Heart Mountain Detachment Research Articles

Episodic Dissolution, Precipitation, and Slip along the Heart Mountain Detachment, Wyoming

AbstractThe Heart Mountain allochthon is among the largest landslide masses in the rock record. The basal fault, the Heart Mountain detachment, is an archetype for the mechanical enigma of brittle fracture and subsequent frictional slip on low-angle faults, both of which appear to occur at ratios of shear stress to normal stress far below those predicted by laboratory experiments. The location of the detachment near the base of thick cratonic carbonates, rather than within subjacent shales, is particularly enigmatic for frictional slip. A broad array of potential mechanisms for failure on this rootless fault have been proposed, the majority of which invoke single-event, catastrophic emplacement of the allochthon. Here, we present field, petrographic, and geochemical evidence for multiple slip events, including cross-cutting clastic dikes and multiple brecciation and veining events. Cataclasites along the fault show abundant evidence of pressure solution creep. Banded grains, which have been cited as evide...

South Fork Fault as a gravity slide: Its break-away, timing, and emplacement, northwestern Wyoming, U.S.A.: COMMENT

Clarey's (2012) model for South Fork (SF) thrusting contains major errors as regards timing of emplacement, number of emplacement events, magnitude of displacement, and geometry of the SF allochthon. A model better supported by data: (1) has SF thrusting taking place before local emplacement of the Heart Mountain (HM) allochthon, rather than after; (2) has emplacement of the SF allochthon by multiple events rather than by a single catastrophic event; (3) envisions only gradual changes in the magnitude of displacement along strike of the SF thrust system, rather than abrupt doubling of displacement across tear faults; (4) regards the SF allochthon as segmented by tear faults only where it has moved across footwall lateral ramps, not in its hinterland; and (5) recognizes that the fault viewed by Clarey (2012) as a break-away to the SF system is instead a fault within the HM allochthon. Clarey's (2012) claim that SF thrusting postdated emplacement of the HM allochthon is based on his assertion that the HM detachment and overlying allochthon are folded above the SF frontal ramp, both on his section A–A′ and near the Castle fault. This argument is disproven by the geologic map of Pierce and Nelson (1969), which presents a much more complete picture of relevant relationships than is shown in Clarey (2012). Pierce and Nelson's (1969) cross section A–A′ is drawn where the preserved HM allochthon and the SF frontal ramp are in …

Vertical injectites of detachment carbonate ultracataclasite at White Mountain, Heart Mountain detachment, Wyoming

Research Article| May 01, 2012 Vertical injectites of detachment carbonate ultracataclasite at White Mountain, Heart Mountain detachment, Wyoming John P. Craddock; John P. Craddock * 1Geology Department, Macalester College, St. Paul, Minnesota 55105, USA *E-mail: Craddock@Macalester.edu. Search for other works by this author on: GSW Google Scholar Jesse Geary; Jesse Geary 1Geology Department, Macalester College, St. Paul, Minnesota 55105, USA Search for other works by this author on: GSW Google Scholar David H. Malone David H. Malone 2Department of Geography, Geology, Illinois State University, Campus Box 4400, Normal, Illinois 61790, USA Search for other works by this author on: GSW Google Scholar Author and Article Information John P. Craddock * 1Geology Department, Macalester College, St. Paul, Minnesota 55105, USA Jesse Geary 1Geology Department, Macalester College, St. Paul, Minnesota 55105, USA David H. Malone 2Department of Geography, Geology, Illinois State University, Campus Box 4400, Normal, Illinois 61790, USA *E-mail: Craddock@Macalester.edu. Publisher: Geological Society of America Received: 10 Aug 2011 Revision Received: 03 Jan 2012 Accepted: 05 Jan 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (5): 463–466. https://doi.org/10.1130/G32734.1 Article history Received: 10 Aug 2011 Revision Received: 03 Jan 2012 Accepted: 05 Jan 2012 First Online: 09 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 P. Craddock, Jesse Geary, David H. Malone; Vertical injectites of detachment carbonate ultracataclasite at White Mountain, Heart Mountain detachment, Wyoming. Geology 2012;; 40 (5): 463–466. doi: https://doi.org/10.1130/G32734.1 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 SocietyGeology Search Advanced Search Abstract Carbonate ultracataclasite (CUC) is found as a veneer along the bedding plane portion of the Eocene Heart Mountain detachment (Wyoming, United States). At White Mountain, where the CUC is thickest, we report the discovery of six dikes, as much as 1 m wide, that were intruded vertically ∼120 m from the detachment into the overlying Madison and Bighorn Formations. The horizontal basal detachment and injectite material is texturally, petrographically, geochemically, and isotopically identical, containing sparse, sand- to pebble-sized andesitic clasts, various quartz-calcite melt spherules, and armored lapilli in a matrix (95%) of fine-grained calcite. The relationship of hanging wall displacement to fault gouge generation, and the presence of vertical, 120 m, fault gouge injectites, makes the Heart Mountain slide anomalous to all fault systems, especially as this fault system had only one episode of motion. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Heart Mountain and South Fork fault systems: Architecture and evolution of the collapse of an Eocene volcanic system, northwest Wyoming

