Three Hotspot Reference Frames – Shallow and Lithospherically Controlled – Reflect “Mesoplates”, a Plate Tectonics Heuristic
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Three Hotspot Reference Frames – Shallow and Lithospherically Controlled – Reflect “Mesoplates”, a Plate Tectonics Heuristic Submitted to: Penrose Conference- Plume IV: Beyond the Plume Hypothesis, Iceland, 2003 May 1, 2003 (Not to be circulated beyond the organizing committee) Rex H. Pilger, Jr. Landmark Graphics Corporation 1805 Shea Center Drive Suite 400 Highlands Ranch, Colorado 80129 United States of America Extended Abstract The hotspot reference frames of the Atlantic-Indian Ocean and the Pacific Ocean are kinematically distinct. Relative motion of the two reference frames in the Late Cretaceous and Early Tertiary corresponds with motion of the American plates in the Atlantic-Indian Ocean frame, suggesting a lithospheric control on the reference frames. A third reference frame, beneath Iceland and Eurasia, cannot be satisfactorily quantified at this time, but the inability to fit Iceland in either of the two other reference frames implies its existence. The two hotspot reference frames can be shown to be relatively shallow. Hotspot traces, cross- grain oceanic gravity lineations of the Pacific Ocean reflect a shallow hotspot reference frame. Intracontinental stress fields of North America correspond with the Atlantic-Indian Ocean reference frame, also reflecting a shallow reference frame. Pacific evidence: Minor hotspot island-seamount chains of the Pacific plate (Figure 1) are controlled by lithospheric structure. Almost every such trace appears to originate on the south side of a fracture zone separating older lithosphere on the north from younger lithosphere on the south. Assuming that older plate is thicker than younger plate, passage of a fracture zone separating lithosphere of contrasting age and thickness over the asthenosphere and deeper mesosphere would result in isostatic rise of affected portions of both layers. Isostatic rise results in enhanced partial decompression melting and plate-penetrating magmatism. Further, relative volume of volcanism along the Hawaiian island seamount chain is controlled by lithospheric structure. Increased volcanism occurs to the south of older-on-north fracture zones, while reduced volcanism is observed to the south of younger-on-north fracture zones. Similarly, cross-grain gravity lineations in the Pacific (Figure 2) appear to be controlled by lithospheric structure; lineations occur on the south side of older-on-north fracture zones. Plate regions to the south of younger-on-north fracture zones do not appear to have cross-grain lineations. Atlantic-Indian evidence: Intracontinental contemporary and paleostress fields within North America (Figures 3-4), especially, and possibly Africa and South America, largely correspond with kinematic models of plate motions in the Atlantic-Indian Ocean hotspot reference frame. Because the stress fields reflect intra-lithospheric and lithospheric-asthenospheric-mesospheric interaction, this implies that the hotspot reference frame is similarly shallow. The minor hotspot factor observed in the Pacific cannot be documented in the Atlantic-Indian hotspot set since relative plate motions tend to parallel hotspot-reference frame motions except for very slowly moving plates (in the hotspot frame). Reference frame interactions: Plate reconstructions indicate that Late Cretaceous and Tertiary motion of the Pacific Ocean hotspot set parallels that of North America in the Atlantic-Indian reference frame (Figure 5). Low angle subduction beneath the Cordillera filled the gap. Subduction boundaries may correspond with hotspot reference frame boundaries. It is proposed that the three hotspot reference frames be identified with distinct plate-like layers termed “mesoplates” (Figures 6-7). The top of each mesoplate corresponds with the asthenosphere-mesosphere boundary; the base is as yet undefined – perhaps seismic tomography will help define it. The Pacific hotspot reference frame corresponds with the “Hawaiian” mesoplate. The Atlantic-Indian Ocean reference frame corresponds with the “Tristan” mesoplate. The possible reference frame beneath Eurasia and northeastern North America is the “Iceland” mesoplate. Where present, subduction zones are the primary boundary separating mesoplates. Other boundaries between mesoplates could well be diffuse, as the apparent “rigidity” of mesoplates is entirely a passive feature – there is little internal deformation of mesoplates because they are not subjected to significant stresses. Mesoplates are significantly larger than lithospheric plates (“lithoplates”). Divergent lithoplate boundaries (spreading centers) are fed by vertical movement of asthenosphere and mesosphere, with conversion of mesoplate to asthenosphere. Lateral movement of mantle material to accommodate vertical movement of mesoplate occurs beneath mesoplates. Geodynamic modeling needs to incorporate the kinematic constraints of hotspot traces and intracontinental stresses into convection parameterizations. Mesoplates are a heuristic device (like lithoplates) that serve as a placeholder until geodynamic models that reproduce observed plate kinematics, stress fields, and anomalous volcanism are successfully generated. Figure 1. Minor hotspot traces of the South Pacific. Gray scale bathymetry from Sandwell and Smith (1997; Sandwell website, 2003); locations of isotopic ages shown by solid diamonds (compilation of Pilger, 2003a); plate isochrons and fracture zones (older-on-north fracture zones are shown as thick lines) from Müller et al. (1997); hotspot loci (connect open circles) from Gripp and Gordon (1991) and Raymond et al. (2000) as interpolated by Pilger (2003a); predicted seamount locations (open diamonds) from Wessel and Smith (1997; Wessel website, 2003). Figure 2. Gravity lineations, eastern Pacific ocean, from satellite observations (Sandwell, 2003), together with magnetic isochrons (black lines; Muller et al., 1995) and calculated hotspot loci (white lines with nodes at 5 m.y. intervals; Raymond et al., 2000; Pilger 2003a). Plate 2 is color version. Figure 3. Contemporary maximum horizontal principal compressive stress (?1) for North America, together with flowlines of North American plate motion in the hotspot reference frame (Gripp and Gordon, 1991). Figure 4. Example of maximum horizontal principal compressive paleostress (?1) for North America (45 to 50 Ma) with flowlines (45 Ma is thin; 50 Ma is thick) of North American plate motion in the hotspot reference frame (Müller et al., 1993; Pilger 2003a). Figure 5. Hotspot frame loci with nodes at 5 m.y. intervals, western North America. Southern loci anchored at proposed Raton and Yellowstone hotspots. Solid circles: North America in Tristan hotspot reference frame. Solid diamonds: North America in Hawaiian hotspot reference frame. Open squares: Hawaiian hotspots in Tristan reference frame. Note subparallelism of loci between ~25 and ~80 Ma. Tristan hotspot parameters from Muller et al. (1993) interpolated by Pilger (2003a); Hawaiian hotspot parameters from Raymond et al. (2000) and Norton (2000) interpolated by Pilger (2003a). Figure 6. Postulated mesoplates with approximate boundaries. Boundaries partially correspond with lithoplate subduction boundaries. Figure 7. Cartoon cross section illustrating mesoplate concept. Subduction boundaries separate mesoplates. Thin asthenosphere separates mesoplates and lithoplates. .