge of North America in the southern Canadian Cordillera

Sr and Nd isotopic compositions of the late Paleozoic metavolcanics and Late Triassic to early Tertiary granitoids from four magmatic episodes in the southern Canadian Cordillera from the Kootenay Arc to the Fraser Fault have been used to (i) identify the sources of these rocks, (ii) constrain the compressive tectonic history from Middle Jurassic to Paleocene, and (iii) constrain the western boundary of the basement in this region. The 215–190 Ma old primitive granitoids (εNd = +3.1 to 8.7; 87Sr/86Sr = 0.7028 − 0.7043) of the Late Triassic and Early Jurassic magmatic episode were emplaced in the Paleozoic oceanic crust of Quesnellia (εNd = +2.9 to +9.3) prior to its obduction over the basement. In contrast, during the younger magmatic episodes (Middle–Late Jurassic, Cretaceous, and early Tertiary), the granitoids from western Quesnellia show primitive isotopic compositions, and those from eastern Quesnellia show eastward-increasing crust-contaminated compositions. The contaminated characters of the Middle–Late Jurassic (180–150 Ma) granitoids from eastern Quesnellia (εNd = +2.8 to −9.1; 87Sr/86Sr = 0.7041 − 0.7083) suggest that by 180 Ma, the eastern part of Quesnellia obducted over the North American cratonic basement by an amount of about 100 km (Eocene extension corrected) measured from westward shifts of the Nd and Sr isopleths. The eastward-increasing crustal-contamination patterns in the Cretaceous (120–80 Ma) and the Paleocene igneous rocks also show westward shifts of these isopleths by 20 and 70 km, respectively. Thus, we observe that a total 190 km of obduction took place, this amount is similar to the amount of shortening measured in the Rocky Mountains Fold and Thrust Belt, and the western boundary of the North American basement presently lies at least 25–75 km east of the Fraser Fault.

The southern Omineca Belt, British Columbia: new perspectives from the Lithoprobe Geoscience Program

Geological, isotopic, and geochronology studies carried out by university and government researchers, concurrently with the Lithoprobe program, have greatly refined our understanding of the regional geology, crustal structure, and tectonics of the Omineca Belt. Sound correlations have been established between surface geology and seismic reflection data. Cretaceous–Eocene thrust faults that are imaged in the subsurface in the Shuswap complex may be part of a break-forward thrust system that feeds into the Purcell Anticlinorium and the Foreland Belt. The Monashee décollement is the western continuation of the sole thrust beneath the Foreland Belt and provides a means of linking shortening across the entire orogen. The thermal peak of metamorphism in the central and southern Shuswap complex is now known to have occurred in the Late Cretaceous–Paleogene in contrast with earlier held views. North American basement rocks are now known to extend beneath the eastern half of the Canadian Cordillera. Geochronology studies have revealed Early Proterozoic and Late Cretaceous–Eocene metamorphism in basement rocks of the Monashee complex, and suggest that these rocks were located to the east of the metamorphic front throughout the Jurassic and Early Cretaceous. Anatectic peraluminous granites were produced in the Shuswap complex between 135 and 52 Ma in response to pulses of crustal thickening and heating, and in some cases served to localize Eocene extensional shear zones and to transfer extensional displacement from one shear zone to another. A flat Moho and other seismic reflection data are consistent with interpretations of lower crustal flow to balance early Tertiary extension in the upper crust. Crustal-scale extension and the Slocan Lake fault zone provided the source and setting for Ag–Pb–Zn–Au mineralization in the Nelson–Silverton area.

The distribution of skarns in British Columbia and the chemistry and ages of their related plutonic rocks

