Interactions of Mechanical Controls on Magma Emplacement with the Petrology of Volcanic Edifice - building Flows on

Introduction: Large volcanic edifices on Venus constitute a primary expression of the thermal evolution of that planet. Over 150 volcanic edifices with diameters > 100 km are distributed heterogeneously over the surface of Venus [1-4]. They are characterized by extensive radially oriented lava flow aprons and generally shallow flank slopes. The compositions of the lava flows that build such structures potentially record variations in the physical conditions (depths, temperatures of melting, etc.) that govern magma ascent and eruption. In this abstract, we discuss the petrological implications of a recently discovered link between volcanic edifice shape and magma ascent paths that is modulated by lithospheric flexural stresses [5-6]. This link suggests that under certain circumstances , magma may ascend directly from the mantle melt region to the surface, rather than collecting and differentiating in reservoirs at intermediate depths. Furthermore, considerations of buoyancy [7] and enhanced lithospheric stress [8] at the crust-mantle boundary may lead to enhanced magma storage at that horizon. These mechanically based scenarios predict specific trends in major element ratios that may be detectable by instruments on future missions to Venus. Mechanics of Magma Ascent: The state of stress in the lithosphere can exert a major control on the ascent of magma from mantle melting zones [9]. Two criteria for magma ascent must be satisfied [5-6]: 1) principal stress orientations (least compressive stress horizontal [10]) and 2) gradients of horizontal differential stresses (positive gradients, i.e., horizontal compression decreasing with increasing height [9]). To account for the mitigating effects of buoyancy and overpressurization, small adverse values of horizontal compression and stress gradients are considered to satisfy the ascent criteria. The " dipole " nature of flexural stress dictates that Criterion 1 will be violated in about half of the litho-sphere (in the absence of vertically uniform membrane stress), but for small enough stress magnitudes the mitigating effects will overcome this " half-barrier ". In particular, large values of elastic lithosphere thickness T e (order 40-50 km) create low enough stress magnitudes to allow ascent via Criterion 1 (see Fig. 1). However , as T e decreases, adverse stress magnitudes and stress gradients tend to shut off magma ascent beneath the centers of large loads; preferred ascent paths migrate toward the outer flanks, favoring formation of more domical-shaped edifices. At the lowest values of