Gravity-Spreading Origin of the Heart Mountain Allochthon, Northwestern Wyoming

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 previously 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

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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 nomenclature 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.

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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.

along the detachment beneath volcanic rocks. Table 1 summarizes the defined west of the detachment area, is considered, in part, equivalent to, features of these areas and others described by Hauge (1983b). and, in part, younger than the Cathedral Cliffs Formation (Smedes and Prostka, 1972). Within the detachment area, the Lamar River Formation Stratigraphic Criteria has been interpreted, in part, as having slid onto and, in part, as having been deposited on the Heart Mountain detachment (Nelson and others, When the Heart Mountain detachment was first mapped in the Absa- 1980, PI. I, legend). Given that the lithologies of these Absaroka volcanic roka Mountains, by Dake (1916) in the Shoshone River drainage and rocks are very similar, distinguishing between the various formations in by Pierce (1941, 1957) in the Clarks Fork drainage, the involvement of outcrop has proven very difficult, particularly in areas of tectonic disrup- volcanic rocks in faulting was not recognized. Mapping by Hague (1899) tion and uncertain stratigraphic relationships: had established an informal terminology for Absaroka volcanic rocks before the detachment was known to exist. Involvement of volcanic rocks "At [some] localities ... it is difficult to distinguish between volcanic rocks of the in Heart Mountain faulting was first documented by Hares (1933) in the Cathedral Cliffs and Lamar River Formations, and the Wapiti Formation." (Pierce, Shoshone River drainage and by Pierce (1958, 1960) in the Clarks Fork 1979, p. 1); "The Lamar River Formation looks very much like the alluvial facies of the Wapiti Formation, and the two are often difficult to distinguish.. . .Vent facies drainage. Subsequent mapping (Pierce, 1963a; Nelson and Pierce, 1968; deposits of both formations are even more difficult to tell apart" (Nelson and others, Pierce and others, 1973) recognized local large volumes of allochthonous 1980, p. 17-18); "Most of the formations [of the Absaroka Supergroup] consist of volcanic rocks, but most volcanic rocks overlying the detachment were repetitious sequences of andesitic volcaniclastic rocks which, by themselves, are not interpreted as being younger than Heart Mountain faulting (Pierce, 1978; distinguishable from one formation to another. For this reason, the recognition and Prostka, 1978; Nelson and others, 1980). Hauge (1983b) discovered the mapping of many formations have depended to a large extent on tracing widespread unconformities and on mapping distinctive marker units, such as lava flows, within only known exposure of lower-plate volcanic rocks (area 10, Fig. 1) in a formations" (Smedes and Prostka, 1972, p. C38). small channel incised through the bedding-plane detachment and overlain by allochthonous Paleozoic rocks. Due to the fact that the formal nomenclature of volcanic rocks of the In establishing formal nomenclature for volcanic rocks in the de- Northern Absaroka Mountains has proven problematical in field mapping tachment area, Pierce (1963a) included all volcanic rocks then believed to of the detachment area (Fig. 2), and because the names "Wapiti Forma- be fault-emplaced in the Cathedral Cliffs Formation, and Nelson and tion" and "Cathedral Cliffs Formation" both connote temporal relation- Pierce (1968) included all volcanic rocks then believed to postdate faulting ships with Heart Mountain faulting that are unresolved, the formal in the Wapiti and younger formations. Implicit in the formal nomenclature nomenclature was abandoned in this study. Detailed stratigraphic and is the assumption that Heart Mountain faulting created an isochronous structural studies of the mostly younger volcanic section in the southern tectonic horizon (the inferred surface of tectonic denudation) which can be Absaroka Mountains, incorporating extensive paleontologic and paleo- used to distinguish lithostratigraphic (and presumably chronostratigraphic) magnetic data, have been conducted by Bown (1982, 1983), Eaton units: "Inasmuch as the volcanic rocks that were deposited on the [approx- (1982), Sundell (1982), and Sundell and Eaton (1982). Similar studies of imately] 650 km2 area of the surface of tectonic denudation ... are the Heart Mountain fault area are required to resolve the stratigraphic unquestionably included in the Wapiti Formation, that designation will be difficulties. followed here because the mechanism of tectonic denudation provides a mechanism for precise time correlation" (Pierce, 1980, p. 276). If, how- Structural Criteria ever, volcanic rocks along the detachment are everywhere allochthonous and the "mechanism of tectonic denudation" envisioned by Pierce is in Allochthonous rocks in many parts of the detachment area are re- error, volcanic rocks now in contact with the detachment need not be markably little deformed. At Heart Mountain, where a minimum of 21 km correlative. of transport is indicated, deformation above the poorly exposed detach- The volcanic formations established in the detachment area subse- ment is largely restricted to discordant moderate dips of Paleozoic strata quently proved incompatible with formations established in Yellowstone and a few throughgoing, mostly high-angle faults. In the area of the National Park west of the detachment area. The Lamar River Formation, bedding-plane detachment, allochthonous Paleozoic rocks locally display

