Geology topics

J. Lawford Anderson

Publications and source records attributed to J. Lawford Anderson.

3 recordsLinked to original sources

USGS2008

Magmatism and tectonics in a tilted crustal section through a continental arc, eastern Transverse Ranges and southern Mojave Desert

This field guide describes a two-and-one-half day transect, from east to west across southern California, from the Colorado River to the San Andreas fault. Recent geochronologic results for rocks along the transect indicate the spatial and temporal relationships between subarc and retroarc shortening and Cordilleran arc magmatism. The transect begins in the Jurassic(?) and Cretaceous Maria retroarc fold-and-thrust belt, and continues westward and structurally downward into the Triassic to Cretaceous magmatic arc. At the deepest structural levels exposed in the southwestern part of the transect, the lower crust of the Mesozoic arc has been replaced during underthrusting by the Maastrichtian and/or Paleocene Orocopia schist.

Arizona, California↗

USGS1994

Conflicting tectonics? Contraction and extension at middle and upper crustal levels along the Cordilleran Late Jurassic arc, southeastern California

Effects of mid-Mesozoic contraction followed closely in time by extension are present in mid- to upper-crustal plutonic rocks in the Chuckwalla Mountains of the eastern Transverse Ranges, California. Late Jurassic movement along a steeply dipping, right-lateral mylonitic shear zone is bracketed between 159 and 147 Ma via U-Pb dated plutons. Depth of emplacement vs. time data based on hornblende geobarometry and U-Pb geochronology of Mesozoic plutons indicate that a period of dramatic uplift affected the Chuckwalla Mountains during the Late Jurassic, contrasting sharply with data from the (then) nearby San Gabriel Mountains. Subsequent latest Jurassic extensional tectonics is suggested by alkalic plutonism, nearly synchronous intrusion of the Late Jurassic Independence dike swarm, and possibly by deposition of the McCoy Mountains Formation. We conclude that both contraction and extension were significant at upper- and mid-crustal depths in the Chuckwalla Mountains region during the Late Jurassic, and speculate that the combined influence of oblique convergence and the opening of the Gulf of Mexico may have caused the contrasting and nearly contemporaneous tectonic modes.

California↗

USGS1993

Early Proterozoic ties between two suspect terranes and the Mojave crustal block of the Southwestern U.S

Southern California and adjacent areas contain two suspect or exotic terranes comprised largely of ancient continental crust, namely the Tujunga (San Gabriel) and Joshua Tree terranes, that have been considered part of a larger displaced terrane, the Santa Lucia-Orocopia allochthon. Paleomagnetic data for the allochthon indicate northward transport in excess of 2000 km and, thus, an origin extraneous to North America. However, Early Proterozoic plutons of the Mojave crustal block and the Joshua Tree and Tujunga terranes have strikingly comparable features, including: (1) crystallization ages of 1.63 to 1.68 Ga; (2) biotite + sphene + magnetite hornblende garnet mineralogy; (3) high LIL and enriched HFS elemental composition; (4) WPG (within-plate granite) trace element chemistry; (5) similar and unique oxygen isotopic compositions; and (6) distinct Pb and Nd isotopic signatures. These features of the Mojave block, which clearly originated as part of native North America, nevertheless distinguish it from crust elsewhere in North America. On the basis of data presented here, we conclude that the Tujunga terrane is a disrupted portion of the Mojave crustal block and is neither far-traveled nor exotic to North America. Its apparent "exotic" nature stems from derivation out of the middle crust. We also conclude that the Joshua Tree terrane is correlative to the Mojave block. We have found no significant evidence for its displacement and consider Joshua Tree to be contiguous with the Mojave block and thus not a valid terrane. The Tujunga (San Gabriel) and Joshua Tree terranes should not be considered as part of, or having shared the same transport as, the Santa Lucia-Orocopia allocthon.

The Journal of Geology↗