Phillip B. Gans

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite-in, staurolite-in, garnet-in, and chloritoid-in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E-W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N-S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post-D1 and pre- to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end-member metamorphic core complex-type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

Characteristics and consequences of flow in the lower crust

In some places, there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening. In order to better understand the processes involved, we investigate the behavior of a fluid layer over a fluid half-space to see how such a system responds to the deformation of its upper and lower boundaries. This simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet. The changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically. The most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front, which may cause a topographic step above it at the Earths surface. Deformation within the channel will be principally by simple shear. The clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur. The smoothing of crustal thickness contrasts leads to differential vertical motions, and is thus a method by which substantial tilting can occur without faulting. This differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the Basin and Range province of the western United States. It is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults.

Rapid Miocene slip on the Snake Range–Deep Creek Range fault system, east-central Nevada

New fission-track data together with 1:24 000-scale geologic mapping and analysis of Tertiary sedimentary deposits provide better constraints on the time and nature of motion along the Snake Range decollement, a classic Basin and Range metamorphic core complex detachment fault in east-central Nevada. Here, the fission-track method provides a particularly effective tool for dating faulting where bracketing or crosscutting relations are not available. These new data suggest that the Snake Range decollement forms part of a more extensive, 150-km-long north-south–trending fault system, the Snake Range–Deep Creek Range fault system. This fault system extends along the eastern flank of the northern and southern Snake Range, Kern Mountains, and Deep Creek Range, and accommodated at least 12–15 km of rapid slip in the Miocene, ca. 17 Ma. This component of motion is distinctly younger (by about 15–20 m.y.) than an earlier episode of slip and extension across the region bracketed stratigraphically and geochronologically as late Eocene–early Oligocene age. Apatite fission-track ages (n = 57) in most parts of the Snake Range and adjacent ranges cluster at 17 Ma, indicating rapid cooling from >125 to 310 to <50 °C. Formation of at least part of the pervasive mylonitic fabrics in the northern Snake Range may have occurred during this Miocene time interval, very late rather than early in the extensional history of the region. Coarse fanglomerate and rock-avalanche deposits in flanking Tertiary basins provide additional evidence for major tectonism at this time. Comparison of the timing of events in the northern Snake Range to that along strike of the fault system indicates that Miocene slip along the low-angle northern Snake Range decollement and exhumation of extensive footwall mylonites were coeval with more typical Basin and Range high-angle rotational faulting in the Deep Creek Range and Kern Mountains to the north and in the southern Snake Range to the south. This suggests that the two styles of faulting (low-angle detachment and high-angle rotational) can occur simultaneously along the length of a single normal fault system. Data from the northern Snake Range also underscore the importance of a vertical component of uplift of the range in Miocene time, leading to the present domal geometry of the northern Snake Range decollement. When considered together with footwall deformational fabrics, the new data are most simply explained as the consequence of higher local geothermal gradients and a shallower brittle-ductile transition zone along the northern Snake Range part of the fault system. It can be speculated that the Snake Range metamorphic core complex represents the top of a stretching welt of hotter, deeper level crust that rose during extension. This rising welt may have been localized by the presence of previously thickened crust beneath the region and could have been triggered by increased regional magmatism and heating accompanying rapid extension in Miocene time.

BASEMENT GEOLOGY FROM THREE KINGS RIDGE TO WEST NORFOLK RIDGE, SOUTHWEST PACIFIC OCEAN : EVIDENCE FROM PETROLOGY, GEOCHEMISTRY AND ISOTOPIC DATING OF DREDGE SAMPLES

Abstract We present new petrographie, X-ray fluorescence, electron microprobe, Ar-Ar radiometric, fission track, and Sr, Nd and Pb isotopic data for igneous and metamorphic rocks from 18 dredges from a number of basins and ridges in the southwest Pacific Ocean. The dredge samples can be divided into four groups: (1) Permian to Cretaceous gabbroids, granitoids and hornfelses from the West Norfolk and Wanganella Ridges; (2) mainly Cretaceous mafic igneous rocks from the Reinga Ridge and Vening Meinesz escarpment; (3) subduction-related Late Oligocene and Early Miocene basalts and shoshonites from the Three Kings and southern Norfolk Ridges; and (4) back-arc basin and intraplate basalts of Early Miocene age in the South and North Norfolk Basins, respectively, and intraplate basalts of Pliocene age in the Reinga Basin. Our dredge data, combined with regional magnetic and gravity data, provide much-needed tests of earlier tectonic interpretations of the region. Correlatives of the Permian Brook Street Terrane and Mesozoic Median Tectonic Zone of onshore New Zealand clearly extend along the West Norfolk and Wanganella Ridges. The Three Kings Ridge was active at least in the 19- to 20-Ma period, and was probably generated above a west-dipping subduction zone. The Norfolk Basin was a back-arc basin to this arc, and had opened by 18–20 Ma. The oldest Cenozoic volcanic rocks in the region are 26-Ma subduction-related volcanic breccias on the southern Norfolk Ridge; they probably predate opening of the Norfolk Basin and associated eastward migration of the Three Kings Ridge.

