Influence of reactivated basement structures on evolving orogens: Along-strike diachronous Himalayan metamorphism in far west Nepal
0
Citation
0
Reference
10
Related Paper
Abstract:
Determining the geometry and evolution of a basal detachment and its influence on orogenesis is a challenging, but important, aspect to understanding orogenic evolution. The basal detachment of the Himalayan orogen in far west Nepal is presently segmented by a documented tear fault. New pressure-temperature-time-deformation paths from the Himalayan metamorphic core along the Seti Khola river transect were integrated to compare the tectonometamorphic evolution on either side of the basal detachment tear fault to outline its history. Peak metamorphic conditions of 645−745 °C and 0.85−1.1 GPa were reached in the Seti Khola Himalayan metamorphic core rocks during the Oligocene to earliest Miocene, 10−14 m.y. prior to equivalent along-strike rocks in the adjacent Karnali valley, which indicates segmentation of the Himalayan metamorphic core across the tear fault. We interpret the segmentation of the orogen to have been caused by the development of the tear fault in the basal detachment of the Himalayan orogen and differing ramp-flat geometries on either side. The segmentation and change in basal detachment geometry is consistent with the reactivation of an underthrusted Indian plate inherited basement structure, the Great Boundary Fault, during the Oligocene to earliest Miocene. The comparison of tectonometamorphic histories along-strike in far west Nepal highlights the basal detachment geometry through time and the need to consider the pre-orogenic structural features of the plates involved in orogenesis. This study reinforces the importance of combining tectonometamorphic studies with geophysical and geomorphological data to fully understand the causes of along-strike segmentation of orogenic systems through time.Keywords:
Diachronous
Basement
Abstract Wireline log correlation panels of palynologically analysed boreholes illustrate lateral facies transitions within Rupelian age strata of the Dutch Rupel Formation across the southeastern North Sea Basin. The middle to upper part of the Rupel Formation consists of clays and silts of the Boom Member and the overlying sandier Steensel Member. In the Mill and Goirle boreholes in the Dutch Province of North Brabant, the Boom Member is thickly developed and represents the middle to upper Rupelian (biozones NSO3 to NSO5a), while the Steensel Member is rather thinly developed and only comprises the uppermost Rupelian (biozone NSO5a). Borehole log correlations show that towards the south (or more proximal to the palaeo-continent) the Rupel Formation becomes sandier and the clayey Boom Member thins in favour of the sandy superjacent Steensel Member. Palynological analyses confirm that the Boom Member is restricted to the middle Rupelian (biozone NSO3) age here, and that the superjacent Steensel Member is of middle to upper Rupelian age (biozones NSO3 to NSO5a). Geological models constructed for northern Belgium propose that this facies transition occurs rather sharply, along a WSW-ENE oriented zone parallel to the presumed shoreline at that time. The results of this study support this interpretation and extend the trend towards the Netherlands.
Diachronous
Cite
Citations (0)
The Paleoproterozoic Torngat Orogen of northern Labrador, a segment of the Trans‐Hudson Orogen, records the consequences of the convergence and collision of the Rae and Nain cratons and the intervening Burwell Domain. To better characterize the ages of the main lithologies and the timing of metamorphism, multiple samples were collected at sites along a transect across northern Torngat Orogen and analyzed for U‐Pb and Sm‐Nd. The results support previous models in which the Burwell Domain is an eastern extension of the Nain Province proper and experienced protracted exhumation between 1877 and 1800 Ma above an east‐dipping Abloviak Shear Zone. The steep Kormaktorvik Shear Zone to the east, which separates the Nain Province proper and the Burwell Domain, facilitated late, rapid exhumation of the Nain Province. Ages from multiple samples, phases, and localities indicate that metamorphism traversed similar PT paths diachronously from west to east and at different rates. The data also demonstrate that metamorphic zircon forms by composition‐dependent reactions over a range of temperature conditions, including below the closure temperature of zircon. This affirms that in deeply exposed orogenic belts, metamorphic zircon ages do not inherently represent the time of peak or near‐peak temperatures, and that a range of ages across a region cannot be assumed to signal diachronous peak conditions. Furthermore, rocks may experience granulite‐facies metamorphism without forming metamorphic zircon. Once formed, zircon may survive subsequent granulite‐grade metamorphism and preclude formation of zircon during later thermal events. Sm‐Nd analyses of minerals from units precisely dated by U‐Pb geochronology show that Nd closes in different minerals over a range of temperatures. In particular, Nd in garnet, monazite, and apatite closes at lower temperatures than in pyroxenes (and perhaps titanite). The use of garnet to derive Sm‐Nd isochrons will, therefore, generally derive an age younger than the true age of mineral formation and peak conditions. Exceptions are predicted in rapidly cooled domains (where all phases close within the time resolution of the chronometer) and in rocks without a second, rare earth element–bearing phase that closes at low temperature.
