James H. Trexler

Late Paleozoic tectonism in Nevada: Timing, kinematics, and tectonic significance

Three late Paleozoic, angular unconformities, each tightly constrained in age by biostratigraphy, are exposed in Carlin Canyon, Nevada. These record deformation as well as erosion. Folding associated with these deformation events is roughly coaxial; all three sets of fold axes trend northeast. Each unconformity represents tectonic disruption of the middle part of the western North American margin between the times of the initiation of the Antler orogeny (Late Devonian–Early Mississippian) and the Permian–Triassic Sonoma orogeny. This paper focuses on one of these unconformities in the Middle Pennsylvanian—the C6 unconformity—and the deformation and age constraints associated with it.nnOur data from Carlin Canyon yield detailed glimpses of how the Antler foreland evolved tectonically in Mississippian and Pennsylvanian time. Middle Pennsylvanian (Desmoinesian) northwest-southeast contraction resulted in thin-skinned folding and faulting, uplift, and erosion. These data require reinterpretation of the tectonic setting at the time of the Ancestral Rocky Mountains orogeny and suggest that plate convergence on the west side of the continent played a significant role in late Paleozoic tectonics of the North American continent.

Paleoseismology of an active reverse fault in a forearc setting: The Poukawa fault zone, Hikurangi forearc, New Zealand

The Poukawa fault zone, on the North Island of New Zealand within the forearc of the Hikurangi subduction zone, consists of a series of en echelon reverse faults and companion hanging-wall anticlines. The geomorphically expressed length of the fault zone is 34 km. However, on the basis of coseismic deformation associated with an M s 7.8 earthquake in 1931 and the presence of blind faults north of the geomorphically expressed fault zone, it appears that the seismogenic length of the fault zone may be as much as 130 km. On the basis of chronostratigraphic horizons identified in each of three trenches evenly distributed along the exposed fault zone, from which a paleoseismological record for the past ∼25 k.y. can be determined, there is not a characteristic rupture length for earthquakes. Some slip events are confined to the ∼10–20-km-long southern part of the fault zone, whereas other slip events may have ruptured the entire 34 km length of the geomorphically expressed fault zone. At least two slip events that occurred in the northern part of the fault zone did not occur in the southern part of the zone. The largest earthquake recorded in the trenches had a maximum reverse slip in excess of 10 m. We infer that this prehistoric earthquake, similar to the 1931 earthquake, entailed slip on faults along the geomorphically expressed fault zone and on blind faults to the north. This prehistoric earthquake may have had a rupture length (surface plus subsurface) in excess of 100 km. Average earthquake repeat times on the fault zone range from 3–7.5 k.y. for the southern and middle part of the zone to 7–12 k.y. for the northern part of the fault zone. Average single-event slip ranges from 3 m to as much as 6 m. Slip was initially accommodated at the surface primarily by folding. With successive slip events, however, coseismic displacements propagated to the surface and surface deformation became increasingly dominated by reverse slip on fault planes. The Poukawa fault zone is part of a foreland-propagating fold and thrust belt in the forearc of the Hikurangi subduction zone. Older, actively eroding hanging-wall anticlines are present to the west of the fault zone toward the volcanic arc, whereas younger folds are developing above blind reverse faults east of the main fault trace. In addition to propagating to the east, the fault zone is propagating northward beneath the Heretaunga Plains. This active propagation testifies to ongoing and evolving contractional forearc deformation in response to oblique plate convergence.

Piggyback basin in the Sevier orogenic belt, Utah: Implications for development of the thrust wedge

Timing data on a frontal thrust system indicate that the Sevier thrust wedge of central Utah advanced rapidly to its ultimate structural front in ∼30 m.y., between early Albian and Campanian time. It then persisted by out-of-sequence thrusting for an additional 25 m.y., until the early Eocene. The key to the history of the frontal thrust is a piggyback basin that formed on the hanging wall. Unconformity-bounded sedimentary units within the basin thin both westward and eastward onto structures generated by thrust faulting. Up to 860 m of upper Campanian to lower Eocene strata accumulated between these structures, which consist of a ramp anticline in the west and a thrust duplex in the east. The newly recognized interaction of coeval deposition and thrust deformation permits revised limits on the inception and longevity of the frontal thrust system.

