Present-day motion of the Sierra Nevada block and some tectonic implications for the Basin and Range province, North American Cordillera

Abstract In order to investigate the large errors which sometimes characterize fundamental-mode attenuation coefficient determinations, we have made many such determinations in the Basin and Range province and have plotted particle motion for the observed three-component seismograms. Rayleigh wave attenuation coefficient values determined for four two-station paths across the Basin and Range province fluctuate between about −2.0 × 10−3km−1 and +3.0 × 10−3km−1 at periods between 6 and 33 s. Partide motion plots indicate that many of those determinations are contaminated by arrivals from non-great circle paths and from waves generated at heterogeneities along the path, factors which lead to systematic errors in the measured attenuation coefficient values. Attenuation coefficient determinations for the path MNV to ELK, which is within 20° of being normal to the structural trends of the Sierra Nevada and Great Valley, are, however, relatively free from such contamination. Mean values for that path decrease rapidly with period from about 3.0 × 10−3 to about 0.7 × 10−3km−1 between 6 and 10 s and then decrease slowly to about 0.3 × 10−3km−1 at 33 s. Standard deviations range between 0.2 × 10−3 and 0.3 × 10−3km−1 for most of the period range, but increase to about 0.4 × 10−3 at periods between 6 and 8 s. These results indicate that careful screening, based upon criteria such as three-dimensional particle motion, are necessary to obtain surface wave attenuation data of sufficient quality to use for inverting for crustal anelasticity in complex regions.

Download Full-text

Minimal net incision of the northern Sierra Nevada (California, USA) since the Eocene–early Oligocene

Geology ◽

10.1130/g47902.1 ◽

2020 ◽

Vol 48 (10) ◽

pp. 1023-1027

Author(s):

Emmanuel J. Gabet ◽

Daniel P. Miggins

Keyword(s):

Sierra Nevada ◽

North American ◽

Past Century ◽

Mountain Range ◽

Late Cenozoic ◽

North American Cordillera ◽

The Past ◽

The North ◽

Early Oligocene ◽

Northern Half

Abstract Significant late Cenozoic uplift (>1000 m) of the northern half of the Sierra Nevada (California, USA), a mountain range in the North American Cordillera, has been a dominant paradigm over the past century. This paradigm has been supported by evidence suggesting that in response to this recent uplift, the range’s deep canyons were incised in the past 3–4 m.y. However, paleochannel elevations compiled from a mining report and geological maps demonstrate that while some modern rivers have incised 560 m below their Eocene–early Oligocene riverbeds, incision by others has been <300 m. For example, Eocene–early Oligocene fluvial gravels can be found just 161 m above the modern channel deep within the canyon of the South Fork American River. We conclude that the initiation of late Cenozoic incision was due to a resumption of a period of downcutting that was interrupted in the Eocene when the rivers were buried by fluvial sediment and by later volcanic deposits. This interpretation challenges the hypothesis that recent uplift was responsible for deep canyon incision. Correctly identifying the causes of recent incision in the northern Sierra Nevada has important implications for understanding the geological history of the North American Cordillera because the range is hypothesized to have been the western ramp of the Nevadaplano.

Download Full-text