British Columbia contains at least 694 recorded skarns (as defined by Burr, 1977) that range from small occurrences to deposits exceeding 20 million metric tons of ore. Skarns have accounted for nearly 90 percent of the iron (magnetite), 80 percent of the tungsten, 16 percent of the gold, and 12 percent of the copper produced by hard-rock mining in the province. Copper skarns make up almost half of the occurrences; the other skarn classes, in decreasing abundance, are Fe (21%), Pb-Zn (11%), W (7%), Au (4%), Mo (3%), and Sn (0.4%). A small group (2%) lack significant metallic minerals but represent potential industrial mineral deposits.Three separate plutonic episodes were responsible for most of the skarns. The earliest and most significant occurred during the Early to Middle Jurassic when over half of the skarns developed. Later important episodes during the Cretaceous and Eocene-Oligocene accounted for 17 and 9 percent, respectively.Skarns in British Columbia are distributed across 19 tectono-stratigraphic terranes. However, there is a marked spatial and temporal relationship between certain metallic classes of skarn, their metal production, and the character and origin of the host terranes. Despite having abundant plutonic rocks, terranes of the Coast belt host only 4 percent of the province's skarns and these have had negligible metal production. The terranes in the North American basement and craton host 5 percent of the skarns, but these have been responsible for all of the W produced. However, terranes dominated by oceanic island-arc rocks contain over 80 percent of the skarns and have accounted for virtually all the Fe, Cu, Au, Ag, and Zn produced.The majority of British Columbia's Fe, Cu, and Au skarns developed during Early to Middle Jurassic oceanic island-arc activity in Wrangellia, Quesnellia, and Stikinia. By contrast, nearly all the W skarns are related to a belt of Cretaceous plutons that intrude sedimentary rocks deposited either close to or on the ancestral North American continent.In many terranes, there is no consistent relationship between the number of skarn occurrences and metal production. Wrangellia hosts nearly all the Fe skarns and has accounted for virtually all the 35 million metric tons of magnetite produced in British Columbia. Yet it also has the largest number of Cu skarns of any terrane but has had only one-fifth the skarn copper production of Quesnellia.Both major and trace element analyses indicate that the plutonic rocks associated with Fe, Cu, Au, and, to a lesser extent, Mo skarns are chemically distinct from those associated with W and Sn skarns. Plutons related to the former group are relatively undifferentiated and Fe rich; their Nb, Y, and Rb contents are characteristic of volcanic arc granites. Those associated with W and Sn skarns are highly differentiated, Fe poor, large ion lithophile elements rich, and represent within-plate intrusions.Copper- and Au-bearing skarns are currently the most economically attractive targets for skarn exploration in British Columbia, and this study indicates that island-arc assemblages in the Quesnellia terrane, or in correlative rocks elsewhere, have the best exploration potential for these metals.

Three-dimensional crustal structure of the Purcell anticlinorium in the Cordillera of southwestern Canada

The Purcell anticlinorium in southwestern Canada and the northwestern United States is situated at the transition from deformed North American rocks of the foreland thrust and fold belt to accreted rocks of the Kootenay arc. Nearly 1000 km of industry seismic reflection data were processed with extended VIBROSEIS correlation and coherency filtering and then combined with LITHOPROBE reflection data to outline the three-dimensional crustal structure of the anticlinorium. Reflection Moho is visible at ∼12.0 s (∼36 km) beneath the anticlinorium and deepens eastward to ∼15.0 s (∼45 km) beneath the Rocky Mountain thrust and fold belt. The transition from thick to thin crust coincides with westward deepening of North American basement and with the southern Rocky Mountain Trench, a basin or series of basins formed during Tertiary extension. Regionally extensive reflections in the upper crust correlate with ca. 1468 Ma gabbroic sills in the Aldridge Formation of the Mesoproterozoic Belt/Purcell Supergroup. Features outlined by the sills support an interpretation that the anticlinorium formed during Mesozoic contraction when imbricate thrust faults carried up to 15 km of Belt/Purcell and Paleozoic margin sedimentary rocks, and basement in some areas, eastward over a prominent basement ramp.

Detrital zircon geochronology and the provenance of the Harmony and Valmy Formations, Roberts Mountains allochthon, Nevada