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only minor faulting and very little tilting, despite transport of a few to, tonic transport on the order of kilometres. Reconnaissance suggests that perhaps, >10 km. As a result, the contact between Paleozoic sedimentary preserved stratigraphic sequences of allochthonous volcanic rocks are lo- rocks and Tertiary volcanic rocks along the detachment was, for many cally hundreds of metres thick and may have approached several kilome- years, mapped as an unconformity (Hague, 1899; Rouse, 1937), and the tres in thickness during faulting. existence of a detachment in this area, first recognized by Pierce (1941), was not immediately accepted (Bucher, 1947, Fig. 1). Pierce (1950) dis- Examples of Tectonic Deformation of Volcanic Rocks covered locally ex tensive high-angle faulting of Paleozoic rocks that does not extend below the base of the Ordovician, requiring the presence of a The north-facing slopes west of Corral Creek (Fig. 1, area 21) pro- décollement parallel to bedding. In well-exposed Paleozoic rocks at several vide an example of tectonic deformation of volcanic rocks along the localities (such as areas 7 and 22, Fig. 1), upper-plate normal faults, detachment and at the edge of a large mass of allochthonous Paleozoic commonly high-aagle, disappear downward into a shatter zone as much as rock. Here, tilted volcanic strata are shattered and truncated along both the ten or more metr» thick at the base of the allochthon. Only locally are detachment and a major upper-plate, high-angle fault. Volcanic rocks in upper-plate Paleozoic rocks appreciably tilted; in areas 10 and 26, for this area were mapped by Pierce (1978) as in situ Wapiti Formation, and instance, fault-bounded blocks as much as several hundred metres across by Prostka (1978) as undifferentiated volcanic rocks. The 10-n-thick are tilted to as mi.ch as 30° discordance with the detachment surface. basal shatter zone overlies the detachment for at least 200 m westward Although Paleozoic rocks exhibit continuous beds, allowing ready from the fault contact with a large mass of allochthonous Paleozoic car- recognition of tectonic deformation, volcanic rocks in many areas lack bonate rocks (Fig. 3). The shatter zone (Fig. 4), which forms gentler slopes obvious primary fabrics. Volcanic rocks along the detachment in some than do overlying or underlying rocks, is bounded below by the detach- areas (9, 14, 16, 21) are coarse, polymict breccias. Planar fabrics in these ment, which is locally marked by a slickensided and striated veneer of rocks, which are locally apparent in distant views, are difficult to discern microbreccia. Numerous, variably oriented clastic dikes containing car- on the outcrop. In such areas, it can be extremely difficult to distinguish bonate and volcanic clasts intrude the shatter zone and locally mark its primary (depositional) from secondary (tectonic) aspects of the fabric of upper boundary, which is a low-angle fault overlain by little-deformed, volcanic rocks immediately above the detachment, particularly when the east-dipping volcanic strata. These relationships require that the volcanic possibility of catastrophic faulting and subsequent volcanism is enter- rocks were transported across the detachment and therefore precate de- tained: "a rapidly accumulating body of volcanic flows, pouring out over tachment faulting. Allochthonous Paleozoic rocks shown in Figure 3 are an irregular layer of fault breccia [on the detachment where subaerially also structurally lowered, but the fault between these and older allochtho- exposed] is likely to produce features similar in some respects to those nous strata adjacent to the east (Pierce and Nelson, 1971) is not exposed. produced by sliding mass[es] of the same material" (Pierce, 1982, p. 181). The amount of offset across the fault shown in Figure 3 is not known. Elsewhere, however, rhythmic sequences of flows and extrusive breccias Similar relationships are exposed west of the allochthonous carbonate display well-developed, laterally continuous primary layering and geopetal mass at Republic Mountain (Fig. 1, area 6), where the generally south- structures, and in. these areas, fabric elements of tectonic origin are easily dipping stratification of indistinctly layered volcanic rocks is obscu re. Here identified. Pierce and others (1973) showed dips as great as 50° in volcanic and in many other places along the detachment (Table 1; Pierce.. 1979), rocks northeast of area 11, and Prostka (1978) recognized the involvement clastic dikes of fault breccia are conspicuous in volcanic rocks. Although of these stratified volcaniclastic rocks in Heart Mountain faulting. Pierce (1979) considered the clastic dikes to be younger than faulting Numerous features revealed by new field studies of the detachment (because they intrude volcanic rocks that he judged as postdating faulting), and overlying volcanic rocks are difficult to ascribe to a depositional clastic dikes observed in the present study (Table 1) are associated with origin. As interpreted here, these features suggest that volcanic rocks im- other features of tectonic origin and, therefore, are better interpreted as mediately overlying the detachment are everywhere allochthonous. Fea- being syntectonic. tures viewed as secondary and indicative of tectonic emplacement of volcanic rocks i nclude (1) shearing, shattering, and brecciation of basal volcanic rocks; (2) faults subparallel to the detachment within basal volcanic rocks; (3) slickensides and striae in volcanic rocks directly above the detachment; (4) a veneer of microbreccia, derived, in part, from volcanic rocks, on the uppermost lower plate; (5) volcanic strata dipping toward inferred source areas; (6) truncation of moderately to steeply dipping

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Figure 4. Sheared and shattered base of allochthonous volcanic rocks of Figure 3. Rela- tively undeformed volcanic rocks overlie basal deformed zone, here 5 m thick, with a network of light colored clastic dikes of fault breccia. Bedding- plane detachment and autochthon are visible at lower right. View is to the southwest.

At Jim Smith Creek (Fig. 1, area 13), features indicating intense strata, disappear downward into a basal shatter zone (Fig. 5). The detach- deformation of lithified volcanic rocks during transport along the detachment is marked by a striated, slickensided microbreccia veneer 0.5 to 2.0 ment are particularly well exposed. These volcanic rocks were mapped by mm thick overlain by sheared, striated volcanic rock (Fig. 6). Light tan Pierce (1968, 1978, 1979) as Wapiti Formation, in depositional contact clastic dikes of fault breccia containing angular clasts of volcanic and with the detachment, and by Nelson and others (1980) as Lamar River carbonate rock intrude the basal shatter zone, as do dark, aphanitic clastic and Cathedral Cliffs Formations (possibly allochthonous). Prostka (1978) dikes composed predominantly of comminuted volcanic rocks. A lens of mapped these rocks as allochthonous Lamar River Formation. The basal steeply dipping volcanic strata is bounded above by a low-angle fault and 10 to 20 m of volcanic rocks in this area are profoundly deformed; steep, below by the shatter zone. Prostka (1978) interpreted the volcanic rocks in inaccessible slopes prevent access to immediately overlying volcanic rocks. this area as having been emplaced on the detachment as unconsolidated Low-angle and high-angle faults, the former truncating tilted primary rubble derived from volcanic rocks atop a nearby block of allochthonous

Figure 5. Technically deformed volcanic rocks along the bedding-plane detachment, area 13, showing basal shatter zone up to 10 m thick, steeply dipping truncated strata (right of center), and internal faulting of volcanic rocks. The detachment and underlying autochthonous strata are also shown. View is westward.

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Figure 6. Bedding-plane detachment with microbreccia veneer and supeijacent, sheared volcanic rock at area 13. Pen and pencil in lower half of picture lie parallel to two striae sets on the microbreccia veneer. Pencil at upper right parallels striae on a sheared surface -10 cm above, and subparallel to, the detachment. View is obliquely downward.

Paleozoic rocks. Pierce (1979) suggested that volcanic rocks in this area were unconsolidated when clastic dikes were injected, indicating that the volcanic rocks arc younger than detachment faulting and flowed across the detachment (and fault breccia) shortly before dike injection. The deformation of volcanic rocks in this area, particularly the truncation (both updip and downdip) of a coherent sequence of tilted volcanic strata, is here viewed as the result of tectonic deformation of lithified volcanic rocks during transport along the detachment. On the east side of Painter Creek (Fig. 1, area 25) is a well-exposed example of a high-angle, upper-plate fault bounded on both sides by volcanic rocks. Well-bedded flows and breccias in the hanging wall dip 15°-25°N and arc truncated downdip both along the upper-plate fault and Contact along the detachment (Fig. 7). Footwall lithologies, less well exposed, are Fault, showing dip, massive porphyries, indicating a complete mismatch of units across the dotted where con- fault. Two hundred metres above the detachment, the fault is oriented cealed. N60°E, 70°SE, flattening downward to N10°E, 50°SE a few metres above Dike the detachment. Striae are steeply oblique, typically trending S70°E. An >- Strike and dip apparent throw of at least 200 m across the high-angle normal fault, the Appro*, strike and downdip truncation of hanging-wall strata along the detachment, and the dip. kinematic data are explained if the upper-plate fault was lowered to the Detachment .dot- detachment horizon along a low-angle fault within the allochthon. Low- ted where con- angle faults have :iot been widely observed within the allochthon, but the cealed. Reef Creek fault (Pierce, 1963b) may be an example of a low-angle

