Showing papers by "Roland Mundil published in 2016"

The precise temporal calibration of dinosaur origins

Abstract: Dinosaurs have been major components of ecosystems for over 200 million years. Although different macroevolutionary scenarios exist to explain the Triassic origin and subsequent rise to dominance of dinosaurs and their closest relatives (dinosauromorphs), all lack critical support from a precise biostratigraphically independent temporal framework. The absence of robust geochronologic age control for comparing alternative scenarios makes it impossible to determine if observed faunal differences vary across time, space, or a combination of both. To better constrain the origin of dinosaurs, we produced radioisotopic ages for the Argentinian Chanares Formation, which preserves a quintessential assemblage of dinosaurian precursors (early dinosauromorphs) just before the first dinosaurs. Our new high-precision chemical abrasion thermal ionization mass spectrometry (CA-TIMS) U–Pb zircon ages reveal that the assemblage is early Carnian (early Late Triassic), 5- to 10-Ma younger than previously thought. Combined with other geochronologic data from the same basin, we constrain the rate of dinosaur origins, demonstrating their relatively rapid origin in a less than 5-Ma interval, thus halving the temporal gap between assemblages containing only dinosaur precursors and those with early dinosaurs. After their origin, dinosaurs only gradually dominated mid- to high-latitude terrestrial ecosystems millions of years later, closer to the Triassic–Jurassic boundary.

Repeated, multiscale, magmatic erosion and recycling in an upper-crustal pluton: Implications for magma chamber dynamics and magma volume estimates

Abstract: The Tuolumne Intrusive Complex, an upper-crustal (7–11 km emplacement depths), incrementally constructed (95–85 Ma growth history) plutonic complex (~1100 km 2 ), preserves evidence from several data sets indicating the repeated, multiscale, magmatic erosion of older units occurred and that some eroded material was recycled into younger magma batches. These include: (1) map patterns of internal contacts (hundreds of kilometers) that show local hybrid units, truncations, and evidence of removal of older units by younger; (2) the presence of widespread xenolith and cognate inclusions (thousands), including “composite” inclusions; (3) the presence of widespread enclaves (millions), including “composite” enclaves, plus local enclave swarms that include xenoliths and cognate inclusions; (4) the presence of widespread schlieren-bound magmatic structures (>9000) showing evidence of local (meter-scale) truncations and erosion; (5) antecrystic zircons (billions) and other antecrystic minerals from older units now residing in younger units; (6) whole-rock geochemistry including major element, REE, and isotopic data; and (7) single mineral petrographic and geochemical studies indicating mixing of distinct populations of the same mineral. Synthesis of the above suggests that some erosion and mixing occurred at greater crustal depths, but that thousands of “erosion events” at the emplacement site resulted in removal of ~35–55% of the original plutonic material from the presently exposed surface with some (~25%?) being recycled into younger magmas and the remainder was either erupted or displaced downward. The driving mechanisms for mixing/recycling are varied but likely include buoyancy driven intrusion of younger batches into older crystal mushes, collapse and avalanching along growing and over-steepened solidification fronts within active magma chambers (1 to >500 km 2 in size), and local convection in magma chambers driven by internal gradients (e.g., buoyancy, temperature, and composition).