Magma supply and storage in volcanic systems: Shallow crustal emplacement processes and causes of the large axial high along the western Galapagos Spreading Center, and, Relation of earthquakes to tectonic and magmatic features near Lassen Peak, northern California.

摘要

Magma storage and supply is investigated in two different tectonic environments: the Galapagos Spreading Center (GSC, a plume-influenced mid-ocean ridge, and Lassen Peak, a subduction-related volcano. Along the GSC multi-channel seismic reflection data are used to infer crustal accretion processes and forward modeling is used to investigate causes of the axial high. At Lassen Peak, catalog earthquakes are relocated using the double-difference method and resulting locations are examined.; Moving westward away from the hotspot along the GSC the magma lens deepens, layer 2A thickens, and the axial high rapidly disappears near 92.7°W. Increasing layer 2A thickness and magma lens depth support the interpretation of layer 2A as the extrusive volcanic layer with thickness controlled by pressure on the magma lens and its ability to push magma to the surface. Off-axis thickening of layer 2A east of 94.0°W suggests narrower magma lenses focus diking close to the ridge axis such that lava flowing away from the axis blankets older flows thickening the extrusive crust off-axis.; Causes of the GSC axial high are investigated using a model that determines the flexural response of the lithosphere to loads resulting from the thermal and magmatic structure. Results reveal that the large axial high requires either that the crust below the magma lens contains a lot of melt (≥35%), or that melt extends into the mantle in a narrow region beneath the axis. Less melt is required for a profile to the west where the axial high is smaller (like the East Pacific Rise).; Earthquake relocation at Lassen Peak shows focusing of events into three clusters 4--6 km beneath the south flank of the volcano. These clusters may be related to movement of magmatic and hydrothermal fluids and may mark the top of a region of hot crust overlying a small magma chamber. Just north of Manzanita Creek (∼14 km northwest of Lassen Peak) is a linear set of earthquakes not corresponding to any mapped faults. A single basaltic vent at the eastern end of this feature indicates magma may have used this weak zone in the crust to make its way to the surface.