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At Squaw Creek (Fig. 1, area 16), it is difficult to resolve whether location—almost invariably within 2 m of a single, lower-plate basal strata of Absaroka affinity are allochthonous or autochthonous. stratigraphic horizon—as well as for the apparent lack of deformation of Here, a 20-m-thick sequence of subhorizontal beds of volcanic sands and the immediately underlying rocks. Where overlain by volcanic rocks, the conglomerates is exposed in a 100-m-long cliff face, and there is no indica- bedding-plane detachment is locally characterized by a 1- to 2-mm-thick tion of tectonic disruption. The detachment immediately beneath this ex- layer of microbreccia veneered to the lower plate. Wherever this micro- posure is concealed, but 200 m to the south, the detachment is marked by breccia veneer was observed (Table 1), its upper surface is slickensided a 2-m-thick breccia zone containing boulder-sized clasts of carbonate and and striated. Examination of microbreccia specimens in thin section with volcanic material and several striated, low-angle faults. Volcanic flows and microscope and electron microprobe revealed a submicroscopic, dark ma- breccias are exposed a few hundred metres farther south. The lack of trix of silicate-carbonate composition in which are suspended clasts of deformation of the epiclastic beds suggests that they are autochthonous, volcanic rock, Paleozoic rock, or both. The veneer is interpreted as being a but the sheared basal volcanics nearby suggest that they may be allochtho- tectonic breccia that formed during emplacement of the overlying volcanic nous. This exposure is the best example (observed in the present study) of a rocks. Pierce (1968) described a "frozen" (p. 195, quotation marks his) possible depositional contact of Absaroka strata upon the detachment. As contact at Jim Smith Creek (area 13) that is probably the same feature, the detachment beneath these strata and the lateral contacts with nearby although he reported no striae and considered the veneer to be a deposi- flows and breccias are not exposed, this issue is unresolved. tional feature. Contacts between upper-plate volcanic and Paleozoic rocks, in addi- Striae along the bedding-plane detachment (Fig. 2) were observed on tion to those described above, are commonly faulted (Table 1), providing the microbreccia veneer, on lower-plate rocks, and in shear zones within evidence that the volcanic rocks overlying them are allochthonous. Con- volcanic rocks along the detachment (Table 1). Their orientations are tacts of this type are spectacularly exposed on the north side of Pilot Creek, variable. Enigmatically, no striae were observed along the detachment at Cathedral Cliffs, and at Logan Mountain (Fig. 1, areas 11, 22, 28). At beneath Paleozoic rocks, despite extensive searching. In summary, tectonic Pilot Creek and Logan Mountain (Fig. 8), hanging-wall volcanic strata dip contacts between volcanic rocks and underlying rocks are present 20°-40° into high-angle, upper-plate normal faults, below which lie Pa- throughout the area of the bedding-plane detachment. The implications leozoic rocks. Striae indicate oblique-normal to strike-slip motion along of this new information are discussed in the next section. the high-angle fault at Pilot Creek, and dip-slip motion along high-angle faults at Logan Mountain. The detachment is not exposed in either area. Characteristics of the bedding-plane detachment commonly indicate tectonic displacement of overlying volcanic rocks. As described by Pierce (1968, 1973), the bedding-plane detachment is remarkable for its Figure 8. (a) Generalized cross section across the Ter- tiary-Paleozoic contact within the allochthon in area 11 (sketch from photo, Hauge, 1983b, Fig. N 26). Relationships in the allochthon are well exposed; detachment is concealed. Drag folding suggests normal faulting, but striae on fault are subhorizontal. Striae also trend east-west ~0.S km farther east, where this fault strikes east-northeast, (b) Cross section of part of area 28. Faults cutting Paleozoic and Tertiary section are tilted with strata in domino-style extension above the concealed detachment. scale Northernmost fault is untilted, with untilted allochthonous footwall. Extension direction is N30E inferred from dip of beds (25°-40° to N) to be north- 7000 south; no striae were observed on major faults. Nonplane strain may account, in part, for loss of basal upper-plate section (see T Hauge, 1983b, Fig. 61). Tv = 6000- T wi volcanic rocks; Twi = Willwood Formation; Mm = Madison Limestone; MDtj = Three Forks and Jefferson Formations; Ob = Bighorn Dolomite; CO = b. o 300m I Cambrian and Ordovician scale autochthon.

