J. D. Walker

The Geological Society of America Geologic Time Scale

The Geological Society of America has sponsored versions of the geologic time scale since 1983. Over the past 30 years, the Geological Society of America Geologic Time Scale has undergone substantial modifications, commensurate with major advances in our understanding of chronostratigraphy, geochronology, astrochronology, chemostratigraphy, and the geomagnetic polarity time scale. Today, many parts of the time scale can be calibrated with precisions approaching less than 0.05%. Some notable time intervals for which collaborative, multifaceted efforts have led to dramatic improvements in our understanding of the character and temporal resolution of key evolutionary events include the Triassic-Jurassic, Permian-Triassic, and Neoproterozoic-Phanerozoic boundaries (or transitions). In developing the current Geological Society of America Time Scale, we have strived to maintain a consistency with efforts by the International Commission on Stratigraphy to develop an international geologic time scale. Although current geologic time scales are vastly improved over the first geologic time scale, published by Arthur Holmes in 1913, we note that Holmes, using eight numerical ages to calibrate the Phanerozoic time scale, estimated the beginning of the Cambrian Period to within a few percent of the currently accepted value. Over the past 100 years, the confluence of process-based geological thought with observed and approximated geologic rates has led to coherent and quantitatively robust estimates of geologic time scales, reducing many uncertainties to the 0.1% level. (Less)

Extension in the Cretaceous Sevier orogen, North American Cordillera

Recent geologic studies in the western United States Cordillera provide evidence of extensional deformation in Cretaceous time, during development of the Sevier foreland fold and thrust belt. This evidence comes from the region roughly bound on the east by the fold and thrust belt and on the west by the Mesozoic continental arc. We postulate that extension in this Internal Zone accommodated gravitational collapse of the. evolving Sevier orogen. Major surface-breaking normal faults and extensional basins that characterize other regions of syncompressional gravitational collapse (such as the Himalayan orogen) have not been documented in the Cretaceous record of the Internal Zone. Consequently, if extension was widespread in the Internal Zone, then it had little surface expression. We propose a conceptual model of the Sevier orogen, consistent with geologic constraints and supported by simplistic rheological arguments, in which westward movement of a mid-crustal extensional allochthon was driven by buoyancy stresses in the over-thickened Internal Zone. Bound at the top by the master decollement of the Sevier fold and thrust belt and at the bottom by a west-dipping, normal-sense shear zone, this extensional allochthon would have been effectively decoupled from the upper and lower crust and free to accommodate gravitational collapse during continued convergence across the orogen.

The role of the mantle during crustal extension: Constraints from geochemistry of volcanic rocks in the Lake Mead area, Nevada and Arizona

One of the fundamental questions in areas of large-magnitude extension and magmatism is the role of the mantle in the extension process. The Lake Mead area is ideally suited for developing models that link crustal and mantle processes because it contains both mantle and crustal boundaries and it was the site of large-magnitude crustal extension and magmatism during Miocene time. In the Lake Mead area, the boundary between the amagmatic zone and the northern Colorado River extensional corridor parallels the Lake Mead fault system and is situated just to the north of Lake Mead. This boundary formed between 11 and 6 Ma during, and just following, the peak of extension and corresponds to a contact between two mantle domains. During thinning and replacement of the lithospheric mantle in the northern Colorado River extensional corridor, the lithospheric mantle in the amagmatic zone remained intact. Contrasting behavior to the north and south of this boundary may have produced the mantle domain boundary. The domain to the north of the boundary is characterized by mafic lavas with a lithospheric mantle isotopic and geochemical signature (ϵ Nd = -3 to -9; 87 Sr/ 86 Sr = 0.706-0.707). To the south of the boundary in the northern Colorado River extensional corridor, lavas have an ocean island basalt (OIB)-mantle signature and appear to have only a minor lithospheric mantle component in their source (ϵ Nd = 0 to +4; 87 Sr/ 86 Sr = 0.703-0.705). Mafic lavas of the northern Colorado River extensional corridor represent the melting of a complex and variable mixture of asthenospheric mantle, lithospheric mantle, and crust. Pliocene alkali basalt magmas of the Fortification Hill field represent the melting of a source composed of a mixture of asthenospheric mantle, high U/Pb (HIMU)-like mantle, and lithospheric mantle. Depth of melting of alkali basalt magmas remained relatively constant from 12 to 6 Ma during, and just after, the peak of extension but probably increased between 6 and 4.3 Ma following extension. Miocene and Pliocene low ϵ Nd and high 87 Sr/ 86 Sr magmas and tholeiites at Malpais Flattop were derived from a lithospheric mantle source and were contaminated as they passed through the crust. The shift in isotopic values due to crustal interaction is no more than 4 units in ϵ Nd and 0.002 in 87 Sr/ 86 Sr and does not mask the character of the mantle source. The change in source of basalts from lithospheric mantle to asthenospheric mantle with time, the OIB character of the mafic lavas, and the HIMU-like mantle component in the source are compatible with the presence of rising asthenosphere, as an upwelling convective cell, or plume beneath the northern Colorado River extensional corridor during extension. The Lake Mead fault system, a major crustal shear zone, parallels the mantle domain boundary. The Lake Mead fault system may locally represent the crustal manifestation of differential thinning of the lithospheric mantle.

