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Douglas M. Morton

Publications and source records attributed to Douglas M. Morton.

52 records · Page 3Linked to original sources
USGS1978

Cyclic landsliding at Wrightwood, southern California: A preliminary report

Recurring landslide and mudflow events in the Wrightwood area of Southern California are parts of a composite cycle of landslide activity that includes three recognizable stages. The three stages are interdependent, occur in sequence, and are of different duration. Deposits of the first stage--the largest in size--are removed to positions further downstream by the activity of second- and third-stage landslides. First-stage landslides are represented by huge slumped masses derived from steep bedrock slopes in the canyon heads; the material moves down the principal stream drainage, which may be completely filled with debris. Second-stage activity develops as streams cut a network of branching channels into the massive first-stage deposit. The second-stage landslides are chiefly slumps from the older slide mass and from adjacent bedrock slopes. The movement of these slides generally is downslope toward actively eroding drainages. Third-stage activity includes mudflows that accompany the spring melting of snowpack. The debris moves down the stream channels to depositional reaches on major fans. Removal of sufficient amounts of the first-stage landslide mass to the fan by second- and third-stage events resets the bedrock slope of the main drainage for another first-stage event. The first-stage landslides in the Wrightwood area are of prehistoric origin, and their recurrence interval in any one canyon is probably several thousand years. The active duration of a first-stage landslide is one to several thousand years. Second-stage landslides last one to several years and are apparently preceded and triggered by a series of high-precipitation winters. The duration of observed third-stage (spring mudflow) sequences ranges from a few days to as much as six weeks; peak mudflow activity apparently results when a-heavy spring snowmelt occurs during a period of second-stage landslide activity.

California↗
USGS1977

Comparison of granitic intrusions in the Pelona and Orocopia Schists, southern California

Dating of some small plutons in the San Gabriel and Chocolate Mountains, southern California, yields Miocene K-Ar ages. A single granodiorite pluton that has been segmented by branches of the San Jacinto fault intrudes the Pelona Schist and yields ages of 14.0 to 18.6 million years. Several quartz monzonite plutons in the Chocolate Mountains, 235 kilometers to the southeast and on the opposite side of the San Andreas fault, yield ages ranging from 20.0 to 23.4 m.y. At least one pluton in the Chocolate Mountains intrudes the Orocopia Schist, a probable equivalent of the Pelona Schist. Chemical, modal, textural, mineralogical, and apparent K-Ar age differences suggest that the two groups of plutons are not offset parts of a single body, but the differences do not preclude the two groups from being related in a general way.

California↗
USGS1977

Chemical variability in the Lakeview Mountains pluton, southern California batholith: A comparison of the methods of correspondence analysis and extended Q-mode factor analysis

An extended method of Q -mode factor analysis that has been described previously offers a number of important advantages over conventional Q -mode factor analysis and correspondence analysis when applied to compositional data. Among these are the ability to compute the compositions, in the original units of the data, represented by the reference axes. The only special requirement of the data is that the values for each observation sum to a constant. Mathematically derived reference axes, such as the principal-components and varimax axes, commonly represent compositions that are partly negative. Even where no negative values are present, these compositions are comparatively difficult to interpret in petrologic terms and impossible or inappropriate to include in petrogenetic models. A model has been developed for the Lakeview Mountains pluton which employs vectors representing the compositions of melanocratic schlieren, leucocratic schlieren, and inclusions as the reference axes. When the effect of the inclusions is removed, the compositional zoning in the pluton is like that found in otherwise similar plutons in the Sierra Nevada batholith. The genetic implication of the model is that the inclusions were separated from the magma at least in its marginal parts. In the central part of the pluton, the inclusions were either separated to a lesser extent than in the marginal parts, or were reincorporated into the magma.

California↗
USGS1977

Surface deformation in part of the San Jacinto Valley, southern California

Ground fissuring and subsidence occur in a deep alluvium-filled graben between the Casa Loma and San Jacinto faults, San Jacinto Valley, southern California. These processes are attributed chiefly to compaction due to withdrawal of ground water. Subsidence due to compaction is estimated to be a maximum of 3.5 centimetres per year; tectonic subsidence of the graben is estimated to be from 0.3 to 0.6 cm per year. Ground fissuring occurs primarily on the west side of a closed depression in the graben. The area of ground fissures has expanded from 1 square kilometre in 1953 to 12 km2 in 1974 and could grow even more. Individual fissures have attained lengths of 850 metres.

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