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GEOMETRY AND RATE OF EMPLACEMENT horn Basin (Rouse, 1937; Love, 1939), probably before faulting began. If OF THE ALLOCHTHON so, volcanic rocks in this area were probably involved in faulting, the western part of the "fault on former land surface" developing within, or Examples of tectonic deformation of volcanic rocks, such as those below, volcanic strata rather than on the land surface. As the extent to just described, are widespread in the detachment area (Table 1), indicating which the allochthon overrode the Eocene land surface is unknown, the that allochthonous volcanic rocks are much more common than was terms "transgressive fault" and "fault on former land surface" are aban- previously believed. Furthermore, features suggestive of a depositional doned (see Fig. 1). In the Shoshone River drainage, allochthonous vol- contact of volcanic rocks upon the detachment, such as post-tectonic, canic rocks were observed in area 28 (Figs. 8b, 10,12), suggesting that the pre-volcanic erosional channels, sedimentary deposits, or contact meta- intact allochthon extended across the Cretaceous-Eocene autochthon in morphism of the lower plate, were not observed in the present study or in this area. earlier studies. In this section, these observations form the basis of a new The thickness of the allochthon is also imprecisely known, but esti- model of Heart Mountain faulting in which the upper plate is viewed as an mates of stratigraphic thicknesses of allochthonous rocks provide general intact, extending allochthon composed largely of volcanic rocks. Con- constraints. The maximum stratigraphic thickness of Paleozoic s :rata in- straints on the dimensions of the allochthon indicate that its maximum volved in faulting is 490 m (Pierce, 1973); however, the average thickness thickness was probably 1 to 4 km, despite an areal extent of as much as of these rocks in the area of the bedding-plane detachment may have been 3,400 km2 or more. Relationships between dikes and faults within the much thinner than this at the onset of faulting. This is indicated by the allochthon are seen as indicating noncatastrophic emplacement coeval lower-plate channel that is filled with volcanic rocks and overlain by with ongoing volcanism. allochthonous Paleozoic rocks in area 10, as mentioned above (see also Hauge, 1983b). This channel deposit may be syntectonic rather than pre- The Upper Plato as an Intact, Extending Allochthon tectonic, and so the relief on the Paleozoic-Tertiary contact at the onset of faulting is poorly constrained. The maximum stratigraphy thickness of In the model proposed here, the upper plate of the Heart Mountain volcanic rocks involved in faulting is also unknown, although a minimum fault is viewed as an intact, extending allochthon rather than numerous of 670 km is indicated at the break-away fault (Prostka, 1978). My recon- slide blocks. Its emplacement may have occurred gradually, over as much naissance suggests that stratigraphic thicknesses of allochthonous volcanic as a million years; or more, rather than at the catastrophic rates inferred by rocks may approach several kilometres in areas 9,10, and 11, but detailed previous workers. The foundation of the model presented here is the field study is required to determine whether this section is repeated across assertion that field relationships are best explained if the bedding-plane normal faults in this area. The total thickness of the allochthon was at least detachment had not been exposed by tectonic denudation. If volcanic 700 m, based on the preserved thickness of strata west of the break-away rocks overlying the detachment are everywhere allochthonous rather than fault (Prostka, 1978) and preserved structural thicknesses in the area of post-tectonic, as :he data described previously suggest, subaerial exposure Pilot Peak. An upper bound on the structural thickness of the allochthon, of the detachment by tectonic denudation need not have occurred. The however, is constrained only by the inferred maximum thickness of ac- lack of direct evidence of subaerial exposure (Pierce, 1973; Prostka, 1978), cumulation of Absaroka volcanic rocks, estimated to be 2 to 4 krn at the signifying to previous workers that subaerial exposure was very brief, is Sunlight volcano (Iddings, 1899; Parsons, 1939). Allowable estimates of viewed here as evidence that such exposure did not occur. If the bedding- maximum thickness of the allochthon thus range from < 1 km to as much plane detachment was not exposed by tectonic denudation, it follows that as 4 km. the upper plate, at least in the area of the bedding-plane detachment, must These estimated dimensions, despite their uncertainties, indicate that have been a single, intact allochthon rather than numerous slide blocks. the allochthon was remarkably thin with respect to its areal extent. A 1- to Furthermore, catastrophic rates of faulting and subsequent volcanism are 4-km thickness is only a few percent of its northwest-southeast leng :h of 60 not required to explain the short duration of subaerial exposure envisioned to 100 km. Figure 9 is a schematic interpretation of the evolution of the in previous studies. I therefore conclude that the detachment was never allochthon, which assumes that it extended as a continuous feature beyond tectonically denuded and that the upper plate was a single, extending the bedding-plane fault. A thickness of several kilometres of the allochthon allochthon rather than numerous slide blocks, with catastrophic emplace- was presumably maintained by ongoing Absaroka volcanism; volcanic ment rates not required. rocks in the detachment area are thought to have been extruded largely from the volcanoes at Sunlight Basin and Hurricane Mesa (Parsons, 1939; Dimensions of the Allochthon Smedes and Prostka, 1972; Nelson and others, 1980). An assumed average initial thickness of Paleozoic rocks of 300 m over the 1,300-km2 area of The areal extent and thickness of the active allochthon are difficult to the bedding-plane detachment indicates that volcanic rocks constitute 2 constrain, due to post-faulting erosion. The present study indicates conti- -95% of an allochthon covering 3,400 km and averaging 2 km in thick- nuity of the allochthon in the area of the bedding-plane detachment, ness. Alternatively, if the allochthon was continuous only in the area of the estimated to be 1,300 km2 by Pierce (1973). Allochthonous rocks, how- bedding-plane fault and was an average of 1 km thick, volcanic rocks ever, are distributed over a 3,400 km2 area (Pierce, 1973), including the constituted —70% of the volume of the allochthon. From this persjDective, bedding-plane fault and areas to the east and southeast, suggesting that the Heart Mountain faulting appears to be predominantly a volcanic phe- intact allochthon may have extended beyond the area of the bedding-plane nomenon, Paleozoic strata at the base of the allochthon providing an detachment. Alternatively, klippe such as Heart Mountain and McCulloch appropriately oriented surface to accommodate detachment anc. trans- Peaks in the Bighorn Basin may be remnants of slide blocks shed from the lation of the allochthon. toe of the allochthon. These alternative possibilities cannot be tested directly because of extensive erosion of the detachment horizon in the Big- Absaroka Dikes and Detachment Faulting horn Basin; nevertheless, several features support the possibility that the intact allochthon extended beyond the bedding-plane detachment. There is The duration of Heart Mountain faulting is not tightly constrained by little doubt that Absaroka volcanic rocks once extended far into the Big- available data. Field relationships, in particular those between upper-plate

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faults and Absaroka dikes, locally support the concept of a protracted dikes within the allochthon that form radial patterns around the volcanic period of faulting coeval with Absaroka igneous activity; however, there is centers at Sunlight Basin and Hurricane Mesa, and therefore postdate most some controversy regarding the relationship of Absaroka dikes to Heart extension-related faulting, are likely to have undergone some transport Mountain faulting, and so the data are briefly reviewed here. along the detachment. Many hundreds of dikes associated with Absaroka volcanism are Crosscutting relationships between dikes and faults in the allochthon apparent in the allochthon, but, despite examination of extensive exposure also indicate that dike intrusion and extension of the allochthon were of the lower plate (Parsons, 1958; Nelson and others, 1972; Hauge, coeval. In areas 7, 17, 18, 20, 22, and 26, dikes are offset across upper- 1983b), only a few lower-plate dikes are known. Dikes are typically 1 to 3 plate faults. In numerous areas, dikes are intruded into fault zones; some m thick, steeply dipping to vertical, and are widely distributed within the show no evidence of deformation (areas 7, 17, 18, 27), and others have allochthon. Many are radially disposed about major volcanic centers (Par- striated margins or are internally sheared (areas 7,15, 16, 18, 19, 22, and sons, 1939; Pierce, 1978; Nelson and others, 1980). Nelson and others 26). In area 7, the center of a dike is foliated, suggesting ductile shearing (1972) declared many of the dikes to be post-tectonic and laterally in- before the dike had cooled. In areas 7 and 22, dikes occupy fault zones and truded, whereas Voight (1974a) and Prostka (1978) considered them al- are offset across other upper-plate faults. Some dikes (areas 7,10,11,13, lochthonous. Unless the lower-plate equivalents of so many dikes are 17) cut across faults with no offset of the dikes. Taken together, these fortuitously concealed, which seems unlikely, the dikes accommodate relationships suggest a protracted period of extension, including one or significant extension that is restricted to the upper plate, and syntectonic more cycles of faulting, magma intrusion (and presumably associated ex- lateral intrusion from Absaroka volcanic centers seems likely. Under such trusion), solidification of intrusions, and renewed faulting. circumstances, however, most dikes would have undergone at least minor Other features, such as possible growth-fault geometries in volcanic transport along the detachment as the allochthon expanded to accommo- strata of area 25 and multiple orientations of striae on the detachment and date subsequent extensional faulting or intrusion. In this view, even the upper-plate faults in numerous areas, may have resulted from multiple

CONTINUED VOLCANISM

0 5 10 KM C= V=H=|:|

Figure 9. Cartoons showing Heart Mountain faulting interpreted as gravitational spreading of an intact, continuous allochthon within an active volcanic field. Before faulting (A), 0 to 300 m of undeformed Paleozoic strata overlay the detachment horizon, which was locally breached by erosion. Volcanic rocks, probably at least 1 km thick and perhaps locally >3 km thick, overlay the Paleozoic strata and younger strata to the southeast. Subsequently (B), rocks along the detachment underwent lateral translation and extension while structurally higher (largely volcanic) rocks were downfaulted, tilted, and translated. Displacement was coeval with local volcanism (feeders out of plane of the section) and was in three dimensions (see Fig. 2), much more complex than this figure indicates. Symbols: - = Cambrian shale; brick = Ordovician to Mississippian carbonates; dot-dash = late Paleozoic and younger sedimentary rocks; dash-v = Eocene volcanic rocks. (C) This is a cartoon of postfaulting relationships without vertical exaggeration.

