| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | December 7, 2016 |
| Latest Amendment Date: | December 7, 2016 |
| Award Number: | 1650411 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Stephen Harlan
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | January 1, 2017 |
| End Date: | December 31, 2019 (Estimated) |
| Total Intended Award Amount: | $52,569.00 |
| Total Awarded Amount to Date: | $52,569.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
660 S MILL AVENUE STE 204 TEMPE AZ US 85281-3670 (480)965-5479 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
781 Terrace Rd Tempe AZ US 85287-6004 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Tectonics |
| Primary Program Source: |
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| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
The McDowell Mountains in south-central Arizona are composed of a newly identified approximately 10 kilometer-thick section of rocks of Middle Proterozoic age. This is a time period in North American geologic history for which very little concrete information exists. Detailed geologic, isotopic age determinations, and structural study of the McDowell Mountains section will greatly increase current knowledge and understanding of geologic events occurring in North America during this period. The results may also provide constraints to evaluate proposed plate tectonic reconstructions during the Middle Proterozoic. The research is unique in that it the field area lies within the McDowell Sonoran Preserve, one of the largest municipal preserves in the nation (greater than 122 square kilometers), located adjacent to the Greater Phoenix metropolitan area. The scientific results of this study will be communicated to the public via the McDowell Sonoran Preserve by the preparation of outreach materials (geologic trail guides) available for download on the web and public lectures by the project team. Research products and samples will be turned over to the McDowell Sonoran Conservancy, the volunteer branch of the Preserve, for archiving and their use in public outreach to visitors of the Preserve. The project will also contribute to the training of a graduate student and will contribute to the broadening of underrepresented groups in an important science, technology, engineering and math (STEM) discipline.
The McDowell Mountains rock section is comprised of metamorphosed sedimentary and volcanic rocks that are intruded by several granite bodies. The age of the section is bracketed by an early Proterozoic uranium-lead age (1630 million years) in the oldest volcanic tuff and a Middle Proterozoic uranium-lead age (1423 million years) for the youngest granite body intruding the section. In the last four years at least two, much smaller, sections in Arizona and New Mexico, were identified as Middle Proterozoic in age. The observations that these rocks are folded and/or are cut by large thrust faults, which were then deeply buried and metamorphosed, led to the identification of a new mountain range building event of regional extent: the Picuris orogeny. The thickness and nature of the McDowell Mountains section provides a unique opportunity to gain a better understanding of the timing, nature, and regional extent of this orogeny. Geologic and structural mapping and rock sampling was recently completed in the McDowell Mountains. A pilot zircon uranium-lead dating study on 3 samples yielded Middle Proterozoic ages for all three, but the most significant age is 1546 million years for the youngest volcanic rocks in the section, confirming the age of the section as Middle Proterozoic. These ages make it the only currently known North American Middle Proterozoic section that contains thick deposits of volcanic rocks. The research proposed here intends to build on the initial work to address the following objectives for the McDowell Mountains section: (1) further constrain its age range, (2) determine the timing and nature of metamorphic events, (3) characterize the age, chemistry, deformation, and mantle model ages of the igneous rocks, (4) constrain the age and nature of deformation, and (5) characterize the detrital zircon uranium-lead populations of the sedimentary rocks. A carefully selected set of samples will be used for this study: 23 for zircon and monazite uranium-lead dating, a subset of which will be analyzed for lutetium-hafnium isotopes, 15 for major and trace element geochemistry of the volcanic and granitic rocks, and 90 for petrographic and micro-structural fabric thin section study of the rocks in the section. The results of the five research objectives will provide a comprehensive geologic, geochronologic and structural characterization of the McDowell Mountains sequence. In addition, the section represents the only significant volcanic and intrusive rocks within the North American "magmatic gap" (between about 1.6 and 1.45 billion years ago) and may represent the "missing link" for plate reconstruction models that have, heretofore, always required the presence of an exotic source terrain e.g. Australia, northwestern Europe, or associated continental fragments to explain the occurrence of 1.5 to 1.6 billion year old zircons in North American sedimentary sequences for which no local source was known.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The results of the detailed geological study of the McDowell Mountains in south central Arizona confirm that the rocks comprising the mountains indeed represent a heretofore unrecognized section of middle Proterozoic age. The age of the section ranges from an early Proterozoic age for the oldest volcanic tuff of 1639 Ma to an age of 1546 Ma for the youngest volcanic rocks within the section. The presence of thick rock sequences of volcanic origin of middle Proterozoic age in the McDowell Mountains is unique for North America and provides important new data for those researching plate reconstruction models throughout geologic history. Since the presence of ~1.5-1.6 billion year old zircons in North American sedimentary sequences now does not require the presence of exotic source terrains any more.
The dating and structural results have allowed correlation of the bottom the rock sequence to other rock sequences of similar age in Arizona and New Mexico and have provided new insights into the Picuris orogeny which affected these rock sequences causing them to be deformed and moderately metamorphosed. The chemistry of the main volcanic deposits with the sequence is rhyolitic and the main intrusive bodies associated with the sequence are granitic. Overall the results of this study are helping to fill in a gap in the complex puzzle that is the geologic history of the North American continent and the United States.
A manuscript for publication in a peer reviewed journal is currently in preparation but results from our study were presented at last year’s (2019) annual meeting of the Geological Society of America, which was held in Phoenix, Arizona. Results were detailed in both a poster and oral presentation, showing the new results and how these results expand our knowledge and understanding of middle Proterozoic geological evolution of the region. In addition, a short half day conference field trip was organized to one of the more accessible areas of the McDowell Mountains to give interested colleagues the opportunity to see parts of the sequence in the field.
The McDowell Mountains fall mostly within a regional preserve: The McDowell Sonoran Preserve and during this project several presentations were given by the project consultant to the membership of the McDowell Sonoran Conservancy (MDSC), which is the volunteer branch of The McDowell Sonoran Preserve. In addition, with help of the citizen scientists and leadership of the MDSC the geologic map of the area that stands at the basis of this project was digitized and turned into a fully georeferenced online geological map, which includes both the occurrence of the geologic rock formations and the structural information that was collected during mapping of the area. When a rock unit is clicked on the map a pop up with a short but detailed geologic description appears, that will be updated with the latest information obtained during this study when the manuscript reporting it is accepted for publication. The map, which also contains all the trails in the area, can be accessed by any member of the public through the MDSC website and can even be accessed using a mobile phone, thus providing visitors to the preserve with the opportunity to learn about the geology they see around them while hiking.
Last Modified: 07/08/2020
Modified by: Matthijs C Van Soest
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