Widespread effects of middle Mississippian deformation in the Great Basin of western North America

Stratigraphic analyses in central and eastern Nevada reveal the importance of a deformation event in middle Mississippian time that caused widespread deformation, uplift, and erosion. It occurred between middle Osagean and late Meramecian time and resulted in deposition of both synorogenic and postorogenic sediments. The deformation resulted in east-west shortening, expressed as east-vergent folding and east-directed thrusting; it involved sedimentary rocks of the Antler foredeep as well as strata associated with the Roberts Mountains allochthon. A latest Meramecian to early Chesterian unconformity, with correlative conformable lithofacies changes, postdates this deformation and occurs throughout Nevada. A tectonic highland—created in the middle Mississippian and lasting into the Pennsylvanian and centered in the area west and southwest of Carlin, Nevada—shed sediments eastward across the Antler foreland, burying the unconformity. Post oro genic strata are late Meramecian to early Chesterian at the base and are widespread throughout the Great Basin. The tectonism therefore occurred 20 to 30 m.y. after inception of the Late Devonian Antler orogeny, significantly extending the time span of this orogeny or representing a generally unrecognized orogenic event in the Paleozoic evolution of western North America.nnWe propose a revised stratigraphic nomenclature for Mississippian strata in Nevada, based on detailed age control and the recognition of unconformities. This approach resolves the ambiguity of some stratigraphic names and emphasizes genetic relationships within the upper Paleozoic section. We take advantage of better stratigraphic understanding to propose two new stratigraphic units for southern and eastern Nevada: the middle Mississippian Gap Wash and Late Mississippian Captain Jack Formations.

Late Paleozoic contractional and extensional deformation at Edna Mountain, Nevada

New mapping, structural analysis, and biostratigraphy of the late Paleozoic Antler overlap sequence at Edna Mountain, Nevada, document several distinct deformational events between the Devonian–Mississippian Antler orogeny and the Permo-Triassic Sonoma orogeny. Twofold sets (F 1 , mid-Pennsylvanian, and F 3 , mid-Permian) and the Iron Point fault are late Paleozoic. These record both shortening and extension in late Paleozoic time, and provide further evidence that western North America was not relatively quiescent between the Antler and Sonoma orogenies. These structures are overprinted by folding thought to be associated with emplacement of the Golconda allochthon (F 4 ), indicating that the rocks at Edna Mountain were in approximately their present location and orientation by that time. Unconformities within the Antler overlap section record periods of uplift and erosion related to late Paleozoic tectonism. They are recognized based on angular truncations of the underlying units, basal conglomerates containing fragments of the underlying units, and local erosional features including channels and karst. Biostratigraphic age control makes it possible to bracket the ages of the unconformities, and to correlate some of them with known regional unconformities. The detailed deformation record at Edna Mountain demonstrates the strength of combined biostratigraphic, stratigraphic, and structural analysis of the Antler overlap assemblage for deciphering previously unrecognized late Paleozoic deformation. The Iron Point fault is a mid-Pennsylvanian, down-to-the-east, low-angle normal fault. Previously mapped as the Iron Point thrust (Erickson and Marsh, 1974a, 1974b, 1974c), this fault places younger rocks over older, truncates folds in both hanging wall and footwall, and cuts downsection to the east in the hanging wall. Reconstruction of the original fault orientation by removing younger deformation events results in a gently east- or northeast-dipping surface. Motion along the Iron Point fault may have been a response to crustal thickening during the development of west-southwest–verging F 1 folds, the most penetrative deformation at Edna Mountain. To our knowledge, the Iron Point fault is kinematically unique for this region and time period. Its presence suggests that the rocks at Edna Mountain were near the locus of mid-Pennsylvanian deformation and that this deformation was probably related to events at the western boundary of the continent.