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episodes of faulting. Data suggest that upper-plate extension occurred in bly occurred coeval with ongoing volcanism over a period of a million or several episodes, but more data, including detailed stratigraphic studies and more years. radiometric dating, are required to further constrain the duration of Heart Mountain faulting. As interpreted here, field relationships permit "normal" KINEMATICS OF EXTENSION AND TRANSLATION rates of fault movement (millimetres to centimetres per year) and argue OF THE ALLOCHTHON against catastrophic rates. In summary, the upper plate of the Heart Mountain detachment is During the present study, kinematic data were collected frori many seen as an intact, extending allochthon, at least in the area of the bedding- localities within the detachment area (Figs. 10-13). In this section, these plane detachment but, perhaps, also across the entire detachment area. It data are summarized in four groups: data from allochthonous Paleozoic was composed predominantly of volcanic rocks, with Paleozoic rocks strata, from allochthonous Tertiary volcanic rocks, from contacts between conspicuous at its base, and was 1 to 4 km thick. Its emplacement proba- Paleozoic and Tertiary volcanic rocks within the allochthon, and from the

Figure 10. Lower-hemisphere, equal-area stereographic projections of orientations of faults (great circles) and striae (dots) in Paleozoic rocks of the Heart Mountain allochthon. See Hauge (1983b) for additional data. Numbers refer to areas in Figure 1.

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detachment and break-away fault. The distribution of pre-volcanic al- tion along the detachment rather than by listric geometries. Listric lochthonous rocks, as noted by previous workers, is also discussed. On the upper-plate faults are uncommon (one example is at the north end of area basis of these data, transport of the allochthon locally to the north, north- 25). Bedding of upper-plate Paleozoic rocks is subhorizontal in most areas, east, east, and southeast is inferred, suggesting that transport was, in although in places (such as parts of areas 10, 18, 24, 26, 28, and 29), general, directed away from active volcanic centers and toward the Big- fault-bounded Paleozoic strata dip 20° to 30°. Slickensides and striae are horn Basin. common on these upper-plate faults (Fig. 10), and Absaroka dikes, usually subvertical, are locally conspicuous in allochthonous Paleozoic rocks Paleozoic Rocks (Fig. 11 A). These features provide locally abundant information on the kinematics of extension of rocks that typically remained along the base of Allochthonous Paleozoic rocks are locally disrupted by numerous the allochthon as faulting progressed. high-angle, normal-oblique faults. Where the base of Paleozoic allochtho- The kinematic patterns displayed by Paleozoic rocks overlying the nous rocks is exposed (areas 7, 22, 26), these faults typically disappear bedding-plane detachment are highly variable between localities, but an downward into a zone of shattering and brecciation at the base of the over-all pattern of generally east-west extension, giving way eastward to allochthon and meet the shattered zone at high angles. Offset on upper- north-south extension, can be discerned. East-west extension was accom- plate faults appears to be accommodated by penetrative brittle deforma- modated by north-south-striking faults with steeply oblique striae (areas 17,

Figure 11. Stereographic projections of orientations of Absaroka dikes (great circles) and striae (dots) on sheared dike margins; A, in Paleozoic rocks of the Heart Mountain allochthon; B, in Absaroka volcanic rocks of the Heart Mountain allochthon. For area 20, only a few of several tens of north-northwest-striking dikes are shown.

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Figure 12. Stereographic projections of orientations of faults (great circles) and striae (dots) in Absaroka volcanic rocks in the Heart Mountain allochthon and (area 1) along the break-away fault. Triangles represent striae on the detachment.

19, 22) and by faults with diverse strikes and east-west-trending striae dikes dip very steeply and strike generally north-south, except in eastern- (areas 7, 15, 18, 21, 24). Less commonly, north-south-trending strike-slip most areas (26, 27), where strikes are east-west. or shallowly oblique striae are evident (areas 10, 18, 22, 24). Along the eastern margin of the bedding-plane detachment (area 26), north-south Tertiary Volcanic Rocks extension, reflected by east-west-striking faults, north-south-trending striae, and dips of strata to the north or south, is dominant. The few The kinematics of emplacement of volcanic rocks is revealed by localities (27, 28, 29) south and east of the bedding-plane fault that were orientations of fault striae, dikes, and rotated volcanic strata. Paterns of studied reflect highly variable extension directions. Orientations of dikes fault and striae orientations (Fig. 12) are typically diffuse within the areas are less variable between areas than are fault and striae orientations. Most studied and are variable between areas, although in some areas (4, 9, 13,

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Figure 13. Stereographic projections of orientations of fault contacts (great circles, dashed where approximate) between Absaroka volcanic rocks and (footwall) Paleozoic rocks within the Heart Mountain allochthon. Dots show orientations of striae on the faults. Stereonets for areas 11 and 22 each depict data collected from a single fault zone. Where small blocks of Madison Limestone occur beneath volcanic rocks along the detachment (such as between areas 1 and 7; Pierce, 1979), hanging walls of upper-plate, faulted unconformities may have reached the detachment horizon.

14), many striae from upper-plate faults trend subparallel to local striae on kinematically most significant. Unfortunately, in general they are deeply the detachment. The dips of volcanic strata, which are commonly toward, weathered or poorly exposed. Where these contacts or their bounding rather than away from, inferred source areas, present a more consistent rocks are exposed (areas 4, 7, 10, 11, 21, 22, 28), their tectonic nature is picture (Fig. 2), being generally to the south in the northwest part of the evident from slickensides, striae (Fig. 13), brecciation, and (or) truncation detachment area and generally to the north and west in the less well of hanging-wall strata. The over-all kinematic picture, like that of the studied eastern part of the area. internal deformation of volcanic rocks, is one of highly variable directions In several areas (4, 5, 8, 9, 14), volcanic rocks along the detachment of relative movement. are massive, matrix-supported, coarse, polymict breccias. Planar fabrics, such as primary stratification, are not apparent where these rocks are The Detachment and the Break-away Fault accessible, and their textures are not clearly either tectonic or primary. Faults and shear zones within these rocks are indistinct and discontinuous, Unlike striae on upper-plate faults, which reflect internal deformation and orientations of kinematic indicators are typically quite variable. Re- of the allochthon, striae on the detachment and the break-away fault connaissance of structurally higher volcanic rocks revealed thick sequences indicate directions of translation of the allochthon. The orientations of of strata with generally southerly dips (Fig. 2), suggesting that the rocks detachment striae (Fig. 2), known only from the bedding-plane detach- supeijacent to the detachment form a zone of tectonic breccia ten or more ment where overlain by volcanic rocks, are consistently northwest- metres thick beneath tilted strata. Basal zones of tectonic breccia up to ten southeast east of area 16, but northwest of area 16, variable orientations or more metres thick are apparent in other areas where their relationship are observed. Striae on the break-away fault (Fig. 12) are either very to higher stratified volcanic rocks was observed directly (areas 9, 13, 21; steeply or very shallowly oblique, both orientations commonly being pres- Fig. 4). As noted above, analogous thick zones of tectonic breccia were ent on the same exposure. This raises the question as to whether this fault is observed in Paleozoic rocks at the base of the allochthon; thus, the basal primarily an extensional (headwall, or "break-away") fault or a strike-slip volcanic rocks in these areas are interpreted as being tectonic breccias. boundary. Striae of both orientations are abundant on both hanging-wall Figure 1 IB shows orientations of Absaroka dikes intruding and footwall sides of the 7-m-thick fault zone at area 1, but no sense of upper-plate volcanic rocks. Orientations of these dikes are consistent superposition was observed. within most areas but are variable between areas. Like those within Paleozoic rocks, dikes most commonly strike north-south. In area 20, Distribution of Allochthonous Pre-volcanic Rocks "volcanic" rocks are actually a sheeted dike complex across an exposure several hundred metres wide. The dikes in area 20, which dip 60°-70°W, The existence of allochthonous Paleozoic rocks in the Bighorn Basin were probably tilted from initial vertical dips as volcanic strata of areas 20 and Shoshone River drainage, with closest possible source areas in the and 21 were tilted from the horizontal to 20°E dips. Elsewhere, dikes are Absaroka and Beartooth Mountains, suggested to Dake (1916) that trans- generally subvertical. port of the upper plate was to the east. Pierce's (1941) discovery of the bedding-plane detachment provided the source area for allochthonous Contacts between Paleozoic and Tertiary Rocks Paleozoic rocks, indicating southeastward transport. Distribution of the within the Allochthon Crandall Conglomerate, a stream-channel deposit here interpreted as pre- dating detachment (compare Pierce and Nelson, 1973), indicates south- These contacts include depositional and tectonic contacts within the eastward translation from lower-plate exposures between areas 16 and 20 upper plate. Boundaries of major horsts, grabens, and half-grabens are toward upper-plate exposures south of area 27, as discussed by Pierce and probably the upper-plate faults with the largest offsets and therefore are Nelson (1973). One upper-plate exposure 5 km southwest of the lower-

