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Jonathan M.G. Glen

Publications and source records attributed to Jonathan M.G. Glen.

43 records · Page 3Linked to original sources
USGS2008

A prominent geophysical feature along the northern Nevada rift and its geologic implications, north-central Nevada

We consider the origin and character of a prominent large-scale geophysical feature in north-central Nevada that is coincident with the western margin of the northern Nevada rift—a mid-Miocene rift that includes mafic dike swarms and associated volcanic rocks expressed by a NNW-striking magnetic anomaly. The geophysical feature also correlates with mid-Miocene epithermal gold deposits and is coincident with the central part of the Battle Mountain–Eureka mineral trend. The Reese River Valley, a 2-km-deep Cenozoic basin, is located along the western margin of this feature and is inferred from the inversion of gravity data to be influenced by, and perhaps in part structurally controlled by, the geophysical feature. Geophysical modeling indicates that the source of the geophysical anomaly must extend to mid-crustal depths, perhaps reflecting a transition from Paleozoic crust in the southwest to Precambrian crust in the northeast, the presence of felsic intrusive rocks in the middle crust, or the edge of mid- to sub-crustal mafic intrusions related to late Tertiary magmatic underplating associated with hotspot magmatism. These cases offer very different possibilities for the age, depth, and origin of the source of the geophysical anomaly, and they present distinct implications for crustal evolution in the northern Great Basin. For example, if the anomaly is due to a pre-Cenozoic basement structure, then its coincidence with the mid-Miocene northern Nevada rift suggests that the trend of the rift was guided by the pre-existing crustal structure. On the other hand, if the anomaly is related to Tertiary mafic intrusions, then the western limit of this magmatism may have been influenced by hotspot fracturing of the crust.

Nevada↗
USGS2007

Three-dimensional geologic model of the northern Nevada rift and the Beowawe geothermal system, north-central Nevada

A three-dimensional (3D) geologic model of part of the northern Nevada rift encompassing the Beowawe geothermal system was developed from a series of two-dimensional (2D) geologic and geophysical models. The 3D model was constrained by local geophysical, geologic, and drill-hole information and integrates geologic and tectonic interpretations for the region. It places important geologic constraints on the extent and configuration of the active Beowawe geothermal system. The geologic framework represented in this model facilitates hydrologic modeling of the Beowawe geothermal system and evaluation of fluid flow in faults and adjacent rock units. Basin depths were determined using an iterative gravity-inversion technique that calculates the thickness of low-density, basin-filling deposits. The remaining subsurface structure was modeled using 2D potential-field modeling software. Crustal cross sections from the 2D models were generalized for use in the 3D model and consist of six stratigraphic layers defined as low-density basin sediments, volcanic rocks, basalt-andesite rocks of the northern Nevada rift, Jurassic and Cretaceous intrusive rocks, and Paleozoic siliceous and carbonate sedimentary rocks of the upper and lower plates of the Roberts Mountains allochthon, respectively. This simplified stratigraphy was combined with mapped surface geology and was extrapolated across the 3D model area. Features along the northern Nevada rift depicted by the model may represent preexisting crustal structures that controlled the locations and character of Tertiary tectonic and magmatic events related to Basin and Range extension and emplacement of the middle Miocene northern Nevada rift. Several of the geologic features represented are important components of the Beowawe geothermal system. Prominent ENE-trending faults (e.g., Malpais fault) that bound the southern edge of Whirlwind Valley, and older NNW-striking faults (e.g., Dunphy Pass and Muleshoe faults) that form major features of the model, are likely important pathways for geothermal fluids and groundwater flow from the Humboldt River, which may recharge the Beowawe system.

Nevada↗
USGS2005

Lava fingerprinting using paleomagnetism and innovative X-ray fluorescence spectroscopy: A case study from the Coso volcanic field, California

[1] We demonstrate an efficient method of rigorously separating difficult-to-distinguish lavas into eruptive units based on paleomagnetic remanence direction and rapid X-ray fluorescence spectroscopy (XRF) for Rb, Sr, Y, Zr, and Nb of polished paleomagnetic core samples (called PC XRF). Combined use of paleomagnetic remanence and PC XRF for lava fingerprinting allows correlation of individual eruptive units from one locality to another, permitting compilation of composite stratigraphy and paleomagentic measurement of relative vertical axis rotation of fault-bounded blocks. We test this lava fingerprinting method on rocks from the Coso volcanic field, California, against similar fingerprinting using XRF analysis by established methods. Resulting unit definitions and correlations are the same by both XRF techniques when coupled with paleomagnetic data, but at great time and cost savings with PC XRF. PC XRF analysis is possible because (1) matrix and grain size effects are minimal for the element set analyzed, (2) moderately phyric to aphyric polished paleomagnetic cores are already homogenous on spatial scales of 2 cm, and (3) use of element ratios cancels out some analytical uncertainties as well as minimizes effects of concentration variations due to fractional crystallization. Paleomagnetic remanence direction is an indispensable part of fingerprinting because it distinguishes flows of similar chemistry and can also place constraints on the duration of emplacement of each eruptive unit.