Thanks to Anders et al. (this volume) for their attention to the legacy of the late Ed Beutner, and to his and my recent Rocky Mountain Geology paper (Beutner and Hauge, 2009). I make no claim of being able to reply to their comment as aptly as Ed would have, but here I'll try to represent my own perspective (and Ed's, as I recall it) on their remarks. Beutner and Hauge (2009) made a case for an early, noncatastrophic phase of displacement along the Heart Mountain (HM) detachment, followed by final catastrophic emplacement of the allochthon. Anders et al. (this volume) argue that this noncatastrophic phase is not supported by the evidence, and they cite both data (calcite twinning; radiometric ages) and models (Aharonov and Anders, 2006) in support of this claim. The models of Aharonov and Anders (2006) and Beutner and Hauge (2009) are fundamentally incompatible because the Aharonov and Anders model requires that the Paleozoic strata of the future allochthon were intact and able to confine pressure that triggered catastrophic failure. In the Beutner and Hauge model, significant extension of the HM allochthon had already taken place when catastrophic failure was triggered. Both models have weaknesses. To varying degrees they require initial conditions that are improbable, explain away conflicting data, inadequately confront alternative models, and make predictions that are not borne out by available data. The resolution of the problems of the initiation, maintenance, and rate of displacement of the HM allochthon has been hampered by the vast scale, rugged terrain, and concealment by younger strata that have made observations difficult. As it has for over a century, the problem remains unresolved. ### Issues Surrounding Beutner and Hauge's (2009) Comments on Aharonov and Anders (2006) The following paragraphs respond directly to the comments of Anders et al. (this volume) in the context of the two competing models. Beutner and Hauge (2009) …

Dynamics of the emplacement of the Heart Mountain allochthon at White Mountain: Constraints from calcite twinning strains, anisotropy of magnetic susceptibility, and thermodynamic calculations

White Mountain is centrally located in the bedding-plane portion of the Eocene Heart Mountain detachment and contains the only upper plate Mississippian Madison Group rocks that have been metamorphosed into marble. The marble rests upon the thickest (1 m) part of a carbonate ultracataclasite that marks the detachment. Thermodynamic and mechanical calculations based on possible frictional melting of calcite and other minerals, geochemical data, the characteristics of the carbonate ultracataclasite, and the geometrical characteristics of White Mountain suggest a possible initial upper plate emplacement rate of 126–340 m/sec and that the duration of the emplacement event was less than 4 min, too brief a time to develop an emplacement-related calcite twinning strain overprint in upper or lower plate carbonates. While the detachment-related carbonate ultracataclasite did not form by melting, it does preserve a magnetic fabric where K max is parallel to the detachment slip direction and records a westward and down paleopole (287° and 27°), where magnetite is the carrier mineral. The Eocene (49.6 Ma) paleopole for this latitude in North America was southerly and upward (0° and 45°). This brief and catastrophic detachment event produced a signifi cant amount of CO 2 by fl ash heating. This report is the fi rst to quantify the emplacement rate of the upper plate of the Heart Mountain detachment based on physical and geochemical parameters.

Hot water: A solution to the Heart Mountain detachment problem?