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plate exposures (fierce and Nelson, 1971, 1973) complicates this picture. be reduced, and perhaps solved, by the continuous-allochthon model pro- These data apply no direct constraints on transport directions in areas west posed here. The following discussion reviews the mechanical enigma of and north of lower-plate exposures of Crandall Conglomerate. Heart Mountain faulting and places the newly proposed hypothesis in a context with previous hypotheses. Directions of Displacement of the Allochthon The mechanics of low-angle faulting has long been problematic (Reade, 1908), and, in this context, the mechanics of Heart Mountain Striae on the detachment and the break-away fault, and the distribu- faulting has, for many years, been particularly puzzling. Recognition that tion of allochthon ous rocks, are the most direct indicators of directions of the Heart Mountain allochthon is extensional and not a thrust sheet pre- transport of the a llochthon. Arrows in Figure 2 are transport directions cludes emplacement by surface stresses (a push from the rear), leaving only inferred from these data. Striae on the detachment east of area 16 probably body forces (the downslope component of gravity) to emplace the upper reflect southeastward motion, in accord with the distribution of Crandall plate (Bucher, 1933). Pierce (1957, 1973) suggested that the apparently Conglomerate and the occurrence of allochthonous rocks in the Bighorn brittle nature of deformation precludes the presence of a low-viscosity Basin. Northwest of area 16, however, a different kinematic pattern is layer along the detachment. The laws of sliding friction, rather than laws of indicated. Detachment striae trending at high angles to the break-away viscous flow (Smoluchowski, 1909; Kehle, 1970), thus were judged ap- fault in areas 5,9, and 13 suggest movement of this part of the allochthon propriate to Heart Mountain faulting, and a mechanism whereby friction to the northeast, east, and north. In light of these striae, southward dips of was reduced along the detachment was required to allow catastrophic strata between areas 1 and 16 may have resulted from back-tilting during gravity sliding down slopes averaging <2° (Pierce, 1973). north-south extension and northward translation. The variable directions Several hypothetical mechanisms to reduce effective normal stress of extension evident in kinematic data from within the allochthon may (total normal stress minus pore-fluid pressure; Terzaghi, 1923) across the have been a response to the variable directions of translation. detachment, and (or) to impart the upper plate with high displacement The over-al. inferred displacement pattern (arrows, Fig. 2) can be velocities, have been suggested (for a partial summary, see Pierce, 1973). viewed as the superimposition of two displacement patterns, one directed These include volcanic explosions (Bucher, 1933), earthquake vibrations generally away from volcanic centers at Hurricane Mesa and Sunlight (Bucher, 1947; Pierce, 1973), acoustic fluidization (Melosh, 1981), and Basin, and the other directed southeastward toward the Bighorn Basin. The various fluid-pressure models, such as Hubbert and Rubey's (1959) "beer volcanoes at Hurricane Mesa and Sunlight Basin were impressive strato- can" model, Hsu's (1969) "air cushion," Hughes' (1970a, 1970b) volcanic volcanoes when active. They are marked by plugs and radial dike swarms gas "hovercraft," Voight's (1972, 1973a, 1973b, 1974a, 1974b) "pneu- (Iddings, 1899; Parsons, 1939; Pierce, 1978; Nelson and others, 1980) and matic-hydraulic plastic wedge," and Straw and Schmidt's (1981a, 1981b) were interpreted as the source of most of the volcanic rocks in the detach- "phreatomagmatic-hydraulic" hypothesis. Earthquake vibrations were en- ment area (Parsons, 1939; Smedes and Prostka, 1972; Nelson and others, visioned as reducing effective normal stress across the detachment by 1980). Iddings estimated that the Sunlight volcano attained an elevation in imparting a greater upward acceleration to the upper plate than to the excess of 6.6 km; Parsons estimated an elevation 600 to 900 m above the lower plate, thereby momentarily reducing total stress across the fault highest current elevation, or >4.3 km. Displacement of the extending (Pierce, 1973). Fluid-pressure models employ various means to increase allochthon thus may have been controlled both by the slopes of Absaroka pore-fluid pressure along the detachment, thereby reducing effective nor- volcanoes, promoting displacement radially away from the volcanoes, and mal stress. Each of these hypothetical mechanisms stands or falls on its by the slope of tie bedding-plane detachment, southeastward toward the ability to sustain motion of slide blocks for long distances across low Bighorn Basin. slopes, a particularly critical constraint on fluid-pressure models, as over- pressured fluids would likely escape from the edges of the blocks (Davis, MECHANICS OF ALLOCHTHON EMPLACEMENT 1965) unless displacements were very rapid (Voight, 1973a, 1974a, 1974b). The kinematic data just described, together with data regarding the Both fluid pressures and earthquake vibrations are more credible geometry and ra :e of emplacement of the allochthon and its activity coeval means of reducing friction in the continuous-allochthon model proposed with Absaroka volcanism, suggest that the mechanism of faulting involved here than in slide-block models. If the Heart Mountain allochthon was spreading in response to the gravitational instability of the flanks of an laterally continuous rather than consisting of numerous slide blocks, fluid active volcanic field. In this view, the Paleozoic bedding plane that under- pressure along the detachment could be more easily maintained because lay the volcanic field provided an appropriately oriented slip surface to the length of "edge" per unit area of allochthon was much less. Of greater accommodate spreading, which relieved the gravitational instability by significance is the possibility that rates of displacement were on the order reducing surface slopes of the volcanic complex. Detachment faulting of millimetres per year to centimetres per year, as is permitted by the progressed as ongoing volcanism renewed the gravitational instability of continuous-allochthon model but not by the slide-block models. Low rates surface slopes. As in the cases of other models of Heart Mountain faulting, of displacement eliminate the requirement that friction was reduced simul- there are two immediate questions regarding this mechanism. (1) Why did taneously across large parts of the 1,300-km2 bedding-plane detachment: the detachment form along a bedding plane within the Bighorn Dolomite the development of transient fluid pressures in different local areas at rather than along a parallel bedding plane within Cambrian shales 10 m different times could have reduced effective normal stresses and facilitated lower? (2) By what means was friction along the detachment overcome to local displacements along the detachment. These displacements cculd have allow slip along its very gently dipping slope? The first question, discussed occurred by stick-slip, creep, or both. As regards earthquake vibrations, by Pierce (1973), remains unresolved in the present study and is not recent studies indicate that historic earthquakes have triggered movement discussed further. The problem posed by the second question is thought to of large, detached masses. The 1975 earthquake of Kalapara, Hawaii (M =

