B.A. van der Pluijm

U-Pb geochronology of the Grenville Orogen of Ontario and New York: constraints on ancient crustal tectonics

Based on lithological, structural and geophysical characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones. In order to constrain the timing of metamorphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites. In addition, U-Pb ages were obtained for monazites from metapelites and for a rutile from an amphibolite. These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen. Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals:Central Granulite Terrane:Adirondack Highlands: 1150 Ma; 1070–1050 Ma; 1030–1000 MaCentral Metasedimentary Belt:Adirondack Lowlands 1170–1130 MaFrontenac domain 1175–1150 MaSharbot Lake domain ca. 1152 MaFlzevir domain: 1240 Ma; 1060–1020 MaBancroft domain: ca. 1150 Ma; 1045–1030 MaCentral Gneiss Belt: ca. 1450 Ma; ca. 1150 Ma; 1100–1050 MaGrenville FrontTectonic Zone ca. 1000 Ma.Combination of mineral ages with results from thermobarometry indicates that metamorphic pressures and temperatures recorded by thermobarometers were reached polychronously in the different domains that are separated by major shear zones. Some of these shear zones such as the Robertson Lake shear zone and the Carthage-Colton shear zone represent major tectonic boundaries. The Grenville Orogen is made up of a collage of crustal terranes that have distinct thermal and tectonic histories and that were accreted laterally by tectonic processes analogous to those observed along modern active continental margins. The subsequent history of the orogen is characterized by slow time-integrated cooling rates of 3±1°C/Ma and denudation rates of 120±40m/Ma.

Paleogeography of the Amazon craton at 1.2 Ga: early Grenvillian collision with the Llano segment of Laurentia

A paleomagnetic, geochronologic and petrographic study was undertaken on the flat-lying gabbros and basalts of the Nova Floresta Formation of Rondo “ nia state, western Brazil in order to constrain the Mesoproterozoic paleogeography of the Amazon craton. Measurement of the anisotropy of magnetic susceptibility on the gabbroic samples reveals a flat-lying foliation with a radiating pattern of lineations, supporting the field evidence that the gabbros are part of a large, undeformed sill. Petrographic observations of oxides in the gabbros reveals two populations of magnetite grains produced during the original cooling of the sill: large, oxyexsolved titanomagnetite grains and fine-grained magnetite in igneous reaction rims. New 40 Ar/ 39 Ar age dating of biotite and plagioclase yield ages of V1.2 Ga, which represent the rapid cooling following emplacement of the mafic magma. Whole rock dating of basalt samples yields total gas ages of 1062 ˛ 3 Ma, similar to the V1.0 Ga K/Ar ages reported by previous workers. However, the strong compositional dependence of the age spectrum renders this younger whole rock age unreliable except as a minimum constraint. A single magnetic component is found in the basalts, indistinguishable from the characteristic remanence found in the gabbros that is oriented WNW and steeply upward. This magnetization is considered to be primary and was acquired during the cooling of the sill and associated lavas. A paleomagnetic pole calculated from the Nova Floresta Formation (n = 16 sites, Plat: = 24.6‡N, Plong: = 164.6‡E, A95 = 5.5‡, Q = 5), the first reported pole for the Amazon craton for the 1200^600 Ma Rodinia time period, constrains the paleogeographic position of Amazonia at V1.2 Ga. Juxtaposition of the western Amazon craton with the Llano segment of the Laurentia’s Grenville margin causes the NF pole to lie on the 1.2 Ga portion of the combined APWP for Laurentia and Greenland, which indicates that a collision with the Amazon craton could have caused the Llano deformation in early Grenvillian times. fl 2002 Elsevier Science B.V. All rights reserved.

The formation of an orocline by multiphase deformation: a paleomagnetic investigation of the Cantabria-Asturias Arc (northern Spain)

The Variscan orogeny in the Cantabria‐Asturias Arc (CAA) of northern Spain represents a multiphase deformation history associated with Pangea’s amalgamation. To determine the timing and kinematic history of deformation, a paleomagnetic investigation was carried out on Devonian limestones in three structural domains of the CAA’s hinge zone. Two characteristic remagnetizations in the CAA are distinguished: an Early Permian (B) component that postdates initial Westphalian‐Stephanian aged folding (F1 and F2), and a Late Carboniferous (C) component that postdates F1 deformation. Both the B and C magnetizations experienced a later F3 folding phase. The kinematics and geometry of post-magnetization deformation are determined by bringing measured magnetic directions into agreement with reference directions. These structural corrections also allow separation of deformation events in time. We conclude that generally east‐west-trending, but variably plunging, fold axes characterize F3 folding in the hinge zone of the CAA. This post-magnetization deformation involved significant amounts of tilting and (sub)vertical axis rotation, which together produced clockwise rotation in the north of the arc and counterclockwise rotation in the south of the arc, to form the horseshoe-shaped orocline that is observed today. F3 fold axes change from near-vertical to moderately inclined between and within structural domains, due to the structural fabric imposed by F1 and F2. Based on calculated F3 fold properties, we can reconstruct pre-existing F1 and F2 structural geometries of the hinge zones. This analysis shows that F1 and F2 structures are the result of Late Carboniferous deformation, as part of the same east‐west but temporally discrete compression regimes. On the other hand, F3 is controlled by Permian, north‐south