Research Article| July 01, 1994 Detrital zircon geochronology and the provenance of the Harmony and Valmy Formations, Roberts Mountains allochthon, Nevada MOIRA SMITH; MOIRA SMITH 1Department of Geosciences University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar GEORGE GEHRELS GEORGE GEHRELS 1Department of Geosciences University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information MOIRA SMITH 1Department of Geosciences University of Arizona, Tucson, Arizona 85721 GEORGE GEHRELS 1Department of Geosciences University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (7): 968–979. https://doi.org/10.1130/0016-7606(1994)106<0968:DZGATP>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MOIRA SMITH, GEORGE GEHRELS; Detrital zircon geochronology and the provenance of the Harmony and Valmy Formations, Roberts Mountains allochthon, Nevada. GSA Bulletin 1994;; 106 (7): 968–979. doi: https://doi.org/10.1130/0016-7606(1994)106<0968:DZGATP>2.3.CO;2 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 A long-standing debate over the provenance of lower Paleozoic sedimentary rocks in the Roberts Mountains allochthon, Nevada, is addressed through abraded single-grain U-Pb detrital zircon analysis of the Harmony and Vahny Formations. Twenty-seven zircon grains from mature quartz arenite of the Middle Ordovician Valmy Formation yielded concordant to a few percent discordant ages of ∼1050 Ma (n = 2),1830-1845 Ma (n = 4),1910-1960 Ma (n = 5),2270-2340 Ma (n = 2), 2650-2750 Ma (n = 8), and ∼2900 Ma (n =1), and moderately discordant ages of ∼2070 Ma (n =1) and ∼3240 Ma (n =1). These ages are consistent with ultimate derivation from the north (Slave craton and Medicine Hat province and central/northern Alberta) and suggest a link between the Valmy Formation and the coeval Eureka Quartzite on the adjacent shelf and miogeoclinal prism. Twenty-seven analyses of zircon from immature quartzo-feldspathic wacke of the Upper Cambrian Harmony Formation include concordant analyses in the range of 695-710 Ma (n = 9), 1015-1225 Ma (n = 12), ∼1330 Ma (n = 2), ∼1745 Ma (n = 1), ∼1915 Ma (n = 1), and 2570 Ma (n = 1). The ∼700 Ma and ∼1100 Ma ages (which represent >80 %of the grains) have no known western North American basement source of reasonable proximity and age, suggesting derivation from an unknown western or southern source, or perhaps from rocks that presently underlie the Roberts Mountains allochthon.A growing body of U-Pb isotopic evidence indicates that 1000-1270 Ma and 670-750 Ma zircon ages are relatively common in offshelf continental margin strata. The latter set of ages is also represented by orthogneiss that intrudes or is overlain by continental margin strata, necessitating the widespread exposure of felsic plutonic rocks of these ages along the Cordilleran margin during early Paleozoic time. Subsequently, such rocks may have been overlain by younger continental margin strata, structurally overlain by allochthonous units, or tectonically removed from the region by younger rifting events. The presence of widespread mid- and late Proterozoic felsic magmatic rocks along the Cordilleran margin is not easily accommodated by popular pre-570 Ma plate reconstructions that place Nevada and the central Cordillera opposite the Transantarctic Mountains or eastern Australia. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Displacement of the Lewis thrust sheet in southwestern Canada: New evidence from seismic reflection data

Research Article| September 01, 1994 Displacement of the Lewis thrust sheet in southwestern Canada: New evidence from seismic reflection data Arie J. van der Velden; Arie J. van der Velden 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar Frederick A. Cook Frederick A. Cook 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Arie J. van der Velden 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Frederick A. Cook 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (9): 819–822. https://doi.org/10.1130/0091-7613(1994)022<0819:DOTLTS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Arie J. van der Velden, Frederick A. Cook; Displacement of the Lewis thrust sheet in southwestern Canada: New evidence from seismic reflection data. Geology 1994;; 22 (9): 819–822. doi: https://doi.org/10.1130/0091-7613(1994)022<0819:DOTLTS>2.3.CO;2 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 SocietyGeology Search Advanced Search Abstract Seismic reflection data across the Rocky Mountain trench in the southern Canadian Cordillera reveal prominent reflections from Mesoproterozoic (Aldridge Formation) sills that allow a direct stratigraphic correlation across the trench when 10 km of extension is restored. Trun cated reflections at ∼11-15 km depth on the west side of the trench are interpreted as a footwall cutoff of the Lewis thrust in Belt-Purcell strata that lies ∼115 km west of the leading edge of the Lewis thrust, exposed in the Waterton area; retrodeformation of the Lewis sheet requires a minimum of 75 km of transport on the Lewis thrust and 40 km of transport by the development of footwall-domain duplexes. The trench is located above a major change in slope of the top of the North American basement that coincides with a pronounced increase in thickness of lower Belt-Purcell strata in their retrodeformed position. This transition may represent part of an ancient margin that was initially established in Paleoproterozoic time. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Electromagnetic images of regional structure in the southern Canadian Cordillera

As part of Lithoprobe's Southern Cordilleran transect investigations, magnetotelluric (MT) soundings were made at 160 sites providing unprecedented coverage from the Rockies to the west coast. Striking lateral variation, which spatially correlates with the morphogeological belt boundaries, is apparent at periods sensing the lower crust (≈10 s). For the Rockies, MT phases are around 35°, indicative of a moderately resistive (100's – 1000's Ω·m) North American Basement. Foreland belt phases are transitional and increase from 60° in the east to 70° in the west. Omineca and Coast belt phases are high (75°), implying a conductive (10–30 Ω·m) lower crust, whereas Intermontane belt phases are more than 10° lower (equivalent to ≈150 Ω·m). The regional variation in conductivity correlates to first order with surface heat flow changes along the profile and is also correlative with coincident seismic reflection sections in some aspects.

