New structural, petrological, and geochronological data from the southeastern San Gabriel Mountains define the amalgamation history of suspect terranes in this region. Four metamorphic terranes constitute the area—the Cucamonga, San Antonio, San Gabriel, and Baldy terranes—and were juxtaposed in two discrete episodes during the Late Cretaceous.
The Cucamonga, San Antonio, and San Gabriel terranes were juxtaposed along subparallel, east-west-trending, left-lateral mylonitic fault zones during the first episode. The Cucamonga terrane consists of Lower Cretaceous granulite-facies gneissic rocks. These rocks were intruded in the Late Cretaceous by granitic rocks as the Cucamonga terrane was faulted ductilely against the southern margin of the San Antonio terrane. Mylonitized units were subjected concurrently to retrograde amphibolite-facies metamorphism. Metasedimentary pendants in the San Antonio terrane were metamorphosed under upper-amphibolite-facies conditions as plutonic rocks were emplaced syntectonically. Along its northern boundary, the San Antonio terrane was juxtaposed concurrently against the San Gabriel terrane, a terrane consisting of Precambrian gneiss and Precambrian and Mesozoic intrusive rocks. The boundary is represented by Middle Fork Complex, a ductile zone intermingling rocks of the two terranes.
After palinspastic restoration of local Cenozoic fault displacements, the left-lateral mylonitic fault zones separating the terranes appear as parts of a major sinistral transform system along the southern margin of the San Gabriel Mountains. Displacement for each of the mylonitic fault zones was approximately several tens of kilometers and was part of a broad left-lateral shear system. This system is modeled as a lateral ramp or tear fault to a west-directed, synplutonic, ductile thrust system underlying the San Gabriel terrane. Crosscutting syn- and post-tectonic intrusives indicate that most of the displacement occurred between ∼88 and 78 m.y. ago. No lithologic, isotopic, or structural data support previous assertions that these terranes were accreted to North America during the Tertiary.
The Late Cretaceous synplutonic juxtaposition of the Cucamonga, San Antonio, and San Gabriel terranes was followed by underthrusting of oceanic rocks of the Baldy terrane (that is, Pelona Schist) along the Vincent thrust. Precise timing of this episode is as yet uncertain.