Yvette D. Kuiper

The interpretation of inverse isochron diagrams in 40Ar/39Ar geochronology

Abstract The concepts involved in the construction and interpretation of inverse isochron diagrams used in 40 Ar/ 39 Ar geochronology are reviewed. The diagrams can be useful as a means of recognising atmospheric argon and excess 40 Ar, incorporated in the mineral lattice, which cannot be recognised from 40 Ar/ 39 Ar spectra. The age established using an inverse isochron plot, unlike that yielded by a spectrum, is not affected by trapped argon 40 Ar/ 36 Ar ratios that are different from the atmospheric argon ratio (e.g. due to excess 40 Ar), and may contribute to a better age interpretation. However, a heterogeneous distribution of excess 40 Ar or heterogeneous argon loss can cause ‘false’ isochrons, with axial intercepts indicating an incorrect age and an incorrect trapped argon composition. Inconsistency between the ages from a spectrum and from the associated inverse isochron plot may indicate the degree of inaccuracy of isochrons. However, it is possible that both the spectrum and inverse isochron yield the same incorrect age. The importance of considering all possible interpretations before assigning an age to a specimen is stressed.

Possibility of channel flow in the southern Canadian Cordillera: a new approach to explain existing data

Abstract Existing structural, metamorphic and geochronological data in and close to the Shuswap Metamorphic Complex in the southern Canadian Cordillera are shown to be consistent with a channel flow model. Four general structural levels (domains) can be distinguished in the region, based on the orientation and vergence of folds. In the lowest three levels folds are mostly recumbent, whereas in the uppermost level they are upright. The lowest three levels are interpreted as a channel flow zone. NE-verging folds of the lowest level (Domain 1, e.g. the Monashee Complex) formed during top-to-the-NE detachment flow and/or in the lower part of a channel flow zone. When detachment flow changed to channel flow, the sense of shear changed in the upper part of the channel flow zone, resulting in overprinting of NE-verging folds by SW-verging folds (Domain 2, e.g. most parts of the Shuswap Metamorphic Complex to the west of the Monashee Complex). Temperature was probably increasing, weakening a progressively larger portion of the crust, and the crustal shear zone therefore widened. Thus, in the highest structural levels within the channel flow zone, SW-verging folds developed in areas where no NE-verging folds originally formed (Domain 3, e.g. the Cariboo Mountains). The channel flow model as presented here is compatible with many of the ductile structures and accommodates existing metamorphic and geochronological data in the part of the southern Canadian Cordillera described.

Isotopic age constraints from electron microprobe U-Th-Pb dates, using a three-dimensional concordia diagram

Abstract Using a three-dimensional U-Th-Pb concordia diagram, electron microprobe (EMP) U-Th-Pb analyses are shown to yield isotopic age constraints on isotopically concordant and discordant data that have not been recognized previously. The three-dimensional U-Th-Pb concordia diagram is discussed. It is demonstrated that a date obtained from an EMP analysis is as old as or younger than the 207Pb/235U and 207Pb/206Pb ages, and as old as or older than the 208Pb/232Th age. EMP analyses always yield a minimum age for the oldest Pb component.

Development of the Norumbega fault system in mid-Paleozoic New England, USA: An integrated subducted oceanic ridge model

A model is presented for what may be the first recognized ancient analogue of migration of a Mendocino-style triple junction, exposed at mid-crustal levels in the eastern New England Appalachians (northeastern United States). In the model, the mid-Paleozoic subvertical crustal-scale dextral transpressive Norumbega fault system formed as a result of subduction of an oceanic ridge and associated transform fault, similar to the evolution of the modern San Andreas fault system in western North America. Ridge subduction in eastern New England started in the latest Silurian at the latitude of the Coastal Volcanic belt in eastern Maine, causing bimodal plutonism and volcanism in the area. Subduction of an associated transform fault, and the transition from convergence between the northern Rheic Ocean plate and Laurentia to dextral transpression between the southern Rheic Ocean plate and Laurentia, resulted in development of the Norumbega fault system starting in the Middle to Late Devonian. The Norumbega triple junction gradually moved ∼200 km southwestward throughout the Devonian, similar to northward migration of the Mendocino triple junction at the northern end of the San Andreas fault system over the past ∼30 m.y. The ridge subduction model explains why convergent tectonics was ongoing until the latest Devonian or earliest Carboniferous in eastern Massachusetts, analogous to motion along the modern Cascadia subduction zone (western North America), while to the north, a progressive southwestward transition from predominantly convergent tectonics to predominantly strike-slip movement along an active Norumbega fault system occurred, reflecting migration of a triple junction similar to the Mendocino triple junction.

