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

We present an interpretation of crustal seismic refraction data from t he 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 i n 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 sedimenta ry rocks lie 1-2 km of 4.0-4.6 km/s materials, correlating with andesi tic volcaniclastic sedimentary rocks and lava flows of the Lower Juras sic 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 j umps to 6.3 km/s and increments to 6.6 km/s by 10-12 km depth. Velocit ies 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 i s poorly resolved. Depth to the Moho discontinuity could not be determ ined from our data. The absence of clear PmP reflections may indicate that Moho is deeper than 40 km. Data from two offset shotpoints northe ast of the line and within the Wrangellia terrane constrain the deep s tructure transition between Peninsular and Wrangellia terranes. The 6. 3-6.6 km/s material thickens to the northeast, toward the suture betwe en Peninsular and Wrangellia terranes, but southwest of its mapped tra ce at the West Fork fault. Peninsular terrane crustal structure appear s dissimilar to that of continental interiors. It is similar to veloci ty structures determined for accreted island are fragments in Californ ia, such as the basement of the Great Valley and the Klamath Mountains .