Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.t21d..06m&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #T21D-06
Physics
8010 Fractures And Faults, 8145 Physics Of Magma And Magma Bodies, 8164 Stresses: Crust And Lithosphere, 8434 Magma Migration, 8450 Planetary Volcanism (5480)
Scientific paper
Ascending magma can load the mechanical lithosphere from below, above, or within, depending on the mode of emplacement (e.g., underplating, intrusion, or extrusion). In turn, stresses induced by the flexural response to such loadings can strongly influence magma emplacement modes, with significant implications for the volcanic and tectonic structures evident at the surface. Finite element models of stress and topography for growing volcanoes allow an exploration of the conditions under which given emplacement modes are favored. In growing volcanic edifices welded to the underlying lithosphere, horizontal compressive stresses in the upper lithosphere (induced by flexural subsidence) are transmitted through the welded boundary into the edifice. The resulting stress state in the upper lithosphere and edifice is incompatible with magma ascent to the surface, but instead favors lateral emplacement of magma in sills. Material above inflating sills will experience uplift and relief of horizontal compression, ultimately re-opening the magma ascent path through the upper lithosphere and edifice. The maximum radial extent of a sill (rsill) exerts significant control over surface observables. Sustained sill inflation will produce a slope break, with associated radial extensional stresses (consistent with circumferential fissures or faults), near rsill. The stress state in the zone above the sill edge (i.e., in the upper lithosphere and edifice) favors the emplacement of circumferential dikes, consistent with the presence of arcuate fissures (and flows emanating from them) near the slope break. Despite the high production rate of biogenic sediments in Galápagos waters, such sediments lack the low hydraulic diffusivity required to generate the high pore pressures required for a basal detachment (as abyssal clay sediments likely provide at the Hawaiian Islands). Thus, Galápagos volcanoes have welded basal boundary conditions, and the models described above are appropriate. For a typical Galápagos volcano, a broad, flat summit region and gently sloping lower flanks are separated by a steep (15-35 degrees) mid-flank slope break. Circumferential fissures occupy an annulus between the summit caldera and the slope break, and radial fissures occur on the lower flanks. Alba Patera, a broad volcano on Mars, exhibits a similar association of a mid-flank slope break with circumferential extensional features (fractures and graben), and also has radial fractures associated with dike emplacement on its shallowly sloped lower flanks. Mid-flank tectonic annuli and slope discontinuities are also observed on a number of large volcanoes on Venus. These structural similarities, and their correspondence to the predictions of volcanic loading models that include intra-lithospheric sills, suggest that sill intrusion played important roles in the evolution of the Galápagos Islands, Alba Patera, and volcanoes on Venus. This suggestion is supported by radar interferometric evidence for the emplacement of sills beneath Galápagos edifices.
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