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- 1. FROM CONTRACTION TO EXTENSION: TECTONOMAGMATIC ARCHITECTURE OF THE SEVIER OROGENIC WEDGE OF WESTERN MONTANA • Tom Kalakay – Rocky Mountain College • David Foster - University of Florida
- 2. What do I do? Field geology Mapping and Structural Analysis • Geometry • Kinematics • Timing I’m cheap labor… ROCKY GEOLOGY MOUNTAIN COLLEGE ROCKY GEOLOGY MOUNTAIN COLLEGE
- 3. Scientific Motivation What is the net effect of orogenesis on Continental Crust?—e.g. modification of the Wyoming Craton Crustal rheology: composition temperature How much do these two important elements vary in space and time? ROCKY GEOLOGY MOUNTAIN COLLEGE ROCKY GEOLOGY MOUNTAIN COLLEGE
- 4. Tectonic map Major Components of the Cordilleran Orogenic Belt. Contemporaneous variations in composition and thermal conditions along strike From Kalakay, 2001
- 5. Tectonic map What were the main controls on wedge taper (geometry), wedge kinematics, and wedge mechanics? From Kalakay, 2001
- 6. Well-studied Central sector of the Cordileran Orogen Basement culminations along the “hingeline” ~75Ma (DeCelles & Mitra, 1995)
- 7. Sevier orogenic wedge developed large culminations along tapered edge of the Mid-Proterozoic Belt Basin. Belt Supergroup Restored Belt Supergroup From Sears 2005 EarthScope Workshop Northern Rockies— Montana Cordillera
- 8. Basement culminations along the “hingeline” Continue into SW Montana Coryell & Spang, 1988 Kalakay, 2001
- 9. NW Montana/Canada sector of Cordilleran Orogen Culminations— comprised of thick, “strong” sequence of Belt Supergroup. ~75Ma (Fuentes et al, 2012) Augusta synclineSawtooth Range Continental Divide synclineSouth Fork Flathead river valley Eldorado thrust Lewis thrust Hoadley thrust Diversion thrust Home thrust Montana Power Co. State 1 (projected) Projected seismic data Seismic coverage of Figure 7 S.L. 1 2 3 –1 –2 –3 –4 –5 –6
- 10. SW Montana sector of Cordileran Orogen “magmatic” culminations replace basement/Belt culminations ~75Ma (Kalakay, 2001)
- 11. Why did arc magmatism migrate so far east? • Leaky lithosphere? • Inherited “flaws”? • Change in subduction angle? • Role of the Lewis & Clark system? ~80-75Ma
- 12. At ~75Ma— Contrasts in composition and rheology along strike Wait… plutons in the thrust belt? Is that normal? Voluminous silicic magmatism
- 13. Geometry: •tabular bodies •coincide with thrust ramps and flats Kinematics & timing: •some plutons emplaced during thrust movement Plutons in the thrust belt
- 14. Boulder batholith Elkhorn Mtns. volcanic field Very large plutons in the thrust belt
- 15. With more field work, comes new ideas… Meta-Belt—pelites, calc-silicates Cretaceous plutons
- 16. Modified from Lonn et al. K-plutons
- 17. Previous interpretation New(er) interpretation Anaconda metamorphic core complex Extreme transposition within high-grade Belt rocks Anaconda footwall
- 18. •nappe-style folds •attenuated section Anaconda footwall High-T strain is Cretaceous (~74.5 +/- 0.4 Ma)
- 19. Middle Belt carbonate Just add heat
- 20. Thick Belt sequence is orogenically “fertile” Relative to Archean and/or Proterozoic crystalline crust.
- 21. Modified from Rey et al, 2001
- 23. Late-K (75 Ma) Represents major change in rheology Led to ramp collapse • Brittle wedge mechanics don’t really work in some orogenic wedges (do they work for ANY orogenic wedges?)
- 24. the MEGA A pitch for the Eocene
- 25. Eocene stretching lineations—consistent extension @ 100-110°
- 27. Major questions: How much does orogenic wedge or crustal rheologic profile vary in space and time? Must consider variations in composition/lithology Must also consider changes in thermal regime (e.g., magmatic input) Do variations in crustal rheology control variations in thrusting, hinterland thinning, and the transition to extension? Are mantle kinematics coupled with kinematics in deforming continental crust? If so, how? What large-scale phenomenon is responsible for Eocene extension with such consistent kinematics? Do field studies test or help refine the dimensions of plate-scale models (e.g., flat slab subduction, slab break-off, slab windows, etc.)? What the hell is the Lewis and Clark line?
