SEISMIC-REFRACTION MEASUREMENTS WITHIN THE PENINSULAR TERRANE, SOUTH CENTRAL ALASKA

We present an interpretation of crustal seismic refraction data from the Peninsular terrane, one of the many exotic terranes that have been accreted to the continental margin of southern Alaska in the past 200 m.y. A seismic refraction line was collected along the Glenn Highway in the Copper River Basin of south central Alaska in 1984 and 1985, as part of the U.S. Geological Survey Trans-Alaska Crustal Transect (TACT) program. P wave velocities of 2.7-3.5 km/s and thicknesses of 1-2 km characterize post-lower Jurassic sedimentary rocks that underlie most of the seismic refraction line. An average crustal velocity structure includes the following five velocity divisions. Beneath the sedimentary rocks lie 1-2 km of 4.0-4.6 km/s materials, correlating with andesitic volcaniclastic sedimentary rocks and lava flows of the Lower Jurassic Talkeetna Formation. Below these rocks, seismic velocity increases rapidly, from 5.0 to 6.1 km/s, in 2-3 km. At 7-8 km depth, velocity jumps to 6.3 km/s and increments to 6.6 km/s by 10-12 km depth. Velocities increase from 6.8 to 7.0 km/s between 12 to 20 km depth. At about 22 km depth, a jump in velocity from 7.0 to 7.4 km/s is inferred but is poorly resolved. Depth to the Moho discontinuity could not be determined from our data. The absence of clear PmP reflections may indicate that Moho is deeper than 40 km. Data from two offset shotpoints northeast of the line and within the Wrangellia terrane constrain the deep structure transition between Peninsular and Wrangellia terranes. The 6.3-6.6 km/s material thickens to the northeast, toward the suture between Peninsular and Wrangellia terranes, but southwest of its mapped trace at the West Fork fault. Peninsular terrane crustal structure appears dissimilar to that of continental interiors. It is similar to velocity structures determined for accreted island are fragments in California, such as the basement of the Great Valley and the Klamath Mountains.