Stephen M. Richard

Spatial and temporal relationships between mid‐Tertiary magmatism and extension in southwestern Arizona

Cenozoic magmatism in southwestern Arizona, which is within the Basin and Range tectonic province, occurred almost entirely between 15 and 25 Ma. Volcanic rocks typically consist, in ascending order, of (1) a thin sequence of mafic to intermediate lava flows, (2) voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows, and (3) basalt and andesite. Volcanic rock sequences rest disconformably on pre-Tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism. Prevolcanic clastic debris is interpreted as a consequence of local early normal faulting. In most regions, tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism. Extension generally ended before about 17 Ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated Transition Zone tectonic province which is adjacent to the Colorado Plateau. Rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 Ma. Extension was extreme in this belt, whereas it was generally moderate to slight in other parts of southwestern Arizona. Large-magnitude extension was not associated with areas of greatest igneous activity, and rapid cooling and exhumation of core complexes postdated local magmatism. These relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust. Except for young extension in this northwest trending belt, there are no apparent regional migration trends for either magmatism or extension within southwestern Arizona. Lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension. The long duration and large magnitude of extension adjacent to the Transition Zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots. Regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally.

A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards

The geologic time scale is a complex data structure composed of abstract elements that represent time intervals and instants and their relationships with specific concrete representations in the geologic record as well as the observations made of those concrete representations. The International Union of Geological Sciences’ International Commission on Stratigraphy guidelines recommends a very precise usage of the relationships between these components in order to establish a standard time scale for use in global correlations. However, this has been primarily described in text. Here, we present a formal representation of the model using the Unified Modeling Language (UML). The model builds on existing components from standardization of geospatial information systems. The use of a formal notation enforces precise definition of the relationships between the components. The UML platform also supports a direct mapping to an eXtensible Markup Language (XML)‐based file format, which may be used for the exchange of stratigraphic information using Web-service interfaces.

Age and tectonic setting of the Mesozoic McCoy Mountains Formation in western Arizona, USA

The McCoy Mountains Formation consists of Upper Jurassic to Upper Cretaceous silt- stone, sandstone, and conglomerate exposed in an east-west-trending belt in southwest- ern Arizona and southeastern California. At least three different tectonic settings have been proposed for McCoy deposition, and multiple tectonic settings are likely over the ~80 m.y. age range of deposition. U-Pb iso- topic analysis of 396 zircon sand grains from at or near the top of McCoy sections in the southern Little Harquahala, Granite Wash, New Water, and southern Plomosa Moun- tains, all in western Arizona, identifi ed only Jurassic or older zircons. A basaltic lava fl ow near the top of the section in the New Water Mountains yielded a U-Pb zircon date of 154.4 ± 2.1 Ma. Geochemically similar lava fl ows and sills in the Granite Wash and southern Plomosa Mountains are inferred to be approximately the same age. We interpret these new analyses to indicate that Meso- zoic clastic strata in these areas are Upper Jurassic and are broadly correlative with the lowermost McCoy Mountains Formation in the Dome Rock, McCoy, and Palen Moun- tains farther west. Six samples of numerous Upper Jurassic basaltic sills and lava fl ows in the McCoy Mountains Formation in the Granite Wash, New Water, and southern Plomosa Mountains yielded initial e Nd values (at t = 150 Ma) of between +4 and +6. The geochemistry and geochronology of this igne- ous suite, and detrital-zircon geochronology of the sandstones, support the interpretation that the lower McCoy Mountains Forma- tion was deposited during rifting within the western extension of the Sabinas-Chihuahua- Bisbee rift belt. Abundant 190-240 Ma zir- con sand grains were derived from nearby, unidentifi ed Triassic magmatic-arc rocks in areas that were unaffected by younger Juras- sic magmatism. A sandstone from the upper McCoy Mountains Formation in the Dome Rock Mountains (Arizona) yielded numer- ous 80-108 Ma zircon grains and almost no 190-240 Ma grains, revealing a major reor- ganization in sediment-dispersal pathways and/or modifi cation of source rocks that had occurred by ca. 80 Ma.

A geologic timescale ontology and service

We have developed an OWL ontology for the geologic timescale, derived from a Unified Modeling Language (UML) model that formalized the practice of the International Commission for Stratigraphy (ICS) (Cox and Richard 2005). The UML model followed the ISO/TC 211 modeling conventions, and was the basis for an XML implementation that was integrated into GeoSciML 3.0. The OWL ontology is derived using rules for generating OWL ontologies from ISO-conformant UML models, as provided in a (draft) standard from ISO/TC 211. The basic ontology is also aligned with SKOS to allow multilingual labels, and to enable delivery through a standard vocabulary interface. All versions of the International Stratigraphic Chart from 2004 to 2014 have been encoded using the ontology. Following ICS practice, the elements of the timescale retain the same identifiers across the multiple versions, though the information describing each geochronologic unit evolves with the versions of the timescale. The timescales are published through multiple web interfaces and APIs.