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7.1 to 7.2; hypocentral depth = 6 to 7 km), initiated downslope movement Davis for introducing me to the enigma of Heart Mountain faulting, help- of an allochthon 750 km2 in area and possibly as thick as 1 km (Tilling ing to define the nature of the problem, and clarifying field relationships; and others, 1976; Cox, 1980). Coastal subsidence of up to 3 m resulted, W. G. Pierce, whose previous work laid the foundations of my studies, for and uplift of a 600-km2 area of seafloor generated a tsunami. This example providing early encouragement, guidance, and hospitality in the field; of detachment faulting on the flank of an active volcano differs from Heart E. Johnson, L. Thurn, and C. Fuller for providing able and pleasant field Mountain faulting in several respects (for example, basaltic shield versus assistance; M. Kenevan, Mr. and Mrs. R. Philips, and T. Frey, whose andesitic stratovolcanoes; largely submarine versus wholly subaerial de- friendship and hospitality made field study more pleasant and efficient; tachments), but it demonstrates the plausibility of seismic energy having and P. O'Day, for performing microprobe analyses. Development of con- facilitated small increments of slip along the Heart Mountain detachment. cepts presented here was aided by discussions with C. Ando, G. A. Davis, As in the case of fluid pressure, earthquake vibrations need not have J. Eaton, E. Frost, L. Thurn, J. Willemin, and others. C. Ando, G. A. caused the entire continuous allochthon to move simultaneously. Absa- Davis, K. Kellogg, P. O'Day, C. Potter, B. Voight, J. Weitz, and roka volcanism and Laramide basement faulting provide convenient B. Wernicke reviewed the manuscript and provided valuable criticism. sources of seismic energy (Pierce, 1973). The study was funded by Penrose Grants from the Geological Society of In addition, these mechanisms are not mutually exclusive. If fluid America (1978, 1980); by the Geological Sciences Graduate Student Re- pressure, earthquakes, and possibly other mechanisms acted in concert, search Fund, University of Southern California (1977, 1978,1980, 1981); coevally or sequentially over protracted periods of time, the mechanical and by grants from Sigma Xi (1977, 1978). problem of Heart Mountain faulting is greatly reduced. A variation of the

mechanics of gravitational spreading (Elliott, 1976), rather than gravity REFERENCES CITED sliding, may apply. Bown, T. M„ 1982, Geology, paleontology, and correlation of Eocene volcaniclastic rocks, southeast Absaroka Range, Sales (1983) recently reinterpreted the Heart Mountain allochthon as Hot Springs County, Wyoming: U.S. Geological Survey Professional Paper 1201-A, 75 p. 1983, Catastrophic large-scale late Cenozoic detachment faulting of Eocene volcanic rocks, southeast Absaroka being a "gigantic rock glacier" (p. 1) in which the laws of flow of a viscous Range, northwest Wyoming: Wyoming Geological Association, Annual Field Conference, 33rd, Guidebook, fluid governed the mechanics of deformation. He suggested that the upper p. 185-201. Bucher, W. H., 1933, Volcanic explosions and overthrusts: American Geophysical Union Transactions, v. 14, p. 238-242. plate was emplaced due to the catastrophic collapse of a flank of the 1935, Remarkable local folding, possibly due to gravity, bearing on the Heart Mountain thrust fault problem [abs.]: Geological Society of America Proceedings for 1934, p. 69. Absaroka volcanic complex. As Hauge (1982a, 1982b, 1983a) had done, 1940, The geology of the Cody region: New York Academy of Sciences Transactions, ser. 2, v. 2, no. 7, p. 1 -4. Sales envisioned a continuous allochthon rather than numerous detached 1947, Heart Mountain problem: Wyoming Geological Association, Annual Field Conference, 2nd, Guidebook. p. 189-197. slide blocks, and he discussed many aspects of the field relationships that Cox, D. C., 1980, Source of the tsunami associated with the Kalapara (Hawaii) earthquake of November, 1975: Hawaii Institute of Geophysics, H1G-80-8,46 p. are better explained by this assumption. Sales' conclusions that the alloch- Dake, C. L., 1916, The Hart Mountain overthrust and associated structures in Park County, Wyoming: Journal of thon was continuous and composed predominantly of volcanic rocks and Geology, v, 26, p. 45-55. Davis, G. A.. 1965, Discussion on 'Role of fluid pressure in mechanics of overthrust faulting': Geological Society of that faulting occurred in response to gravitational instability of a volcanic America Bulletin, v. 76, p. 463-474. Eaton, J. G., 1982, Paleontology and correlation of Eocene volcanic rocks in the Carter Mountain area. Park County, complex support the interpretations described here. Sales, however, envi- southeastern Absaroka Range, Wyoming: University of Wyoming Contributions to Geology, v. 21, p. 153-194. Elliott, D„ 1976, The motion of thrust sheets: Journal of Geophysical Research, v. 81, p. 949-963. sioned a catastrophic emplacement event, whereas data described here and Hague, A., 1899, Description of the Absaroka quadrangle (Crandall and Ishawooa quadrangles): U.S. Geological Survey by Hauge (1982a, 1982b, 1982c, 1983a, 1983b) suggest noncatastrophic Geologic Atlas, Folio 52, 6 p. Hares, C. J., 1933, Relative age of the Heart Mountain overthrust and the Yellowstone Park Volcanic series [abs.]: emplacement. Geological Society of America Proceedings, 1932, p. 84. Hauge, T. A., 1982a, The Heart Mountain detachment fault: Role of Absaroka volcanic rock: Geological Society of America Abstracts with Programs, v. 14, p. 314. CONCLUSIONS 1982b, The Heart Mountain detachment fault, northwest Wyoming: Involvement of Absaroka volcanic rock: Wyoming Geological Association, Annual Field Conference, 33rd, Guidebook, p. 175-179. 1982c, Style and kinematics of extension above detachment faults: The Heart Mountain fault, northwest Wyo- ming: Geological Society of America Abstracts with Programs, v. 14, p. 510. New field data suggest that Heart Mountain faulting involved noncat- 1983a, The Heart Mountain detachment fault: gravitational spreading from an active volcanic center: Geological Society of America Abstracts with Programs, v. 15, p. 374. astrophic, gravity-induced spreading of an intact, extending allochthon 1983b, Geometry and kinematics of the Heart Mountain detachment fault, northwestern Wyoming and Montana rather than catastrophic gravity sliding of numerous detached slide blocks. [Ph.D. thesis]: Los Angeles, California, University of Southern California, 265 p. Hewett, D. F., 1920, The Heart Mountain overthrust, Wyoming: Journal of Geology, v. 28, p. 536-557. These data support the inference that volcanic rocks along the detachment Hsu, K. J., 1969, Role of cohesive strength in the mechanics of overthrust faulting and of landsliding: Geological Society of America Bulletin, v. 80, p. 927-952. are everywhere allochthonous and that the detachment was not subaerially Hubbert, M. K., and Rubey, W. W., 1959, Role of fluid pressure in mechanics of overthrust faulting. I. Mechanics of exposed by tectonic denudation. Relationships between dikes and faults fluid-filled porous solids and its application to overthrust faulting: Geological Society of America Bulletin, v. 70, p. 115-166. within the allochthon suggest that faulting occurred over a protracted Hughes, C. J., 1970a, The Heart Mountain detachment fault—A volcanic phenomenon?: Journal of Geology, v. 78, no. I, p. 107-116. period of time, perhaps a million or more years, coeval with Absaroka 1970b, Reply to Comment on 'The Heart Mountain detachment fault—A volcanic phenomenon?': Journal of volcanism. Kinematic data indicate that displacement of the allochthon Geology, v. 78, p. 629-630. Iddings, J. P., 1899, The dissected volcano of the Crandall basin, Wyoming: U.S. Geological Survey Monograph 32, Part was not simply downdip toward the Bighorn Basin but was also generally 2, p. 215-269. Kehle, R. O., 1970, Analysis of gravity sliding and orogenic translation: Geological Society of America Bulletin, v. 81, away from Absaroka volcanic centers. Heart Mountain faulting is per- p.1641-1664. ceived as having resulted from gravitational instability of the flanks of Love, J. D., 1939, Geology along the southern margin of the Absaroka Range, Wyoming: Geological Society of America Special Paper 20, 134 p. these volcanic centers. At noncatastrophic displacement rates, fluid pres- Melosh, H. J., 1981, Acoustically activated decollement: Mechanics of the Heart Mountain fault [abs.]: EOS (American Geophysical Union Transactions), v. 62, p. 1046. sures or earthquake vibrations may have reduced friction along the gently Nelson, W. H., and Pierce, W. G., 1968, Wapiti Formation and Trout Peak Trachyandesite, northwestern Wyoming: U.S. dipping detachment to facilitate gravitational spreading of the allochthon. Geological Survey Bulletin 1254-H, p. Hl-Hll. Nelson, W. H., Pierce, W. G„ Parsons, W. H„ and Brophy, G. P., 1972, Igneous activity, metamorphism, and Heart Mountain faulting at White Mountain, northwestern Wyoming: Geological Society of America Bulletin, v. 83, p. 2607-2620. ACKNOWLEDGMENTS Nelson, W. H., Prostka, H. J., and Williams, F. E., 1980, Geology and mineral resources of the North Absaroka Wilderness and vicinity, Park County, Wyoming, with sections on mineralization of the Cooke City mining district by James E. Elliott and aeromoagnetic survey by Donald L. Peterson: U.S. Geological Survey Bulletin 1447, This study was undertaken in partial fulfillment of requirements for 101 p. Parsons, W. H., 1939, Volcanic centers of the Sunlight area, Park County, Wyoming: Journal of Geology, v. 47, p. 1-26. the doctoral degree at the University of Southern California. I thank G. A. 1958, Origin, age, and tectonic relationships of the volcanic rocks in the Absaroka-Yellowstone-Beartooth region.

