CONSTRAINTS ON THE TIMING OF DEFORMATION IN PINE CREEK AGE DACITE LAVA DOMES, MOUNT ST. HELENS

Past deformation of volcanic edifices may cause crack damage that can be assessed by analysis of elastic moduli. At Mount St. Helens the northeast and northwest sides of the crater wall, exposed since the infamous 1980 debris avalanche, are highly fractured. Assessment of the degree of deformation within Pine Creek age (2.90-2.55 ka) lava domes and Castle Creek age (2.30-1.70 ka) dykes from these areas enables us to determine whether deformation pre or post-dates Castle Creek intrusion. The Pine Creek age samples consist of competent dacite from Northeast dome, and both competent and incompetent dacites from Breach and Kid domes within the northwest flank. Castle Creek age samples are taken from a basaltic dyke cutting Northeast dome. Deformation may have been caused by (1) shearing during flank failure and avalanche in Pine Creek times or (2) shearing during cryptodome growth and the subsequent debris avalanche in 1980. Ultrasonic wave velocities combined with density and porosity measurements were used to establish dynamic elastic properties for the Pine and Castle Creek samples. Anisotropic deformation and acoustic emissions were recorded during cyclic uniaxial compressive strength tests. A 21-44 % decrease in Young’s modulus concomitant with an increase in Poisson’s ratio by a factor of 1.25-3 at the same stress on the first and last loading cycles occurs as a result of cyclic induced cracking in all samples. Acoustic emissions confirmed the nucleation and propagation of cracks, and an increase in output by several orders of magnitude immediately prior to failure represents crack coalescence. Dynamic and static Young’s moduli in Castle Creek age basalt (11.9-30.4 GPa) are much higher than in Pine Creek age dacite (1.1-19.5 GPa). The basalt also failed at higher stress (182 MPa v. 36 MPa), which can be attributed to a lesser amount of crack damage in the younger basalt. More extensive crack damage in the Pine Creek dacite suggests that the main phase of deformation took place during late Pine Creek age (~2.70-2.55 ka) prior to the Castle Creek dyke intrusion. The precise cause of this deformation remains unclear.