Nanocoatings of clay and creep of the San Andreas fault at Parkfield, California

Mudrock samples were investigated from two fault zones at ∼3066 m and ∼3296 m measured depth (MD) located outside and within the main damage zone of the San Andreas Fault Observatory at Depth (SAFOD) drillhole at Parkfield, California. All studied fault rocks show features typical of those reported across creep zones with variably spaced and interconnected networks of polished displacement surfaces coated by abundant polished films and occasional striations. Electron microscopy and X-ray diffraction study of the surfaces reveal the occurrence of neocrystallized thin film clay coatings containing illite-smectite (I-S) and chlorite-smectite (C-S) minerals. 40 Ar/ 39 Ar dating of the illitic mix-layered coatings demonstrated Miocene to Pliocene crystallization and revealed an older fault strand (8 ± 1.3 Ma) at 3066 m MD, and a probably younger fault strand (4 ± 4.9 Ma) at 3296 m MD. Today, the younger strand is the site of active creep behavior, reflecting a possible (re)activation of these clay-weakened zones. We propose that the majority of slow fault creep is controlled by the high density of thin (

Growth and retrograde zoning in garnets from high-grade, metapelites: Implications for pressure-temperature paths

A previously unrecognized pattern of garnet zoning has been preserved in garnets from the upper amphibolite facies Britt domain, Ontario Grenville province. Some large garnets contain discrete regions of high grossular content as well as variations in the Mg/(Mg+Fe) and Mn nearly parallel to grain boundaries. The latter are superimposed on and cut across regions of high grossular content. Such crosscutting zoning patterns have not been previously recognized in garnets. The Ca zoning involves changes of 4 to 10 mol% grossular and is interpreted to represent growth zoning, whereas the decreasing Mg/(Mg+Fe) and increasing Mn from core to rim are believed to be a diffusional retrograde effect. The Ca zoning indicates a pressure drop of ca. 5 kbar at 700 ±50°C (

The Carthage-Colton Mylonite Zone (Adirondack Mountains, New York): The Site of a Cryptic Suture in the Grenville Orogen?

U-Pb ages were determined on metamorphic sphenes and monazites from the Late Proterozoic Adirondack Highlands and Lowlands in the vicinity of the Carthage-Colton mylonite zone. Monazites were extracted from metapelites, and sphenes were separated from marbles, calc-silicate gneisses, and granite gneisses in order to determine the timing and the duration of metamorphism as well as the cooling histories for rocks on either side of the mylonite zone. Monazite ages from the Lowlands range from 1171-1137 Ma; sphene ages in the Lowlands range from 1156-1103 Ma, those from the Highlands immediately to the east of the mylonite zone range from 1050-982 Ma. The ages indicate that the last high-grade metamorphism in the Highlands is ca. 100 m.y. younger than in the Lowlands and that both terranes had separate cooling histories at least until ca. 1000 Ma. Sphenes from within the Carthage-Colton mylonite zone yield ages of about 1098 Ma, which are distinct from sphene ages on either side of the shear zone. The mineral ages, structures, and metamorphic histories suggests that the Carthage-Colton mylonite zone is a fundamental tectonic boundary within the Proterozoic Grenville Orogen of North America.

Chlorite-smectite clay minerals and fault behavior: New evidence from the San Andreas Fault Observatory at Depth (SAFOD) core

Segments of the modern San Andreas fault experience creep behavior, which is attributed to various factors, including (1) low values of effective normal stress, (2) elevated pore-fluid pressure, and (3) low frictional strength. The San Andreas Fault Observatory at Depth (SAFOD) drill hole in Parkfield, California, provides new insights into frictional properties by recognizing the importance of smectitic clay minerals, as demonstrated by analysis of mudrock and fault gouge samples from zones between 3186 and 3199 m and 3295 and 3313 m measured depths. X-ray diffraction (XRD) results show illite, chlorite, and mixed-layered illite-smectite and chlorite-smectite minerals in the faulted mudrock, whereas serpentine, Mg-rich smectite, and chlorite-smectite minerals are concentrated in the southwest deformation zone and the central deformation zone of the two actively creeping sections in the San Andreas fault. These rocks are abundantly coated by shiny clay mineral layers in some cases, reflecting mineral formation during creep. Secondary- and transmission-electron microscopy (SEM/TEM) and XRD studies of these slip surface coatings reveal thin films of neoformed chlorite-smectite phases, similar to previously described illite-smectite microscale precipitations. The abundance of chlorite-smectite minerals within fault rock of the SAFOD borehole significantly extends the potential role of mineralogic processes to depths up to 10 km, with cataclasis and fluid infiltration creating nucleation sites for neomineralization on displacement surfaces. We propose that localization of illitic to chloritic smectite clay minerals on slip surfaces from near the surface to the brittle-ductile transition promotes creep behavior of faults.