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Wyoming-Montana: Billings Geological Society, Annual Field Conference, 9th Guidebook, p. 36-43. Reade, T. M., 1908, The mechanics of overthrusts: Geological Magazine, v. 5, p. 518. Pierce, W. G., 1941, Heait Mountain and South Fork thrusts. Park County, Wyoming: American Association of Rouse, J. T., 1937, Genesis and structural relationships of the Absaroka volcanic rocks, Wyoming: Geologia 1 Society of Petroleum Geologist; Bulletin, v. 25, p. 2021-2045. America Bulletin, v. 48, p. 1257-1296. 1950, Source and movement of the Heart Mountain thrust blocks. Park County, Wyoming: Geological Society of Sales, J. K., 1983, Heart Mountain—Blocks in a giant volcanic rock glacier Wyoming Geological Associai on. Annual America Bulletin, v. 61, p. 1493. Field Conference, 34th, Guidebook, p. 117-165. 1957, Heart Mountain and South Fork detachment thrusts of Wyoming: American Association of Petroleum Smedes, H. W., and Prostka, H. 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Terzaghi, K., 1923, Die Berechnung der Durchlassigkeitsziffer des Tones aus dem Verlauf der hydrodynamiichen Span- 1966, Geologic map of the Cody quadrangle. Park County, Wyoming: U.S. Geological Survey Geologic Quad- nungserscheinungen: Sitzungberichte Akademie der Wissenschaften Wien, Mathematisch-naturwissc nschaftliche rangle Map GQ-542. scale 1:62,500. klasse, part 112, v. 132, p. 105. 1968, Tectonic denudation as exemplified by the Heart Mountain fault, Wyoming: in International Geological Tilling, R. I., Koyanagi, R. Y., Upman, P. W„ Lockwood, J. P., Moore, J. G., and Swanson, D. A., 1976, Earthquake and Congress, 23rd, Orojjenic Belts, Prague, Czechoslovakia, Report, Section 3, Proceedings, p. 191-197. related catastrophic events, island of Hawaii, November 29, 1975: A preliminary report: U.S. Geological Survey 1973, Principal features of the Heart Mountain fault and the mechanism problem, in DeJong, K. A., and Schölten, Circular 740,33 p. R., eds.. 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Rod: mechanics: Quadrangle Map G<}-935, scale 1:62,500. The American Northwest, 33rd Congress International Society of Rock Mechanics Expedition Guidebook, Special 1973, Crandall Conglomerate, an unusual stream deposit, and its relation to Heart Mountain faulting: Geological Publication, Experiment Station, College of Earth and Mineral Sciences, The Pennsylvania Stau University, Society of America Bulletin, v. 84, p. 2631-2644. p. 115-122. Pierce, W. G., Nelson, W. H., and Prostka, H. J., 1973, Geologic map of the Pilot Peak quadrangle, Park County, Wyoming, and Pari: County, Montana: U.S. Geological Survey Miscellaneous Geological Investigations Map 1-861, scale 1:62,500. Prostka, H. J., 1978, Heart Mountain fault and Absaroka volcanism, Wyoming and Montana, U.S.A., in Voight, B., ed., MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 1,1984 Rockslides and avalanches, 1, natural phenomena: Amsterdam, Oxford, New York, Elsevier Scientific Publishing REVISED MANUSCRIPT RECEIVED APRIL 29,1985 Company. MANUSCRIPT ACCEPTED MAY 31,1985

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