Search

Southwest Spitsbergen, Wedel Jarlsberg Land, consists of two Proterozoic crustal blocks with differing metamorphic histories. Both blocks experienced Caledonian greenschist-facies metamorphism, but only the southern block records an earlier pervasive M1 amphibolite-facies metamorphism and strong deformational fabric. In situ EMPA total-Pb monazite geochronology from both matrix and porphyroblast inclusion results indicate that the older M1 metamorphism occurred at 643 [+ or -] 9 Ma, consistent with published cooling ages of c. 620 Ma (hornblende) and 580 Ma (mica) obtained from these same rocks. This region thus contains a lithostratigraphic profile and metamorphic history which are unique within the Svalbard Archipelago. Documentation of a pervasive late Neoproterozoic Barrovian metamorphism is difficult to reconcile with a quiescent non-tectonic regime typically inferred for this region, based on the occurrence of rift-drift sequences on the Baltic and Laurentian passive margins. Instead, our new metamorphic age implies an exotic origin of the pre-Devonian basement exposed in SW Spitsbergen and supports models of terrane assembly postulated for the Svalbard Archipelago. Keywords: Svalbard, Caledonides, terranes, geochronology, tectonics.

Abstract: The Laramide Black Hills uplift of southwest South Dakota exposes a Precambrian crystalline core of ~2560-2600 Ma basement granitoids nonconformably overlain by two Paleoproterozoic intracratonic rift successions. In the [...]

We propose that the late Paleoproterozoic igneous and deformational history preserved in the northern midcontinent United States can be explained by a change in subduction-polarity from geon 18 south-dipping subduction during Penokean accretion to geon 17 north-dipping subduction as convergence continued after Penokean orogenesis. New U-Pb zircon ages indicate that late to post-Penokean magmatism occurred at ca. 1800, 1775, and 1750 Ma and generally migrated southeastward across the newly accreted Penokean terrane. We suggest that geon 17 Yavapai slab rollback caused continental arc magmatism to step southeastward between 1800 and 1750 Ma. As the slab steepened, reduced compressional stresses and magma-induced thermal weakening allowed for collapse of the overthickened portions of the Penokean crust. Postcollapse crustal stabilization (the 1750-1650 Ma Baraboo interval) was followed by geon 16 Mazatzal arc accretion further south. The 1900-1600 Ma tectonic history of the north-central United States, not surprisingly, records events related to the southward growth and tectonic development of the southern Laurentian margin. New and published [sup.40]Ar/[sup.39]Ar mineral ages delineate the northern and western extent of geon 16 Mazatzal deformation. Interestingly, only little exhumed crust intruded by a small volume of shallow-level ca. 1750 Ma plutons (and associated rhyolites) was deformed significantly during geon 16. In contrast, more deeply exhumed crust and crust pervasively invaded by a large volume of post-Penokean magma (i.e., East-Central Minnesota Batholith) were largely unaffected by Mazatzal deformation and reheating. We suggest that posttectonic intrusions and crustal thinning were an important step in strengthening and stabilizing the crust in the southern Lake Superior region. Keywords: Penokean orogen, U-Pb zircon ages, geon 17 magmatism, Yavapai slab rollback, crustal stabilization.

A two-time metamorphism of the Early Proterozoic sediments in central Minnesota is proposed, initially during the Penokean orogeny and the second time during an episode of post-orogenic magmatism at 1770 to 1760 Ma. According to new thermochronologic, thermobarometric and geobarometric data, widespread staurolite-grade metamorphism and intrusion of post-Penokean plutons occurred simultaneously within the internal zone of the orogen. Stabilization of continental crust occurred following the sudden and rapid collapse of the overthickened Penokean orogen.

The 40Ar/39Ar evidence for middle proterozoic slow cooling of the southern Black Hills, SD, was examined to determine its implications for early proterozoic P-T evolution and posttectonic magmatism. The research utilized proterozoic mica dates from metasedimentary rocks. Results revealed that uplift and crustal thinning before the Harney Peak Granite intrusion was due to differential thinning of the subcrustal lithosphere beneath the Black Hills.

Gneiss domes preserved throughout the Paleoproterozoic (1870-1820 Ma) Penokean orogenic belt are commonly viewed as collisional features. However, 1700 Ma mica 40Ar/39Ar ages from Archean rocks of the McGrath gneiss dome, Minnesota, are in marked contrast with 1750-1760 Ma mica ages obtained everywhere outside the dome, suggesting the dome may have formed well after compression had ceased. We hypothesize that the bimodal age pattern is a result of crustal excision between Archean basement and Paleoproterozoic cover during extensional collapse of the over-thickened orogen. Removal of structural section and progressive unroofing may have led to the development of a domed basement-cover shear zone. In this respect, the McGrath gneiss dome would represent one of the oldest analogues of a metamorphic core complex. Postorogenic collapse of the Penokean orogen was apparently long-lived, with significant pulses occurring at [approximately]1755 Ma and at [approximately]1700 Ma The sudden and widespread crustal fusion at 1770-1760 Ma that preceded the proposed collapse suggests that rapid sub-crustal lithospheric thinning may have triggered collapse as has been proposed recently for several Phanerozoic collapsed orogens.