Sequence stratigraphy and evolution of the Antler foreland basin, east-central Nevada

The Mississippian Antler foreland basin contains siliciclastic sedimentary rocks that record a series of orogenic events along the western margin of North America from about 350 to 320 Ma. Our new stratigraphic and sedimentologic studies in Nevada indicate that the strata are not generally progradiational as previously described, and that uplift played a large role in basin evolution. We have recognized three unconformity-bounded stratigraphic sequences in the Antler basin in central Nevada: the Diamond Range submarine-fan system, the Newark Valley fluvial and delta-plain system, and the Green Springs deltaic and shelf-carbonate system. They propose a two-phase history for the antler orogeny: (1) collision of the western edge of North America with the Antler allochthon and downwarping of the continental margin (360-350 Ma), which resulted in deposition of the Diamond Range submarine-fan system; and (2) uplift and low-amplitude folding of the basin (350-320 Ma), accompanied by deposition of a thin veneer of reworked siliciclastic sediments (Newark Valley and Green Springs sequences) across a shallow-marine shelf. Siliciclastic sedimentation waned in the Late Mississippian and Early Pennsylvanian, and gradually gave way to carbonate sedimentation.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Roberts Mountains allochthon: New insights into the early Paleozoic tectonics of western North America

Detrital zircon U-Pb geochronology and Hf isotope geochemistry provide new insights into the provenance, sedimentary transport, and tectonic evolution of the Roberts Mountains allochthon strata of north-central Nevada. Using laser-ablation inductively coupled plasma mass spectrometry, a total of 1151 zircon grains from six Ordovician to Devonian arenite samples were analyzed for U-Pb ages; of these, 228 grains were further analyzed for Hf isotope ratios. Five of the units sampled have similar U-Pb age peaks and Hf isotope ratios, while the ages and ratios of the Ordovician lower Vinini Formation are significantly different. Comparison of our data with that of igneous basement rocks and other sedimentary units supports our interpretation that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units sampled, as well as Ordovician passive margin sandstones of the western Laurentian margin, had a common source in the Peace River Arch region of western Canada. We propose that the Roberts Mountains allochthon strata were deposited near the Peace River Arch region, and subsequently tectonically transported south along the Laurentian margin, from where they were emplaced onto the craton during the Antler orogeny.

Stratigraphic trends in detrital zircon geochronology of upper Neoproterozoic and Cambrian strata, Osgood Mountains, Nevada, and elsewhere in the Cordilleran miogeocline: Evidence for early Cambrian uplift of the Transcontinental Arch

U-Pb detrital zircon geochronology provides insight into the provenance of the upper Neoproterozoic–lower Cambrian Osgood Mountain Quartzite and the upper Cambrian–lower Ordovician Preble Formation in the Osgood Mountains of northern Nevada (USA). We analyzed 535 detrital zircon grains from six samples of quartz arenite by laser ablation–multicollector–inductively coupled plasma–mass spectrometry. The detrital zircon age data of these Neoproterozoic–lower Paleozoic passive margin units record a provenance change within the Osgood Mountain Quartzite. Comparison of these data with the work of others reveals that this change in provenance occurred in correlative strata throughout an east-west transect of the Great Basin. From latest Neoproterozoic through earliest Cambrian time, most grains were shed from the 1.0–1.2 Ga Grenville orogen. After that time, drainage patterns changed and most grains were derived from the 1.6–1.8 Ga Yavapai and Mazatzal provinces; very few grains from the Grenville orogen were found in the younger strata. We suggest that this shift records the uplift, in early Cambrian time, of the Transcontinental Arch. Our data also support our interpretation that the Osgood Mountain Quartzite and the Preble Formation are correlative to other contemporaneous passive margin strata in western Laurentia.