Pre- to post-Cordilleran transposition history of Joss Mountain: Insights into the exhumation of the Shuswap complex, southeastern Canadian Cordillera

We present new multiscale structural, mineral chemical, and U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data in order to unravel part of the tectono-metamorphic evolution of the Shuswap complex in the southern Canadian Cordillera. We reconstructed the pressure-temperature-deformation-time ( P - T - d - t ) history of the Joss Mountain domain within the Shuswap complex. The west-dipping Greenbush shear zone separates the Joss Mountain domain from the structurally lower Thor-Odin culmination to the east, the southern culmination of the Monashee complex, and one of the structurally deepest parts of the Shuswap complex. At Joss Mountain, the protolith of an orthogneiss crystallized at ca. 360 Ma which is consistent with Late Devonian arc magmatism along the western paleomargin of North America. Joss Mountain metasedimentary rocks and orthogneiss were transposed at ∼21–29 km depth over a period of at least 20 m.y., and possibly more than 38 m.y., during Late Cretaceous to Paleocene mature stages of Cordilleran continental collision. This mature collision took place while slow detachment of the subducted oceanic lithosphere occurred and thermal conditions were approaching those of a crust undergoing postorogenic thermal relaxation. Transposition at Joss Mountain ended earlier and exhumation started earlier than in the Monashee complex. Exhumation occurred under conditions of near-isothermal decompression and geothermal gradients consistent with lithospheric thinning. Earlier and slower exhumation of the Joss Mountain domain than of the adjacent northwestern Thor-Odin culmination may have resulted from normal movement along the Greenbush shear zone contributing to the exhumation of the Shuswap complex.

Geochemical evidence for a Ganderian arc/back-arc remnant in the Nashoba Terrane, SE New England, USA

New geochemical data including Sm/Nd isotopic data show evidence for an early Paleozoic arc/back-arc complex in the Nashoba terrane of southeastern New England. The Nashoba terrane lies between rocks of Ganderian affinity to the northwest and Avalonian affinity to the southeast. It consists of early Paleozoic mafic to felsic metavolcanic and metasedimentary rocks that were intruded by intermediate to felsic plutons and metamorphosed to upper amphibolite facies conditions in the mid-Paleozoic. Major and trace element geochemical data indicate that the early Paleozoic igneous rocks contain a mix of arc, MORB, and alkaline signatures, and that the terrane formed as a primitive volcanic arc/back-arc complex built on thinned continental crust. Amphibolites have +4 to +7.5εNd(500) values consistent with formation in a primitive volcanic arc with minimal crustal contamination. Intermediate and felsic gneisses have εNd(500) values between +1.2 and –0.75 indicating a mixture of juvenile arc magmas and an evolved (likely basement) source. Depleted mantle model ages of 1.2 to 1.6 Ga point to a Mesoproterozoic or older age for this source. Metasedimentary rocks yielded –6 to –8.3 εNd(500) values and 1.6 to 1.8 Ga model ages, indicating an isotopically evolved source (or sources) that included Paleoproterozoic or older material. The εNd(500) values and model ages of the intermediate and felsic and metasedimentary rocks indicate that the basement to the Nashoba terrane is Ganderian rather than Avalonian. The Nashoba terrane therefore represents a Ganderian arc/back-arc complex similar to the Cambrian Penobscot arc/back-arc seen in Maritime Canada and Newfoundland, and particularly in the Annidale and New River terranes of southern New Brunswick. This correlation has not previously been recognized in southeastern New England. The Ganderian affinity of the Nashoba terrane also extends Ganderia farther SE in New England than previously established and indicates that the Nashoba terrane did not originate as a separate oceanic arc/back-arc complex or microcontinent.

