G&G Colloquium Fall 2018

Aug. 31

Sep. 7

Rebecca Flowers

(University of Colorado Boulder)

Title: Cratonic surface histories, kimberlites, and mantle dynamics: Insights from deep-time (U-Th)/He thermochronology

Abstract:

Fundamental questions exist about the Neoproterozoic and Phanerozoic deep-time surface evolution of vast cratonic interior regions. In this talk I will summarize examples of our work using advances in (U-Th)/He thermochronology to place constraints on these surface histories as far back as ~1 Ga, and consider the causes and potential links with other records. First, I will describe our integration of (U-Th)/He data with geologic information to decipher the thickness, spatial extent, and evolution of missing sections of the Phanerozoic stratigraphic record across the North American cratonic interior. The results reveal that substantial burial events are missing from the cratonic geologic record, but their thermal imprint can be detected using low-temperature thermochronology. The inferred history of burial and erosion is similar to the evolution of dynamic topography predicted by a 3D global model of thermochemical convection. This suggests that Pangea supercontinent assembly and breakup may have exerted first-order control on the elevation change history of the North American interior. The results also reveal an intriguing relationship between cratonic burial phases and gaps in the kimberlite record for which we consider possible explanations. Second, I will briefly outline a new collaborative effort to push these techniques deeper into the past to decipher the timing, magnitude, and spatial heterogeneity of erosion that lead to formation of the Great Unconformity with the goal of testing competing models for oxygenation and the Cambrian explosion.

Sep. 12

(Wed.)

Roberta Rudnick

(UC Santa Barbara)

Sep. 14

Sarah Brownlee

(Wayne State University)

Demystifying seismic anisotropy in the continental crust by talking to the rocks

Seismic anisotropy, the directional dependence of seismic velocity, has been an invaluable tool for understanding strain and flow in the upper mantle. The utility of seismic anisotropy in the upper mantle can be attributed in part to a wealth of studies characterizing the properties of mantle rocks and minerals. In contrast, the continental crust is not nearly as well-characterized due in large part to its very small volume in relation to global seismic raypaths. The continental crust also poses numerous complexities in mineralogy and structure making it significantly more difficult to characterize. Recently, a number of studies have been focused on characterizing the full anisotropic elasticity of rocks from the continental crust. These studies have motivated efforts to predict how these rocks will appear in seismic observations, and thus to recalibrate the assumptions used in seismic inversions in order to improve our ability to distinguish various rock types and deformation in the continental crust. I will begin by discussing the basics of seismic anisotropy and how it is observed. Then we will delve into the catalogue of crustal rock properties, reviewing some of the trends in elastic symmetry with deformation and rock type in the continental crust. I will present a simple scaling scheme to allow for more realistic non-elliptical hexagonal elastic tensors in seismic inversions, and discuss how real crustal rocks might appear in seismic data. The take home message is that while the continental crust is complicated, it cannot be ignored, because even when the focus of study is the mantle, most of our observations are made through the window of the continental crust. Further characterization of the elastic properties of crustal rocks, and how these rocks are expressed in seismic data will improve our ability to use seismic methods to understand deformation in and beyond the continental crust.

Sep. 21

Colleen Dalton

(Brown University)

Sep. 28

Friedhelm von Blanckenburg

(Potsdam)

Luc Deike

(Princeton University)

Oct. 12

Anat Shahar

(Carnegie Geophysical Institute)

Oct.  

19

Oct. 26

Roger Benson

(Oxford)

Patterns of tetrapod diversification on land, and possible explanations

Tetrapods - mammals, birds and other reptiles - comprise around 30,000 species today and play important roles in terrestrial ecosystems. They evolved from a single ancestor species that lived c.360 million years ago. Patterns of both species richness and phenotypic disparity, and how they have changed through time, have been subject to debate, and the prevailing hypothesis has been one of near-continual expansion through time, driven by frequent innovation. This hypothesis is central to macroevolutionary hypotheses about biotic diversification on Earth. However, resolving the actual patterns of tetrapod diversification is challenging in light of their fossil record, in which most intervals are characterised by just a handful of well-sampled regions at best.

    My work has focussed on quantitative approaches to characterising patterns of phenotypic diversification and of tetrapod species richness in a spatially-patchy fossil record. Dynamic evolutionary radiations, involving increases in phenotypic diversity, and expansion of species richness occur frequently on shorter timescales (~ 10 Ma) and local phylogenetic scales. However, longer-term, large-scale patterns are almost static, with species diversification rates of approximately zero over extended intervals. This background of little net change in species richness is punctuated by abrupt episodes of dramatic radiation, that substantially elevated species richness on land beyond previous levels. These are associated with post-extinction radiations, and with a small number of truly exceptional evolutionary innovations. Explanations for this pattern may be rooted in macroecology and energetics. 

Nov. 1

Kurt Stüwe

(Institute for Earth Science, Austria)

GEOLOGY OF THE ALPS - SEEN FROM ABOVE

Geologically, the European Alps are often considered to be one of the most complicated orogens in the world.  One of the reasons for this lies in human nature: many of the tectonic features that formed the Alps occur on space and time scales that can barely be observed from the ground. Moreover, vegetation, rugged mountains and human influence obscure many of the critical geological features.   In an unparalleled  adventure, Kurt Stüwe and photographer Ruedi Homberger have used a bush-plane and an ultralight aircraft to cover the entire range from Vienna to Nice  and  produced  an aerial photo catalogue of some 20000 mostly panoramic pictures, illustrating  geological features  of the Alps (see: www.alpengeologie.org ).  On hand of these photos,  the geology, the plate- and the neo-tectonic  history of the Alps can be explained  intuitively and spectacularly. This presentation presents  highlights of the Alpine Geology that will startle the curiosity of any Earth scientist.

Nov.  9

Ian Hewitt

(Oxford)

(Wed.)

Nov. 14

Melanie Hopkins

(American Museum of Natural History)

Morphological Evolution and Disparity in Some Marine Invertebrates

Morphological disparity is a measure of morphological variation among species and higher taxa, and is a distinct measure of biodiversity complementing taxonomic proxies.  Disparity studies have led to improved understanding of the evolutionary history of major clades and fostered new research on adaptive radiations, rates of evolution, and morphological innovations.  One limitation of traditional disparity studies, however, is that different evolutionary processes may generate similar disparity profiles.  This limitation can be overcome by estimating rates in the context of phylogenetic trees and by teasing apart patterns of morphological evolution within the context of morphospaces.  In this talk, I will demonstrate the dynamic nature of morphological evolution and diversification through studies of early bursts and trends in echinoids and trilobites.   

Simon Poulton

(University of Leeds)

Dec.  7

Doug Erwin

(Smithsonian Museum of Natural History)