Research Article| December 01, 1997 Grenvillian extensional tectonics in northwest Scotland: Comment and Reply I. S. Sanders; I. S. Sanders 1Department of Geology, Trinity College, Dublin 2, Ireland Search for other works by this author on: GSW Google Scholar Stephen Temperley; Stephen Temperley 2Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Brian F. Windley Brian F. Windley 2Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information I. S. Sanders 1Department of Geology, Trinity College, Dublin 2, Ireland Stephen Temperley 2Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom Brian F. Windley 2Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (12): 1151–1152. https://doi.org/10.1130/0091-7613(1997)025<1151:GETINS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation I. S. Sanders, Stephen Temperley, Brian F. Windley; Grenvillian extensional tectonics in northwest Scotland: Comment and Reply. Geology 1997;; 25 (12): 1151–1152. doi: https://doi.org/10.1130/0091-7613(1997)025<1151:GETINS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Peak and retrograde P – T conditions of Grenville-age eclogites from the Glenelg–Attadale Inlier of the northwest Highlands of Scotland are presented. Peak conditions are estimated as c . 20 kbar and 750–780°C, in broad agreement with previous work. The eclogites subsequently followed a steep decompression path to c . 13 kbar and 650–700°C during amphibolite facies retrogression. Peak eclogite facies metamorphism occurred > 1080 Ma and retrogression at c . 995 Ma, suggesting fairly sluggish uplift rates of < 0.3 km/Ma and cooling rates of < 1.25°C/Ma, when compared with other parts of the Grenville orogeny and/or modern orogens. However, current poor constraints on the timing of peak metamorphism mean that these rates cannot be used to interpret the geodynamic evolution of this part of the orogen. The P – T – t data, together with petrology and the field relationships between the basement rocks of the Glenelg–Attadale Inlier and the overlying Moine Supergroup, mean that it is difficult to support the currently held view that an unconformable relationship exists between the two. It is suggested that more data are required in order to re-interpret the Neoproterozic tectonic evolution of the northwest Highlands of Scotland.
The Glenelg–Attadale Inlier is the largest basement inlier within the Caledonian Moine nappe of NW Scotland. In the eastern part of the inlier amphibolite-facies retrogression of the eclogites is associated with tectonic fabrics, and P – T estimates indicate significant decompression ( c . 20 km). Previous Sm–Nd mineral–whole-rock dates indicated that peak eclogite-facies metamorphism occurred around c . 1.08 Ga, which was correlated with the Grenvillian orogeny. However, the middle REE enrichment of the analysed garnets suggests the influence of apatite inclusions. It is therefore likely that the interpretation of the c . 1.08 Ga age is complex, possibly reflecting re-equilibration at lower temperatures. Sampled eclogites contain zircon in a number of distinct textural forms that are mainly associated with pargasite and plagioclase, and are part of the retrograde amphibolite-facies assemblages. Titanite extensively replaces rutile, and is clearly associated with the retrograde amphibolite-facies event. A second textural type of titanite forms anhedral grains with plagioclase and pargasite, which is developed where the retrograde amphibolite-facies assemblage overprints the eclogite mineralogy. U–Pb dating has yielded the following ages: zircon age of 995 ± 8 Ma, and variably discordant rutile ages between 416 and 480 Ma. U–Pb and Pb–Pb isochrons on titanite and plagioclase/quartz separates yielded ages of 971 ± 65 Ma and 945 ± 57 Ma, respectively, in agreement with the zircon age. Analysed zircons and titanites are texturally part of the amphibolite-facies assemblage. The new zircon age demonstrates that amphibolite-facies metamorphism during exhumation occurred at 995 ± 8 Ma; the titanites could have closed with respect to Pb at this time or alternatively at some time between c . 1000 and 900 Ma. These data clearly demonstrate that parts of the Scottish basement underwent major thick-skinned tectonics during the Grenvillian orogeny. Rutile is part of the eclogite-facies paragenesis, and yet has young ages; these data are best explained by reheating producing near-total Pb loss related to emplacement of the late- to post-tectonic Ratagain Granite Complex at c . 425 Ma, at the end of the Caledonian orogeny.
Abstract On the basis of the effective scale-independence of brittle structures, from microcracks to regional fault systems, we have used analogue centrifuge models to provide insights into the initiation and evolution of continental active rifts, with a single dilational fracture segment representing a prototype rift segment. In the models, which are physically and dynamically scaled, a semi-brittle compound material, with flexural rigidity, was devised to simulate the lithosphere as a single layer. This is justified by the fact that, at the largest scale, the lithosphere behaves as a single viscoelastic, flexurally rigid unit. Silicon polymers of two different viscosities and densities represent the asthenosphere. A parallelepiped of lower viscosity and lower density material is embedded within and near the base of the second polymer that fills up most of the model box. The former is activated as a plume-like diapir that rises, spreads laterally and ultimately generates extensional stress in the semi-brittle layer on top. In terms of model fracture distributions, both narrow and wide failure modes were achieved, analogous to narrow and wide modes of rifting. The processes of fracture initiation, propagation and coalescence are the same for each mode. During the early stages of model runs, fracture initiation is more important than the propagation of existing segments in relieving stress, although the latter dominates during later stages. The key parameters of overlap, offset, obliquity and propagation angle, defined and illustrated, are used to distinguish different ways in which pairs of fractures coalesce. We recognize three distinct types of coalescence (type 1, type 2 and type 3) involving both tip-to-tip and tip-to-sidewall linkages. These are described and both graphically and statistically discriminated. Whether narrow or wide mode, an intracontinental rift system consists of a number of discrete fault-bound rift segments. Similar types of interactions to those in the models have been identified between pairs of rift segments from the Cenozoic Baikal and East African rift systems, and the Mesozoic to early Tertiary Central African rift system. The factors that control the type of coalescence between natural rift segments would appear to be precisely those that govern linkages of fracture segments in the models.
