The Kontum massif of south-central Vietnam has long been classified as lower continental crust of Precambrian (Archean) age. U-Pb zircon and 40 Ar/ 39 Ar biotite ages from charnockites and amphibolite facies rocks from this basement, however, yield much younger ages and imply at least two Paleozoic thermal events. Syntectonic charnockites from the Kannack complex and the southern Kontum massif record Permo-Triassic magmatism (U-Pb: [2j] Ma and Ma). The former sample yields an 40 Ar/ 39 Ar age of Ma (2j), implying rapid cooling 249 ע 2 253 ע 2 2 4 3 ע 4 to the 250ЊC isograd following emplacement into the lower crust. The ages confirm the importance of Permo-Triassic magma-generating orogenesis throughout a large part of eastern Indochina probably related to the closing of the Paleo-Tethys Sea. Older ages in amphibolite facies igneous and metamorphic rocks from the western Kontum massif record magmatic emplacement at Ma (U-Pb) and possibly a Paleozoic tectonometamorphic event (∼340 Ma) that 451 ע 3 partially to completely reset the argon systems (40 Ar/ 39 Ar: Ma, Ma, and Ma). Crustal thick-424 ע 5 380 ע 3 339 ע 4 ening and regional heating during suturing of the Indochina block to the South China block along the Song Ma suture zone may have produced the 340 Ma low-temperature thermal event, although slow variable cooling could also explain the argon data. The Kontum massif and Kannack metamorphic complex should henceforth be considered exposures of deep crustal levels of the Permo-Triassic orogeny in contact with Paleozoic midcrustal rocks. Evidently, the Kontum massif did not rift from a Precambrian granulite belt in Gondwana.
Accurate and detailed paleomagnetic data can be obtained from continuous measurements of U-channels extracted from sedimentary cores using a small-diameter passthrough magnetometer and high resolution signal-response pick-up coils. The spatial resolution is not significantly lower than that obtained by measuring standard paleomagnetic samples and features on the order of 3-5 cm can be confidently resolved. We propose a method to analyze the demagnetization data based on the angular dispersion of the directions between successive demagnetization steps and the downcore changes in the rate of intensity decrease. This technique allows us to distinguish very rapidly between intervals with well-identified characteristic magnetic components and zones characterized by a more complex behavior during demagnetization. It is also possible to directly measure the magnetic susceptibility, and to induce and measure most rock magnetic properties along the U-channels. Thus all parameters necessary for a complete paleomagnetic study of sediment cores can be confidently obtained directly from U-channels without extracting individual samples. Finally this equipment makes possible the measurement and complete demagnetization of up to 10 single samples at a time in less than one hour by using techniques similar to those developed for the U-channels.
Abstract. In the Gulf of California the Pacific-North America plate boundary changes character from an oceanic-type spreading center and transform fault system (to the south) to a region of diffuse continental deformation (to the north). The presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric, heat flow, gravity, or seismic data which indicate significant differences north and south of latitude -30øN. We suggest instead that north of-30øN a continent-ocean transition begins which we name the Wagner Transition Zone (WTZ). Diffuse deformation characterizes the WTZ where slip occurs along reactivated north to NNW striking normal faults developed during late Miocene or Pliocene ENE directed extension. Transtensional deformation varies from ENE directed extension along dip-slip faults in the west to dextral shear along the coast to dextral-oblique slip along inferred north to NNW striking faults submerged in the northern gulf. By accounting for rotational and extensional plate motion deformation in northeastern Baja California, vector constraints require that submerged structures accommodate -30 mm/yr of slip in a direction slightly clockwise of the relative plate motion direction. The juxtaposition of the discrete spreading center system in the central gulf with the diffuse WTZ appears to have been a stable configuration since 4-6 Ma, perhaps controlling the evolution of spreading center jumps between Upper and Lower Tibur6n and Delfin basins due to the juxtaposition of kinematically partitioned structural domains. Different histories of prerift extension and subduction-related arc magmatism along the length of the gulf, partly related to the migration of the Rivera triple junction, may explain the location of the continent-ocean transition.
Two granitoid intrusions within the Bu Khang extensional complex in central Vietnam have been dated by U–Pb and Rb–Sr geochronology. A monazite U–Pb age of 26.0 ± 0.2 (2σ) Myr was obtained for the Bu Khang pluton and 23.7 + 1.6/–1.7 Myr for monazite, allanite and zircon from the Dai Loc intrusion. These ages date crystallization of magmas previously assigned Precambrian to Devonian. Rb–Sr analyses of K‐feldspar and biotite fractions from the samples yield ages of 19.8 ± 0.6 (2σ) Myr and 19.6 ± 0.5 Myr, respectively. The thermal history recorded by the different geochronometers implies an average exhumation rate of ∼2 mm yr−1 corresponding to ∼9 km of unroofing. Magmatism was either (i) induced passively by lithospheric thinning driven by changes in regional tectonic stresses, or (ii) triggered actively by an ascending plume. Tertiary exhumation and magmatism documented elsewhere in Indochina (e.g. Ailao Shan‐Red River and Wang Chao shear zones) favours a regional tectonic cause for extension and granitoid magmatism in the Bu Khang complex. On the other hand, the presence of an upwelling thermal anomaly since at least 35 Ma, causing mantle melting below Indochina, is supported by shear‐wave velocity variations in the mantle, and source geochemistry of both the Bu Khang plutons and the Red River belt intrusions. In either case, Tertiary exhumation of the Bu Khang complex can account for previously undocumented NE–SW‐directed extension, which is required in northern Vietnam to account for structural changes related to the opening of the South China Sea.
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