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Papers by Richard Schweickert

Seismological Research Letters, 1999

INTRODUCTION On Friday, October 30, 1998, at 1:53 am local time (9:53:31 UT), an earthquake of ma... more INTRODUCTION On Friday, October 30, 1998, at 1:53 am local time (9:53:31 UT), an earthquake of magnitude Mw 4.9 (UNR) took place near the California-Nevada state border. The epicenter of the event was 7 km north-northwest of Incline Village, Nevada and the north shore of Lake Tahoe (39°18.14′N, 119°58.76′W) (see Figures 1 and 2). A focal depth of 10.5 ± 0.3 km and strike-slip focal mechanism suggest it is similar to other recent eastern Sierra Nevada-Western Basin and Range Extensional Province earthquakes (Rogers et al., 1991; Ichinose et al., 1998a). The earthquake occurred 4 km northwest of the surface trace of the northeast-striking North Tahoe-Incline Village Fault Zone, which is the largest fault zone in the area (Figure 2). This is a system of south-east-dipping normal faults, so the earthquake is well within the footwall of this fault zone. The Seismological Laboratory received felt reports, from which we assigned the...

Geosphere, 2013

Age, chemical, and isotopic data from late Cenozoic volcanic rocks in the northern Sierra Nevada,... more Age, chemical, and isotopic data from late Cenozoic volcanic rocks in the northern Sierra Nevada, California (USA), from Lake Tahoe north to the southern end of the modern Cascades volcanic arc, were obtained to investigate the evolution of the upper mantle beneath this continental margin during the transition from active subduction to the opening of a slabless window, and to test the possibility that the foundering of mantle lithosphere proposed for the southern Sierra Nevada extended to the northern reaches of the mountain range. Our data are consistent with previous work in the region and illustrate that volcanism shifted from widespread intermediate composition magmatism to small volume, localized trachybasalts to trachyandesites ca. 3 Ma. Similar to southern Cascades volcanism, 87 Sr/ 86 Sr and 206 Pb/ 204 Pb decrease, and ε Nd increase in the older (older than 3 Ma) volcanic rocks with increasing proportions of a slab component, as measured by increasing (Sr/P) N , where N is primitive-mantle normalized. We interpret these observations as evidence that the older volcanic rocks are subduction related and represent the products of basaltic melts derived from fl ux melting of mantle wedge that interacted to varying degrees during ascent with lower ε Nd and higher 87 Sr/ 86 Sr sub-Sierra Nevada continental mantle lithosphere. The younger volcanic rocks lack evidence for the involvement of a slab component in their generation, but have ranges of Nd, Sr, and Pb isotopic compositions similar to those of older volcanic rocks interpreted to have interacted to the greatest extent with the continental mantle lithosphere. However, the younger volcanic rocks have higher high fi eld strength element (HFSE) and higher phosphorus abundances, and higher (La/Yb) N , than their older counterparts, suggesting that they are not simply the products of smaller degrees of partial melting of the same mantle lithosphere involved in the older magmatism. The high HFSE and P contents were more likely controlled by metasomatic accessory carrier phases such as rutile and apatite, the stabilities of which control the abundance of these elements in melts produced from the lithospheric mantle after 3 Ma. One possibility is that the accessory phases were introduced to lithosphere during melt-wall-rock interaction associated with the older magmatic episode. These phases were then purged as a result of conductive heating of the remaining lithospheric mantle triggered by postsubduction upwelling of the sublithospheric mantle. Our data are consistent with lithospheric mantle serving as a melt reactor during the earlier subduction-related magmatism that was baked out during later conductive heating, a process that may be relevant to the production of immediately postsubduction magmatism along other continental margins.

Geological Society of America Bulletin, 2012

Http Dx Doi Org 10 1080 00206810903123481, Aug 12, 2009

Beheaded west-flowing drainages are identified in the northern Sierra Nevada west and north of th... more Beheaded west-flowing drainages are identified in the northern Sierra Nevada west and north of the Lake Tahoe Basin. These include the canyons of the modern South Fork American River, the Rubicon River, the South Yuba River and their tributaries. These drainages were beheaded, east-side-down displacements along faults of the Tahoe-Sierra frontal fault zone as Cenozoic normal faults encroached into the

