Insight from thermochronology, magnetotellurics, and structural analysis into the tectonic evolution of the Ozark Dome-Illinois Basin region, Midcontinent USA

DeLucia, Michael Sanderson

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Description

Title

Insight from thermochronology, magnetotellurics, and structural analysis into the tectonic evolution of the Ozark Dome-Illinois Basin region, Midcontinent USA

Author(s)

DeLucia, Michael Sanderson

Issue Date

2020-07-14

Director of Research (if dissertation) or Advisor (if thesis)

Marshak, Stephen

Doctoral Committee Chair(s)

Marshak, Stephen

Committee Member(s)

• Guenthner, William R
• Anders, Alison M
• Best, James L
• Christie, Max

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Midcontinent
• Tectonics
• Structural Geology
• Low-Temperature Thermochronology
• Magnetotelluric imaging
• Great Unconformity
• Ozark Plateau
• Ozark Dome
• Illinois Basin

Abstract

The north-central Midcontinent of the United States lies within the cratonic platform of North America, a geological province characterized by a thin veneer of Phanerozoic sedimentary cover strata overlaying Precambrian basement. (The boundary between the cover and the basement is the Great Unconformity.) This region includes both the vast plains of Illinois, which display barely any relief, as well as the Ozark Plateau, an area of elevated land (up to 600 m above sea level) dissected by streams. This plateau is distinctive from other, better known, plateaus in the world (e.g., the Colorado Plateau, the Tibetan Plateau, and the Altiplano) in that it lies far from any plate boundaries or active collisional orogenic belts. Geologically, the Illinois Basin (IB) underlies the plains of Illinois. Here, Phanerozoic strata are warped downward, and strata thicken substantially toward the center of the basin, indicating that subsidence was syntectonic. The Ozark Dome (OD) underlies the Ozark Plateau. Here, cover strata are relatively thin, and thicken in all directions away from the apex of the dome. The oldest rocks of the dome (~1.47 Ga granite and rhyolite) crop out in the St. Francois Mountains at the northeast corner of the Ozark Plateau. Here, the Great Unconformity is exposed at an elevation of about 550 m. This unconformity occurs as much 7.5 km lower in the adjacent Illinois Basin. Several first-order geological questions about the Ozark Dome and Illinois Basin, and the boundary between them, have not yet been definitively answered, including: When and how did the Great Unconformity form in a cratonic interior? How did the IB/OD boundary become established and why is there so much structural relief across it, as manifested by depth changes in the Great Unconformity? What is the surface manifestation of the IB/OD boundary, and does it correlate with deeper structures? What is the character of fault-and-fold zones (both at the surface and at depth in the lithosphere) in the Illinois Basin and Ozark dome region, and how are they related to the IB/OD boundary? Why does the topographic signature of the Ozark Plateau persist today? This dissertation approaches these questions, both back in time and down in crust, using an interdisciplinary approach. Chapter 1 applies thermochronologic-derived constraints on the timing and amount of exhumation that produced the Great Unconformity. Results suggests that continent-wide exhumation (of 4 - 6 km), related to the supercontinent cycle, took place between 850 and 680 Ma which produced this distinct erosional horizon. This uplift may reflect the isostatic response to delamination of dense lithospheric mantle, made possible when the asthenosphere warms and weakens sufficiently to allow delamination to take place. Chapter 2 applies magnetotelluric (MT) imaging (a method of measuring subsurface electrical conductivity) to characterize the lithosphere-scale structures in the Midcontinent region, providing insight into the nature of faulting at depth. Specifically, modeling of data from the EarthScope MT array delineates a steeply dipping zone of NW-trending high-conductivity, named the “Missouri High-Conductivity Belt” (MCHB), can be traced across the Moho and into the lithospheric mantle. Notably, this belt parallels one of the major fault sets of the Midcontinent, aligns with NW-trending magnetic anomalies, and coincides with the extensive, yet cryptic, NW-trending Missouri Gravity low. If such high-conductivity zones represent graphite- and/or sulfide-infused shear zones, then their existence suggest that at least some of the NW-trending fault zones of the Midcontinent are trans-lithospheric, and thus may have originated as Proterozoic transform faults. A study focused on the IB/OD boundary (Chapter 3), incorporating new field structural analysis and the production of a high-resolution structure-contour map, together with a review of literature concerning Midcontinent fault-and-fold zones, geophysical anomalies, and EarthScope results concerning Moho depth and earthquake distribution, emphasizes that the IB/OD boundary has a long and complex tectonic history. It is manifested at the ground surface by a topographic step as well as by structurally controlled reaches of the Mississippi River Valley. These features coincide with the trace of the Ste. Genevieve fault and the nearby Eureka-House-Springs Valmeyer structure (EHSVS), both of which were subject to multiple reactivation events, with varying kinematics. Significantly, the ground-surface manifestation of the IB/OD boundary overlies a ~5 km-high step in the Great Unconformity as well as a 15 to 20 km-high step in the Moho, and it coincides with a distinct NW-trending seismic belt. These features suggest that the IB/OD boundary may be a trans-crustal discontinuity. In fact, by analogy with the MCHB, the IB/OD boundary may, speculatively, be a trans-lithospheric discontinuity. Conceivably, it initiated as a transform fault associated with Proterozoic rifting, that may have been localized by the Moho step. Subsequently, it has behaved as a weak zone, reactivated at times when Paleozoic orogenic activity on the margin of North America transmitted stress into the continental interior. It remains a weak zone today delineated by contemporary seismicity. Taken together, the studies described in this dissertation emphasize that the Ozark Dome-Illinois Basin region of the Midcontinent provides insight into long-term tectonic evolution of a cratonic platform. Timing of events in the Midcontinent support models in which stresses from marginal orogenies reactivate long-lived crustal, and possibly trans-lithospheric weaknesses.

Graduation Semester

2020-08

Type of Resource

Thesis

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Copyright 2020 Michael DeLucia

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