Paleomagnetism and 40Ar/39Ar ages from volcanics extruded during the Matuyama and Brunhes Chrons near McMurdo Sound, Antarctica

Maps of virtual geomagnetic poles derived from international geomagnetic reference field models show large lobes with significant departures from the spin axis. These lobes persist in field models for the last few millenia. The anomalous lobes are associated with observation sites at extreme southerly latitudes. To determine whether these features persist for millions of years, paleomagnetic vector data from the continent of Antarctica are essential. We present here new paleomagnetic vector data and Ar-40/Ar-39 ages from lava flows spanning the Brunhes and Matuyama Chrons from the vicinity of McMurdo Sound, Antarctica. Oriented paleomagnetic samples were collected from 50 lava flows by E. Mankinen and A. Cox in the 1965-1966 austral summer season. Preliminary data based largely on the natural remanent magnetization (NRM) directions were published by Mankinen and Cox [1988]. We have performed detailed paleomagnetic investigations of 37 sites with multiple fully oriented core samples to investigate the reliability of results from this unique sample collection. Of these, only one site fails to meet our acceptance criteria for directional data. Seven sites are reversely magnetized. The mean normal and reverse directions are antipodal. The combined mean direction has (D) over bar =12, (I) over bar=-86, alpha=4, kappa=37 and is indistinguishable from that expected from a GAD field. We obtained reproducible absolute paleointensity estimates from 15 lava flows with a mean dipole moment of 49 ZAm(2) and a standard deviation of 28 ZAm(2). Ar-40/Ar-39 age determinations were successfully carried out on samples from 18 of the flows. Our new isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales.

The chronology of Cenozoic volcanism and deformation in the Yerington area, western Basin and Range and Walker Lane

High-precision 40 Ar/ 39 Ar isotopic ages obtained from Cenozoic volcanic rocks and subvolcanic intrusions document the age of initiation and the temporal evolution of extensional and strike-slip faulting in the western Basin and Range Province. In the northern Wassuk Range, faulting began between ca. 26 and 24.7 Ma; both normal and strike-slip faults are bracketed between 23.1 and 22.2 Ma, and between 15 and 14 Ma. These ages document inception of the Ancestral Walker Lane, a northwest-trending zone of right-transtensional faulting in western Nevada that separated extending crust on the east from the unextended Sierra Nevada block on the west at lat 39°N. We speculate that the southwesterly migrating, episodic Oligocene–early Miocene, east-west extensional faulting in the Basin and Range thinned and weakened the crust, allowing right-slip faults to develop in the Walker Lane in response to San Andreas right-shear in California. Southwest of the Walker Lane there was no faulting prior to 15 Ma. Here, in the Yerington district, andesitic magmatism began at ca. 15 Ma and was followed by >150% east-west extension along closely spaced (1–2 km) normal faults with up to 4 km of offset each (Proffett, 1977). These faults tilted older Cenozoic rocks 35°–40°W. Our new 40 Ar/ 39 Ar ages substantially revise earlier K-Ar ages of the timing of extension and establish that andesite lava flows cut by normal faults are 13.8–15 Ma, and that these faults are intruded by 12.6–13.0 Ma dacites. Rapid extension is thus bracketed to a 0.7–1.7 m.y. interval at 95% confidence, indicating local, east-west strain rates of 2–4 × 10 −14 /s (5–10 mm/yr). Following this period, lower rates of extension prevailed near Yerington along more widely spaced normal oblique-slip faults that localized clastic sedimentation of the Wassuk Group between 11 and 8 Ma. These faults and sedimentary rocks are more abundant southwest of Yerington in a belt parallel to the Walker Lane in previously little-extended crust. From 7 Ma to present, normal right-oblique slip faults with a lower rate of extension than the previous two periods produced the modern ranges near Yerington and extend 100 km southwest of the Walker Lane, which continues to be the locus of strike-slip faulting. Thus, since 15 Ma the margin of the Basin and Range has moved progressively 100 km west creating the broad Walker Lane belt and lower strain rates near Yerington.

Protolith ages and exhumation histories of (ultra)high-pressure rocks across the Western Gneiss Region, Norway

The timing of protolith formation, ultrahigh-pressure (UHP) subduction, and subsequent exhumation for the ultrahigh-pressure to high-pressure units across the eastern part of the Western Gneiss Region, Norway, were assessed using U/Pb zircon, Th/Pb monazite, and 40 Ar/ 39 Ar white mica ages. U/Pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the Baltica basement before the entire mass was subducted to (ultra)high pressure. Eclogites within the allochthons across the entire Western Gneiss Region are Caledonian and show a degree of zircon (re)crystallization that increases with peak pressure, permitting the interpretation that the entire region underwent synchronous subduction. 40 Ar/ 39 Ar white mica ages of 399 Ma indicate that the eastern part of the Western Gneiss Region had been exhumed to shallow crustal levels while UHP metamorphism was ongoing farther west, indicating a westward dip to the slab. The 40 Ar/ 39 Ar white mica ages also show a clear east-to-west gradient across the entire Western Gneiss Region, indicating that the Western Gneiss Region rose diachronously to crustal levels from east to west between 399 and 390 Ma.