The Heart Mountain block slide of northwestern Wyoming and southwestern Montana is one of the largest slides known to have occurred in Earth9s history. This early Eocene block slide covered an area of over 3400 km 2 and moved a minimum of 45 km across open terrain. The initial 2- to 4-km-thick Heart Mountain block slide moved on a slope of about 2°, detaching for half its length on a nondescript bedding plane in the Ordovician Big Horn Dolomite (BHD). Given our current understanding of fundamental mechanics, such a great mass of rock should not have begun sliding on such a gentle slope without some special condition. Here we suggest that a special condition existed during the interval between extensive upper-plate dike injections and the initial movement phase. In our model, the dike injections increased horizontal stresses and heated the surrounding layers. Both the increased stresses and the heat input elevated fluid pressure of water trapped within the BHD. In addition, vertical hydrofracturing was retarded as horizontal stress approached vertical, thus allowing a critical buildup of fluid pressure. Fluid overpressuring is a mechanism that can overcome the mechanical problem of initiating movement on a low-angle surface. Moreover, this mechanism explains the observed fluidized features found along the basal contact of the slide block as well as the observed lack of deformation in the lower plate.

Anisotropy of magnetic susceptibility (AMS) analysis of basalt dikes at Cathedral Cliffs, WY: implications for Heart Mountain faulting

Mafic dikes pervade the upper plate of the Heart Mountain Detachment (HMD), yet the dike concentration in the lower plate is sparse. Previous workers interpreted that these dikes were emplaced either coeval with or subsequent to the emplacement of the upper plate. The magnetic fabrics of 32 mafic dikes at Cathedral Cliffs were analyzed using low-field anisotropy of magnetic susceptibility (AMS) as a proxy for flow. These dikes intrude Ordovician–Mississippian carbonate and overlying Eocene volcanic rocks and are truncated along the nearly horizontal HMD. The dikes trend between N10°W and N20°E, are all steeply dipping, and range in width between 0.5 and 3 m. Flow directions for the dikes were determined by the bearing and plunge of the K max (maximum principal susceptibility) axes relative to the dike orientation. About 66% of the dikes sampled show typical dike AMS patterns with K max and K int in the plane of the dike and K min normal to the dike plane. About 66% of the dikes sampled have K max inclinations >45° and thus were emplaced upward; 16% of the dikes have K max inclinations of <10° and thus were emplaced laterally. The remaining dikes have intermediate K max inclinations. With numerous dikes showing vertical to sub-vertical emplacement directions and with no magmatic source immediately below the detachment the dikes must predate emplacement of the upper plate. Therefore, upper plate dilation by dike intrusion could not be a driving force for protracted extension. Our date is consistent with a single catastrophic emplacement event, and inconsistent with an extensional allochthon model of incremental emplacement over long intervals of time.

Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana, USA

The mechanism that allowed many tens of km of movement of the enormous block slide floored by the rootless Heart Mountain detachment fault in NW Wyoming has long been a puzzle. Carbonat-rich microbreccia that is widespread along the fault and in dikes in the upper plate contains accreted grains indistinguishable from those observed as fallout from volcanic eruption clouds (accretionary lapilli) and impact ejecta clouds and in intrusive diatremes. In these settings and also in industrial processing, accreted grains form when particles in a turbulent gaseous suspension containing limited water adhere to a nucleating grain or to each other. Elongate grains in thick microbreccia have strong but diverse shap-preferred orientations unlike those reported from other fault rocks but instead suggestive of turbulent flow, and the microbreccia contains layering and other features of sedimentary character that appear to record deposition from suspension rather than frictional processes along a fault. We suggest that frictional heating led to dissociation of carbonate rock along the fault, producing supercritical CO 2 as the suspending medium. High CO 2 pressure drastically reduced friction along the fault and allowed continuation of catastrophic movement, probably initiated by a volcanic or phreatomagmatic explosion, resulting in very large displacement on a low-dipping surface. Earlier slower sliding may have occurred but final emplacement was rapid (minutes) and spectacular.

