A pinboard by
Shelly Wernette

Graduate Research Assistant, University of California, Riverside


Eastern Myanmar, western Thailand, western Yunnan, and northern malaysia collectively form a small continental terrane called Sibumasu. The late Cambrian trilobite fossils on Sibumasu are interbedded with volcanic ashes and related to trilobites found on the North China block and in Australia. The volcanic ashes contain zircon crystals which are datable using uranium and lead isotopes. The trilobite fossils are part of the better known biostratigraphic sequences in north China and Australia. Applying the radiometric dates to the fossil sequence will produce the first calibrated late Cambrian geochronology known anywhere in the world. The dates can be correlated from Sibumasu to the rest of the world using similarities in fossil assemblages. The similarities between trilobite assemblages and the ages of detrital zircon grains found in sandstones can also be used to reconstruct the paleogeographic relationships between Gondwanan terranes during the late Cambrian. These datasets will also reveal whether Sibumasu has always been a single unified terrane or whether it was later assembled from even smaller blocks that had a separate history during the early Paleozoic. Resolution of paleogeography and geochronology are essential to anyone studying rates of evolution, tectonic movements, paleoclimatic changes, ocean chemistry changes, speciation patterns, or biogeographic dispersals at the Cambrian-Ordovician boundary. Knowledge of the rates and nature of atmospheric and ocean chemistry changes in the past, particularly in this period of intermittent extinction preceding a major diversification, are essential to understanding the effects of ocean and atmospheric chemistry changes today.


Provenance and tectonic evolution of Lower Paleozoic–Upper Mesozoic strata from Sibumasu terrane, Myanmar

Abstract: The provenance of Sibumasu terrane sedimentary rocks and their tectonic relationships with surrounding terranes exposed in Southeast Asia record separation and accretion of Gondwana-derived terranes during Late Paleozoic and Mesozoic time. This paper reports sandstone petrographic and U–Pb detrital zircon geochronologic data from Ordovician to Lower Jurassic strata within the Sibumasu terrane in Shan State, Myanmar. The Ordovician strata are composed of limestone and siltstone. The Lower Silurian Linwe and Upper Silurian Namhism Formations are comprised of limestone, silty sandstone, conglomerate and sandstone, respectively. Sandstones from both Ordovician and Silurian strata are dominated by 567–470 Ma and 982–917 Ma detrital zircons that are interpreted to be sourced from the eastern Gondwana supercontinent. The Carboniferous unit is composed of metasedimentary rocks (phyllite, slate, quartzite, and meta-marl). Sandstones from Carboniferous units show a strong 1165–1070 Ma detrital zircon age peak that is not present in the Ordovician and Silurian strata. These zircon grains were most likely derived from both the Albany–Fraser Province in Southwest Australia and Maud Province in Antarctic. The Upper Triassic to Lower Jurassic, shallow marine Loi-an Group consists of thin- to medium-bedded sandstone and mudstone that unconformably overly the Permian to Middle Triassic Plateau Limestone Group. Sandstones from the Loi-an Group contain abundant Permian to Triassic detrital zircons that are interpreted to have been derived from the Sukhothai Arc of the western Indochina terrane. Formation of this arc is attributed to eastward (present coordinates) subduction of Paleo-Tethyan oceanic lithosphere beneath Indochina. Therefore, we propose that the Sibumasu terrane was juxtaposed against northwestern Australia as part of the Gondwana supercontinent during Paleozoic to Early Permian time. During the Late Triassic and Early Jurassic, Sibumasu strata record an abrupt influx of Permian and Triassic zircon grains, signifying a change in provenance from Gondwana to the Sukhothai Arc. These data are consistent with a tectonic model involving rifting of the Sibumasu terrane from Gondwana and subsequent docking with the Indochina/Simao terranes during Mesozoic time.

Pub.: 18 Apr '15, Pinned: 01 Aug '17

Closure of the Proto-Tethys Ocean and Early Paleozoic amalgamation of microcontinental blocks in East Asia

Abstract: As a complex paleo-ocean located between the Tarim-North China and the Sibumasu/Baoshan blocks, the Proto-Tethys Ocean was opened from the rifting of the Supercontinent Rodinia and mainly closed at the end of the Early Paleozoic. The known studies suggest that there were many continents and/or micro-continents in the Proto-Tethys Ocean. During closure of the Proto-Tethys Ocean and assembly of these continents/micro-continents, some Early Paleozoic ophiolites and HP-UHP metamorphic rocks developed in East Asia similar to those Early Paleozoic orogens in Gondwana. However, some academic debates still remain on the boundaries of the Proto-Tethys Ocean and the nature, relationships and assembly processes of these continents/micro-continents to the Tarim-North China Continent to the north. These problems are important for revealing and reconstructing tectonic processes before the closure of the Proto-Tethys Ocean and the initial assembly of the Supercontinent Pangea. Not surprisingly, the Proto-Tethys tectonic domain is characterized by complex ocean-continent configurations, assemblies and dispersals of continents, from the rifting and drifting of the Supercontinent Rodinia to the assembly of the Supercontinent Pangea. Therefore, this paper mainly focuses on summarizing and discussing the northern part of the Proto-Tethys tectonic domain based on field geology, structural geology, magmatism, sedimentary formations, geochemical records, geochronology and tomography, in order to reveal three key aspects: 1) identifying the southern and northern boundaries of the Proto-Tethys Ocean; 2) establishing affinities of continents/micro-continents within the Proto-Tethys Ocean and its ocean-continent configuration; and 3) clarifying the temporal sequence and styles of micro-continental assembly and the closure of the Proto-Tethys Ocean. Integrated analysis results show that to the north the region is bounded by the paleo-Luonan-Luanchuan Suture (or Kuanping Suture) and its extension to West Kunlun; the southern boundary is marked by the Longmu Co-Shuanghu-Changning-Menglian Suture. The Tarim-Alax-North China Block to the north of the Proto-Tethys Ocean had a southward subduction polarity and collided with Gondwana along the northern margin of Gondwana in the Early Devonian. The southern branch of the Proto-Tethys Ocean may be closed, making the Greater South China Block, including the northern Qiangtang, Ruoergai, Yangtze and Cathaysia, Bureya-Jiamusi and Indochina blocks, southward subduction and accretion to the northern margin of Gondwana in the Early Devonian. The results also show that the North China Block had no clear affinity to Gondwana, whereas the other continental/micro-continental blocks, such as the Yangtze, Cathaysia, Tarim, Qaidam, Alax, North Qinling, Qilian, Oulongbuluke, South Qiangtang, Lhasa, Lanping-Simao and Indochina all have an affinity to Gondwana in the earlier part of the Early Paleozoic. During the interval 480–400 Ma these series of continental blocks/micro-continental blocks experienced gradual southward subduction and accretion to the eastern segment of the northern margin of Gondwana, resulting in the closure of the Proto-Tethys Ocean and formation of the supercontinent called Proto-Pangea. The Greater South China Block and the Tarim-North China Block separated and drifted from Greater Gondwana of the Supercontinent Proto-Pangea since 380 Ma, resulting in the formation of the Paleo-Tethys and the Mianlue oceanic crusts. After this minor adjustment and until 240–220 Ma, they assembled northward gradually to develop Laurasia, which in turn resulted in the final formation of the Supercontinent Pangea.

Pub.: 28 Jan '17, Pinned: 01 Aug '17