The Form and Growth of Fingered Sheet Intrusions

Research Article| March 01, 1975 The Form and Growth of Fingered Sheet Intrusions DAVID D. POLLARD; DAVID D. POLLARD 1Department of Geological Science, University of Rochester, Rochester, New York 146272Present address: National Center for Earthquake Research, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025. Search for other works by this author on: GSW Google Scholar OTTO H. MULLER; OTTO H. MULLER 1Department of Geological Science, University of Rochester, Rochester, New York 14627 Search for other works by this author on: GSW Google Scholar DAVID R. DOCKSTADER DAVID R. DOCKSTADER 1Department of Geological Science, University of Rochester, Rochester, New York 14627 Search for other works by this author on: GSW Google Scholar Author and Article Information DAVID D. POLLARD 1Department of Geological Science, University of Rochester, Rochester, New York 146272Present address: National Center for Earthquake Research, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025. OTTO H. MULLER 1Department of Geological Science, University of Rochester, Rochester, New York 14627 DAVID R. DOCKSTADER 1Department of Geological Science, University of Rochester, Rochester, New York 14627 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (3): 351–363. https://doi.org/10.1130/0016-7606(1975)86<351:TFAGOF>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DAVID D. POLLARD, OTTO H. MULLER, DAVID R. DOCKSTADER; The Form and Growth of Fingered Sheet Intrusions. GSA Bulletin 1975;; 86 (3): 351–363. doi: https://doi.org/10.1130/0016-7606(1975)86<351:TFAGOF>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 The form of several sheet intrusions in a sandstone-shale host rock is regular, but much more complex than the term "sheet intrusion" implies. The periphery of a sill emanating from the Shonkin Sag laccolith in Montana is composed of fingers of igneous rock with lengths of about 100 m, widths of 3 to 5 m, and thicknesses which vary from about 1.2 m near their points of coalescence to about 0.6 m near their terminations. At finger terminations, the host rock is wedged aside and compacted, and along the sides of fingers, buckling and shearing of strata are common. Energy expended by magma to dilate and flow through a sheet is small compared to that for fingers. Initiation of fingers is attributed to instability of the advancing interface between a viscous magma and a more viscous host rock. Fingers coalesce into a sheet such that cusp-shaped grooves filled with deformed host rock remain in the contact. Systematic offsets of sheet-intrusion contacts form where offset fingers coalesce. Several large dikes (5 to 20 m thick) near La Veta, Colorado, exhibit contact offsets of as much as 3 m in magnitude, which give the dike the appearance of a clapboard wall. Small dikelets, horns, often project from the outer edge of offsets. If fingers grow in a surface perpendicular to the least principal compressive stress, offsets may develop as the intrusion propagates through a region where this stress changes orientation. Grooves and offsets form parallel to finger lengths and thus indicate the propagation direction of the intrusion. Flow of magma subsequent to finger coalescence may not be parallel to the propagation direction. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.