ABSTRACT

Before coastal California began moving along the San Andreas fault, strike-slip faults displaced two other large terranes, thereby affecting petroleum maturation and migration. During the early Eocene, the Pacific plate converged against North America, because at that time it moved parallel to the Emperor Seamounts hot-spot trace. Later, when the Pacific plate began to move parallel to the Hawaiian Ridge (43 m.y. ago), the Pacific-Farallon spreading axis was contacting North America at Oregon. Strike-slip faulting, as evidenced by displaced Tertiary paleolatitudes in Alaska, then detached the Wrangellia-Chugach terrane from the Pacific Northwest and began to move it northward. Today, this block, with a pointed leading edge at the Alaska Peninsula, lies between the Aleutian Trench and the Denali fault. Local deformation indicates that during the Pliocene it curved to the west and came to rest along an ancestral Aleutian subduction zone. Magnetic lineations show that in the middle Tertiary another segment of the Pacific-Farallon spreading axis intersected the continent at northern Baja California. A terrane from Vizcaino Bay to the Olympic Peninsula, which included the Sierra Nevada and Klamath Mountains, moved northward along a fault that joined the earlier strike-slip fault near Vancouver Island. This terrane juxtaposed Obispo-block Franciscan rocks from Vizcaino Bay against Salinian-block potassium-rich plutons at the back side of the Sierra Nevada batholith. At the end of the Miocene, the East Pacific rift jumped east and established the present plate boundary, which is detaching continental rocks only from the Gulf of California to Cape Mendocino.

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