Post-2.6 Ma tectonic and topographic evolution of the northeastern Sierra Nevada: The record in the Reno and Verdi basins

A coarse conglomerate, known as “the Gravel of Reno,” fills a deep channel incised into a 2.6 Ma sedimentary section a few km west of Reno, Nevada. The canyon and its conglomerate fill record an abrupt shift in both provenance and paleocurrent direction compared with the underlying lake-marginal Neogene strata. Notably, the intermediate volcanic provenance of the Neogene section is supplemented in the overlying conglomerate by large plutonic clasts derived from the Sierran batholith. The syntectonic Gravel of Reno signals the initiation of Pleistocene faulting along the eastern edge of the Sierra Nevada near latitude 40° N. Structures within the Neogene and Quaternary rocks reveal the progressive deformation of the Sierra Nevada9s eastern margin. There is no discordance between the basal Gravel of Reno conglomerate and the underlying Neogene sedimentary section, and both are presently tilted 23° east. Therefore, significant tilting did not occur until after channel incision and deposition of the basal conglomerate. The dip within the Gravel of Reno decreases with stratigraphic height, documenting ongoing tilting during deposition. Several pervasive fault sets cut the Neogene rocks; one set of normal faults probably predates much of the tilting, but strike-slip faults appear to have been active synchronously with it. Fault sets include early west- and northwest-dipping normal faults, and two mutually cross-cutting sets of strike-slip faults: northwest-striking dextral faults and northeast-striking sinistral faults. The most continuous mappable fault surfaces, with probably much of the most recent movement, are north-striking faults with normal or oblique-slip motion. Overall, these faults accommodate east-west extension. In summary, the structural style at the northern termination of the Carson Range is characterized by distributed slip along many minor faults, and faulting was synchronous with tilting of the sedimentary section. Gravity studies constrain the location and geometry of the main structures as they project eastward under the Reno basin. A negative anomaly extends eastward from the east-tilted Gravel of Reno. This gravity low (and the Gravel of Reno it represents) terminates eastward against a steep, north-striking gravity gradient under central Reno; we interpret this to mark a west-dipping normal fault, the “Virginia Street fault,” which was active throughout deposition of the coarse clastic section. A more localized and pronounced gravity low in west Reno corresponds to the eastward projection of the east-dipping Neogene diatomite that is exposed at the ground surface. The abrupt termination of this negative gravity anomaly requires that the diatomite terminates eastward at depth, therefore documenting the eastern margin of the Neogene lacustrine environment. Other gravity anomalies document both the relief on the sub-Neogene unconformity and a complex pattern of faults that offset Neogene and younger rocks in the Reno basin, consistent with the multiple fault sets seen in outcrop west of Reno.

Constraints on the history and topography of the Northeastern Sierra Nevada from a Neogene sedimentary basin in the Reno-Verdi area, Western Nevada

Neogene (Miocene–Pliocene) sedimentary rocks of the northeastern Sierra Nevada were deposited in small basins that formed in response to volcanic and tectonic activity along the eastern margin of the Sierra. These strata record an early phase (ca. 11–10 Ma) of extension and rapid sedimentation of boulder conglomerates and debrites deposited on alluvial fans, followed by fluvio-lacustrine sedimentation and nearby volcanic arc activity but tectonic quiescence, until ∼ 2.6 Ma. The fossil record in these rocks documents a warmer, wetter climate featuring large mammals and lacking the Sierran orographic rain shadow that dominates climate today on the eastern edge of the Sierra. This record of a general lack of paleo-relief across the eastern margin of the Sierra Nevada is consistent with evidence presented elsewhere that there was not a significant topographic barrier between the Pacific Ocean and the interior of the continent east of the Sierra before ∼ 2.6 Ma. However, these sediments do not record an integrated drainage system either to the east into the Great Basin like the modern Truckee River, or to the west across the Sierra like the ancestral Feather and Yuba rivers. The Neogene Reno-Verdi basin was one of several, scattered endorheic (i.e., internally drained) basins occupying this part of the Cascade intra-arc and back-arc area.