31 Geologic and Structural Evolution of the Transition Zone

Wednesday, 16 May 2018
10:20 AM - 12:20 PM
Humphreys/Mt. Elden Room

ANATOMY AND CHARACTERISTICS OF MEGA-SCALE VOLCANIC LANDSLIDES OF THE MARYSVALE VOLCANIC FIELD, SOUTHWESTERN UTAH

Paper number: 31-1
Program: Theme Sessions
Time: 10:20 AM – 10:40 AM

Authors

Abstract

Continued geologic mapping of the polygenetic Oligocene to Miocene Marysvale volcanic field, which straddles the Basin and Range and High Plateaus in southwest Utah, has revealed at least two gigantic catastrophic landslide events resulting from the southward collapse of the south flank of the volcanic field. The previously identified Markagunt gravity slide (MGS, >5000 km2) was emplaced at 22 to 21 Ma, while the newly discovered Sevier gravity slide (SGS, >2000 km2) was emplaced at 25 to 23 Ma. The MGS and SGS each consist of three distinct structural areas: (1) a slide zone containing both a breakaway segment and a bedding-plane segment (~65 km long for the MGS and ~55 km long for the SGS), the latter typically within the Eocene-Oligocene Brian Head Formation, (2) a ramp zone ~1-2 km wide where the slide masses cut up section from the slide zone, and (3) a former land surface zone with runouts > 35 km over the Miocene landscape. These volcanic mass movements remain relatively coherent in the slide zone where largely undeformed blocks retain most of their original stratigraphy with some degree of internal deformation by minor faults and folds. Deformation is much greater in the ramp and former land surface zones, where chaotic tilted blocks of volcanic rocks commonly occur in a brecciated and sheared matrix; this deformation represents a debris-avalanche style of movement. Each mega-scale volcanic landslide is a continuous catastrophic emplacement event, with the resulting mass being a gradational combination of slide and debris-avalanche movements. Characteristics of these astonishingly large gravity slides suggest that these types of volcanic fields are preconditioned for collapse by virtue of their: (1) large volume of available slide material that rapidly accumulated in a thick volcanic wedge, (2) a substrate of weak subhorizontal strata, (3) underlying intrusive complexes whose growth tilts the volcanic pile, and (4) development of pre-collapse summit fractures and/or normal faults by intrusive doming or during gradual lateral spreading on deep-seated thrust faults that weaken the structural integrity of the volcanic field.

Geological Society of America Abstracts with Programs. Vol. 50, No. 5

doi: 10.1130/abs/2018RM-314106

© Copyright 2018 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.