The Deschutes Formation: evidence of extension-enhanced explosivity in the early High Cascades Public Deposited


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  • The eruptive history of the Quaternary Cascades arc has been relatively well characterized. However, much less is known about the frequency and sizes of explosive eruptions produced by earlier stages of the arc. The Late Neogene Deschutes Formation of Central Oregon preserves a remarkable record of heightened pyroclastic activity during the initial stages of High Cascades volcanism, following an eastward shift in volcanic activity ~7.5 Ma. Extensive fieldwork, 40Ar 39Ar geochronology, and geochemical analyses allow us to reconstruct this unusually explosive phase of the earliest Central Oregon High Cascades.Plagioclase 40Ar 39Ar ages for eight laterally-extensive marker ignimbrites that stratigraphically bracket the many pyroclastic deposits exposed within the Deschutes Formation, indicate that almost all of these explosive eruptions occurred within only ~800 k.y., between 6.25± 0.07 and 5.45± 0.04 Ma. Combining these age data with multivariate statistical tephra correlation methods, I establish a comprehensive tephrostratigraphy of the Deschutes Formation. These correlations suggest that at least 67 distinct explosive eruptions (possibly as many as 120) occurred within the 800 k.y. explosive pulse.Using a new ArcGIS-based method that I developed for calculating ignimbrite volumes, I find that a total volume of 82 km3 (62 km3 DRE) for just 26 of the ignimbrites deposited distally into the Deschutes Basin. If these ignimbrites also deposited an equal volume to the west and had a tephra fall:flow ratio of betweenii0.5:1 and 1.9:1 (similar to Mount Pinatubo and Valley of Ten Thousand Smokes), the total volume of all 26 eruptions was likely between 246 and 475 km3, or a rate of 6-12 km3 m.y. km. This rate is approximately 2-8 times higher than the production rate of all compositions over the entire Quaternary Cascades and is the highest rate in Oregon over at least the last 17 Ma.The unique timing and location of this pulse, approximately 1 m.y. after an eastward shift of the arc axis, and in a region undergoing extension, may explain the anomalous explosivity recorded in the Deschutes Formation. I suggest that such extension allowed for penetration of hot, low-K tholeiitic basalt magmas into shallow levels of the crust, which induced a period of enhanced shallow crustal melting and the production of large volumes of hot-dry-reduced rhyolites (high Fe, Na, Y, MREE, and low Eu Eu* and Sr). Thus, the anomalously high production of silicic magma and rate of explosive volcanism recorded in the Deschutes Formation is mirrored by the unusual geochemistry of the eruptive products, and are together indicative of magmatic processes driven by extension, that no longer operate during the Quaternary.In addition to constraining changes in geochemistry and style of volcanism through time, I used rigorous statistical methodology to assess the geochemical variability along-arc for the Quaternary Cascades. To do this, I compiled a dataset of over 11,000 samples and utilized a Monte Carlo approach with weighted bootstrap resampling to reduce the bias that over-sampled volcanoes have on overall trends. In in doing so, I assessed regional, rather than local processes. Our study develops a novel approach to assessing along-arc geochemical variability using entirely objective and statistically-based methodology. Using this new approach, I separated the Cascades arc into 5 segments such that the geochemical differences between each is maximized. This new segmentation scheme, which includes the North, Washington, Graben, Mazama, and South Segments is more statistically robust than previous segmentation schemes. By separating the arc into the most statistically distinct regions, one can better assess the spatially disparate processes that lead to geochemical heterogeneity. This, in turn, provides a better understanding of the fundamental processes involved in arc magma generation.
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  • description.provenance : Submitted by Bradley Pitcher ( on 2017-07-07T23:54:15ZNo. of bitstreams: 1PitcherBradleyW2017.pdf: 7556828 bytes, checksum: 95154287f79d47a38dc23fb319af51fa (MD5)
  • description.provenance : Made available in DSpace on 2017-07-11T20:53:28Z (GMT). No. of bitstreams: 1PitcherBradleyW2017.pdf: 7551193 bytes, checksum: 0adb3943f99b536384fb0a3e4fb7a150 (MD5) Previous issue date: 2017-06-09
  • description.provenance : Submitted by Bradley Pitcher ( on 2017-07-08T20:35:02ZNo. of bitstreams: 1PitcherBradleyW2017.pdf: 7551193 bytes, checksum: 0adb3943f99b536384fb0a3e4fb7a150 (MD5)
  • description.provenance : Approved for entry into archive by Julie Kurtz( on 2017-07-08T21:06:33Z (GMT) No. of bitstreams: 1PitcherBradleyW2017.pdf: 7551193 bytes, checksum: 0adb3943f99b536384fb0a3e4fb7a150 (MD5)
  • description.provenance : Rejected by Julie Kurtz(, reason: Hi Brad,I had to reject your dissertation for the following revisions -- Change the commencement date on the bottom of the title page to read – Commencement June 2018- Approval page – 1st signature line change to – Major Professor, representing Geology 2nd signature line change to – Dean of the College of Earth, Ocean, and Atmospheric Sciences- Remove the page numbers in the pretext pages- LIST OF APPENDICES there is no page numbers and your last page of your dissertation just has the heading – Appendices with nothing after. Should there be additional pages with these appendices?Everything else looks good. Once revised, log back into ScholarsArchive and go to the upload page. Replace the attached file with the revised PDF and resubmit.Thanks,Julie on 2017-07-08T00:27:25Z (GMT)
  • description.provenance : Approved for entry into archive by Steven Van Tuyl( on 2017-07-11T20:53:28Z (GMT) No. of bitstreams: 1PitcherBradleyW2017.pdf: 7551193 bytes, checksum: 0adb3943f99b536384fb0a3e4fb7a150 (MD5)


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