The Blacktail thrust-fold, Crandall Conglomerate, and Heart Mountain detachment fault, northwestern Wyoming

The Blacktail anticline in northwestern Wyoming was interpreted by Pierce and Nelson (1973) to be a pop-up anticline which rose into a pull-apart chasm created during an early stage of movement of the Heart Mountain detachment fault (HMD). The Crandall Conglomerate was thought to be the sedimentary fill of that chasm. Alternatively, Hauge (1990) suggested that the channel formed by normal erosional processes and that the Blacktail structure is a late Laramide thrust-fold localized along this channel. Based upon new field observations, we interpret the Blacktail structure to be a northeast-verging, early Laramide thrust-fold, similar to nearby structures. All Crandall Conglomerate exposures in the HMD footwall are always just northeast of the crest of the thrust-fold, a geometry which suggests that a canyon incised in a plateau surface was initially localized along fractured rock on the crest of the thrust-fold. As the canyon eroded vertically, it departed with depth from the southwest-inclined thrust-fold. Crandall Conglomerate deposited in this canyon and the adjacent thrust-fold were then decapitated by movement of the HMD. This history negates the need for an early period of HMD movement.

Fluid flow and the Heart Mountain fault: a stable isotopic, fluid inclusion, and geochronologic study

AbstractNumerous studies have proposed that movement along the 3400‐km2 low‐angle (&lt;3°) Heart Mountain detachment fault of north‐western Wyoming and south‐western Montana was facilitated by the presence of lubricating fluids. A recent stable isotope study suggested that the fluids along the Heart Mountain fault originated from large hydrothermal systems associated with Eocene intrusive centers. Herein, we present results from a combined stable isotope, fluid inclusion, and 40Ar/39Ar geochronology study of the relationship between shallow crustal fluids from Eocene intrusive centers in the New World Mining district and fluids associated with the Heart Mountain fault. Our results suggest that: (i) Eocene intrusive bodies within the New World Mining District focused hydrothermal fluids along specific units and structures from the ore deposit centers along the Heart Mountain fault detachment surface; (ii) hydrothermal waters were focused along the Heart Mountain fault from the breakaway region near Silvergate, Montana toward Heart Mountain in Cody, Wyoming; and (iii) hydrothermal activity along the Heart Mountain fault occurred at the same time as emplacement of the Eocene intrusives, between 50.1 and 48.1 Ma. These data, taken together, suggest that the hydrothermal activity generated by intrusion of Eocene rhyodacites and dacites provided the source of fluids found along the Heart Mountain fault plane.

Calcite twinning strain constraints on the emplacement rate and kinematic pattern of the upper plate of the Heart Mountain Detachment

Two models that address the emplacement rate of the upper plate of the Heart Mountain Detachment (HMD) have been advanced. These are the catastrophic tectonic denudation model and the slow, incremental continuous allochthon model. We compared the two models by analyzing upper and lower plate rocks for a detachment-related overprint of the older, layer-parallel Laramide–Sevier calcite twinning strain fabric. Lower plate Pilgrim Limestone samples, even those collected a few meters below the detachment, show no evidence of a detachment-related twinning strain overprint (negative expected values, NEVs, avg. 8.3%) of the Laramide–Sevier layer-parallel fabric. The allochthonous upper plate carbonate rocks preserve the layer-parallel Laramide–Sevier shortening strain, but the shortening axis ( e 1) for each block occurs in different, non-EW and both subhorizontal and non-subhorizontal orientations. For the six upper plate blocks that preserve chaotic shortening axis orientations, no evidence of any detachment-related twinning strain overprint (NEVs avg. 7.5%) was found. In the absence of any HMD-related twinning overprint, the upper plate allochthon motion must have been rapid enough to be accommodated by fracturing of upper plate rocks without additionally twinning any calcite. We conclude that the emplacement of the upper plate of the HMD was not accompanied by an overprint of the Laramide–Sevier twinning fabric and that models that require catastrophic, rather than slow, incremental emplacement rates are best supported by these data.

Volcanic fluidization and the Heart Mountain detachment, Wyoming

The presence along the Heart Mountain detachment in Wyoming of microbreccia containing volcanic glass grains with primary shapes and accreted grains equivalent to accretionary and armored lapilli supports the concept that injection of volcanic gases along the fault produced fluidization. The probable source of the volcanic contribution was a fixed feeder pipe, now beneath the Crandall intrusive complex, which left a trail of intrusives akin to a hotspot trace in the moving allochthon. Volcanic gas carrying glass and fluidized microbreccia was injected in sill-like fashion along a bedding horizon near the base of the Ordovician Bighorn Dolomite, resulting in gravitative collapse and spreading, probably catastrophic, of the overlying carbonate and volcanic massif.

