Estimation of confidence limits on net tectonic rotation by an ‘overlapping circles’ method can lead to values which are too large. A simple method based upon an F‐ratio test is presented which yields more accurate constraints on net rotation. An example illustrates that the limits calculated by the ‘overlapping circles’ method can be in error by more than 20%.
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.
Abstract New 40 Ar/ 39 Ar ages are presented from the giant Sulu ultrahigh‐pressure (UHP) terrane and surrounding areas. Combined with U‐Pb ages, Sm‐Nd ages, Rb‐Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K‐feldspar 40 Ar/ 39 Ar ages. Subsequent unroofing at rates of 5–25 km Myr −1 brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by 40 Ar/ 39 Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr −1 . Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr −1 , suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr −1 , suggesting relatively shallow crustal depths.
Abstract Subduction‐related volcanism in the northern part of the North Island of New Zealand shifted abruptly during the late Pliocene. This study focuses on the transition, in time and space, from the NNW‐oriented Miocene‐Pliocene Coromandel Volcanic Zone to the northeast‐oriented active Taupo Volcanic Zone. The volcanic rocks marking this transition are exposed in the Tauranga Basin and adjacent Kaimai Range, and associated here with the recently defined Tauranga and Kaimai Volcanic Centres, respectively. New 40Ar/39Ar age determinations indicate that the transition occurred between 1.90 and 1.55 Ma, that is between the youngest age of silicic volcanism in the Tauranga‐Kaimai area, and the age of the oldest silicic volcanism in the Taupo Volcanic Zone. This interpretation is generally consistent with recent plate models and with the initiation of the Kermadec Arc within the last 2 m.y.
Research Article| July 01, 1994 40Ar/39Ar geochronology and exhumation of high-pressure to ultrahigh-pressure metamorphic rocks in east-central China Elizabeth A. Eide; Elizabeth A. Eide 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115 Search for other works by this author on: GSW Google Scholar Michael O. McWilliams; Michael O. McWilliams 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115 Search for other works by this author on: GSW Google Scholar Juhn G. Liou Juhn G. Liou 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115 Search for other works by this author on: GSW Google Scholar Geology (1994) 22 (7): 601–604. https://doi.org/10.1130/0091-7613(1994)022<0601:AAGAEO>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 Elizabeth A. Eide, Michael O. McWilliams, Juhn G. Liou; 40Ar/39Ar geochronology and exhumation of high-pressure to ultrahigh-pressure metamorphic rocks in east-central China. Geology 1994;; 22 (7): 601–604. doi: https://doi.org/10.1130/0091-7613(1994)022<0601:AAGAEO>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 40Ar/39Ar ages from rocks in the high-pressure and ultra-high-pressure (HP-UHP) metamorphic complex in the Hong'an and Dabie Mountains areas, east- Central China, document a two-phase cooling and exhumation history following Triassic continental collision and metamorphism. Phengite ages from blueschist through kyanite-bearing eclogite facies rocks in Hong'an record initial exhumation from the collision zone between 230 and 195 Ma. Biotite and hornblende ages from migmatites and eclogite-bearing gneisses from the Dabie Mountains record a cooling event between 128 and 117 Ma, corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons.The Triassic through Early Jurassic 40Ar/39Ar cooling ages corroborate U-Pb and Sm-Nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid. Emplacement of granitic melts within the HP-UHP sequences ∼80 m.y. after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial, rapid exhumation episode. 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.
New structural, geochronological, and petrological data highlight which crustal sections of the North American–Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortís Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American–Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortís Block record the Late Cretaceous–Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortís Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction–accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacús complexes), and the Laramide foreland fold–thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortís Block contains a significant part of the history of the eastward migration of the Chortís Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortís Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene–Recent translation of the Chortís Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic–Cenozoic evolution of the North American–Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortís Block, and the terranes of southern Mexico: (i) in the Early–Middle Palaeozoic, the Acatlán complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacús complex, and the Chortís Block were part of the Taconic–Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite–facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacús complex and the Chortís Block. The separation of North and South America also rifted the Chortís Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic–Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian (c. 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortís Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction–accretion complex was emplaced onto the Chortís Block probably in the late Early Cretaceous and the Chortís Block collided with southern Mexico. Related arc magmatism and high-T/low-P metamorphism (Taxco–Viejo–Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortís Block shows continuous Early Cretaceous–Recent arc magmatism.
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