CRETACEOUS SHEAR ZONES, ZONED PLUTONIC COMPLEXES, AND DISPLACED TERRANES, NORTHERN PINE FOREST RANGE AND PUEBLO MOUNTAINS, NV-OR

The northern Pine Forest Range (NPFR) and Pueblo Mountains (PM) form an important link between the western Idaho region and the Sierra to Sahwave (NV) region. I here summarize key relations now known from this area and discuss regional implications.

(1) Cretaceous intrusive rocks are widespread in the NPFR and form the zoned Mahogany Mtn. plutonic complex (MMPC). U-Pb zircon dating (SHRIMP-RG at Stanford Univ.) indicates intrusion from 113-104 Ma . Most of the MMPC consists of quartz diorite to granodiorite: core rocks are 2-mica granite; flanking rocks are mostly monzodiorite and gabbro. Early Cretaceous age of intrusion and intermediate composition makes these rocks most similar to plutonic suites of western Idaho.

(2) There is widespread evidence for Cretaceous ductile shearing in the NPFR-PM, in the form of three NNE-trending shear zones: the Pueblo shear zone (PSZ) which deforms allochthonous Jurassic continental arc rocks of the PM; the Antelope Valley shear zone (AVSZ) which deforms MMPC intrusive rocks; and the intervening Baltazor shear zone (BSZ) which is localized along an older, possibly dextral, fault boundary in the NPFR between disparate rock assemblages of unknown terrane affinity. Mylonitic fabrics are well-developed in the shear zones and provide clear evidence for shear sense. The PSZ and BSZ dip SE and record top-NW shear, while the AVSZ dips NW and records top-SE shear. U-Pb zircon dating of cross-cutting MMPC intrusives confines BSZ offset to ca. 113 Ma, and AVSZ offset to ca. 108 Ma. There are no Cretaceous plutons near the PSZ, but 40Ar/39Ar dating of syntectonic micas provides a minimum offset age of 100 Ma. Prominent evidence for ductile shearing in the NPFR-PM, and Albian age of shearing is most similar to relations seen in western Idaho.

(3) There is clear evidence for dextral displacement of terranes in the NPFR-PM area: PM Jurassic continental arc rocks bear no relation to the well-established marine terranes that dominate geology elsewhere in NW NV, but are instead more like SW Cordillera continental arc assemblages. It is not clear whether any of the needed offset is recorded by the local shear zones. More likely, terrane displacement was accomplished along older, less obvious structures, and the exposed shear zones formed during a second tectonic event. This two-part history is also most similar to relations in western Idaho.