Heart Mountain, Wyoming, detachment lineations: Are they in microbreccia or in volcanic tuff?

Research Article| September 01, 1991 Heart Mountain, Wyoming, detachment lineations: Are they in microbreccia or in volcanic tuff? WILLIAM G. PIERCE; WILLIAM G. PIERCE 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar WILLIS H. NELSON; WILLIS H. NELSON 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar ANTONI K. TOKARSKI; ANTONI K. TOKARSKI 2Polish Academy of Science, Krakow, Poland 31-002 Search for other works by this author on: GSW Google Scholar EWA PIEKARSKA EWA PIEKARSKA 2Polish Academy of Science, Krakow, Poland 31-002 Search for other works by this author on: GSW Google Scholar Author and Article Information WILLIAM G. PIERCE 1U.S. Geological Survey, Menlo Park, California 94025 WILLIS H. NELSON 1U.S. Geological Survey, Menlo Park, California 94025 ANTONI K. TOKARSKI 2Polish Academy of Science, Krakow, Poland 31-002 EWA PIEKARSKA 2Polish Academy of Science, Krakow, Poland 31-002 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1991) 103 (9): 1133–1145. https://doi.org/10.1130/0016-7606(1991)103<1133:HMWDLA>2.3.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 WILLIAM G. PIERCE, WILLIS H. NELSON, ANTONI K. TOKARSKI, EWA PIEKARSKA; Heart Mountain, Wyoming, detachment lineations: Are they in microbreccia or in volcanic tuff?. GSA Bulletin 1991;; 103 (9): 1133–1145. doi: https://doi.org/10.1130/0016-7606(1991)103<1133:HMWDLA>2.3.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 concept of tectonic denudation followed by deposition of lower middle Eocene Wapiti Formation volcanic rocks on the exposed Heart Mountain detachment has been challenged by Hauge. His "extending allochthon" interpretation requires that the Wapiti Formation be fault emplaced and that lineations in a volcanic tuff overlying the fault north of Jim Smith Peak be fault striae in "microbreccia." Our re-examination of the field evidence in minute detail indicates that these lineations were produced by flowage of volcanic rocks on a thin layer of air-fall tuff. The following lines of evidence indicate that this tuff was deposited on the detachment surface during the brief interval that the denuded surface was exposed. (1) Field relations indicate that the Wapiti volcanic rocks are younger than the detachment, and so they could not have been fault emplaced. (2) Lineations occur only on the upper surface of Wapiti Formation tuff; none were found at the contact of the tuff with the detachment surface. (3) The reported direction of translation indicated by the lineations is not consistent. Many lineations in the tuff indicate north and northeast movement—a direction that is consistent with lineations produced by lava flowing from a known vent but inconsistent with a southeast direction of detachment movement. (4) The lineations are restricted to the northwestern part of the area covered by the Wapiti Formation, which is also consistent with their derivation as volcanic flow lineations from a known vent. If the Wapiti had been tectonically emplaced, fault striae should occur elsewhere on the detachment surface, but none have been observed. (5) The lower contact of the tuff with the detachment surface, as seen in thin sections, is irregular, suggesting that it is a depositional rather than a fault contact. (6) The contact between the tuff and overlying Wapiti, as seen in thin sections, is also irregular, as would be expected if the Wapiti volcanic breccia had been deposited on the tuff. Thin sections of the "microbreccia" suggest that it is a sedimentary rock, not a "fault rock." (7) A remnant of the tectonic carpet of all-carbonate fault breccia lying on the detachment surface, unmixed with the overlying Wapiti volcanic rock, is compatible with the depositional emplacement of the Wapiti but is not compatible with Wapiti Formation being moved many kilometers across the carpet of all-carbonate fault breccia and not removing or becoming mixed with it. 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.

