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Geology of the southern Sierra Calamajue, Baja California Norte, Mexico

By
Roger Griffith
Roger Griffith
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John Hoobs
John Hoobs
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Published:
January 01, 1993

The boundary between the western Jurassic-Cretaceous arc terrane and the eastern, pre-Cretaceous cratonally derived metasedimentary rocks extends along the axis of the Peninsular Ranges. In the southern Sierra Calamajue (lat. 29°25′N), where this boundary is exposed, detailed stratigraphic and structural analyses of these lower greenschist grade rocks indicate that the western and eastern terranes are structurally interleaved in a 5- to 7-km-wide zone. Five northwest-trending tectonostratigraphic units are mapped in this area. The structurally lowest (southwesternmost) unit consists of 1,300 m of chert and fine-grained clastic rock, with minor limestone of Late Mississippian age. These rocks are structurally overlain by 2,600 m of Upper Jurassic basaltic to andesitic volcanic and volcaniclastic rock and Lower Cretaceous (Alisitos Group) volcaniclastic and epiclastic rocks. Medium- to thick-bedded quartz sandstones in the upper part of the Jurassic sequence appear to be redeposited in the overlying Alisitos Group, suggesting a depositional contact between these rocks. Two of the three structurally overlying units consist of variably tectonized limestone and fine- to medium-grained clastic rocks of unknown (Cretaceous?) age. These units are in sharp fault contact with, and are separated by, Lower Cretaceous volcaniclastic and pyroclastic rocks.

Three phases of deformation have been identified in these rocks. The oldest deformation, D1, produced steeply east dipping reverse faults (which separate the rock units), tight to isoclinal folds, and an associated, northwest-trending axial plane cleavage. Progressive flattening strain during D1 rotated fold hinge lines and the X-axis of deformed lapilli into the direction of tectonic transport, which is defined by a steeply plunging extension lineation. Strain analyses indicate that shortening perpendicular to D1 foliation probably exceeded 60 to 70% in some areas. The timing of this deformation is constrained by a 103 ± 4-Ma Rb/Sr whole-rock age for deformed metavolcanic rocks (Alisitos Group), and a U-Pb zircon age of circa 100 Ma for an undeformed granite that crosscuts the early fabric. The second recognizable deformation, D2, produced a conjugate set of kink bands that indicate shortening parallel to the earlier formed fabric. A map-scale sinistral flexure, which occurs at the join between the Sierra Calamajue and the Sierra la Asamblea, deforms structures produced during D1 and D2.

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Contents

GSA Special Papers

The Prebatholithic Stratigraphy of Peninsular California

R. Gordon Gastil
R. Gordon Gastil
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R. H. Miller
R. H. Miller
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Geological Society of America
Volume
279
ISBN print:
9780813722795
Publication date:
January 01, 1993

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