- 31. Augusta synclineSawtooth Range Continental Divide synclineSouth Fork Flathead river valley Eldorado thrust Lewis thrust Hoadley thrust Diversion thrust Home thrust Montana Power Co. State 1 (projected) Projected seismic data Seismic coverage of Figure 7 S.L. 1 2 3 –1 –2 –3 –4 –5 –6 –7 –8 5 10 15 20 km0 From Fuentes et al, 2012
- 32. volcanic units Key elements exposed near Bannack, MT
- 33. basement involved faulting at depth progressive rotation of Beaverhead folds cut by Ermont thrust late-stage plutons follow fault geometry Sequence of events
- 34. pre-magmatic basement duplexing ~79-75 Ma Probably began earlier out of sequence motion on Ermont thrust Bannack stock synkinematic to late-stage emplacement ~73.2 Ma 1 2 3
- 36. pre-magmatic wedge growth basement duplexing out of sequence thrusting magmatic wedge growth extensional collapse Orogenic sequence
- 37. What was going on in the hinterland?
- 39. Metamorphism and deformation in the footwall domain continued… melt pods • No muscovite • Pods of leucocratic melts in low strain zones • Parts of footwall domain passed through the MS-out isograd. suggest that footwall deformation formed during peak metamorphism and anatexis. Boudin neck
- 40. Mr. Ugly: a deformed qtz. diorite sill along a thrust fault within the attenuated section 40Ar/39Ar hb cooling age of 84.12 ± 3.20 Ma. (Grice, 2005) Storm Lake stock: synkinematic overprints quartz diorite bi K-Ar age of 78.7 ± 0.8 Ma (Wallace et al., 1992). 79 Ma. (Grice, 2005) Solid-state deformation of “Mr. Ugly.” Age of high-T deformation in the footwall:
- 41. Interpretations: Mid-crustal ductile flow and deformation was contemporaneous with upper crustal thrusting in western Montana during the late- Cretaceous. Footwall deformation is the result of high T attenuation, mid- crustal collapse of the Sevier orogenic wedge during late-Cretaceous time. The upper and lower crust were potentially decoupled in the late- Cretaceous. Later, Eocene development of the Anaconda mylonite and detachment overprinted the earlier, mid-crustal ductile attenuation now exposed in the footwall of the Anaconda metamorphic core complex.
Notes de l'éditeur
- Tectonic map of the western United States, showing the major components of the Cordilleran orogenic belt. Structural culminations in Archean-Proterozoic crystalline basement (purple) developed along a basement step formed by Neoproterozoic rifting. The initial Sr ratio line is taken to represent the approximate western edge of North American cratonic basement.
- Tectonic map of the western United States, showing the major components of the Cordilleran orogenic belt. Structural culminations in Archean-Proterozoic crystalline basement (purple) developed along a basement step formed by Neoproterozoic rifting. The initial Sr ratio line is taken to represent the approximate western edge of North American cratonic basement.
- Tectonic map of the western United States, showing the major components of the Cordilleran orogenic belt. The initial Sr ratio line is taken to represent the approximate western edge of North American cratonic basement. Structural culminations in Archean-Proterozoic crystalline basement (purple) developed along a basement step formed by Neoproterozoic rifting.
- Tectonic map of the western United States, showing the major components of the Cordilleran orogenic belt. The initial Sr ratio line is taken to represent the approximate western edge of North American cratonic basement. Structural culminations in Archean-Proterozoic crystalline basement (purple) developed along a basement step formed by Neoproterozoic rifting.
- Schematic diagrams illustrating wedge geometry induced by 3 different mechanisms. a) (From Fuentes et al, 2012) The Cordilleran thrust belt of northwestern Montana (United States) has received much less attention than its counterparts in the western interior of USA and Canada. The structure of the thrust belt in this region is well preserved and has not been strongly overprinted by Cenozoic extension. The thrust belt in this region consists of a frontal part of highly deformed Paleozoic, Mesozoic, and Paleocene sedimentary rocks, and a western region dominated by a >15-km-thick succession of Proterozoic Belt Supergroup strata b) In southwest Montana basement duplex structures and Belt culminations were replaced by large volumes of silicic intrusive and volcanic rocks (Schmitt, et al., 1995; Kalakay, 2001; Lageson, et al.,2001). c) Utah and Wyoming, critical taper maintained by uplift of basement culminations coincident with thrust sheets being accreted to the toe (Yonkee, 1992; DeCelles, 1994; DeCelles and Mitra, 1995; Mitra and Sussman, 1997).
- In SW Montana, at ~75 Ma, basement-dominated and Belt-dominated culminations were replaced by silicic plutons, including the large volume Boulder batholith, that invaded the fold and thrust belt at all crustal levels. Late Cretaceous plutons are dominantly laccolithic and particularly localized along thrust ramps. Most intruded along major or local thrust systems and thus inflated the hanging wall sections.
