T.A. Dumitru
E. L. Miller
C.F.I. Savage
J. L. Wooden
A.E. Egger
2003
<p><span>The </span>Grouse<span> </span>Creek<span>-Albion-Raft River metamorphic core complex </span>in<span> northwestern </span>Utah<span> and southern Idaho is characterized by several </span>Tertiary<span> plutons with a range of ages and crosscutting relations that help constrain the </span>timing<span> of extensional deformation. </span>In<span> the </span>Grouse<span> </span>Creek<span> </span>Mountains<span>, at least three distinct, superimposed, </span>extension<span>-related </span>Tertiary<span> deformational events are bracketed by intrusive rocks, followed by a fourth event: motion on range-bounding faults. The Emigrant Pass plutonic complex was emplaced at depths of less than 10 km into Permianage rocks. SHRIMP U-Pb zircon analysis indicates a three-stage intrusion of the complex at 41.3 ± 0.3 Ma, 36.1 ± 0.2 Ma, and 34.3 ± 0.3 Ma. The two youngest phases represent distinctly younger intrusive event(s) than the oldest phase, separated by more than 5 m.y. The oldest phase cuts several metamorphosed and deformed younger-on-older faults, providing a pre-41 Ma age bracket for oldest </span>extension<span>-related deformation </span>in<span> the region. The youngest phase(s) are interpreted to have been intruded during development of a map-scale, N-S-trending recumbent fold, the Bovine </span>Mountain<span> fold, formed during vertical shortening of roof rocks during intrusion. This second event folded older normal faults that are likely pre-41 Ma. Zircons from the youngest part of the pluton show inheritance from Archean basement (~2.5 Ga) and from its Proterozoic sedimentary cover (~1.65 Ga). The Red Butte pluton, emplaced at 15-20 km depth, intruded highly metamorphosed Archean orthogneiss at 25.3 ± 0.5 Ma; cores of some zircons yield latest Archean ages of 2.55 Ga. The pluton is interpreted to have been intruded during a third deformational and metamorphic event that resulted </span>in<span> vertical flattening fabrics formed during NW to EW stretching, ultimately leading to thinning of cover and top-to-the west motion on the Ingham Pass fault. The Ingham Pass fault represents an important structure </span>in<span> the </span>Grouse<span> </span>Creek<span> </span>Mountains<span>, as it juxtaposes two parts of the crust that apparently resided as much as 10 km apart (</span>in<span> depth) at times as young as the Miocene. The varied structural, metamorphic, and intrusive relations observed </span>in<span> the </span>Grouse<span> </span>Creek<span> </span>Mountains<span> reflect their formation at different levels within the crust. Data from these various levels argue that </span>plutonism<span> has been a key mechanism for transferring heat into the middle and upper crust, and localizing strain during regional </span>extension<span>. Interestingly, events documented here correlate </span>in<span> a broad way with cooling events documented </span>in<span> the Raft River </span>Mountains<span>, although plutons are not exposed there. Major and trace element geochemistry imply a crustal component </span>in<span> all of the studied plutons, indicating significant degrees of crustal melting at depth during </span>extension<span>, and point to mantle heat sources during the timespan of Basin and Range </span>extension<span> as the cause of melting. Basin and Range faulting and final uplift of the range is recorded by apatite fission track ages, averaging 13.4 Ma, and deposition of about 2 km of syn-faulting basin fill deposits along the </span>Grouse<span> </span>Creek<span> fault mapped along the western flank of the range. Similar apatite ages from the Albion </span>Mountains<span> to the north indicate that the western side of the Albion-Raft River-</span>Grouse<span> </span>Creek<span> core complex behaved as a single rigid crustal block at this time.</span></p>
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10.2747/0020-6814.45.6.497
en
Taylor & Francis
Timing and nature of tertiary plutonism and extension in the Grouse Creek Mountains, Utah
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