The South Harris Complex is a domain of largely Palaeoproterozoic rocks within the late Archaean, tonalite–trondhjemite–granodiorite (TTG)-dominated Lewisian gneisses of the Outer Hebrides, NW Scotland. The complex is distinguished by a high proportion of metasedimentary rocks and distinctive meta-igneous units, in part representing the remnants of a continental volcanic arc. The Langavat Belt defines the NE border zone of the South Harris Complex, separating the latter from the Archaean gneisses further NE, and has been repeatedly interpreted as a discrete supracrustal unit. However, detailed mapping and petrographic analysis reveals that up to 60% of the belt may be composed of highly deformed felsic orthogneiss. The metasedimentary rocks that are present have a late Archaean zircon provenance signature, implying that they are younger than, and possibly derived from the TTG gneisses to the NE. Previous work on zircon populations in the Leverburgh metasediments, towards the southern flank of the South Harris Complex, indicates very different provenance and we repudiate correlation of the two sequences. The disposition of units of metasediment, amphibolite and felsic orthogneiss is not a primary, pre-tectonic feature and is very unlikely to be the result of folding. Although direct evidence in the form of early fabrics has been wiped out by later, penetrative ductile shearing and metamorphic annealing, we propose that the Langavat Belt was assembled as a zone of thrust imbrication during early Proterozoic contraction. After imbrication, ductile shearing occurred in two distinct periods, separated by intrusion of granite pegmatites dated at c . 1660 Ma.
The work examines relationships between faulting and the physical properties of the host rock based on a study from the active rift zone in Iceland. Although compositionally identical, hyaloclastites, pillow lavas, and columnar‐jointed lavas have different mechanical properties that exert strong controls on the distribution of faults and strain within fault zones and the mechanical processes associated with displacement. In hyaloclastite, a fault zone comprises a braided array of lower‐order shear fractures. Veins of silica within gouge zones imply periodic dilational shear. In pillow lavas, reactivation of the near‐random arrays of primary fractures results in wide fault zones with diffuse margins. Displacement is achieved by the accumulation of small increments of slip on primary joints and by the development of through‐going shear fractures. Columnar lavas have the highest bulk strength and the fewest primary weaknesses. The subvertical cooling joints are favorably oriented for dilational‐shear reactivation. Frictional slip criteria on vertical fault planes indicate that shear must often have preceded dilation. Faulting in consolidated columnar lavas is likely to be velocity weakening and therefore seismogenic at shallow crustal levels. Both pillow lavas and hyaloclastites are weak, so that displacement is diffused within fault zones. Such properties are compatible with velocity‐strengthening behavior and quasi‐stable, nonseismogenic slip.
Research Article| January 01, 1997 Grenvillian extensional tectonics in northwest Scotland Stephen Temperley; Stephen Temperley 1Department of Geology, Leicester University, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Brian F. Windley Brian F. Windley 1Department of Geology, Leicester University, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information Stephen Temperley 1Department of Geology, Leicester University, Leicester LE1 7RH, United Kingdom Brian F. Windley 1Department of Geology, Leicester University, Leicester LE1 7RH, United Kingdom Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (1): 53–56. https://doi.org/10.1130/0091-7613(1997)025<0053:GETINS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Stephen Temperley, Brian F. Windley; Grenvillian extensional tectonics in northwest Scotland. Geology 1997;; 25 (1): 53–56. doi: https://doi.org/10.1130/0091-7613(1997)025<0053:GETINS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract New structural data indicate that eclogite-bearing gneisses in the Glenelg-Attadale inlier of the Caledonian Moine thrust nappe, northwest Scotland, were involved in a major pre-Caledonian system of extensional top-to-the-east ductile shear zones. These shear zones coalesce upward to constitute a major extensional detachment below metasedimentary rocks of the Moine Supergroup. Both the eclogite-bearing gneisses and the Moinian metasedimentary rocks display evidence of extensional deformation, but the latter contain structures indicative of a lower intensity of bulk finite shear strain and may have experienced only the later of two discrete phases of extension that we recognize in the inlier. Coplanar Caledonian brittle-ductile, top-to-the-west thrusting has resulted in only localized and limited reactivation. Timing of the extensional deformation is bracketed by a published Sm/Nd age of 1.08 Ga for eclogite equilibration and by a minimum age of deposition for the Moine Supergroup of 840 Ma. We propose that the eclogite-bearing lower crust was exhumed as a result of collapse of the Grenville Orogen, and that the evolution of the Moinian basin was controlled by this extensional event. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Volcanic systems in Iceland comprise a central volcano linked to magmatic fissures, whose subsurface architecture remains enigmatic. The surface dimensions and trends of active fissures show their spatial relation with fault systems. Structure of the Fremri‐Namur and Hengill volcanic systems was analyzed. The linear vent arrays marking recent fissures are tightly bound by vertical segmented faults, while normal faults accommodating extension are common on the flanks of the fissure swarm. Evidence of normal frictional slip on the fissure‐bounded faults suggests that failure occurred in response to subsidence with subsequent dilation. This is supported by inward tilting of strata adjacent to faults compatible with a downsag phase. We propose that magmatic fissures have vertical feeders with lateral offshoots extending along the rift zone. Their inflation/deflation during an eruptive cycle causes subsidence. Such magmatically generated faults can be subsequently modified by tectonic extension.
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