Synorogenic Collapse: A Perspective from the Middle Crust, the Proterozoic Grenville Orogen

Structural, petrological, and geochronological studies of the middle to late Proterozoic Grenville orogen in Ontario, Canada, indicate that a major extensional fault developed synchronously with late thrusting. This fault zone was initiated during peak metamorphism and extended into the crust to depths of at least 25 kilometers. The temporal and spatial relations among faulting, metamorphism, and regional compression indicate that synorogenic collapse initiated because the crust exceeded the maximum physiographic height and thickness that could be supported by its rheology. Comparison of Grenville with recent Himalayan orogenic activity suggests that during Proterozoic times physiographic height, crustal thickness, and crustal strength were similar to modern conditions in orogenic belts.

Analysis of Variscan dynamics; early bending of the Cantabria^Asturias Arc, northern Spain

Calcite twinning analysis in the Cantabria^Asturias Arc (CAA) of northern Spain provides a basis for evaluating conditions of Variscan stress and constrains the arc’s structural evolution. Twinning typically occurs during earliest layer-parallel shortening, offering the ability to define early conditions of regional stress. Results from the Somiedo^ Correcilla region are of two kinds: early maximum compressive stress oriented layer-parallel and at high angles to bedding strike (D1c1) and later twin producing compression oriented sub-parallel to strike (D2c1). When all D1 compressions are rotated into a uniform east^west reference orientation, a quite linear, north^south trending fold^ thrust belt results showing a slight deflection of the southern zone to the south^southeast. North^south-directed D2c1 compression was recorded prior to bending of the belt. Calcite twinning data elucidate earliest structural conditions that could not be obtained by other means, whereas the kinematics of arc tightening during D2 is constrained by paleomagnetism. A large and perhaps protracted D2c1 is suggested by our results, as manifested by approximately 50% arc tightening prior to acquisition of paleomagnetic remagnetizations throughout the CAA. Early east^west compression (D1c1) likely resulted from the Ebro^Aquitaine massif docking to Laurussia whereas the north-directed collision of Africa (D2c1) produced clockwise bending in the northern zone, radial folding in the hinge, and rotation of thrusts in the southern zone. fl 2000 Elsevier Science B.V. All rights reserved.

Late Mesoproterozoic Deformation of SW Amazonia (Rondônia, Brazil) : Geochronological and Structural Evidence for Collision with Southern Laurentia

Proposed assembly of the Rodinia supercontinent in the late Mesoproterozoic involved the collision of the Amazon craton with some portion of the southern or eastern margin of cratonic North America. Previously reported paleomagnetic data from the SW Amazon craton suggest a paleogeographic link between “Grenvillian” deformation of the SW Amazon craton and late Mesoproterozoic tectonometamorphism in southern Laurentia. A structural, geochronological, and petrological investigation of the western Amazon basement rocks (Rondônia, Brazil) was carried out in order to document evidence of a Grenvillian collision connecting the Amazon to Laurentia. Integration of 40Ar/39Ar data and feldspar thermometry data from regionally extensive strike‐slip mylonitic shear zones (Ji‐Paraná shear zone network) indicates that deformation took place at 450°–550°C between 1.18 and 1.15 Ga. An older, ca. 1.35‐Ga event found exclusively in less‐deformed basement rocks is interpreted as recording cooling from an earlier metamorphic episode (650°–800°C indicated by feldspar thermometry) unrelated to the Grenville collision. The style of deformation in the SW Amazon craton contrasts with that observed in southern Laurentia, where extensive crustal thickening accommodated by deep‐seated thrust sheets resulted in widespread thermal resetting of isotopic systems during exhumation and postorogenic cooling. In contrast, the predominantly strike‐slip activity observed in the Amazon resulted in age resetting through strain‐induced recrystallization, not regional‐scale thermal resetting. Consequently, the ages recorded by hornblende in the SW Amazon craton are slightly older than the cooling ages preserved in southern Laurentia. Differences in structural style and geochronological record are interpreted as indicative of an exhumed, asymmetric crustal structure similar to that of modern orogens.