Geology of Southern California.pdf - Grossmont College

Frank DeCourten
directly from the water of the ancestral Pacific Ocean, accumulated
over the rifted edge of the continent from its
inception throughout the early periods of the Paleozoic Era
(Figure SC.10). These marine sediments built up a shallow
continental shelf that extended into southeastern
California. The thick sequences of Paleozoic limestone,
sandstone, and shale that can be seen in the mountains of
the eastern Mojave Desert and Basin and Range provinces
(Figure SC.10) were deposited in shallow ocean waters
along the passive continental margin. Similar deposits
extend westward into the Sierra Nevada and Transverse
Ranges, but have been highly deformed and metamorphosed
by later events in those areas.
Late Paleozoic-Mesozoic Plate Convergence: In mid-
Paleozoic time, about 300 million years ago, the passive
western margin of Laurentia began to change. The continental
masses formed during the fragmentation of Rodinia
had started to reassemble into the supercontinent know as
Pangaea. The early stages in this process involved collisions
between Laurentia and several plates to the east to create
larger continental masses. When Laurentia collided with
the Eurasian plate, Laurasia, the large northern portion of
Pangaea was formed by the combined masses. The trailing
western margin of Laurentia was not directly involved in
Southern California.2 The Plate Tectonic Framework of Southern California 7
the construction of Pangaea, but the shift from a passive to
an active continental margin occurred at about the same
time. In western North America, there is clear geologic evidence
that convergent plate interactions existed in the later
periods of the Paleozoic early. Oceanic plates approaching
the western edge of Laurasia carried island arcs and other
rock bodies that eventually collided with the continental
margin. Each collision resulted in a pulse of mountain
building in western North America. It is clear that the
serene passive continental margin of the early Paleozoic in
western North America was replaced in mid-Paleozoic time
by a raucous convergent plate boundary with rising
mountains, erupting volcanoes, and violent earthquakes.
When Pangaea began to break apart in the late Triassic
period, the fragment that became modern North America
began moving west as the Atlantic Ocean basin opened.
Geologic activity along the convergent continental margin
intensified as the convergence increased. This era of rapid
plate convergence, intense igneous activity, and widespread
mountain building during the Mesozoic Era is generally
known as the Cordilleran orogeny. There were several
phases of geologic unrest during the Cordilleran Orogeny,
but in the California region, the most prominent was the
Nevadan orogeny, a period of mountain building that
� Figure SC.10 Early Paleozoic limestone and shale in the Funeral Mountains near Death Valley accumulated on the passive margin of
Laurentia.