- Major hinterland extension, in the form of core complexes, began around 54-53 Ma (Foster et al., 2001, 2007) and overlapped with ongoing contractile deformation in the foreland. The Eocene Anaconda detachment, which translated the Boulder batholith tens of kilometers eastward, exposes a footwall showing high-temperature deformation, metamorphism, crustal anatexis, and plutonism in the middle crust of the Montana hinterland (Foster et al, 2007). Eocene extensional structures and fabrics are superimposed on hinterland structures that developed much earlier, during Late Cretaceous contraction and metamorphism. Late Cretaceous, high temperature strain is heterogeneously distributed within the footwall of the Anaconda core complex, but extreme tectonic attenuation of Middle Proterozoic Belt stratigraphy dominates. The age of this attenuation is estimated to be ~74.5 +/- 0.4 Ma (Foster, unpublished data) as constrained by the syn-kinematic Storm Lake pluton.
- Here’s a map, modified from the Philipsburg map, showing some of the footwall features. Doming Attenuated strata and older plutons
- Eocene extensional structures and fabrics are superimposed on hinterland structures that developed much earlier, during Late Cretaceous contraction and metamorphism. Late Cretaceous, high temperature strain is heterogeneously distributed within the footwall of the Anaconda core complex, but extreme tectonic attenuation of Middle Proterozoic Belt stratigraphy dominates. The age of this attenuation is estimated to be ~74.5 +/- 0.4 Ma (Foster, unpublished data) as constrained by the syn-kinematic Storm Lake pluton.
- Eocene extensional structures and fabrics are superimposed on hinterland structures that developed much earlier, during Late Cretaceous contraction and metamorphism. Late Cretaceous, high temperature strain is heterogeneously distributed within the footwall of the Anaconda core complex, but extreme tectonic attenuation of Middle Proterozoic Belt stratigraphy dominates. The age of this attenuation is estimated to be ~74.5 +/- 0.4 Ma (Foster, unpublished data) as constrained by the syn-kinematic Storm Lake pluton.
- The period of mid-crustal thinning described here appears to have been long-lived and coupled to continuous contraction in the overlying brittle upper crust. In the Flint Creek range (not shown), the transition from middle to upper crust is marked by west-verging recumbent folds. We interpret this apparent back-folding to be the result of underflow of ductile middle crust beneath a brittle upper crustal layer We interpret the tectonic history as follows: 1) early tectonic wedge growth through basement culminations 2) construction of a “magmatic culmination” in conjunction attenuation and lateral mid-crustal flow and continued contraction along the thrust front 3) Eocene detachment-style extension possibly related to changes in plate configurations at the continental margin.
- The period of mid-crustal thinning described here appears to have been long-lived and coupled to continuous contraction in the overlying brittle upper crust. In the Flint Creek range (not shown), the transition from middle to upper crust is marked by west-verging recumbent folds. We interpret this apparent back-folding to be the result of underflow of ductile middle crust beneath a brittle upper crustal layer.
- This is a Google Earth image looking north back toward the basement cored Armstead anticline. Here you can see the key elements exposed near Bannack. -The Madigan Gulch anitcline (northern extension of the Armstead anticline) -Synorogenic conglomerates of the Beaverhead, progressively rotated on the east limb of the anitcline. -Volcanic rocks that stratigraphically lie at the bottom and at the top of the Beaverhead sediments. -The out-of-sequence Ermont thrust that cuts both the anticline and overrides the rotated Beaverhead. -The Bannack granodiorite pluton that intruded along the Ermont thrust.
- Here’s a cross section that shows essentially the same thing as we saw in the Google Earth image. Here, however, we can see the interpretation of imbricated basement at depth. From this slide, we can put together a sequence of events based on cross-cutting relationships. 1. Basement imbrication along a thrust ramp. 2. Folding above the basement stack. 3. Progressive deposition and rotation of the Beaverhead and volcanics on the flank of the fold. 4. Out of sequence thrusting associated with the Ermont thrust. 5. Late stage emplacement of the Bannack plutons along existing faults.
- Another summary of the sequence.
- The McCartney Mtn. pluton and nearby structures epitomize the Geology of western Montana. -Well exposed folds and thrust… even folded thrusts. -A granodiorite pluton intruded along a thrust ramp in the system. -And last but not least, a low angle normal fault or detachment that cuts and overprints the earlier formed structures. Recognizing these younger extension –related features anytime you work in western Montana.
- More summary: A contrast in wedge growth processes… Premagmatic… thin skinned thrusting and formation of basement culminations along ramps. Belt rocks are involved in such culminations elsewhere. Sometime around 75 Ma, significant volumes of magma were intruded along the ramps and thereby replacing the basement (or otherwise) culminations that had developed. By the Eocene, the orogenic wedge was in demise and collapsing via regional crustal-scale extension.
- To look at the link between thrusting, magmatism and eventual collapse, you need to find deeper exposures. In Western Montana, those exposures exist in the footwalls of metamorphic core complexes.
- A photo looking west up Mill Creek, showing the east dipping anaconda mylonite zone and footwall underneath.
- Footwall deformation occurred at high temperature—upper amphibolite to granulite facies metamorphism. The lack of muscovite and numerous leucocratic pods indicate the rocks were undergoing partial melting. Leucosomes concentrated in low strain zones such as boudin necks are evidence that melting and deformation were contemporaneous.