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2004 Denver Annual Meeting (November 7– 10, 2004) Paper No. 217-10 Presentation Time: 8:00 AM-12:00 PM (Wednesday) CORDILLERAN TECTONICS: THE ROLES OF LITHOPLATE AND MESOPLATE BOUNDARIES PILGER, Rex H. Landmark Graphics Corporation 1805 Shea Center Drive, Suite 400 Highlands Ranch, Colorado 80004 USA [email protected] CORDILLERAN TECTONICS: THE ROLES OF LITHOPLATE AND MESOPLATE BOUNDARIES Mesoplates are a new heuristic for quantifying the motion of lithoplates relative to the mesosphere. The three major mesoplates are Hawaiian (primarily beneath the oceanic plates of the Pacific), Tristan (beneath most of the plates of the Atlantic and Indian Oceans), and Icelandic (beneath Eurasia, the northernmost Atlantic, Arctic Ocean). While lithospheric plate interaction is the principal control on mountain belt evolution, mesoplate interactions also contribute to the overall “style” of deformation. The principal evidence for mesoplates is distinctiveness of hotspot reference frames for each of the three regions (Hawaiian, Tristan, and Icelandic). While hotspots within each frame show very little relative motion, the three frames demonstrate significant relative motion among themselves over the past 130 to 80 my. Shallowness of the reference frames is indicated by correspondence of stress fields with motions in the hotspot frames, as well as lithospheric thickness controls on the origin of minor hotspot traces in the Pacific and South Atlantic. Because the three reference frames are shallow, interaction with subduction zones is implied. Continuity of deep subduction zones implies that the zones are the probable boundaries between adjacent mesoplates. Where medium-to-deep subduction zones are absent, the boundaries between mesoplates are likely determined by kinematics. For the North American Laramide (~80 to 48Ma), lithoplate and mesoplate kinematics imply North America moved to the southwest relative to Tristan, while motion between the Hawaiian and Tristan mesoplates was divergent, and relative motion of North America and the Hawaiian mesoplate was minimal. The growing “gap” between the two mesoplates was “filled” by the lowangle subducting Farallon (or Kula) plate. Pacific plate motion at ~48 Ma (Hawaiian-Emperor bend) changed relative to the Hawaiian mesoplate and North America. Concurrently, the boundary between the Hawaiian and Tristan mesoplate was gradually unconstrained as younger plate began to be subducted, and then subduction progressively ceased. Thus, the mesoplate boundary began to move beneath western North America, contributing to the complex extensional tectonics and uplift of the western United States in the Late Cenozoic. 2 What are Mesoplates? Mesoplates and overlying lithoplates are separated by the asthenosphere. The base of mesoplates may correspond with either the 410 or 600 km discontinuity. 3 What are Mesoplates? Mesoplates: Kinematically rigid layers in the upper mesosphere. Three are defined: Hawaiian, Tristan, Icelandic. Boundaries are based on hotspot sets, intracontinental stress field coherence, and subduction zones. Original mesoplate boundaries in blue (Pilger, 2003). Recently revised in green. 4 Evidence for Mesoplates (1): North American lithoplate and Hawaiian mesoplate moved in concert 75-25 Ma. Western boundary of North American lithoplate (bold) and Hawaiian mesoplate (light) reconstructed to Tristan mesoplate (Atlantic-Indian Ocean hotspot set). Motion of North American lithoplate and Hawaiian mesoplate may be correlated. 5 Evidence for Mesoplates (1): Myth: Hawaiian-Emperor bend has no kinematic correspondence in relative plate motions. (Bend age is 47-48 Ma.) Note bends (~47 Ma) in loci of North and South American lithoplates and Hawaiian hotspot (mesoplate) relative to Pacific lithoplate. Pacific Loci @ Hawaii 50 Latitude (deg) 40 Data PC-EU PC-NA PC-SA PC-AN PC-AU PC-NZ PC-FA PC-HW 30 20 10 -240 -210 -180 -150 Longitude (deg) Hawaiian-Emperor bend is kinematically significant. 6 Note parallelism of North American lithoplate (circles) and Hawaiian mesoplate (squares) loci relative to Tristan mesoplate. Note lesser motion of North America relative to Hawaiian mesoplate. 7 Imagining the North American Cordillera and adjacent Pacific in the Early Cenozoic. 8 Evidence for Mesoplates (3): Isotopic ages of basalts, western United States. Contemporary and paleostresses (s1): colored bars. Loci of North American lithoplate motion relative to Hawaiian (diamonds) and Tristan (circles) mesoplates, anchored at “hotspots”. 9 Isotopic ages of rhyolites, western United States. Contemporary and paleostresses (s1): colored bars. Loci of North American lithoplate motion relative to Hawaiian (diamonds) and Tristan (circles) mesoplates, anchored at “hotspots”. 10 Basalts appear to reflect both hotspots and Basin Range extension. Rhyolites seem to have less relation to hotspots. North American-Hawaiian locus fits Yellowstone better. North AmericanTristan locus better fits Raton. 11 Yellowstone 25 15 NOAM-HAWA NOAM-TRIS Basalt Rhyolite 10 Loci of North American lithoplate relative to Hawaiian (blue) and Tristan (lavender) mesoplates. 5 0 0 5 10 Raton Distance (deg) 25 Volcanic and stress patterns imply that Yellowstone hotspot has been embedded within Hawaiian mesoplate since 25 Ma; Raton hotspot is within Tristan mesoplate. Late Cenozoic Mesoplate boundary corresponds with tectonic boundaries of the Cordillera. 20 Age (Ma) Age (Ma) 20 Distance from Yellowstone (left) and Raton (right) versus isotopic ages from basalts and rhyolites and loci. Inception of activity was followed by persistence almost to the present. 15 NOAM-HAWA NOAM-TRIS Basalt Rhyolite 10 5 0 0 5 Distance (deg) 10 12 Evidence for Mesoplates (4): 0-10 Ma 10-20 Ma Bars: s1. Small circles: Instantaneous flow-lines. Color is age-coded. 13 20-30 Ma 30-40 Ma Bars: s1. Small circles: Instantaneous flow-lines. Color is age-coded. 14 40-50 Ma 50-60 Ma Bars: s1. Small circles: Instantaneous flow-lines. Color is age-coded. 15 60-70 Ma 70-80 Ma Bars: s1. Small circles: Instantaneous flow-lines. Color is age-coded. 16 80-90 Ma Bars: s1. Small circles: Instantaneous flow-lines. Color is age-coded. Contemporary and paleo- stresses (s1) within stable (cratonic) North America are parallel with motion of North American lithoplate relative to Tristan mesoplate. Intraplate stress fields reflect interaction with thin asthenosphere and underlying mesosphere. 17 Correspondence with Tristan hotspot frame implies hotspots are embedded within kinematically rigid upper mesosphere (mesoplate). Contemporary Hawaiian-Tristan boundary beneath western United States corresponds with boundary between stable continental interior and actively extending Cordillera. 18 Key References: Mϋller et al. (1993) Geology, 21, 275-278. Pilger (2003) Geokinematics: Prelude to Geodynamics, Springer-Verlag. Raymond et al. (2000) AGU Monograph 121, 359-375. Zartman et al. (1995) USGS DDS 14. 19