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346 MESOZOIC/Triassic $240 Ma, whereas tuffs within the underlying Reitziites reitzi ammonite zone in Hungary give maximum ages closer to 241 Ma The Upper Triassic stages are poorly constrained radioisotopically The Ladinian– Carnian boundary approximates 237 Ma based on U/Pb ages from Italy and Hungary Hence, the Upper Triassic is over half the length of the duration of the Triassic Period Estimates for the Carnian–Norian boundary have been based on extrapolation from lithological cyclicity recognized in the non-marine Newark Supergroup of eastern North America These cycles are interpreted as a response to Milankovitch orbital periodicities and can thus be measured in absolute terms This astronomically calibrated numerical scale is set against the magnetostratigraphic polarity profile, which, in turn, can be correlated with successions elsewhere However, there is no current agreement either on a position for a marine-based boundary or on precise correlations of the fossil scale with the Newark succession Independent data come from an Ar/Ar age of a tuff that underlies Adamanian (¼Late Carnian) tetrapods in Argentina This gives an age of $228 Ma A Triassic–Jurassic boundary age of 202 Ma has been based on the ages of Newark Supergroup basalts that overlie the palynologically determined boundary However, the palynological correlation of the boundary is debated and a more recent U/Pb age from British Columbia gives a Triassic–Jurassic boundary age very close to 200 Ma There is essentially no preserved Triassic seafloor, so there is no agreed geomagnetic polarity time-scale for the Triassic However, a composite polarity timescale is now becoming available, based on successions being cobbled together from non-marine and marine sections in North America, Europe, and Asia With continued refinement, this scale will be an important supplement to that provided by biostratigraphy in both the marine and non-marine realms Palaeogeography At the onset of the Triassic, the world’s continents were assembled into a single supercontinent called Pangaea (see Pangaea) (Figure 3) The rest of the globe comprised a single vast ocean called Panthalassa, with a westward-extending arm called Tethys This followed the Late Palaeozoic assembly of the continents when Laurentia, Asia, and Gondwana collided along the Alleghanian–Variscan–Ural mountain chains The nearly hemispheric Pangaean supercontinent was encircled by subduction zones that dipped beneath the continents while the Panthalassan and Tethyan plates carried island arcs and oceanic plateaus that were destined to become accreted to the continental margins Figure Triassic Pangaea, showing major tectonic elements After Lucas (2000) The epicontinental Triassic, an overview Zentralblatt fuăr Geologie und Palaăontologie Teil I 8: 475 496 The supercontinent drifted northward and rotated clockwise throughout the Triassic, so there was considerable latitudinal spread to the landmass, which was nearly symmetrical about the equator (Figure 3) However, no sooner had the supercontinent been assembled than significant fragmentation began Thus, Gondwana and Laurasia began to separate in Late Triassic time with the onset of rifting in the Gulf of Mexico basin It was not until the Early Jurassic, though, that significant marine sedimentation took place in the nascent Atlantic Ocean basin Tectonics and Sedimentation At the broadest level, the tectonics of Triassic Pangaea were simple The accreted supercontinent was simply surrounded by convergent margins (Figure 3) However, these margins were actually complex belts of magmatic arcs and terranes moving in various directions to produce compressional and transpressional tectonics Late Triassic Pangaea was the site of widespread extensional tectonism, especially the initial opening of the Atlantic Ocean basin by rifting of the North American and African plates During the Late Triassic, in the Tethys, North Atlantic, and Arctic, multidirectional rift systems developed (Figure 3) Rifting also took place along a zone of transforms that extended well into the Gulf of Mexico basin and, punctuated by volcanism, dominated the northern border of western Tethys This rifting in the North Atlantic and Tethyan regions subjected western and central Europe to progressive regional extension, culminating in the development there of complex multidirectional systems of troughs and grabens During the Early–Middle Triassic, terminal thrusting took place along the entire Gondwanan margin of Pangaea, which was followed in the Carnian by extension in southern South America and eastern Australia
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