Evidence for post-early Miocene initiation of movement on the Garlock fault from offset of the Cudahy Camp Formation, east-central California

The Cudahy Camp Formation, located in the El Paso basin, east-central California, consists of a 350-m-thick volcanic and sedimentary sequence of early Miocene age. Previous investigators determined from paleocurrent indicators that the source for these rocks was to the south and southeast, but failed to suggest a specific location for that source. Restoration of 64 km of left slip on the Garlock fault places Cudahy Camp rocks north and northwest of the early Miocene Eagle Crags volcanic field. The unique geochemical nature of basalts found in both locations, coincident ages of volcanic rocks, identical clast content of volcanic tuffs, and identical stratigraphic sequence indicate that the source for the Cudahy Camp Formation was the Eagle Crags volcanic field. This is the first documented occurrence of Miocene rocks offset by the Garlock fault. Because the Cudahy Camp rocks and their sources appear to be offset by the full displacement on the Garlock fault, we conclude that movement on the Garlock started after earliest middle Miocene time. Further constraining initiation of movement on the Garlock fault adds significantly to fully understanding the evolution of the western North American plate boundary in the middle to late Cenozoic.

The Mount Perkins block, northwestern Arizona: An exposed cross section of an evolving, preextensional to synextensional magmatic system

The steeply tilted Mount Perkins block, northwestern Arizona, exposes a cross section of a magmatic system that evolved through the onset of regional extension. New 4oAr/39Ar ages of variably tilted (0-90 o) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensional intrusive activity included emplacement of a composite Miocene laccolith and stock, trachydacite dome complex, and east striking rhyolite dikes. Related volcanic activity produced an -18-16 Ma stratovolcano, cored by trachydacite domes and flanked by trachydacite-trachyandesite flows, and -16 Ma rhyolite flows. Similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith. Magmatic activity synchronous with early regional extension (15.7-14.5 Ma) generated a thick, felsic volcanic sequence, a swarm of northerly striking subvertical rhyolite dikes, and rhyolite domes. Field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic. Modes of magma emplacement changed during the onset of extension from subhorizontal sheets, east striking dikes, and stocks to northerly striking, subvertical dike swarms, as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east-west trending, horizontal least principal stress. Preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system, which involved the ponding of mantle-derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels. Major extension halted this system by generating abundant pathways to the surface (fractures), which flushed out preexisting crustal melts and hybrid magmas. Remaining silicic melts were quenched by rapid, upper crustal cooling induced by tectonic denudation. These processes facilitated eruption of mafic magmas. Accordingly, silicic magmatism at Mount Perkins ended abruptly during peak extension -14.5 Ma and gave way to mafic magmatism, which continued until extension ceased.

