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Incipient breakup of the Isla Ángel de la Guarda microcontinent, Gulf of California, México -&- Structure and Properties of Hydrous Stishovite


Oct. 21, 2020, noon - 1 p.m.
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Presented By:
Justing Higa,
&
Travis Gilmore

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Justin Higa: Incipient breakup of the Isla Ángel de la Guarda microcontinent, Gulf of California, México Faults on microcontinents record the dynamic evolution of plate boundaries. However, most microcontinents are submarine and difficult to study. Here, we show that the southern part of the subaerial Isla Ángel de la Guarda (IAG) microcontinent is the likely site of an incipient plate boundary reorganization in the Gulf of California that may lead to future microcontinent breakup. To characterize the kinematics of this reorganization, we integrated remote fault mapping using high-resolution satellite- and drone-based topography with neotectonic field-mapping and 13 luminescence ages from sediment deposits offset or trapped by faults. Onshore, N-S striking normal faults occur along strike of a nascent offshore spreading center in the North Salsipuedes Basin, west of IAG. Late Pleistocene and Holocene luminescence ages indicate onshore fault activity in the last ~50 ka. These observations imply that the North Salsipuedes Basin is kinematically linked with active faults onshore IAG. Thus, crustal extension across southern IAG may connect to and reactivate extinct plate boundary structures east of IAG in the Upper Tiburón basin and play an important role in reorganizing the Pacific-North America plate boundary. /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////// Travis Gilmore: Structure and Properties of Hydrous Stishovite Stishovite is known to accept significant amounts of structurally-bound water. As the major phases of the lower mantle (bridgmanite, ferropericlase) appear to accept very little water, stishovite, occurring in locally enriched portions of the lower mantle, may be a globally significant host for water. However, the structural mechanisms by which water is incorporated in stishovite, particularly in Al-free compositions, is unclear. A better understanding of the structure of hydrous stishovite has important implications for its occurrence and physical properties at lower mantle conditions. We have investigated proposed H incorporation mechanisms in stishovite using Density Functional Theory (DFT) at lower mantle conditions. We use a combination of structural relaxation and determination of experimentally observed physical properties (elastic constants, vibrational frequencies, equation of state, and the CaCl2 phase transition) to better constrain the structure of hydrous stishovite. We focus on charge balancing of H incorporation via Si vacancies and find that some proposed structures are implausible. We investigate the elastic signature of the stishovite to CaCl2 transition in hydrous compositions and discuss comparisons with seismically observed lower mantle reflectors.