Kinematic model of a continuous Heart Mountain allochthon

Two fundamentally different models describing the geometry, kinematics, and mechanism of emplacement of the allochthon of the Heart Mountain detachment have been advocated. The traditional denudation comprises catastrophic gravity sliding of numerous detached blocks, with widespread subaerial exposure of the detachment by tectonic denudation. In this view, catastrophic volcanism covered the allochthonous blocks and the denuded detachment immediately after emplacement of the slide blocks. The alternative allochthon comprises noncatastrophic gravity spreading of a continuous allochthon, without tectonic denudation of the detachment. In this view, volcanism was noncatastrophic and predominantly coeval with, rather than subsequent to, faulting, and volcanic rocks constitute a large volume of the allochthon. Recent field studies indicate that the continuous allochthon model is geometrically and kinematically in accord with field relationships, analog models, geometric models, and relationships characterizing other detachment complexes, whereas the tectonic denudation model is not similarly substantiated. In particular, the tectonic denudation model is disproven by field relationships which demonstrate that volcanic rocks once thought to postdate faulting were instead tectonically emplaced. In addition, theoretical studies indicate that emplacement of the allochthon envisioned in the continuous allochthon model is mechanically plausible, whereas the catastrophic emplacement of detached blocks envisioned in the tectonic denudation model is mechanically implausible.

The case for tectonic denudation by the Heart Mountain fault—A response

Two basic concepts pertaining to the history of the Heart Mountain fault of northwestern Wyoming have recently been challenged; one, that there was tectonic denudation, and two, that volcanic rock of the Wapiti Formation was deposited on the exposed fault surface. Tectonic denudation is believed to have occurred as a consequence of the upper plate having broken into numerous blocks that separated as movement progressed along a nearly horizontal fault surface, thus leaving the fault surface exposed between blocks. Volcanic rocks of the Wapiti Formation were then deposited both on the exposed fault surface and against and over the upper-plate blocks. Two formations of Eocene volcanic rocks are involved. The older volcanic unit, the Cathedral Cliffs Formation, and the Paleozoic carbonate rocks are part of the upper plate of the Heart Mountain fault and moved with it, whereas the younger Wapiti Formation was deposited on the fault surface after movement had ceased. In an alternate interpretation recently advanced by T. A. Hauge, subdivisions of the Absaroka Volcanic Supergroup, of which the Cathedral Cliffs and Wapiti Formations are units, are not recognized. The upper plate of the Heart Mountain detachment is interpreted as having been a single, continuous allochthon composed largely of volcanic rocks with small amounts of Paleozoic rocks. During Heart Mountain faulting, extension of the once-continuous slab of Paleozoic sedimentary rock is alleged to have been accompanied by the formation of ten or more grabens, now filled predominantly by Absaroka volcanic rocks. This interpretation further proposes that the volcanic rocks were emplaced while the separating blocks of Paleozoic strata were still moving and that the basal part of the volcanic rock between these blocks is in fault contact rather than depositional contact with the strata beneath the Heart Mountain fault. Many lines of geologic field evidence indicate that the Wapiti Formation is younger than the Heart Mountain fault and was deposited on the technically denuded fault surface. (1) Wapiti rocks bury the break-away fault. (2) Fault breccia at the base of the upper-plate carbonate blocks is composed entirely of carbonate fault breccia and has no volcanic component. (3) Small blocks of upper-plate rocks have been displaced by gravity from the upper part of the allochthon to the detachment fault surface. (4) Eocene stream-channel deposits locally cut into the surface of tectonic denudation and also have been displaced on the Heart Mountain fault. (5) The volume of Wapiti Formation filling the spaces between allochthonous blocks in proportion to the volume of those blocks is much too great for the Wapiti to have been allochthonous. (6) Clastic dikes of carbonate fault breccia penetrate Wapiti volcanic rocks. (7) Some of these clastic dikes of fault breccia contain Precambrian xenoliths and wood phenoclasts requiring surface exposures of the fault breccia before injection as dikes. (8) Wapiti volcanic rocks having chilled borders are in tightly bonded contact with upper-plate Paleozoic rocks. (9) Faults present in the upper-plate blocks do not penetrate the overlying Wapiti Formation. (10) Volcanic fault breccia is absent where volcanic rocks overlie carbonate fault breccia. (11) A mound of carbonate fault breccia is not mixed with overlying Wapiti Formation. The continuous allochthon interpretation is based on several erroneous assumptions that cannot be supported by field observations. (1) Faults to transport and emplace the Wapiti Formation onto and along the Heart Mountain fault do not exist. (2) The contact between volcanic rocks and the allochthon west of Corral Creek at the west end of Cathedral Cliffs, cited by Hauge as a fault in an extending allochthon, is a depositional contact. (3) The volcanic rock adjoining allochthonous Paleozoic rocks north of Pilot Creek cannot be part of an extending allochthon because (a) it is Cathedral Cliffs Formation, which is pre–Heart Mountain fault, and (b) its direction of movement is horizontal rather than down dip, as required in an extending allochthon. (4) Most of the igneous dikes were intruded after the Heart Mountain fault movement ceased, and so they could not accommodate significant extension of the upper plate. (5) Striae reported as indicating fault emplacement of volcanic rock (Wapiti Formation) on the Heart Mountain fault actually lire flow features, formed as the Wapiti Formation was deposited on the exposed fault surface. Tectonic denudation is the only model that is consistent with evidence observable in the field. Although the process by which tectonic denudation was accomplished remains enigmatic, tectonic denudation remains a constraining fact in any model for the origin of the Heart Mountain fault.

