MECHANISMS AND RATES OF NON TECTONICALLY-ASSISTED MAGMA EMPLACEMENT IN THE UPPER-CRUST: THE BLACK MESA BYSMALITH, HENRY MOUNTAINS, UTAH

A new study of Black Mesa bysmalith (Henry Mountains, Colorado Plateau, Utah) by the investigation of the structure and geometry of the intrusion and wallrocks, shows that it constitutes a classic example of an upper-crustal small pluton assembled over a few years by incremental amalgamation of small magma pulses. The geometry of the internal fabric pattern supports construction of the intrusion by injection of new magma batches from below, by under-accretion. Our data are in general agreement with a published construction model that involves a sill – laccolith – bysmalith succession, mainly based on wallrock geometry, but we question the existence of a true laccolithic phase (simultaneous vertical and lateral growth). Rather, our data support a new growth mechanism, the “sheeted emplacement model”, or sheeted laccolith model, which explains pluton growth through vertical stacking of sill-like magma sheets. Moreover, the association of an apparent asymmetry in pluton geometry and construction with a symmetrical internal fabric within the pluton shows that bending and lifting could have occurred simultaneously, and not necessarily successively. Consequently, the bysmalith stage is not necessarily the record of an increase of the infilling rate, but could be simply the expression of a greater volume of magma intruded in the eastern part of the pluton. One-dimensional thermal models of the pluton were constructed to provide maximum limits on the duration of its growth. We have constrained the number, thickness, and frequency of magma pulses with our structural observations, including: 1) the construction of the pluton by under-accretion of successive magma pulses, 2) the absence of solid-state deformation textures at internal contacts, and 3) the apparent absence of significant recrystallization in the wallrocks. We propose that the emplacement of the Black Mesa bysmalith was a geologically extremely rapid event, with a maximum duration on the order of 100 years, which requires a minimum vertical displacement rate of the wallrocks immediately above the pluton greater than 2 m/yr. Finally, our data show that the rates of plutonic and volcanic processes could be similar, a result of great importance for the general knowledge and interpretation of magma transfer in arc systems.