Late Cretaceous extensional unroofing in the Funeral Mountains metamorphic core complex, California

New field and geochronologic data document the existence of Late Cretaceous extensional structures in the Death Valley region, California-Nevada. We have mapped two major, low-angle, ductile shear zones that omit stratigraphy in the footwall of the Funeral Mountains metamorphic core complex. Intervening strata have been strongly attenuated. Although stratigraphic offset across the shear zones is only 1.5 km, the presence of a large metamorphic discontinuity suggests that the amount of unroofing must be much greater. The timing of shear-zone formation, attenuation, and subsequent northwest-vergent folding is constrained by U-Pb geochronology on (1) prekinematic or synkinematic and (2) postkinematic pegmatites. Deformation was taking place by 72 Ma and had ended by 70 Ma. These results support earlier petrologic and geochronologic data that suggested substantial unroofing of the Funeral Mountains in Late Cretaceous time and add to a growing body of evidence for widespread Mesozoic extension in the hinterland of the Sevier thrust belt.

Development of three genetically related basins associated with detachment-style faulting: Predicted characteristics and an example from the central Mojave Desert, California

Three types of sedimentary basins may develop in large-magnitude detachment-type extensional systems. Possible early Miocene examples of these basin types have been identified in association with northeast-directed extension on the central Mojave metamorphic core complex, southern California. From west to east the basins are (1) the Tropico basin, a flexural basin formed on the unextended footwall behind, and bounded by, the detachment breakaway zone; (2) the Pickhandle basin, a supradetachment half graben bounded to the southwest by the detachment breakaway and to the northeast by the hanging wall; and (3) the Clews basin, an intra-hanging-wall basin that formed on the upper plate of the detachment system. Different basin types may be recognized by the nature and geometry of their strata. The footwall flexural basin should be shallow and areally extensive, with basin fill dominated by fine-grained, low-energy deposits. The supradetachment basin typically will be elongate normal to the extension direction and characterized by a thick sequence of volcanic and coarse-grained deposits that reflect its fault-controlled margins. Intra-hanging-wall basins may be of half- or full-graben geometry and will vary in dimension depending on the spacing of the transfer elements. Recognition of these basin types is potentially useful in delineating features of the extensional system that are important to its reconstruction, including the detachment breakaway zone and the boundaries of the extensional system parallel and normal to the extension direction.

Paleogeographic and tectonic implications of the geology of the Tiefort Mountains, northern Mojave Desert, California

Geologic mapping, petrologic and structural analyses, and U-Pb dating in the Tiefort Mountains, northern Mojave Desert, California, provide data on Paleozoic paleogeography and Mesozoic intra-arc tectonics. Metasedimentary rocks consist primarily of quartzose schists and gneisses that are provisionally correlated with late Precambrian Cordilleran passive-margin facies. Recognition of Precambrian augen gneiss and metasedimentary rocks with craton- margin protoliths suggests that the craton- miogeocline boundary is in or near the Tiefort Mountains; the presence of eugeoclinal rocks in ranges to the west implies truncation or telescoping of the miogeocline in this area. The metasedimentary units are intruded by Middle Jurassic granite and diorite orthogneisses dated by U-Pb as ca. 164 and ca. 160 Ma. Deformed rocks record amphibolite facies metamorphism and mylonitization during Middle to Late Jurassic southeast-vergent ductile shear. Pervasively annealed microstructures indicate that recrystallization outlasted deformation. At South Tiefort Mountain, the mylonitic fabric is deformed by northwest-trending folds and crosscut by a swarm of mafic and felsic dikes dated as 148 ± 14 Ma. At Tiefort Mountain synkinematic biotite granite gneiss is dated as ca. 105 Ma, and mylonitic fabrics are folded about north-trending axes. The presence of the Cretaceous orthogneiss suggests that two deformational events, Jurassic and Cretaceous in age, may have affected part of the study area. Dikes dated as ca. 82 Ma that crosscut all fabrics are interpreted to be part of a regional intrusive event. The timing and vergence of shear zones at South Tiefort Mountain are similar to contractional shear zones in adjacent areas, forming a belt of contractional deformation from the central Mojave Desert to the East Sierran thrust system. This deformation may be related to a period of late Middle to Late Jurassic sinistral-oblique subduction.