Role of bending in gravity-driven overthrusts

The decrease in potential energy of gravity-driven overthrusts due to change in position can be equated to the sum of strain energy required for deformation, the work done against friction, and the kinetic energy. An analytical model based on this relationship allows the strain energy in pure bending to be expressed as a function of displacement. The model applies to an elastic material, and assumes frictionless slip for the multilayered case. Minimization of potential energy leads to an estimate of the minimum initial length required for gravitational sliding, and to the apparent coefficient of friction, displacements, and velocities for given slide geometries and properties. An example problem is worked out for the case of the Heart Mountain detachment of Wyoming, U.S.A. Minimum initial length for frictionless sliding is about 5.9 km, ignoring the effect of bending, and 8.5 km for a case of multilayered bending. Blocks of this size would move only a short distance as the increase in potential energy experienced in the riser prevents further sliding. But multilayered bending seems to have been necessary for sliding, compared to bending of the overthrust sheet as a single layer. Kinetic energy relationships suggest that the apparent coefficient of sliding friction for the 45 km long block was less than 0.031; e.g., for a friction coefficient of 0.025, a velocity of 19 m/sec is predicted at a characteristic displacement of 7 km. The model thereby illustrates the need for drastic friction reduction, but does not attempt to address the question of mechanism by which friction was reduced. The energy required to deform the slide mass is thought to be small compared to the decrease in potential energy due to changing position.

Gravity-spreading origin of the Heart Mountain allochthon, northwestern Wyoming

Field studies of the Heart Mountain detachment and overlying volcanic rocks reveal that “autochthonous” Eocene volcanic rocks previously reported as postdating faulting are tectonically emplaced and, in many places, far traveled. Evidence of tectonic deformation of these volcanic rocks is documented in areas where volcanic rocks directly overlie the detachment, as well as at structurally higher levels within the allochthon. Such evidence includes shearing, brecciation, and faulting of volcanic rocks, and tilting and truncation of volcanic strata. This widespread deformation casts doubt concerning earlier suggestions that volcanic rocks were deposited upon the detachment and upon numerous detached slide blocks after detachment faulting. In light of these new data, the upper plate of the Heart Mountain detachment is interpreted as having been a single, continuous allochthon composed largely of volcanic rocks, rather than as having consisted of numerous separate slide blocks as was previously envisioned. Crosscutting relationships between dikes and faults within the allochthon suggest that allochthon emplacement occurred coeval with volcanism and, contrary to earlier suggestions, need not have been catastrophic. The mechanism of emplacement of the allochthon, which once may have covered >3,400 km 2 , is viewed in terms of gravity-induced spreading on the flanks of an active volcanic field. Kinematic data indicate that transport of the allochthon was locally directed to the north, northeast, east, and southeast, generally away from active volcanoes, as well as downslope toward the Bighorn Basin. Transport was accompanied by variably directed extension of the allochthon, accommodated by normal, oblique-normal, and strike-slip faults, tilting of fault-bounded blocks within the allochthon, and dike intrusion.

