Gravity-spreading origin of the Heart Mountain allochthon, northwestern Wyoming THOMAS A. HAUGE Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0741 ABSTRACT catastrophically as individual slide blocks. Subsequent mapping, primarily by Pierce (1941,1950,1957,1960,1965,1966; Pierce and Nelson, 1969, Field studies of the Heart Mountain detachment and overlying 1971; Pierce and others, 1973), extended the detachment area to the volcanic rocks reveal that "autochthonous" Eocene volcanic rocks northwest into the Clarks Fork River drainage and led to his recognition of previously reported as postdating faulting are tectonically emplaced a "break-away" fault, marking the western margin of the detachment area; and, in many places, far traveled. Evidence of tectonic deformation of a "bedding" fault, where the detachment follows a bedding plane within these volcanic rocks is documented in areas where volcanic rocks the Ordovician Bighorn Dolomite in an estimated 1,300 km2 area of the directly overlie the detachment, as well as at structurally higher levels northeast Absaroka Mountains; a "transgressive" fault, where the detach- within the allochthon. Such evidence includes shearing, brecciation, ment ramps upsection; and a "fault on former land surface," recogn ized by and faulting of volcanic rocks, and tilting and truncation of volcanic previous workers, where allochthonous blocks were thought to have over- strata. This widespread deformation casts doubt concerning earlier ridden the Eocene land surface (Fig. 1). Pierce's mapping and interpreta- suggestions that volcanic rocks were deposited upon the detachment tions (for summaries, see Pierce, 1963b, 1973) supported Bucher's concept and upon numetous detached slide blocks after detachment faulting. of upper-plate rocks having been emplaced as individual slide blocks In light of these new data, the upper plate of the Heart Mountain rather than as a continuous allochthon. detachment is interpreted as having been a single, continuous alloch- Although the involvement of volcanic rocks was suggested early in thon composed Largely of volcanic rocks, rather than as having con- the study of Heart Mountain faulting (Hewett, 1920; Hares, 1933), agree- sisted of numerous separate slide blocks as was previously envisioned. ment that at least minor volumes of volcanic rock were involved in fault- Crosscutting relationships between dikes and faults within the alloch- ing was not reached until much later (Pierce, 1958). Prostka's (1978) thon suggest tha.t allochthon emplacement occurred coeval with vol- mapping recognized greater volumes of allochthonous volcanic rocks than canism and, comtrary to earlier suggestions, need not have been did other previous workers'. Recent publications of previous workers catastrophic. (Pierce, 1978,1979, 1980; Prostka, 1978; Nelson and others, 1980) indi- The mechanism of emplacement of the allochthon, which once cated that large areas of the bedding-plane detachment were subaerially may have covered >3,400 km2, is viewed in terms of gravity-induced exposed by tectonic denudation during Heart Mountain faulting, and, in spreading on the flanks of an active volcanic field. Kinematic data such areas, volcanic rocks are shown as being in depositional contact with indicate that transport of the allochthon was locally directed to the the detachment (Fig. 2). The present report, based on field studies con- north, northeast, east, and southeast, generally away from active vol- ducted between 1977 and 1982, describes previously unknown tectonic canoes, as well as downslope toward the Bighorn Basin. Transport deformation of these volcanic rocks that is evident in many area; and is was accompanied by variably directed extension of the allochthon, locally severe. It also confirms earlier observations (Pierce, 1973; Prostka, accommodated by normal, oblique-normal, and strike-slip faults, tilt- 1978) that there is no direct evidence of subaerial exposure of the autoch- ing of fault-bounded blocks within the allochthon, and dike intrusion. thon by tectonic denudation. These observations suggest that the contact between volcanic rocks and the autochthon along the detachment is INTRODUCTION everywhere tectonic and was never depositional. Volcanic rocks previous- ly thought to postdate detachment faulting are reinterpreted as being The Heart Mountain allochthon is exposed in a 3,400 km2 area of allochthonous. northwestern Wyoming and adjacent Montana (Fig. 1) that includes parts The concept, presented here, of the relationship of Absaroka volcanic of the northeast foothills of the Absaroka Mountains and the western rocks to Heart Mountain faulting suggests a radically different view of margin of the Bighorn Basin. Dake (1916) first recognized the presence of Heart Mountain faulting: the upper plate was an intact, extending alloch- a low-angle fault (the "Hart Mountain thrust") separating Paleozoic sedi- thon, not numerous slide blocks. In this view, the detachment horizon was mentary rocks up to 200 