U–Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic–granodioritic bodies in central portions of the batholith yield ages of 59–58 Ma, coeval with younger phases of the tonalitic sheets. Large granite–granodiorite bodies in central and eastern portions of the batholith were emplaced at 51–48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Variations in the age, shape, location, and degree of fabric development among the various plutons indicate that Late Cretaceous – Paleocene tonalitic bodies were emplaced into a steeply dipping, dip-slip shear zone that was active along the western margin of the batholith. Postkinematic Eocene plutons were emplaced at shallow crustal levels. Inherited zircon components in these plutons range in age from mid-Paleozoic to Early Proterozoic and are coeval with detrital zircons in adjacent metasedimentary rocks. These old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons.
Rocks of the SE Alaska subterrane of the Yukon-Tanana terrane (YTTs) consist of regionally metamorphosed marine clastic strata and mafic to felsic volcanic-plutonic rocks that have been divided into the pre-Devonian Tracy Arm assemblage, Silurian–Devonian Endicott Arm assemblage, and Mississippian–Pennsylvanian Port Houghton assemblage. U-Pb geochronologic and Hf isotopic analyses were conducted on zircons separated from 23 igneous and detrital samples in an effort to reconstruct the geologic and tectonic evolution of this portion of YTT. Tracy Arm assemblage samples are dominated by Proterozoic (ca. 2.0–1.6, 1.2–0.9 Ga) and Archean (2.7–2.5 Ga) zircons that yield typical cratonal εHf(t) values. Endicott Arm assemblage samples yield U-Pb ages that range from Late Ordovician to Early Devonian and εHf(t) values that range from highly juvenile to moderately evolved. Port Houghton assemblage samples yield similar Ordovician–Devonian ages and εHf(t) values, and also include early Mississippian zircons with highly evolved εHf(t) signatures.
Various plate reconstructions predict that the Alexander terrane, a Neoproterozoic–Jurassic crustal fragment now located in the North American Cordillera, evolved in proximity to the northern Appalachian-Caledonian convergent margin during assembly of supercontinent Laurussia. To test stratigraphic connections with Laurussia that are implied by these plate reconstructions, we measured the Hf isotopic compositions of 176 detrital zircons from two relevant sedimentary sequences of the Alexander terrane. An older, Upper Silurian–Lower Devonian terrestrial to shallow-marine molasse sequence yields 405–490 Ma detrital zircons with negative εHf(t) values and Mesoproterozoic to Paleoproterozoic Hf model ages. In combination with paleomagnetic and biogeographic constraints, these Hf data argue for the molasse strata to be now-displaced equivalents of the Old Red Sandstone and primarily sourced from crustally contaminated granitoids in the Greenland, Svalbard, or British Caledonides. Late Silurian–Early Devonian orogenesis in the Alexander terrane is therefore likely related to the Scandian-Salinic phase of Appalachian-Caledonian mountain building. Younger, Middle Devonian sequences of the Alexander terrane are endowed in 390–490 Ma detrital zircons with positive εHf(t) values and Neoproterozoic Hf model ages. These isotopic signatures are consistent with the erosion of local basement rocks during the opening of the Slide Mountain–Angayucham backarc rift and tectonic separation of the Alexander terrane from northern Laurussia.
Research Article| September 01, 2008 Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho Scott Giorgis; Scott Giorgis † 1Department of Geological Sciences, State University of New York–Geneseo, 1 College Circle, Geneseo, New York 14454, USA †E-mail: giorgis@geneseo.edu Search for other works by this author on: GSW Google Scholar William McClelland; William McClelland 2Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA Search for other works by this author on: GSW Google Scholar Annia Fayon; Annia Fayon 3Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455-0219, USA Search for other works by this author on: GSW Google Scholar Brad S. Singer; Brad S. Singer 4Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Basil Tikoff Basil Tikoff 4Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2008) 120 (9-10): 1119–1133. https://doi.org/10.1130/B26291.1 Article history received: 25 May 2007 rev-recd: 21 Feb 2008 accepted: 14 Mar 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Scott Giorgis, William McClelland, Annia Fayon, Brad S. Singer, Basil Tikoff; Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho. GSA Bulletin 2008;; 120 (9-10): 1119–1133. doi: https://doi.org/10.1130/B26291.1 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 SocietyGSA Bulletin Search Advanced Search Abstract The western Idaho shear zone is one of several Cretaceous high-strain zones in the Cordillera that are thought to have been associated with the northward translation and/or docking of terranes presently in British Columbia. Located in west-central Idaho, this zone of intense deformation consists of a mid-crustal exposure of a lithospheric-scale, intra-arc dextral shear zone that overprints the Salmon River suture zone along the western edge of the Idaho batholith. U/Pb zircon geochronology constrains the main phase of deformation to between ca. 105 and 90 Ma. Cessation of movement on the shear zone occurred by 90 Ma, as determined by dating of the syntectonic Payette River tonalite