A Geophysical Study of North Park and the Surrounding Ranges, Colorado

Abstract

Research Article| August 01, 1969 A Geophysical Study of North Park and the Surrounding Ranges, Colorado JOHN C BEHRENDT; JOHN C BEHRENDT U.S. Geological Survey, Federal Center, Denver, Colorado Search for other works by this author on: GSW Google Scholar PETER POPENOE; PETER POPENOE U.S. Geological Survey, Federal Center, Denver, Colorado Search for other works by this author on: GSW Google Scholar ROBERT E MATTICK ROBERT E MATTICK U.S. Geological Survey, Federal Center, Denver, Colorado Search for other works by this author on: GSW Google Scholar Author and Article Information JOHN C BEHRENDT U.S. Geological Survey, Federal Center, Denver, Colorado PETER POPENOE U.S. Geological Survey, Federal Center, Denver, Colorado ROBERT E MATTICK U.S. Geological Survey, Federal Center, Denver, Colorado Publisher: Geological Society of America Received: 13 Jun 1968 Revision Received: 27 Jan 1969 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1969, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1969) 80 (8): 1523–1538. https://doi.org/10.1130/0016-7606(1969)80[1523:AGSONP]2.0.CO;2 Article history Received: 13 Jun 1968 Revision Received: 27 Jan 1969 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JOHN C BEHRENDT, PETER POPENOE, ROBERT E MATTICK; A Geophysical Study of North Park and the Surrounding Ranges, Colorado. GSA Bulletin 1969;; 80 (8): 1523–1538. doi: https://doi.org/10.1130/0016-7606(1969)80[1523:AGSONP]2.0.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 A geophysical study in the North Park basin and surrounding mountains, Colorado illustrates the structural relationship of various sedimentary, metamorphic, and igneous rock units. Bouguer anomalies from 1330 gravity stations range from −210 mgal over Precambrian metamorphic rocks in the mountains to −260 mgal in the Walden syncline and —280 mgal in the North Park syncline. Steep gradients delineate a fault which strikes west-northwest along the north flank of the North Park syncline. Two models fitted to the gravity data show 1 to 2 km relief on this steeply dipping fault. Density contrasts between Precambrian metamorphic and igneous rocks produce anomalies of as much as 25-mgal amplitudes in the Park and Medicine Bow Ranges.A 30-km-long seismic refraction profile, parallel to the most negative Bouguer anomaly values in the North Park basin, shows velocities increasing from 2.5 to 3.4 km/sec within Tertiary rocks at depths ranging from 1.2 to 2.0 km. Mesozoic sedimentary rocks have a velocity of 4.0 to 4.5 km/sec, a very high velocity in view of the predominance of Upper Cretaceous rocks. Precambrian basement with a velocity of 6.25 km/sec underlies the profile at depths ranging from 3.5 to 4.5 km. Strong second arrivals across the profile, observed at distances of more than 14 km from the shotpoints and interpreted as SP reflections, verified the refraction model.An aeromagnetic survey shows numerous anomalies ranging from 100 to 200γ in the Park and Rabbit Ears Ranges and in the Never Summer Mountains, to 400γ in the Front Range, and to 1200γ over the Medicine Bow Range. Positive anomalies in the Park, Medicine Bow, and Front ranges overlie metamorphic rocks. Magnetic and gravity data suggest that the Never Summer Mountains are separated from the Front Range by a north-trending, steeply east-dipping reverse fault, extending beneath the Front Range along the Colorado River valley. The magnetic data indicate that this fault may connect with a possible fault that is parallel to the Laramie River valley. In the Rabbit Ears Range, a series of magnetic anomalies show that igneous rocks are present in the eastern part of the range.A northeast-trending positive magnetic anomaly, which is parallel to foliation trends reported in Precambrian rocks, extends from the Park Range across the North Park basin to the Medicine Bow Range. On the basis of this anomaly, the high seismic velocity of the Precambrian basement, and computed profiles fitted to the gravity and magnetic data, we infer that much of the basin is underlain by high-density metamorphic rock. As shown by gravity data, the deepest part of the basin is 2.7 km below sea level, resulting in a maximum relief of 6.7 km on the basement, relative to the Medicine Bow Range.A 25-mgal negative gravity anomaly and a zone of negative magnetic anomalies outline a large granitic intrusion in the Park Range, which probably extends northeast beneath the North Park basin and connects with granitic rocks in the Medicine Bow Range. 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