the week of Sunday, March 4th, 2018

Wednesday, March 7th, 2018, Wednesday Seminar

4:10 PM, 55 Roessler
Tea and cookies at 3:45 in the aviary - (2110 EPS)


      – by Dr. Mitch Mihalynuk

Quantitative plate reconstructions rely on ocean isochrons, but constant recycling of ocean lithosphere ensures that only ~50% of ocean floor predates Cenozoic, and almost none is older than Jurassic. Such isochron subduction hampers efforts to reconstruct Cordilleran North America, but fortunately, subducted ocean lithosphere is not destroyed. It persists within the mantle as a folding mass, cooler than ambient mantle, and seismically visible, as fast domains called slabs.

Subducted slabs beneath North America can now be highly resolved thanks to USArray, and we use them in combination with existing isochrons to reveal evolving arc/trench-plate geometries (tomotectonic analysis) back to Pangea breakup. Resultant paleogeographies bear on fundamental questions about the assembly of North America. For example, geological relations show that eastward, Andean-type subduction formed both the native Late Triassic-Early Jurassic arc (rooted in continental crust of southwest USA and shedding clastic strata as far inboard as the Colorado Plateau) and the Cascades, but what about times in between? Most workers embrace the simplicity of an always Andean margin. An alternate viewpoint dates to the advent of Plate Tectonics (Moores, 1970, at UC Davis). It explains multiple arc and ophiolite belts as a product of westward subduction beneath an offshore volcanic archipelago.

We bring Tomotectonic analysis to bear on this debate and show that the archipelago interpretation is correct. Two massive arc complexes originated in the seas west of Pangea during its early fragmentation (~190-170 Ma), at a time when east-directed subduction beneath the continental margin arc was shutting down. Subduction zones reconfigured from EAST-directed beneath the continental margin (during final growth of the Intermontane Superterrane, IMS, also known as AltaBC), to WEST-directed, beneath an intraoceanic, massive arc chevron (MAC). East-pointing MAC was >10,000 km long and located 2000-4000 km off the west coast. Within the mantle reference frame, MAC was stationary, as indicated from the near-vertical slab walls 4-7x as thick as the mature ocean lithosphere. Between west-drifting North America and the MAC apex, all ocean lithosphere was consumed by ~155 Ma, causing a microcontinent that extended >2600 km southwards from the MAC apex, the Insular Superterrane (INS, also known as 'BajaBC'), to collide with the leading edge of North America (IMS). Initial collision of INS, which was comparable in length to the Indian subcontinent, generated 'Nevadan' deformation. As MAC was driven farther into, and raked southward along the continental margin, diachronous Sevier deformation migrated eastward and newly accreted terranes were offset sinistrally with respect to the continent. By ~130 Ma, MAC geometry was breaking down, large segments had accreted to the new North American margin, and subduction was forced to jump outboard, forming the nascent Franciscan accretionary complex: a return to eastward/Andean subduction (now the Farallon plate). Arrival of the Shatsky conjugate plateau on the Farallon plate ~90 Ma strongly coupled with Cordilleran crust, transporting it rapidly northward along a new transform margin (BajaBC > AltaBC), as recorded by upward slab truncation, an extinguished Sierra Nevada arc (80 Ma), subducted sediments underplated far inboard of the continental margin, paleomagnetic measurements, and Laramide deformation.

A remarkably complete analogue for MAC collision at ~130Ma (and nascent Shatsky conjugate collision) can be found in modern Australia's override of the Sunda-Banda / Solomon-New Hebrides arcs.

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Thursday, March 8th, 2018, CIG Webinar

2:00 PM, online via zoom

“Where have all the dimensions gone? Hands on methods for introducing students to non-dimensional numbers in laboratory and numerical modeling”

      – by Eric Mittelstaedt, University of Idaho

Experienced modelers are familiar with how non-dimensionalizing mathematical systems can help improve numerical stability, reduce the number of free variables needed to explain a physical system, capture the essential driving processes of a problem of interest, and scale laboratory experiments to the Earth. However, when first introduced to non-dimensional numbers, students often have difficulty understanding how the mantle can have a depth of 1, or how numbers such as the Nusselt number or the Rayleigh number are derived. In this webinar, I will discuss a hands-on, in-class experiment involving a simple oscillator (mass on a spring) that I have used to introduce students to non-dimensionalizing equations, deriving non-dimensional numbers, and scaling experimental results. The mass-spring system is familiar to many students from their introductory physics classes and the mathematical system is simpler than many problems of interest in geodynamics. The combination of familiarity, basic mathematics, and a hands-on experiment facilitate student comprehension and future application of non-dimensional numbers. These methods are aimed at an introductory graduate course or senior level undergraduate course on modeling.

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Friday, March 9th, 2018, Friday Brown Bag

12:10 PM, 1316 Earth and Physical Sciences
PHD Exit Talk

“The Chemistry of Late Paleozoic Conodonts”

      – by Julie Griffin

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Friday, March 9th, 2018, Origins Group Special Seminar

3:10 PM, 1119 Earth and Physical Sciences (Moores Conference Room)

“Experimental Discovery of Superionic Water with dynamic compression”

      – by Marius Millot, LLNL

The pressure-temperature phase diagram of water exhibits a striking degree of polymorphism with more than 15 polymorphs of molecular ice and the pressure-induced transition to the ionic ice X near 70 GPa. Upon further compression and at elevated temperature, several molecular dynamics studies have predicted that water becomes superionic, an extraordinary state with liquid-like hydrogen ions diffusing within a solid lattice of oxygen. The higher entropy of superionic ice is expected to rise its melting temperature to several thousand Kelvin and to favor the transition to new ice structures having a close-packed oxygen lattice.

We will report experimental evidence for superionic electrical conductivity above 100 GPa and 2000 K using velocimetry, pyrometry and optical property measurements of shock compressed water ice VII, as well as in-situ x-ray diffraction of ice up to 4 Mbar using reverberation compression showing that the superionic conduction indeed exists in the presence of a solid oxygen lattice as predicted three decades ago. The new XRD data also suggest the discovery of a new solid ice phase having a face-centered-cubic oxygen lattice.

In addition to providing new benchmarks for quantum theory of condensed matter, our laboratory experimental study suggests that Neptune, Uranus and many icy exoplanets may contain a significant amount of superionic water ice, in contrast with the prevalent picture of fluid interiors for these objects.

Marius Millot obtained his PhD from Université Paul Sabatier in Toulouse (France) in 2009. After a postdoc at the Earth and Planetary Science Department of UC Berkeley, he is now Research Scientist at the Lawrence Livermore National Laboratory, in Livermore, CA.
His research is focused on the study of matter properties at extreme pressures and temperatures for planetary science and astronomy, materials science, High Energy Density science and Inertial Confinement Fusion using dynamic compression methods and ultrafast optical diagnostics.

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