Provenance and U-Pb geochronology of the Mesoproterozoic Belt Supergroup (northwestern United States): implications for age of deposition and pre-Panthalassa plate reconstructions

The Mesoproterozoic Belt-Purcell Supergroup is a thick succession of siliciclastic and carbonate sedimentary rocks in the northwestern United States and adjacent Canada. Recent stratigraphic studies provide strong evidence for an enclosed basin setting with a tectonically active western margin. The age of the western (present coordinates) landmass is unknown, but has been inferred to be different from North American basement that borders the Belt basin on the basis of whole rock Sm-Nd studies. We report U-Pb isotopic analyses of individual detrital zircon, monazite, and xenotime grains separated from westerly derived clastic units in the Belt Supergroup as well as Sm-Nd isotopic data for the same grains where appropriate. These data provide new constraints on the age and isotopic character of the western landmass. Most U-Pb data are concordant or slightly discordant; exceptions are noted below. The Revett Formation (Ravalli Group, lower Belt Supergroup) contains grains with concordant ages of 1590–1600 Ma and a grain with a discordant 207Pb/ 206Pb age of 1780 Ma: the ca. 1600 Ma age is very unusual in North America and effectively dates “Belt Island”, long postulated to be the Revett source in what are now terranes of southeast Washington and northeast Oregon that were accreted in the Mesozoic. Detrital zircons from the Missoula Group (Bonner and Mount Shields formations; upper Belt Supergroup) form a distinct group at 1670–1859 Ma with two ca. 2.6 Ga grains. U-Pb ages from monazite range from 1642 to 1786 Ma, and together with Nd isotopic data ( T DM model ages of 2.05–3.36 Ga) suggest variable interaction with previously differentiated crust. The Buffalo Hump Formation in northeast Washington may correlate with the Bonner Formation or may lie unconformably on the Belt Supergroup; it contains detrital zircons of ca. 1840 Ma and, surprisingly young, 1070–1244 Ma (several grains). Monazites have U-Pb ages of 1753–1774 Ma and Nd signatures similar to the Bonner Formation data. Several major conclusions can be drawn from these data. The ages of 1590–1600 Ma have no known possible source within western North America (although 1576 Ma augen gneiss is present in the Priest River Complex northwest of the Belt basin), and the 1642–1786 Ma ages are uncommon to rare in Precambrian rocks to the immediate south and east; together these data support previous Sm-Nd whole rock and stratigraphic conclusions of a dominantly western source for the Belt basin. The ages in the Buffalo Hump Formation can be interpreted in two ways. Either the Buffalo Hump correlates with the Bonner Formation and therefore the top of the Belt Supergroup is substantially younger (< 1070 Ma) than previously inferred (1250 Ma) and may overlap with Grenville-age tectonic activity. Alternatively, the Buffalo Hump Formation may represent a new unit that is younger than the Belt. Irrespective of this issue, these data help to constrain proposed pre-Paleozoic plate reconstructions; from the 1070–1244, 1590–1600 and 1642–1786 Ma source ages and related Nd data, we suggest that the basement blocks of south-central Australia (Gawler, Musgrave, Willyama, Arunta and possibly Mount Isa) were joined to the western side of Laurentia, adjacent to the Belt basin, prior to Neoproterozoic formation of the Panthalassa Ocean.

GEOLOGIC RATIONALE FOR HYDROCARBON EXPLORATION IN THE CARIBBEAN AND ADJACENT REGIONS

Sedimentary basins in the Caribbean and adjacent areas are assessed in terms of the plate‐tectonic and paleogeographic history of the regions. Primary phases of development were rifting and passive margin development during Jurassic‐Cretaceous drift of North from South America; and the Late Cretaceous to Recent relative eastwards migration of the Caribbean Plate from the eastern Pacific area to its present position between North and South America.Two primary stratigraphic suites of rock occur in the Caribbean region: (I) autochthonous Jurassic, Cretaceous and Cenozoic passive margin sediments deposited along the rifted margins of North and South American basement: and (2) allochthonous oceanic crustal and magmatic arc rocks and overlying sedimentary units of the migrating Caribbean Plate. The tectonic boundary between these suites coincides roughly with the limit of circum‐Caribbean thrusted metamorphic/mafic rocks above the formerly passive Proto‐Caribbean shelf rocks (episutural foredeep basins), and juxtaposition youngs from west (latest Cretaceous in southern Yucatan) to east (late Neogene in Trinidad). Although Jurassic rift/early drift‐related source rocks occur in some areas. such as in Cuba—Bahamas. the primary source rocks in both suites were deposited well after rifting in the “medial” Cretaceous. Basin development during the Late Cretaceous to Recent tectonic juxtaposition of the two suites (i.e. Caribbean migration) directly controlled both clastic reservoir facies deposition and hydrocarbon maturation in many areas around the Caribbean. Basin development, source and reservoir rock deposition, and maturation of source rocks in all basins in the region can be directly related to plate‐tectonic evolution. A satisfactory understanding of the hydrocarbon potential of the Caribbean/northem South American province therefore requires a full appreciation of the region's plate‐tectonic evolution.