Paleomagnetic data have been obtained from Miocene intrusions, including crosscutting mafic and felsic dikes, and Proterozoic crystalline rocks to evaluate deformation during Miocene and younger extension and unroofing of the Black Mountains, Death Valley, California. Synrift intrusions contain a well-defined and, at the site level, well-grouped magnetization, interpreted to be of dual polarity, the in situ direction of which is discordant in declination and inclination with an expected late Cenozoic reference direction. The oldest of these intrusions (11.6 + or - 0.2 Ma) gives a magnetite-dominated remanent magnetization of high coercivity and high laboratory unblocking temperatures (about 550 to 580 degrees C) that is interpreted to predate unroofing from mid-crustal depths. In situ site mean directions of this magnetization are directed toward the west and west-northwest with moderate to shallow positive (down) and negative (up) inclinations. The variation in direction of magnetization, particularly inclination, with site locality around the turtleback structures along the western flank of the Black Mountains, is interpreted to result from folding of the intrusion after remanence acquisition. Younger intrusions (less than or equal to 8.7 Ma) generally give magnetizations with inclinations similar to expected Miocene values. Two populations of in situ site means are identified: one with southwest declination and negative inclination; the other with northward declination and positive inclination. A preferred interpretation for footwall deformation involves, from oldest to youngest: (1) southwest-side down tilting of the entire range block of some 20 degrees to 40 degrees to possibly 70 degrees and, at least locally, folding of the crystalline rocks, on a trend parallel to the Death Valley turtlebacks, between 11.6 and 8.7 Ma; (2) progressive east-to-west footwall unroofing between 8.7 and ca. 6.5 Ma; and (3) clockwise rotation (50 degrees to 80 degrees) of much of the Black Mountains after the core detached from stable terrane to the west. We interpret late rotation of the Black Mountains as oroflexure related to right-lateral shear along the Death Valley fault zone.

Time-depth constraints on intrusion and unroofing during the Miocene are revealed by cooling age patterns and geobarometric data from the Black Mountains in Death Valley, California. Northwest diachronous cooling withdike intrusion show decreasing ages, suggesting a 10-15 km denudation along a northwest-oriented detachment system, with an average initial dip of 20 degress Footwall flexure migration under a scoop-shaped hanging wall block explains this northwestward unroofing and dike intrusion. A signifcant conclusion is that middle crust ductile fabrics steepen during unloading in the upper crust.

A question regarding the 1900-1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40Ar/39Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes ([F.sub.1]) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest-trending upright [F.sub.2] folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40Ar/39Ar horn-blende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is [approximately]50-60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810-1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780-1740 Ma interval of island-arc accretion along the southern margin of the growing craton.

Previous attempts to explain Paleoproterozoic post-tectonic magmatism in the Penokean and southern Trans-Hudson orogens inadequately account for the entire tectonic setting and geologic history of these areas. Key characteristics of post-tectonic plutonism in both orogens include the similar time-lag of emplacement after orogenesis (50-60 m.y.), the close association in both time and space between post-orogenic uplift and intrusion, the depth of emplacement, and the long-lived tectonic quiescence (crustal stabilization) that followed magma emplacement. Post-tectonic magmatism appears closely associated with post-orogenic uplift (collapse) and crustal stabilization. I propose that convective removal of the mantle lithospheric root may best explain post-tectonic magma genesis. Such a model suggests that there is a simple and related evolutionary history that began with construction of the Penokean and southern Trans-Hudson orogens (crustal thickening, metamorphism, plutonism, etc.) and ended with their destruction (crustal thinning, widespread uplift/cooling, plutonism, etc.), thus stabilizing the crust. Post-tectonic plutons may be an indelible mark of orogenic demise, not the culmination of terminal collision. Delamination-magmatism probably played a key role in the differentiation and stabilization of crust after orogenesis and may have promoted its transformation into craton. [ABSTRACT FROM AUTHOR]

Discusses the results of a study which aim was to present evidence for a metamorphic event which affected the Precambrian rocks of central Minnesota. Description of how the study was conducted; What data collected suggests; Analysis of data.

Schists exposed in the central Southern Alps, New Zealand, 1-5 km east of the Alpine fault zone, have been rapidly uplifted during the late Cenozoic. A regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle-ductile transitional behaviour to ductile deformational features. Kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction, indicating their relation to late Cenozoic uplift of the Southern Alps. Analysis of deformed veins suggests a minimum shortening strain of about 50% perpendicular to the foliation. The veins crosscut upright mesoscopic to macroscopic folds which commonly develop high-strain zones on their limbs. Within such zones a strong stretching lineation plunges gently SW, approximately perpendicular to the stretching lineation in the mylonites along the Alpine fault. The folds and high-strain zones do not appear to be related to late Cenozoic uplift but may have originated during an earlier phase of dominantly strike-slip motion. Fluid inclusion studies give depth-temperature estimates of around 3 km and 285°C for the development of the brittle structures and 6-8 km at 310°-350°C for the brittle-ductile transition. The deformed veins show evidence of extensive fluid infiltration during the later phases; data from fluid inclusions and metamorphic assemblages give an estimate of 15-20 km and 400°-450°C for their deformation. These data, combined with uplift rates determined by other studies, allow the construction of a depth-temperature-time path which indicates nearly isothermal decompression associated with rapid uplift. A shallow brittle-ductile transition (6-8 km) is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area. Because of the small temperature change over a large amount of decompression, pressure may be an important factor in controlling the depth of the brittle-ductile transition. The marked temperature drop above the brittle-ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism. [ABSTRACT FROM AUTHOR]