and a crosscutting pegmatite dike in the Little Goose Creek complex. The 40Ar/39Ar thermochronology indicates that the shear zone passed through both the hornblende (~550 °C) and biotite (~325 °C) closure temperatures between 85 and 70 Ma. The 40Ar/39Ar biotite dates from an outcrop-scale, crosscutting shear zone are indistinguishable from that of the host rock, indicating that deformation occurred above the closure temperature of biotite. Apatite fission-track analysis suggests that exhumation to shallow crustal levels occurred ca. 40 Ma during mid-Tertiary regional exhumation or renewed tectonic activity along the Salmon River suture zone. Taken together, the 40Ar/39Ar results and apatite fission-track analyses indicate a two-stage uplift history for the western Idaho shear zone. Overall, the geochronology indicates that dextral transpressional movement on the western Idaho shear zone was temporally distinct from the Early Cretaceous suturing event. Additionally, the first stage of exhumation recorded by the western Idaho shear zone immediately followed transpressional deformation. Cessation of displacement on the western Idaho shear zone by ca. 90 Ma indicates that the exhumation did not solely occur as a result of ductile deformation on the shear zone itself. Moreover, dextral strike-slip movement on the western Idaho shear zone had also ceased by 90 Ma, indicating that terrane translation models for the Cordillera can only use the western Idaho shear zone to accommodate northward translation up to ca. 90 Ma. Lastly, the timing of movement on the western Idaho shear zone and contractional deformation recorded in the Insular terrane suggests a correlation between these events. This hypothesis implies that the deformation recorded in the western Idaho shear zone may have been linked to the oblique collision of the Insular superterrane with North America. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Paleoproterozoic gneisses of the Ellesmere–Devon crystalline terrane on southeastern Ellesmere Island are deformed by metre-scale, east-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the 40 Ar/ 39 Ar step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape-preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 ± 12 Ma, based on previous U–Pb dating of zircon. 40 Ar/ 39 Ar analysis of biotite from the same sample gave a plateau age of 1929 ± 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have 40 Ar/ 39 Ar ages in the range of ≈1870–1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred at ≈1870–1840 Ma, or ≈90 million years after cooling from regional metamorphism. Although the 2.0–1.9 Ga gneisses of southeastern Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny.
Abstract New U-Pb zircon, monazite, 40 Ar/ 39 Ar, and apatite fission track ages provide constraints on the timing of formation and exhumation of the Mabja Dome, southern Tibet, shed light on how this gneiss dome formed, and provide important clues on the tectonic evolution of middle crustal rocks in southern Tibet. Zircons from a deformed leucocratic dyke swarm yield a U-Pb age of 23.1 ± 0.8 Ma, providing the first age constraint on the timing of middle crustal ductile horizontal extension in the North Himalayan gneiss domes. Zircons and monazite from a post-tectonic two-mica granite yield ages of 14.2 ± 0.2 Ma and 14.5 ± 0.1, respectively, indicating that vertical thinning and subhorizontal stretching had ceased by the middle Miocene. Mica 40 Ar/ 39 Ar ages from schists and orthogneisses increase structurally down-section from 12.85 ± 0.13 Ma to 17.0 ± 0.19 Ma and then decrease at the deepest structural levels to 13.29 ± 0.09 Ma. Micas from the leucocratic dyke swarm and post-tectonic two-mica granites yield similar 40 Ar/ 39 Ar cooling ages of 13.48 ± 0.13 to 12.84 ± 0.08 Ma. The low-temperature steps of potassium feldspar 40 Ar/ 39 Ar spectra yield ages of c. 11.0–12.5 Ma and apatite fission track analyses indicate the dome uniformly cooled below c. 115°C at 9.5 ± 0.6 Ma. Based on these data, calculated average cooling rates across the dome range from c. 40–60°C/million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm, to c. 350°C/million years following emplacement of the two-mica granites. The mylonitic foliation, peak metamorphic isograds, and mica 40 Ar/ 39 Ar chrontours are domed, whereas the low-temperature step potassium feldspar 40 Ar/ 39 Ar and apatite fission track chrontours are not, suggesting that doming occurred between 13.0 and 12.5 Ma and at temperatures between 370 and 200°C. Our new ages, along with field, structural and metamorphic data, indicate that the domal geometry observed at Mabja developed by middle-Miocene southward-directed thrust faulting upward and southward along a north-dipping ramp above cold Tethyan sediments. The structural, metamorphic and geochronologic histories documented at Mabja Dome are similar to those of Kangmar Dome, suggesting a common mode of occurrence of these events throughout southern Tibet. Vertical thinning and horizontal stretching, metamorphism, generation of migmatites, and emplacement of leucogranites in the domes of southern Tibet are synchronous with similar events in the Greater Himalayan Sequence that underlie the high Himalaya. These relations are consistent with previously proposed models for a ductile middle-crustal channel bounded above by the South Tibetan detachment system and below by the Main Central thrust in the High Himalaya that extended northward beneath southern Tibet.
