Steven B Shirey

 
 

A fundamental part of the Wilson cycle of plate tectonics is the break-up of continents by rifting. In this setting, there is a unique interplay between magmatic sources that may reside in the passively upwelling asthenospheric mantle, the dynamically rising plume mantle and the rigid subcontinental lithospheric mantle. Although not all rifts go to completion as is evident in the 1100 million year old Midcontinent Rift System of the United States (left), they can still illuminate the architecture of the deep continent.


Proposed geologic terrane map of Precambrian basement rocks in the northern U.S. continental interior. WRB: Wolf River batholith. Underlying gray-toned base map is the newly compiled regional aeromagnetic anomaly map  “Craton margin domain” represents sedimentary and volcanic rocks deposited during the interval 2.3–1.77 Ga; stippled pattern represents area affected by Penokean deformation; cross-hatched pattern represents area termed ‘gneiss dome corridor’ which was affected by Yavapai-interval deformation (Schneider et al., 2004). GIPB: Green Island plutonic belt; BS: Baraboo syncline. Figure and caption from Holm et al., Pre-C Res., 2007.

Nelson, W. R., Furman, T., van Keken, P. E., Shirey, S. B., and Hanan, B. B., 2012, Os-Hf isotopic insight into mantle plume dynamics beneath the East African Rift System. Chemical Geology, 320-321, 66–79.

Ding, X., Ripley, E.M., Shirey, S.B., and Li, C., 2012, Os, Nd, O, and S isotopic constraints on country rock contamination in the conduit-related Eagle Cu-Ni-(PGE) deposit, Midcontinent Rift System, Upper Michigan: Geochimica et Cosmochimica Acta, v. 89, p. 10-30.

Shirey, S.B., 1997, Re-Os isotopic compositions of Midcontinent Rift System picrites; implications for plume-lithosphere interaction and enriched mantle sources: Canadian Journal of Earth Sciences, v. 34, p. 489-503.

Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent Rift System basalts; implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504-520.

Hart, W.K., Carlson, R.W., and Shirey, S.B., 1997, Radiogenic Os in primitive basalts from the northwestern U.S.A.; implications for petrogenesis: Earth and Planetary Science Letters, v. 150, p. 103-116.

Nicholson, S.W., and Shirey, S.B., 1992, Nd and Pb isotopic evolution of basalts of the 1.1 Ga Midcontinent Rift; evidence for a region-wide model for plume-lithosphere-asthenosphere interaction, in Ayuso Robert, A., and Schultz Klaus, J., eds., Informal notes; workshop on the Application of isotope systems to geological problems.: Open-File Report - U. S. Geological Survey: Reston, VA, United States, U. S. Geological Survey, p. 155-161.

Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent Rift system: Tectonophysics, v. 213, p. 33-40.

Ellam, R.M., Carlson, R.W., and Shirey, S.B., 1992, Evidence from Re-Os isotopes for plume-lithosphere mixing in Karoo flood basalt genesis: Nature, v. 359, p. 718-721.

Klewin, K.W., Berg, J.H., Shirey, S.B., Carlson, R.W., and Anonymous, 1991, Isotopic and trace element evidence for time progressive changes in the source region of flood basalts: Eos, Transactions, American Geophysical Union, v. 72, p. 280.

Nicholson, S.W., and Shirey, S.B., 1990, Midcontinent Rift volcanism in the Lake Superior region; Sr, Nd, and Pb isotopic evidence for a mantle plume origin: Journal of Geophysical Research, v. 95, p. 10,851-10,868.

Relevant Publications

Midcontinent Rift System (MRS) of the central United States

East-African rift system in Kenya and Tanzania

The 1100 million year old, Midcontinent Rift System magmatism is important as prime example of one of the largest failed rifts in the world, one the thickest accumulations of volcanic rocks and derived sediments (25km), and an important source of Cu and other base metals. This arcuate magmatic province cuts across 1.1 to 3.6 billlion year-old blocks of continental crust and lithosphere (left) and contains volcanic rocks with well preserved volcanic features in certain regions (below).

Current work on rift-related magmatism is going on with the work of  DTM postdoc, Wendy Nelson. The goal of her project is to look at 20 to 30 million year old pre-rifting volcanic rocks that will help pin down the mantle melting processes that closely precede the initiation of rifting. This one of the few volcanic provinces in the worlds where a magmatic time-line prior to rifting exists. The work is comparing volcanic rocks from the Afar Rift, the Main Ethiopian Rift and the northern part of the Eastern Rift.

Geochemical work on MRS rocks was spurred on the GLIMPCE (Great Lakes Mutidisciplinary Program on Crustal Evolution) project which focussed on the deep structure of the rift system and its growth faults and was a precursor to Lithoprobe and EARTHSCOPE. In the context of the thinking of the 1990, a plume model was widely advocated for the rift magmatism. The main evidence for the plume model is substantial uplift, continental extension (2X), prolific volcanism, the isotopic and trace element composition of the lavas (right) and their chemostratigraphy. The main evidence against the plume model is the lack of a plume track.

Ropy pahoehoe lava flow (left) and 1.5 meter diameter lava tube in the Mamainse Point area exposed during the exceptionally low water stand in lake Superior in 2007.

Variation in epsilonNd(1100) vs Ce/Yb and  epsilonNd vs gammaOs for basal S-W transitional basalts and Mamainse Point Formation lavas and postulated mantle reservoirs. Figure from Shirey (1997).

A new aspect of MRS magmatism has emerged with the discovery of Voisey’s Bay type magmatic massive sulfide flow-through deposits such as the Eagle Deposit (left). These systems not only provide a way to study ore-concentrating mechanisms but access magmatic processes involving country rock and deeper levels in the system.

 

Geologic map of the midcontinent showing the location of the Eagle Deposit (red star). Figure from Ding, 2010, PhD dissertation and manuscript in preparation.

 

Seismotectonic setting of the East African Rift. A portion of Fig. 1 from Stamps et al., 2008.