STRUCTURAL GEOLOGY
Lectures 40
Mesozoic Geology of the Cordillera
(Rocky Mountain Geology exclusive of Laramide Deformation)

Early and Middle Triassic (245-230 Ma) - accretion of a major Paleozoic island-arc terrane in northwest Nevada-northern California (the McCloud belt).  This was the same period as the termination of the Sonoma Orogeny. On the stable continent deposition is marked by the Moenkopi Formation  -  Fluvial near its eastern limits but represents an intertidal environment over most of the Colorado Plateau.  Has marine limestone and gypsum near its western margins.

Late Triassic (230-208 Ma) - Blue Mountains arc (a belt of highly disrupted oceanic rocks in Oregon).  Klamath Mountains (Permian-Triassic subduction complex).  During this period the Chinle Formation accumulated on the stable continental interior. The Chinle is characterized by fluvial channel-fill sandstones and conglomerates form the basal members.  Throughout most of the formation rock types vary from mudstone to siltstone to sandstone with bentonitic clays and volcanic ashes.  Some channel fill deposits are known as the Shinarump Member.  On top of the Chinle we find eolian dunes of the Wingate sandstone encroaching from the north.

Figure 40-1:  Stratigraphic section of the Jurassic System along the Arizona-Utah state line (adapted from Peterson, 1988).

The Jurassic is characterized by three major rock units.  From base to top these are the Glen Canyon Group, the San Rafael Group and the Morrison Formation (Figure 40-1). 

Early Jurassic - (208-187 Ma)  Aztec-Navajo-Nugget dune fields.These dune fields covered an area equivalent to the modern Saharan dune fields of Africa.

Middle Jurassic - (197-163 Ma) Carmel seaway.This was a precursor to the Cretaceous interior seaway.

Late Jurassic - (162-144 Ma).  Morrison Formation may record the first evidence of a broad western orogenic highland flanked to the east by a regional foreland basin.  Possible this detritus was shed from the sheets of the Sevier belt.

Jurassic northward migration of the North American plate resulted in a latitudinal path where the dry trade-wind belt was traversed during the Early and Middle Jurassic deposition of the Navajo-Aztec-Nugget dune fields.  The more cooler westerly belt was then encountered during the deposition of the Upper Jurassic Morrison Formation.

Nevadan orogeny -basis for the collided exotic-arc model for the western Sierra-Klamath belt.  Manifested by the tight folding and slaty cleavage development on the upper Oxfordian Mariposa Formation.  A major Nevadan structure includes the Foothills Fault system and the major east-dipping thrust faults that bound the Josephine ophiolite in the western Klamaths. (approximate age = 162 Ma).

Back-arc tectonic elements along the Cordillera-  Areas east of a narrow Triassic to Middle Jurassic magmatic arc that developed along the western edge of the continental United States had been subject to no early Mesozoic tectonism.  The Middle to Late Jurassic archipelago included a western region of small, locally ocean-floored, intra-arc basins that acted as a tectonic buffer zone between Pacific ocean plates to the west and the North American continent to the east.  During Callovian/Oxfordian collapse of the offshore arc region, coupling between the subducting oceanic plate(s) and western North  America is believed to have increased as the intra-arc buffer zone was eliminated.  Such a region of strong coupling is believed to have occurred from the central Sierra Nevada northward through the Blue Mountains of Oregon and into central Canada.  With the onset of opening of the North Atlantic in Middle Jurassic time (ca 175 Ma), convergence rates along the western continental margin increased and remained relatively high for most of Mesozoic time.

-  Regional setting of back-arc convergence: Weak coupling between subducting oceanic plates and the north American Plate led to little or no back-arc deformation.  During late Mesozoic time, increased convergence rates produced increased coupling between oceanic plates and the North American Plate, possibly due to the active overriding of the offshore subducting plates by the west-moving continental plate.  Events such as the development of Nevadan and pre-Nevadan west-directed thrusts in the Klamath Mountains, the closure of the small intra-arc ocean basins (e.g., Galice), east-directed thrusting in the northern Sierra (Taylorsville thrust), and the beginning of closure of the Star Peak/Luning basin in the late Middle Jurassic to early Late Jurassic time are viewed as expressions of the onset of strong coupling along the western margin of the Cordillera.  This coupling was accompanied by an east west broadening of arc magmatism.  Deformation began in the eastern Sevier belt only after deformation in the Luning-Fencemaker belt had sufficiently thickened the crust beneath the Star Peak/Luning basin to permit shortening stresses to be tranmsmitted farther east.	

- Late Jurassic through Cretaceous foreland fold-thrust belts:  Starting in the Middle Jurassic time tectonism started moving eastward to eventually extend 1000 km.  Tectonism north of Las Vegas divided into three deformation belts  (Figure 40-2):

- Luning-Fencemaker belt:  Thrust system moved basinal sediments onto shelf sediments to the east in western Nevada.

- Eureka belt:  Cretaceous Newark Canyon Group formed within an active fold-thrust belt in Central Nevada.

Figure 40-2: Paleogeographic and tectonic map of the Middle Jurassic to Late Cretaceous time exclusive of Laramide basement block uplifts (adapted from Burchfiel et al., 1992).  Dark areas are largely plutons and volcanic areas of this period.

- Sevier belt;  Late Jurassic to Late Cretaceous.  A foredeep developed along its eastern margin and received up to 6 km of debris eroded from the rising mountains.  Thrusting is thin skinned with changes in their style along strike (Figure 40-2).

-  Montana:  bedding anisotropy of sedimentary strata of the Middle Proterozoic Belt Group controlled thrusting

- Idaho:  thrusts carry crystalline basement rocks where they cross a paleogeographic high (Salmon or Lemhi arch) that marked the southern uplands for the Belt basin.

-  South of Snake River Plane:  thrusts develop within the eastern portion of the Late Proterozoic and Paleozoic miogeocline.  Each more westerly thrust fault carried a thicker miogeoclinal section.

-  southeastern California the thrust belt turns away from the miogeoclinal hinge and crosses into crational strata and their underlying Precambrian basement.Sevier thrusts ultimately involve crystalline basement rocks.

150 Ma - inception of the subduction-related tectonic elements with magmatism switching off in the Sierra Nevada batholith and jumping to more restricted centers in the Rocky Mountains.Subduction along western margin of the US continental margin gave rise to the magmatic arc (Sierran batholithic belt); forearc-basin deposits (The Great Valley Group); and subduction complex (Franciscan Complex).

80 Ma - The mid-Late Cretaceous also witnessed the waning of thin-skinned deformation in the Cordilleran fold and thrust belt and the inception of Laramide deformation.