Large-magnitude Extension Research Articles

Kinematics and timing of Tertiary extension in the western Lake Mead region, Nevada

Explanation of the origin of the complex array of structures in some extensional terranes (including folds and normal, strike-slip, and reverse faults) includes many models that implicitly assume kinematic compatibility between and contemporaneous operation of these structures. We present new stratigraphic and age data from the highly extended western Lake Mead region, Nevada, together with an analysis of fault kinematics (technique of Marrett and Allmendinger, 1990) to test the assumptions of kinematic compatibility and contemporaneity of structures in an area of excellent exposure and superb stratigraphic control. Our analysis indicates an overlapping but clearly distinct chronology of deformation. Early regional extension (>18?-13.5 Ma) is marked by development of a basin into which the middle Miocene lower Horse Spring Formation was deposited. In the western Lake Mead region, this basin was disrupted by more areally restricted, post-13 Ma normal and kinematically coupled right-slip faulting along the Las Vegas Valley shear zone. Kinematic analysis of faults indicates an average regional extension direction of nearly due west for the middle and late Miocene. Extension and right-slip faulting was followed by development of dominantly southvergent contractional structures including tight, east-plunging folds and east-striking reverse faults. These structures deform the post 8.5 Ma Muddy Creek Formation; the Muddy Creek Formation is not cut by the Las Vegas Valley shear zone. Older faults, including the eastern Las Vegas Valley shear zone, reactivated as south-vergent reverse faults. Northeast-striking left-slip faults cut folds and reverse faults. These observations show that north-south shortening and left-slip faulting postdate the major phase of extension and right-slip faulting in the western Lake Mead area. Dynamic models that invoke either a single stress field or rotating stress fields to explain development of structures in the western Lake Mead area are inconsistent with the kinematic and age data. Similarly, kinematic models that view all structures in the context of a single strain field are precluded by systematic cross-cutting relationships that demonstrate at least partial diachroneity of deformational styles. Large-magnitude extension south of the Las Vegas Valley and Lake Mead fault zones appears to have been followed by north-south contraction that was highly localized near the region of greatest extension. We suggest that lateral pressure gradients arising from differential crustal thinning at the northern end of the Colorado River extensional corridor may have provided the driving mechanism for localized contractional deformation.

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.

Tertiary magmatism and mylonitization in the Ruby-East Humboldt metamorphic core complex, northeastern Nevada: U-Pb geochronology and Sr, Nd, and Pb isotope geochemistry

Research Article| July 01, 1993 Tertiary magmatism and mylonitization in the Ruby-East Humboldt metamorphic core complex, northeastern Nevada: U-Pb geochronology and Sr, Nd, and Pb isotope geochemistry JAMES E. WRIGHT; JAMES E. WRIGHT 1Department of Geology and Geophysics, Rice University, Houston, Texas 77251 Search for other works by this author on: GSW Google Scholar ARTHUR W. SNOKE ARTHUR W. SNOKE 2Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1993) 105 (7): 935–952. https://doi.org/10.1130/0016-7606(1993)105<0935:TMAMIT>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JAMES E. WRIGHT, ARTHUR W. SNOKE; Tertiary magmatism and mylonitization in the Ruby-East Humboldt metamorphic core complex, northeastern Nevada: U-Pb geochronology and Sr, Nd, and Pb isotope geochemistry. GSA Bulletin 1993;; 105 (7): 935–952. doi: https://doi.org/10.1130/0016-7606(1993)105<0935:TMAMIT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract U-Pb analyses of 48 zircon and/or monazite fractions from ten intrusive rocks that range in composition from quartz diorite to leucogranite yield crystallization ages between 40 and 29 Ma. The new data indicate that Tertiary intrusive rocks constitute a widespread and significant magmatic component of the Ruby-East Humboldt metamorphic core complex. Some of the intrusive rocks are gneissic to mylonitic and have been penetratively deformed in a shear zone that can be traced for >100 km along the west flank of the core complex. These data, when coupled with previously published geochronologic data, indicate that an important episode of mylonitization occurred between ∼29 and 23 Ma (late Oligocene). Some field relations and geochronologic data, however, also suggest an earlier history of mylonitization in parts of the core complex. This earlier mylonitic history may have occurred in the Eocene or even late Mesozoic time, although more-detailed geochronologic data are needed to clearly define the exact time interval.Sr, Nd, and Pb isotopic data from the dated intrusive units indicate that a relatively sharp, approximately east-west-striking crustal boundary separating predominantly Archean basement in the north from Proterozoic basement in the south transects the complex. We speculate that the crustal boundary represents the continuation of the Cheyenne belt into this part of northeastern Nevada. These new data cast doubt on the existence of the proposed 2.0-2.3 Ga crustal province in this part of the northern Great Basin. The isotopic data from the plutonic rocks coupled with published data from at least in part coeval volcanic rocks indicate that mantle-derived magmas were important in their genesis. Collectively, these data indicate that significant crustal growth, via the addition of mantle-derived magmas, occurred during large-magnitude extension in this part of the northern Great Basin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evidence for Large-Magnitude Cretaceous or Early Tertiary Extension in the Pequop Mountains, Nevada: Implications for the Early Tertiary Unconformity in Northeast Nevada

Evidence for Large-Magnitude Cretaceous or Early Tertiary Extension in the Pequop Mountains, Nevada: Implications for the Early Tertiary Unconformity in Northeast Nevada