Effects of Paleogene faults on the reconstruction of the metamorphic history of the northwestern Thor-Odin culmination of the Monashee complex, southeastern British Columbia

Differences in U-Pb metamorphic monazite ages in the northwestern Thor-Odin culmination of the Monashee complex, southern Canadian Cordillera, are explained in the context of the NNW-trending subvertical transcurrent Paleogene Victor Creek fault. Similar faults are present throughout the Canadian Cordillera. We demonstrate their potential importance in the interpretation of the history of Cordilleran deformation and metamorphism. A pervasive transposition foliation (S T ) is present throughout the Thor-Odin culmination as a result of Cordilleran and possibly earlier deformation. A pre-S T (or early S T ) foliation is preserved as aligned inclusion trails in porphyroblasts such as garnet and kyanite. Monazite U-(Th-)Pb isotope dilution−thermal ionization and secondary ion microprobe mass spectrometry (ID-TIMS, SIMS) ages are used to relate monazite growth to pre- and syn-S T fabrics and associated metamorphism. The ages of both pre- and syn-S T fabrics, and the gap between pre- and syn-S T ages decrease toward the east, in an apparently continuous manner. While monazite west of the Victor Creek fault is latest Cretaceous to earliest Eocene in age, monazite east of the Victor Creek fault is exclusively Eocene. Correlation of rock types across the faults is difficult because the same rock units are repeated many times on either side. However, distinctly different retained ages of metamorphism, and previously recognized differences in structures and detrital zircon signatures across the fault indicate 5−60 km offset along the fault. Similar trends occur across other faults along the western Monashee complex, and faults here and elsewhere in the Canadian Cordillera may have similar geological significance.

Age and provenance of a Paleoproterozoic to Devonian Canadian Cordilleran sequence of metasedimentary rocks, Thor–Odin dome, southeastern British Columbia

We present a U-Pb detrital zircon age and provenance study of a sequence of metasedimentary rocks in the northwestern Thor–Odin high-grade gneiss dome within the Omineca crystalline belt of the Canadian Cordillera. Despite strong overprint by deformation and metamorphism, we successfully analyzed the age and provenance of six samples collected at various structural levels, using U-Pb detrital zircon laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis. The Thor–Odin dome consists of Paleoproterozoic basement and a metasedimentary cover sequence of previously unknown age and tectonic significance. Our results indicate that the oldest units of this sequence may be Paleoproterozoic, and some of the oldest known metasedimentary rocks in the Canadian Cordillera, originally deposited on top of Laurentian basement rocks. The youngest rocks are Devonian and deposited shortly before the onset of widespread Late Devonian to early Mississippian igneous activity in the Selkirk Domain or Kootenay arc. The cover sequence of the Thor–Odin dome thus preserves some of both the oldest and the youngest (meta)sedimentary rocks deposited between the formation of supercontinent Columbia and the onset of igneous activity and convergence that marked the beginning of Cordilleran deformation and metamorphism. Parts of as many as ∼1.4 b.y. of sedimentary history are preserved in the Thor–Odin dome, implying that much information on the sedimentary history of the Canadian Cordillera may be hidden in other Cordilleran gneiss domes.

The Dry-Erase Cube: Making Three-Dimensional Visualization Easy

The 10 cm dry-erase cube is an effective tool for illustrating the three-dimensionality of geological features in maps, cross sections and block diagrams, and it aids in the understanding of stereographic projections and orientated samples. It is ideal for use in classes and labs of structural geology and geological field methods courses. The cube is useful for demonstrating the three-dimensional relationships among any planar or linear features in geoscience courses at any level.