m thick from subjacent Eocene basin-fill deposits not exposed by tectonic denudation, so that catastrophic rates of faulting at Heart Mountain and from subjacent Mesozoic strata along the Sho- and subsequent volcanism, previously inferred from lack of erosion of the shone River west of Rattlesnake Mountain. The predominance of features detachment horizon, are not required. Instead, fault displacement may reflecting extension, rather than shortening, within allochthonous rocks led have occurred noncatastrophically during as much as one or more million Bucher (1933,1935,1940,1947) to suggest that the allochthonous masses years, coeval with Absaroka volcanism that produced both the bulk of the had never been part of a continuous thrust sheet but had been emplaced allochthon and the gravitational instability responsible for its movement. Kinematic data indicate variable directions of upper-plate exten sion and Present address: Exxon Production Research Co., P.O. Box 2189, Houston, translation, which are consistent with displacement directed away from Texas 77252-2189. volcanic centers. An emplacement mechanism involving gravity-induced Geological Society of America Bulletin, v. 96, p. 1440-1456, 13 figs., 1 table, November 1985. 1440 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/11/1440/3445049/i0016-7606-96-11-1440.pdf by guest on 02 October 2021 ORIGIN OF HEART MOUNTAIN ALLOCHTHON, WYOMING 1441 Figure 1. Generalized geologic map and schematic cross section of the Heart Mountain detachment area. Modified from Pierce (1979), whose "transgressive fault" and "fault on former land surface" are labeled "detachment climbs section to Eocene," because data presented here suggest that, when active, the detachment was largely within or beneath Eocene strata rather than upon the land surface; see text. Locality numbers denote areas described in the text and in Table 1. spreading on the flanks of an active Absaroka volcanic field is envisioned. the volcanic rocks. In earlier published reports, formations of Absaroka Concepts discussed here were summarized, in part, by Hauge (1982a, volcanic rocks were defined in terms of a concept of faulting here seen as 1982b, 1982c, 1983a), and additional data are described by Hauge being in error, so that stratigraphic classification of volcanic units was (1983b). inseparable from a structural interpretation with regard to the detachment. In this section, the history of stratigraphic classification by previous EVIDENCE FOR TECTONIC EMPLACEMENT workers is reviewed; structural criteria used in this study for evaluating the OF VOLCANIC ROCKS involvement of volcanic rocks in faulting are defined; and examples of tectonic deformation of volcanic rocks are described. These examples In this study, the relationship of volcanic rocks to Heart Mountain include features observed within the volcanic rocks, at contacts between faulting was assessed on the basis of evidence of tectonic deformation of the volcanic rocks and allochthonous Paleozoic sedimentary rocks, and Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/11/1440/3445049/i0016-7606-96-11-1440.pdf by guest on 02 October 2021 Figure 2. Generalized geologic map of Absaroka volcanic rocks in the detachment area, showing orientations of volcanic strata and detachment striae and summarizing mapping of volcanic rocks by previous workers (Pierce, 1978; Prostka, 1978; Nelson and others, 1980; and, in Montana only, Elliott, 1976). Ti = Absaroka intrusives; Tw = Wapiti Fm; Tu = rocks of contested or uncertain affinity, mapped variably as Tw, Tic, and (or) undifferentiated; Tic = Lamar River and Cathedral Cliffs Formations of all of these workers; Pza = allochthonous Paleozoic rocks. Tw is considered post-tectonic to these previous workers; Tic is all, or mainly, allochthonous. The nomen- clature of volcanic rocks used by previous workers was abandoned in the present study; see text, (inset) Stereonet showing representative orientations of striae observed on the detachment. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/11/1440/3445049/i0016-7606-96-11-1440.pdf by guest on 02 October 2021 ORIGIN OF HEART MOUNTAIN ALLOCHTHON, WYOMING 1443 TABLE I. SUMMARY OF TYPES OF TECTONIC DEFORMATION OF VOLCANIC ROCKS ALONG THE BEDDING-PLANE DETACHMENT Locality Type of Deformation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 20 21 22 23 25 26 28 Tilted strata X X X x X X X X Faults, shear zones x X X X X X X X X X X X X X X X Strata truncated along detachment X X X X Strata truncated along upper-plate faults X X X X X X Basal volcanics sheared s X s s X Basal volcanics shattered or brecciated x X X X X X Microbreccia veneer on autochthon T T.P T.P X T,P T.P p Striae on microbreccia veneer X X X X X Striae on autochthon X X X X Absaroka dikes X X X X X X X X X X X X Clastic dikes X X X X X X X X X X Faulted contact with allochthonous Paleozoic rocks X X X X X X X X X X Symbols: s = striated; T = contains clasts of Tertiary volcanic rocks; P = contains clasts of Paleozoic rocks.