Diachronous
Closure temperature
Orogeny
Geochronology
Lithology
Cite
Citations (28)
Paleozoic rocks in the Catalonian Coastal Ranges are in their largest part affectedby alow- tovery-low grade Hercynian metamorphism. Amphibolite facies conditions are only found in restricted areas such as the southwestern part of the Guilleries massif where upper amphibolite facies conditions are reached. Metamorphic grade increases from top to bottom of the Paleozoic stratigraphic sequence and the metamorphic peak is diachronous, being progressively older in the lower grade metamorphic zones. The isograd pattern, mineral assemblages, mineral chemistry and preserved reaction textures are consistent with a low pressure metamorphism possibly evolving from a previous Barrovian type event. The metamorphic climax in the high grade zone was reached after the seconddeformational phase. Calculatedpeak P-Tconditions are 620-640 OC and around 3.5 Kb . A latter episode of decompression from the maximum conditions to 1-2 Kb, with an associated temperature decrease to 530-550 OC, is recognized. The intrusion of late Hercynian granitoids produced contact metamorphic aureoles where the pyroxene-hornfels facies is locally reached.
Diachronous
Isograd
Massif
Hornfels
Pyroxene
Cite
Citations (9)
ABSTRACT Following the Middle Devonian Acadian deformation an extensive belt of high grade metamorphism was formed in New England. In south‐western Maine, at the northern end of this belt, there occurs a transition along the strike from regional low‐pressure/high‐temperature metamorphism to contact metamorphism in low‐grade rocks. Petrological studies indicate that this transition occurs along a surface plunging to the north‐east at about 3.5°, with respect to the Middle‐to‐Late Devonian erosion surface. In addition, detailed petrological mapping has defined a history of temporally separate, localized metamorphic events associated with plutonism and occurring at increasingly deeper levels to the south‐west. Geochronological studies constrain ambient temperatures in the transition zone at the time of metamorphism to be less than 300° C in the north‐east and between 350° C and 500° C in the south‐west. They also establish a pattern of diachronous cooling due to differential uplift and erosion, with cooling occurring later and most rapidly to the south‐west. Geophysical evidence suggests that along with this spatial variation in metamorphic style the shapes of the plutons in Maine undergo a transition from laterally extensive sheet‐like bodies in the high grade terrane to more equant‐shaped bodies in the low‐grade terrane. Using the results of these petrological, geochronological and geophysical studies, as well as those of stratigraphical and structural studies we construct a thermal model for the transition zone. The model suggests that the Acadian metamorphism in south‐western Maine is a result of deep‐level contact metamorphism near laterally extensive granitic sills dipping to the north‐east with respect to the present erosion surface. The plutons themselves are interpreted to be a result of lower crustal melting in response to crustal thickening in the presence of normal or slightly augmented mantle heat flux.
Diachronous
Plutonism
Devonian
Orogeny
Cite
Citations (95)
Abstract The metamorphism of Pre-Cambrian dolerites in the Northwest Highlands is described and compared with that of basic rocks in the Southwest Highlands, Banffshire and other regions. The first products of metamorphism are not the same in every area. It is suggested that the trend of regional metamorphism differs according to the environment in which it took place, and that a sequence of changes established in one area cannot be used as a standard by which to judge the changes in other areas. In particular, rocks of high metamorphic grade have not invariably passed through stages characterized by low-grade minerals.
Sequence (biology)
Cite
Citations (20)
Diachronous
Cite
Citations (3)