California↗
USGS2004

A kinematic model for the southern Alaska orocline based on regional fault patterns

Among the most prominent physiographic features of southern Alaska are a series of nested arcuate lineations, including the Denali fault, that parallel the concave-southward southern coastline of the state. These features are generally interpreted as major dextral shear zones that formed in the Late Cretaceous to early Tertiary in response to stresses imposed on the western edge of North America by transcurrent motion and oblique subduction along the North American margin. South-central Alaska consists of a collage of Paleozoic and Mesozoic tectonostratigraphic terranes and overlap assemblages. Following accretion to the continent, these terranes were transported northward along its margin along strike-slip faults such as the ancestral Denali fault that formed by oblique subduction. The terranes would have arrived at about their present position by Eocene time. It is commonly held that southwestern Alaska rotated into its present configuration by the middle Eocene, in response to impingement of northeast Asia against western Alaska, to form the southern Alaska orocline. Subsequent to this rotation during the middle and late Tertiary, southern Alaska terranes were presumably transported through the Alaska orocline by continued dextral movement along faults on the east limb of the orocline, such as the Denali and Tintina. Both initial bending of the crust to form the orocline and subsequent transport of crust through the orocline would result in significant crustal shortening within the bend. A model is suggested herein whereby shortening is accommodated by a system of secondary, northeast-trending thrust faults. The distribution of these faults shows a consistent pattern within the bend: the faults appear to splay off at or near the major dextral shear zones and generally occur west of the orocline’s axis. That these faults occur where deformation would be greatest to crust driven through the bend suggests that the faults are directly related to crustal dynamics within the bend. If this model is correct, one may infer the sense and timing of motion along many faults that otherwise lack or have limited documented histories. The interaction of strike-slip and thrust faults suggested by the model is reflected in the rupture sequence of the November 3, 2002, M7.9 Denali earthquake, which involved both initiation of slip along a previously unknown east-northeast–trending thrust fault and subsequent strike-slip motion along the McKinley strand of the east-west–trending Denali fault. This event is likely due, in part, to stresses imposed by accretion of the Yakutat terrane that is presently working its way into the bend of the orocline and deforming as a result of collision. Faulting along the western margin of the Yakutat terrane resembles that seen in central Alaska within the hinge of the bend. As such, it likely represents a present-day analog for crustal deformation associated with the orocline and may therefore provide clues to earlier stages of crustal deformation in central Alaska.

Alaska↗
USGS2004

Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October–November 2002, M = 7.9 Denali fault earthquake

The aftershock zone of the 3 November 2002, M = 7.9 earthquake that ruptured along the right-slip Denali fault in south-central Alaska has been investigated by using gravity and magnetic, magnetotelluric, and deep-crustal, seismic reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska Range orogen north of the Denali fault dip as steeply as 25°N and extend to depths as great as 20 km. These reflections outline a relict crustal architecture that in the past 20 yr has produced little seismicity. The Denali fault is nonreflective, probably because this fault dips steeply to vertical. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a rock body, with low electrical resistivity (>10 Ω·m), that is at depths below ∼10 km. Aftershocks of the Denali fault earthquake have mainly occurred shallower than 10 km. A high geothermal gradient may cause the shallow seismicity. Another possibility is that the low resistivity results from fluids, which could have played a role in locating the aftershock zone by reducing rock friction within the middle and lower crust.

Alaska↗
USGS2004

Geophysical data reveal the crustal structure of the Alaska range orogen within the aftershock zone of the Mw 7.9 Denali fault earthquake

Geophysical information, including deep-crustal seismic reflection, magnetotelluric ( mt ), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 M w 7.9 Denali fault earthquake. These data and aftershock seismicity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25° north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seismicity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonreflective, but modeling of mt , gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of mt data indicates a rock body, having low electrical resistivity (>10 Ω·m), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maximum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the aftershock zone.

Alaska↗