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The Case for NE-SW Extension in Northeast Oregon

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The Case for NE-SW Extension in Northeast Oregon AN ABSTRACT OF THE THESIS OF Jim E Essman for the degree of Master of Science in Geology presented on February 18, 2003. Title: The Case for NE-SW Extension in Northeast Oregon Abstract Approved: Andrew J. Meigs A zone of diffuse deformation -600 km-wide extending from northern California to Washington has developed resulting from the oblique subduction of the Juan de Fuca plate beneath North America, and the northwestward migration of the Pacific plate. This zone is marked by a change in structural style from transtension in the southern Oregon and western Nevada to transpression in southern and western Washington. The transition occurs across a relatively inactive zone in northeastern Oregon. New geologic mapping near the northern edge of this transitional zone suggests that extensional deformation persists into northeastern Oregon. The study area on the Oregon-Idaho border, is a complex zone characterized by NNW- trending primarily sinistral-oblique normal faults linking the NW-trending Halfway and Sturgill Peak (both down to the NE) normal faults to the NW and SE, respectively. The Halfway fault, -20 km-long - 730 m dip-slip separation, marks the southern boundary of a half-graben; whereas the Sturgill Peak fault, - 10 to 15 km long and not associated with a half-graben, has a minimum dip-slip separation of - 660 m. In contrast, the transfer zone informally named the Powder River Peninsula Fault Zone, or PRPFZ, houses numerous -NNW-striking, 2 to 3 km-long normal and sinistral-oblique normal faults with dip-slip separations of < 130 m. Evidence of late Pleistocene-Holocene deformation within the transfer zone is found in at least four places, and at two of those involves sinistral-oblique normal faults (-10-15 m vertical separation on a bedrock scarp) cutting late Pleistocene Bonneville flood gravels, -14 ka, with flood gravels both entrained in the fault plane and interbedded with Holocene colluvium implying significant activity in the late Pleistocene and possibly the early Holocene. Additionally, one fault in the PRPFZ cuts Holocene (?) soils/clays and deforms Pleistocene ash beds. Slip rates calculated from slickenlines and separations in the PRPFZ range from 0.01 mm/yr to possibly as high as 2.8 mm/yr to. This detailed slip data for the PRPFZ can be used as a proxy for the more regional deformational styles. Although the Halfway and Sturgill Peak faults have been interpreted to represent the southeastern extension of the Olympic-Wallowa Lineament (OWL), the tectonic and structural significance of the OWL remains poorly understood. It is suggested here that these structures reflect the continuation of Basin- and-Range style deformation into northeastern Oregon, and western Idaho, and any connection to the OWL to be circumstantial. Furthermore, the presence of these structures poses a potential seismic hazard to local hydroelectric facilities. © by Jim E. Essman February 18th, 2003 All Rights Reserved The Case for NE-SW Extension in Northeast Oregon By Jim E. Essman A THESIS Submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented February 18, 2003 Commencement June 2003 Master of Science thesis of Jim E. Essman presented on February 18, 2003 APPROVED: Major Professor, Representing Geology Chair of Department of Geosciences Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Jim E. Essman, Author ACKNOWLEDGEMENTS I would like to thank my parents Jim and Joan Essman for their motivation, encouragement and support. Thanks to my Father for being the world's best field partner, Dr. Andrew J. Meigs and Ian P. Madin as well as all of my committee members for their guidance, direction and friendship, thanks to Caroline Bruno for keeping me motivated in the end and thanks to Sam VanLaningham and Chris Krugh for always setting the bar high and supporting the "Dream". A final thanks to Idaho Power Company for allowing me extended access to their property. TABLE OF CONTENTS INTRODUCTION .................................................................. 1 METHODS .......................................................................... 7 REGIONAL TECTONIC FRAMEWORK ...................................... 7 FAULT TOPOLOGY IN OBLIQUE/NORMAL RIFTS ...................... 11 REGIONAL FAULTS ..............................................................14 Mapping............... ........................14 Northwest Striking Faults................................................. 15 North Trending Faults (Powder River Peninsula FaultZone)..........17 QUATERNARY ACTIVITY .....................................................18 Quaternary Stratigraphy..................................................... 19 Structural and Evidence for Late Quaternary Faulting..................20 Seismicity..................................................................... 30 TRANSFER ZONE MODEL FOR THE POWDER RIVER PENINSULA FAULT ZONE.....................................................32 Physical Arrangement of Faults withinTransfer Zones.................32 Strike-Slip in thePRPFZ.................. 36 Slip Rates......................................................................43 SEISMIC HAZARD..............................................................43 Previous Studies..............................................................43 Activity........................................................................45 New Calculations.............................................................45 NEW DATA........................................................................ 46 DISCUSSION.......................................................................47 CONCLUSIONS....................................................................49 QuaternaryActivity..........................................................49 Tectonic Model...............................................................50 Transfer Zone Hypothesis................................................... 51 Seismic Hazard...............................................................51 REFERENCES CITED............................................................57 Figure LIST OF FIGURES Page 1. Generalized late Cenozoic faults of the northwestern United States........................................................................ 3 2. Digital elevation model of the Pacific Northwest ..................... 6 3. Regional tectonic map of northeast Oregon........................... 9 4. Surface traces of faults produced by oblique rifting.................. 13 5. Geologic map of active faults in the PRPFZ........................... 16 6. Location map of figures 7-11............................................ 22 7. Outcrop photo of Peninsula fault........................................ 23 8. Outcrop photo of Juniper fault and related soil/colluvium deformation................................................................. 24 9. Aerial view of tilted Pleistocene ash beds in hanging wall of Juniper fault................................................................ 26 10. Outcrop photo of Coyote fault and offset Pleistocene Bonneville flood gravels................................................................ 27 11. Outcrop photo of the Tarter Gulch fault and associated geology... 28 12. Bedrock Scarps of the Coyote and Tarter Gulch faults............... 29 13. Map of major faults in the study area, local hydroelectric facilities and composite focal mechanisms for local earthquakes... 31 14. Map of major faults in the study area, local hydroelectric facilities and historic seismicity......................................... 33 15. Classifications of different types of transfer zones................... 35 16. Proposed models for local deformation styles and types of transfer zones............................................................... 37 Figure LIST OF FIGURES (Continued) Page 17. Schematicdiagram illustratingderivation of formula used to estimate strike-slip in thePRPFZ fromextensionof Halfway- Posy Valley andSturgill Peakfaults.................................... 39 18. Block diagram illustrating derivation of formula used to calculate strike-slip in the PRPFZ from extension measured in the PRPFZ.. 42 19. Geologic cross sections across theHalfway-Posy Valley faults and PRPFZ.................................................................. 44 Table LIST OF TABLES Page 1. Fault slip data for major northwest striking faults and active faults in thePRPFZ ........................................................ 53 2. Estimation of surface wave magnitudes of maximum credible earthquakes ................................................................ 54 3. Estimations of peak ground accelerations for maximum credible earthquakes ................................................................. 55 4. Estimation of seismic moment magnitudes and surface wave magitudes from fault parameters......................................... 56 Plate LISTOF PLATES I. geologic map of portions of the Posy Valley and Sturgill Creek 7.5 min. quadrangles, Oregon-Idaho The Case forNE-SWExtension in Northeast Oregon Introduction Deformation of the western United States is characterized by a broad zone -600 km wide that extends from northern California and Nevada to southern Washington (Fig. 1). It is widely believed that this deformation is the result of the oblique
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