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Description
The Little Maria Mountains form a complex northwest-trending range within Riverside County, California. Highly deformed and moderately metamorphosed (middle to upper greenschist grade) Paleozoic cratonal strata, correlative with the Grand Canyon sequence, and Mesozoic island-arc metasedimentary rocks overlie a Precambrian granite and gneissic basement. The initiation of subduction beneath southwestern North America in Latest Triassic time caused widespread deformational and magmatic activity to occur across the western United States. This affected the rocks of the Little Maria Mountains by Latest Jurassic-Earliest Cretaceous time, initiating a regional metamorphic and deformational event which produced east-west trending folds which verge to the south-southwest. That the basement rocks are included in the folding indicates that crustal softening by high temperatures extended into the craton, producing a plastic style of deformation. The Little Maria syncline formed during this deformational event. A Hansen fold analysis was conducted in two locations in the syncline within the Muav Formation, a Cambrian metasedimentary unit consisting largely of calcitic and dolomitic marbles, locally interlayered with chert. The resulting slip lines, or direction of transport, trend roughly north-south with a shallow northerly plunge. Tight clustering of data points suggests that one continuous deformation is responsible for all the east-west trending folds which verge to the south-southwest, as well as the sub-parallel foliation. Attenuation and thrusting may be a late-stage feature of this deformational event. The close parallelism of the thrust fault plane and the fold structures indicates that the failure of the folds to further flatten and attenuate resulted in fold separation along a thrust plane. Much of the folding and thrusting occurred prior to the emplacement of Late Cretaceous granitic rocks of the northern and southern Midland plutons, which cross cut these deformational features. A well-developed foliation, present in both of the Midland plutons, parallels the fold structure and thrust faults, supporting a theory of an ongoing deformation during the emplacement of igneous rocks. This indicates that metamorphism and deformation initiated in the Latest Jurassic continued until the Latest Cretaceous to Earliest Tertiary. Numerous K-Ar ages from the range cluster around 60 m.y. B. P., indicating completion of this orogenic event. Emplacement of leucogranites of the Little Maria pluton in the northern part of the range give K-Ar dates of 55 m.y. This pluton post-dates the major deformational event and, as a result, lacks a well-developed foliation. Normal dip-slip faults with offset generally less than 10 meters riddle the range and may have been formed during mid-Tertiary extension of this region.