Magmatically induced metamorphism and deformation in the Kigluaik gneiss dome, Seward Peninsula, Alaska

Field observations and U-Pb isotopic data from plutonic and high-grade metamorphic rocks within the Kigluaik gneiss dome on the Seward Peninsula, Alaska, document a Late Cretaceous age of peak metamorphism and shed light on the relationship between fundamentally mantle-derived magmatism and the development of the gneiss dome. The dome consists of upper-amphibolite-facies to granulite-facies metasedimentary rocks mantling the granitoid Kigluaik pluton. The main deformational fabric in the dome is a well-defined, moderately dipping foliation and compositional layering that contains a pervasive, east-west trending mineral-elongation lineation defined by sillimanite and hornblende. Leucosomal segregations in migmatite are boudinaged and isoclinally folded, and late-stage tension gashes are filled with melt, demonstrating that most deformation occurred at peak metamorphic temperatures. The large pluton in the core of the dome consists of a granitic cap overlying a mafic to intermediate root. Mafic pillows with crenulate margins and spectacular magma mingling textures indicate that the two magmas were coeval. The Kigluaik pluton is largely undeformed and discordant, although some dikes possess a weak deformational fabric. The lack of quench textures at the margins of the pluton and very limited alteration in adjacent wall rock suggest that the pluton was emplaced while country rocks were still at high temperatures. U-Pb analyses of three fractions of zircon from a highly strained and concordant garnet-bearing granite orthogneiss yield an intrusive age of 105 Ma; therefore significant deformation in the dome must have occurred after 105 Ma. Several U-Pb analyses of monazite from metapelite and pegmatite (derived from partial melting of metapelite) yield an age of 91 ± 1 Ma for high-temperature metamorphism and deformation. U-Pb analysis of eight fractions of zircon from the Kigluaik pluton shows that it crystallized at 92 ± 2 Ma. Thus there is both field and geochronologic evidence for coeval magmatism, metamorphism, and deformation at about 92 Ma, which may represent the latest stages of a more protracted tectonic event. We present a model for gneiss dome development wherein a large, silicic to intermediate magmatic diapir with its mantling gneisses ascended from ∼35 km to ∼15 km during an event associated with crustal extension in northern Alaska. We emphasize the close link between plutonism and gneiss dome development and the fundamentally mafic character of this plutonism.

Tectonic implications of early Miocene extensional unroofing of the Sierra Mazatán metamorphic core complex, Sonora, Mexico

Sierra Mazatan in northwest Mexico is the southernmost metamorphic core complex in the North American Cordillera. New geologic, structural, and 4 0 Ar/ 3 9 Ar thermochronologic data demonstrate that the core complex detachment fault accommodated 15-35 km of slip at a rate of 3.3-7.7 mm/yr between 20.5 and 16 Ma. These data also suggest that the lower plate was tilted eastward 20°-50° since 20 Ma, indicating that the detachment fault initially dipped 30°-60°. Rapid slip on the fault occurred concurrently with subduction in Sonora and thus was not related to the ca. 12 Ma plate boundary change at this latitude or the late Miocene opening of the Gulf of California. Rapid extension at Sierra Mazatan was synchronous with core complexes along the length of the Cordillera, suggesting a distinct 20-15 Ma core complex event, the fundamental causes of which remain a mystery.

Diapiric ascent and cooling of a sillimanite gneiss dome revealed by 40Ar/39Ar thermochronology: the Kigluaik Mountains, Seward Peninsula, Alaska

Abstract The upper amphibolite to granulite facies Kigluaik gneiss dome cooled rapidly in late Cretaceous time as it rose through the crust and was emplaced against the older blueschist-greenschist facies Nome Group. 40Ar/39Ar data from metamorphic rocks reveal prolonged moderately rapid cooling histories that vary somewhat with structural depth and geographical position. Hornblende ages (Tc c. 535°C) range from 86 to 82 Ma, mica ages (Tc c. 300–400°C) range from 85 to 83 Ma, and K-feldspar spectra yield age gradients that record cooling from c. 300° to c. 150°C between 82 and 65 Ma. These high-precision cooling ages on hornblende, white mica and biotite, and detailed temperature-time curves obtained from diffusion modelling of K-feldspar age spectra allow us to reconstruct the 3D geometry of isotherms during cooling and local reheating of the gneiss dome. Isotherms were parallel to lithological contacts during high-temperature cooling, then became subhorizontal in present-day coordinates as layering was domed by c. 84 Ma. Low-temperature cooling (300–150°C) of the gneiss dome is asymmetrical, with the north side cooling several million years before the south side. The contact between high-grade gneisses of the Kigluaik Mountains and surrounding lower-grade rocks is not a fault, but rather a steep metamorphic field gradient where closely spaced Barrovian isograds and partially reset mica ages document progressive thermal overprint of the older blueschist-greenschist facies Nome Group rocks. These combined geological and thermochronological data are most compatible with a model wherein the Kigluaik gneiss dome rose diapirically from mid-crustal levels between 91 Ma and c. 82 Ma and cooled through low temperatures differentially as it was tilted gently southward between 83 and 65 Ma.