Mesozoic Earth and Life History - The Changing Earth: Exploring Geology and Evolution - page 601

Lowlands

Uplands

Nevadan
mountain
building

a

Land
Mountains
Volcanoes
Epeiric sea
Deep ocean

Newly forming
Gulf of Mexico

◗

Figure 22.5 Jurassic Paleogeography of North America

against the craton margin ( Figure 22.8). This event, known

as the Sonoma orogeny, occurred at or near the Permian-
Triassic boundary and resulted in the suturing of island-arc
terranes along the western edge of North America.
Following the Late Paleozoic-Early Mesozoic destruction
of the volcanic island arc during the Sonoma orogeny, the
western margin of North America became an
oceanic-continental convergent plate boundary. During the
Late Triassic, a steeply dipping subduction zone developed
along the western margin of North America in response to
the westward movement of North America over the Farallon
plate. This newly created oceanic-continental plate boundary
controlled Cordilleran tectonics for the rest of the Mesozoic
and for most of the Cenozoic Era; this subduction zone

◗ marks the beginning of the modern circum-Pacifi c orogenic

system (see Chapter 23).
Two subduction zones dipping in opposite directions
from each other formed off the west coast of North America
during the Middle and early Late Jurassic (

Figure 22.9).
The more westerly subduction zone was eliminated by the
westward-moving North American plate, which overrode the
oceanic Farallon plate.
The Franciscan Complex, which is up to 7000 m thick, is
an unusual rock unit consisting of a chaotic mixture of rocks
that accumulated during the Late Jurassic and Cretaceous.
The various rock types—graywacke, volcanic breccia, silt-
stone, black shale, chert, pillow basalt, and blueshist meta-
morphic rocks—suggest that rocks of the continental shelf,

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