Geological Society of America Bulletin, 1991

Aapg Bulletin, Feb 1, 1972

Geological Society of America Bulletin, 1975

Prebatholithic rocks of Mesozoic age in the Sierra Nevada can be interpreted as remnants of ancie... more Prebatholithic rocks of Mesozoic age in the Sierra Nevada can be interpreted as remnants of ancient volcanic arcs, subduction complexes, and sequences of oceanic lithosphere. Two partly coeval subparallel volcanic arcs, one in the western foothills and the other in the northern and eastern Sierra Nevada, are juxtaposed. The western arc was an east-facing island-arc complex that evolved through a series of steps including formation of a remnant arc and interarc basin. The eastern arc was a west-facing marginal arc that was constructed on the edge of North America. Both arc-subduction complexes consumed intervening oceanic lithosphere and collided during the Late Jurassic Nevadan orogeny. Generation of magmas in both arcs apparently ceased at about this time, and renewed subduction was initiated west of the island arc in latest Jurassic time, giving birth to the Franciscan-Sierran arctrench complex. Fault zones and mélanges in the western Sierra Nevada reflect the complex suturing at the collision boundary.

Tectonics, 1992

Detailed studies in the CP Hills and Mine Mountain area of the Nevada Test Site (NTS), together w... more Detailed studies in the CP Hills and Mine Mountain area of the Nevada Test Site (NTS), together with analysis of published maps and cross sections and a reconnaissance of regional structural relations, indicate that the CP thrust of Barnes and Poole [ 1968] actually comprises two separate, oppositely verging Mesozoic thrust systems: (1) the west vergent CP thrust, which is well exposed in the CP Hills and at Mine Mountain, and (2) the east vergent Belted Range thrust located northwest of Yucca Flat. Regional structural relations indicate that the CP thrust forms part of a narrow sigmoidal belt of west vergent folding and thrusting traceable for over 180 km along strike. The Belted Range thrust represents earlier Mesozoic deformation that was probably related to the Last Chance thrust system in southeastern California, as suggested by earlier workers. A reconstruction of the pre-Tertiary geometry of the Cordilleran fold and thrust belt in the region between the NTS and the Las Vegas Range bears a close resemblance to other regions of the Cordillera and suggests that west vergent deformation developed in the hinterland of a part of the Sevier fold and thrust belt characterized by substantial structural relief. Reconstruction of the fold and thrust belt also suggests that previous estimates of upper crustal Tertiary extension north of the Las Vegas Valley shear zone (e.g., 80% [Guth, 1981]) are 2 or 3 times too large. INTRODUCTION The Nevada Test Site (NTS) (Figure 1) of the southern Great Basin lies within the dominantly east vergent Mesozoic Cordilleran thrust belt near one of the thickest known parts of the Paleozoic Cordilleran miogeocline. In this region, Barnes and Poole [1968] and Hinrichs [ 1968] interpreted several thrust faults that place upper Precambrian and lower Paleozoic rocks over upper Paleozoic rocks as remnants of a single, regional, east to southeast vergent thrust system. They named this thrust system the CP thrust

Geological Society of America Special Papers, 1976

Geosphere, 2014

A gigantic ~12 km 3 landslide detached from the west wall of Lake Tahoe (California-Nevada, USA),... more A gigantic ~12 km 3 landslide detached from the west wall of Lake Tahoe (California-Nevada, USA), and slid 15 km east across the lake. The splash, or tsunami, from this landslide eroded Tioga-age moraines dated as 21 ka. Lake-bottom short piston cores recovered sediment as old as 12 ka that did not reach landslide deposits, thereby constraining the landslide age as 21-12 ka. Movement of the landslide splashed copious water onto the countryside and lowered the lake level ~10 m. The sheets of water that washed back into the lake dumped their sediment load at the lowered shoreline, producing deltas that merged into delta terraces. During rapid growth, these unstable delta terraces collapsed, disaggregated, and fed turbidity currents that generated 15 subaqueous sediment wave channel systems that ring the lake and descend to the lake fl oor at 500 m depth. Sheets of water commonly more than 2 km wide at the shoreline fed these systems. Channels of the systems contain sediment waves (giant ripple marks) with maximum wavelengths of 400 m. The lower depositional aprons of the system are surfaced by sediment waves with maximum wavelengths of 300 m. A remarkably similar, though smaller, contemporary sediment wave channel system operates at the mouth of the Squamish River in British Columbia. The system is generated by turbidity currents that are fed by repeated growth and collapse of the active river delta. The Tahoe splash-induced backwash was briefl y equivalent to more than 15 Squamish Rivers in full fl ood and would have decimated life in low-lying areas of the Tahoe region.