Verdugo Mountains - Geology

Geology

The Verdugo Mountains consist of an east-west-trending antiformal fault block, bounded on south by the Verdugo Fault, a north-dipping reverse fault, and on the north by the Sierra Madre thrust fault near the front of the San Gabriel Mountains, thus including the sediment-covered Crescenta Valley within the Verdugo Mountains Block. The Verdugo Fault lies slightly south of the topographic range front and is completely covered by sediments.

The rocks within the Verdugo Mountains block are almost entirely igneous and metamorphic rocks similar to the crystalline basement rocks exposed to the north in that portion of the San Gabriel Mountains south of the San Gabriel Fault. These rocks consist of gneiss, and gneissic diorite and quartz diorite, intruded by irregular bodies of equigranular granitic rocks, predominantly quartz diorite and granodiorite, with accompanying pegmatite and aplite. Exposed rocks in the Shadow Hills neighborhood at the extreme northwestern end of the Verdugos are typically marine sedimentary rocks of Miocene age, predominantly sandstone and shale.

The Verdugo Mountains are part of the western Transverse Ranges, which have risen in the last 7 million years as the result of contractional deformation resulting from transpressional motion and rotation of crustal blocks in the "Big Bend" region of the San Andreas Fault. The amount of crustal shortening since the beginning of the Pliocene has been estimated to be on the order of 7 kilometers (4.3 mi). The Verdugo fault and Sierra Madre thrust are part of a complex system of faults that accommodate some of this shortening and generally become younger to the south, with the Verdugo Fault possibly being the youngest member of this system and forming the current boundary between this portion of the western Transverse Ranges and the Los Angeles basin. Uplift along the Verdugo fault may total approximately 2.5 km (1.6 mi), at a minimum rate of 1.1 km (0.68 mi) per million years since 2.3 million years ago, moving the crystalline rocks of the Verdugo Mountains up and over younger Tertiary and Quaternary sediments to the south. The Verdugos are thus a young and rapidly rising mountain range, reflected in their steep topography and rapid rates of erosion.