Crustal structure of the southern Rocky Mountains from seismic measurements

Prodehl, C.; Pakiser, L. C.

doi:10.1130/0016-7606(1980)91<147:csotsr>2.0.co;2

Cited by 50 publications

(29 citation statements)

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“…We do not, however, observe distinct P-wave and S-wave Moho reflections, although we reach the critical distance of approximately 120 km for Moho reflections from a presumed 40 km deep crust, depending on the velocity distribution, particularily in the lowermost crust. Our observation is consistent with earlier refraction surveys in northern Colorado and southeastern Wyoming [14,15]. These earlier studies reveal no critical Moho reflections from the area just south of the Archean province, in contrast to high-amplitude wide-angle reflections from the Archean in Wyoming and the Proterozoic in northern-central Colorado.…”

Section: Discussionsupporting

confidence: 93%

“…The lack of Moho reflections does not permit us to verify the crustal model of southwest Wyoming derived from gravity and low-resolution refraction data [3,15] in which the crust deepens from the Archean craton to the Proterozoic accreted crust. Although the inversion and forward modelling results might be biased to a small degree by a dipping structure, our data provide an estimate of crustal composition and formation of an early Proterozoic island arc complex whose compositional character might have been preserved through time.…”

Section: Discussionmentioning

confidence: 86%

“…The large time difference of about 200-400 Ma between the formation of the metavolcanics and the emplacement of the granitic and anorthositic plutons, however, suggests an extended history of possibly several accretional events. Widely spaced seismic refraction data indicate a crust that thickens from 37-41 km in southeastern Wyoming to about 48-54 km beneath the Colorado Front Range [13][14][15]. COCORP reflection profiles across the Laramie Range exhibit low-amplitude, discontinuous, multicyclic arrivals that are observed over the Denver and Laramie Basins [16,17] …”

Section: Geology and Previous Geophysical Studiesmentioning

confidence: 99%

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Seismic wide-angle study of accreted Proterozoic crust in southeastern Wyoming

Gohl

Smithson

1994

Earth and Planetary Science Letters

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A seismic wide-angle experiment was conducted in southeastern Wyoming, USA to investigate the seismic character of a postulated Proterozoic magmatic arc south of the suture (Cheyenne Belt) to the Archean Wyoming Province. Recordings from vibrator and dynamite sources with offsets between 34 and 126 km reveal no evidence for Moho reflections. The large-offset recordings contain multicyclic bands of reflective phases from the middle to lower crust. The data were transformed into the intercept time-ray parameter (~--p) domain to estimate local depth bounds. A subsequent 1D inversion using high-amplitude ~'-p arrivals shows that the reflective part of the crust ranges from the depths of 25 to 40 km. This part of the crust exhibits velocities increasing from about 6.5 to 7.5 km/s. Reflectivity modeling shows that the lower crust might consist of a zone of alternating low-and high-velocity layers with average velocity increasing. The average lower crustal velocity of about 6.9 km/s suggests a predominantly mafic composition with interlayered intermediate to felsic components generating impedance contrasts that cause observable amplitudes from reflections at large offsets but not at clearly pre-critical and near-vertical distances. Our model is consistent with observations of interlayered sequences of gabbroic to ultramafic rocks with more felsic anorthositic and charnockitic rocks in the exposed lower crust of magmatic arc complexes. The lack of wide-angle Moho reflections might be explained by a gradational compositional boundary, or a transitional phase change from granulite to eclogite facies.

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Section: Discussionsupporting

confidence: 93%

Section: Discussionmentioning

confidence: 86%

Section: Geology and Previous Geophysical Studiesmentioning

confidence: 99%

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Seismic wide-angle study of accreted Proterozoic crust in southeastern Wyoming

Gohl

Smithson

1994

Earth and Planetary Science Letters

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“…Johnson et al (1984) favour the former interpretation as it is more consistent with the results of refraction studies which suggest a 12 ? 5 km increase in crustal thickness to the south (Prodehl & Pakiser, 1980). The Cheyenne belt is clearly a profound lithological, geochronological, and geophysical discontinuity.…”

Section: Geological Settingmentioning

confidence: 98%

Evidence for an inverted metamorphic gradient associated with a Precambrian suture, southern Wyoming

Duebendorfer

1988

Journal Metamorphic Geology

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An inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100" C over a km-wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup.Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite-grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet-biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation at minimum temperatures of 480" C and pressures of 350-400 MPa (3-5-4.0 kbar). This implies tectonic burial or upper plate thickness of 13-15 km.The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkinematic growth of garnet and staurolite, preclude rotation of pre-existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low-angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless 'Present address: De artment of Geoscience, University of Nevada, Las%egas, Nevada 89154, USA.high shear stresses are postulated. Up-dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present.

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“…Locations were computed using the P-wave velocity model derived by Wong and Humphrey (1985), which incorporated data from a crustal structure study of the southern Rocky Mountains by Prodehl and Pakiser (1980). The model consists of three layers overlying a 7.9 km/sec half-space (Table 5).…”

Section: Instrumentation and Data Analysis Of Locally-recorded Earthqmentioning

confidence: 99%

The Carbondale, Colorado, earthquake swarm of April - May, 1984

Goter¹,

Presgrave²,

Henrisey³

et al. 1988

Open-File Report

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During the Carbondale, Colorado, earthquake swarm (Mi <3.2) of April -May 1984, a total of 34 earthquakes were located using regional data or local data from a temporary network installed in the epicentral area. These earthquakes were relocated with the method of Joint Hypocenter Determination (JHD) in order to better delineate the source zone of the swarm. The majority of the earthquakes occurred about 7 km south-southwest of Carbondale, at the northern terminus of the Elk Mountain anticline. The most precisely determined hypocenters of the swarm have focal depths in the 2 to 7 km range. Two composite focal mechanisms were constructed from local first-motion data. One of these mechanisms corresponds to normal faulting and the second suggests reverse faulting.

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Crustal structure of the southern Rocky Mountains from seismic measurements

Cited by 50 publications

References 0 publications

Seismic wide-angle study of accreted Proterozoic crust in southeastern Wyoming

Seismic wide-angle study of accreted Proterozoic crust in southeastern Wyoming

Evidence for an inverted metamorphic gradient associated with a Precambrian suture, southern Wyoming

The Carbondale, Colorado, earthquake swarm of April - May, 1984

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