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Potassium-argon dating of five basalts from the three main islands of the Juan Fernández (or Robinson Crusoe) Islands of Chile in the southeastern Pacific gives ages of 1.01 +/- 0.12 and 2.44 +/- 0.14 million years for Masafuera, 3.79 +/- 0.20 and 4.23 +/- 0.16 for Masatierra, and 5.8 +/- 2.1 for Santa Clara. These ages are much younger than that of the underlying oceanic plate and are consistent with the origin of the island-seamount chain from a mantle hot spot beneath the eastward moving Nazca plate. The young age for the archipelago suggests that speciation within endemic genera has occurred within the past 4 to 5 million years. Endemic genera of apparently more ancient origins, such as Lactoris and Thyrsopteris, have apparently dispersed to the islands and survive refugially.

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Stepwise 40Ar/39Ar degassing experiments of 57 mineral separates of hornblende, muscovite, biotite, and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the Grenville Orogen in Ontario. The 40Ar/39Ar data support an interpretation of slow, nearly uniform cooling (1 ø-4 ø C/m. y.) from temperatures of ~500øC to below ~ 150øC. In the Central Gneiss Belt (CGB) hornblendes cooled through Ar closure between 930 and 1025 Ma, whereas in the Central Metasedimentary Belt (CMB) hornblendes record the following range in 4øAr/39Ar cooling ages: 1104 Ma in the Frontenac terrane, 1007-1067 Ma in the Sharbot Lake terrane, 919-1026 Ma in the Elzevir terrane, and 972 Ma in the Central Metasedimentary Belt Boundary Zone. Regional uplift/erosion rates of 0.03-0.14 km/m.y. have been estimated for the Grenville Orogen in Ontario based on the 4øAr/39Ar data, a model retrograde P-T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambrian and Ordovician sediments. These erosion rates are consistent with rates estimated for other Proterozoic or Archcan granulite terranes but are an order of magnitude slower than active orogens such as the Alps and Himalayas. A regular variation in hornblende 40Ar/39Ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the CMB. The pattern of 40Ar/39Ar ages is interpreted to reflect late-tectonic extension, consistent with field observations in the Central Metasedimentary Belt Boundary Zone and elsewhere in the CMB. Up to 13 km of vertical displacement is inferred for some rocks in the CMB between the time they cooled below closure to argon diffusion in hornblende (~500øC) and their exposure at the surface (~25øC).1Now at Copyfight 1991 by the American Geophysical Union. Paper number 91TC00859. 0278-7407/91/91 TC-00859510.00 U/Pb, Sm/Nd, Rb/Sr, K/Ar, or 4øAr/39Ar), for which retention of the daughter isotopes or fission tracks is temperature dependant [e.g., Cliff, 1985]. The 4øAr/39Ar technique is especially useful for cooling studies because potassium-bearing minerals are abundant in many igneous and metamorphic rocks and because these minerals begin to quantitatively retain argon over a wide range of distinct temperatures. The 4øAr/39Ar dating technique has been successfully applied in many cooling history studies of regionally metamorphosed rocks [e.g., Dallmeyer et al., 1975; Dallmeyer and Sutter, 1976; Berger and York, 1981b; Harrison and McDougall, 1981; Sutter et al., 1985; Horton et al., 1987]. The Grenville Province (or Orogen, as used throughout this paper) refers to the 1.0-1.2 Ga old, mostly high-grade metamorphic terrane, including the Adirondacks, exposed from Georgian Bay to the Labrador coast [Moore, 1986; Woussen et al., 1986]. Rocks considered to have genetic affinities with the Grenville Orogen have been reported from the Appalachian...

Summary In recent years considerable attention has focused on metamorphic core complexes of the Basin and Range Province of the western US Cordillera. These highly extended areas are characterized by an upper plate that has been brittlely attenuated by normal faults separated by a sub-horizontal detachment surface from a lower plate that has been ductilely thinned and stretched. A study of mesoscopic structures, finite strain, microstructures, quartz c -axis fabrics and 40 Ar/ 39 Ar geochronology was undertaken in order to characterize the nature, geometry, kinematic history and timing of ductile extension in the lower plate of the northern Snake Range metamorphic core complex in E-central Nevada. These data provide new insights into the processes of deep-seated ductile strain beneath supracrustal normal fault mosaics in highly extended regions. Mesoscopic structures and finite strain analyses indicate that the lower plate underwent plane strain with a sub-vertical Z-axis and a sub-horizontal WNW-ESE X -axis. The magnitude of strain increases dramatically from W-to-E, and is constant vertically. Bedding and foliation are everywhere parallel and bedding is thinned 30–90%. The nature and geometry of microstructures and c -axis fabrics changes progressively from W-to-E. Lower strain rocks on the W flank of the range are characterized by a single bedding-parallel foliation, defined by flattened detrital grains, globular grains which have their c -axes parallel to the Z -axis and symmetrical cross-girdle c -axis fabrics. Further E to slightly higher strains, the rocks are characterized by asymmetrical cross-girdle c -axis fabrics, although globular grains with their c -axes parallel to Z are still present. The high-strain rocks on the E flank of the range are characterized by C -planes, asymmetrical mica ‘fish’ and oblique quartz foliations, and asymmetrical single-girdle c -axis fabrics. These data suggest that the lower plate has not deformed entirely by either coaxial or non-coaxial strain, and we propose an evolutionary model whereby the lower plate deformed during an early period of coaxial strain followed by a later component of non-coaxial strain accompanied by coaxial strain on the E flank of the range. 40 Ar/ 39 Ar geochronology on lower-plate hornblendes and micas indicates that temperatures of deformation were >280°C, but <530°C, and increased with depth. Geochronological data suggest that lower-plate ductile strain is post-latest Cretaceous, and biotite and muscovite data indicate that ductile deformation was probably ongoing 22–26 Ma. Our data and regional geological relations suggest that ductile extensional deformation in the northern Snake Range occurred because of localized heat input to shallow levels of the crust, and together with seismic reflection data, indicate that lower-plate rocks probably represent the top of a regional metamorphic terrane of Tertiary age.