The Heart Mountain break-away fault, northwestern Wyoming

The Heart Mountain break-away fault was the last of the four phases of the Heart Mountain detachment fault to be discovered and is the only fault of this type yet recognized and described. The descriptive term, “break-away fault,” was introduced in 1960 for the steep surface along which the rocks of the Heart Mountain decollement fault mass separated from the stationary block on the west. The break-away fault, located near the northeast entrance to Yellowstone National Park, has a known linear extent of 37 km. The northern one-third trends north, and the remainder trends southeast and south. Rocks on its western side are Paleozoic carbonate rocks and Eocene volcanic rocks; the volcanic rocks are pre–Heart Mountain fault in age. The rocks on its eastern side are younger and consist of Eocene volcanic rocks which were deposited against and over the fault. Thus the break-away fault is unusual in that one side is a fault surface and the other side is a depositional surface, and in that respect is similar to parts of the bedding-plane phase of the Heart Mountain detachment fault. Slickensides and brecciation in the Madison Limestone adjoining the break-away indicate horizontally directed compressive shear. Movement was probably right lateral, and then laterally southeastward away from the fault at an angle of about 45°. To the north, in Wolverine Pass and in the valley of Soda Butte Creek, the break-away fault extends down to a bed in the basal part of the Bighorn Dolomite where it joins or terminates at the bedding plane phase of the Heart Mountain detachment fault; movement of upper-plate rocks away from the break-away fault increases from about 0.25 km at the northernmost trace of the fault to more than 1.5 km on the south side of Soda Butte Creek valley. Several kilometres southeastward, in Cache Creek valley, the Heart Mountain break-away fault terminates downward in volcanic rocks of the undivided Lamar River and Cathedral Cliffs Formations and joins a low-angle fault in those rocks. This low-angle fault probably steps downward to the east where it reaches the Heart Mountain detachment fault surface. Three kilometres farther to the southeast, the break-away fault is presumably represented by a low-angle fault in these volcanic rocks that probably also steps down to the northeast beneath the volcanic cover to join the Heart Mountain detachment fault. Erosion in this area probably has removed rocks that contained the high-angle part of the fault. The south end of the break-away fault is overlapped and concealed by the Wapiti Formation. It presumably continues southward, but it probably does not join a normal fault near Black Mountain, although Voight (1974) has suggested that it does.

Crandall Conglomerate, An Unusual Stream Deposit, and Its Relation to Heart Mountain Faulting

The Crandall Conglomerate (Eocene) is a channel deposit, more than 350 ft (100 m) thick, believed to have formed as a result of preliminary movement of the Heart Mountain detachment fault in northwestern Wyoming. Initial movement of the Heart Mountain fault opened a deep rift in which the conglomerate was deposited. The rift was less than a mile (1.6 km) wide and was bordered by 2,000-ft (600 m) cliffs, mostly of Paleozoic limestone. Before the gravel was deposited, unconfined Cambrian shale below the rift was deformed into the Blacktail fold, a sharp anticline without apparent roots, while streams carried away the upwelling shale and cut a channel several hundred feet deep. The debris that accumulated in this channel is the Crandall Conglomerate. Deposition of the conglomerate was followed by Cathedral Cliffs volcanism, by movement on the Reef Creek detachment fault, and by the main movement on the Heart Mountain detachment fault. The main movement on this fault left the lower part of the conglomerate in place but carried the upper part with deposits of the upper plate roughly 15 mi (24 km) southeastward. Most of the deposits of the lower plate rest directly on the Blacktail fold. Of the 15 known deposits of Crandall Conglomerate, five are in place but have been overridden by the upper plate of the Heart Mountain fault, and ten have been transported as part of the upper plate. After this movement, volcanic rocks of the Wapiti Formation blanketed the region.

Igneous Activity, Metamorphism, and Heart Mountain Faulting at White Mountain, Northwestern Wyoming: Reply

The spire of brown volcanic rock at White Mountain, long regarded as a filled volcanic conduit associated with eruption of the Eocene Wapiti Formation, is actually an erosional remnant of volcaniclastic sedimentary rocks of the Eocene Cathedral Cliffs Formation that has been transported 10 to 15 mi southeastward as part of a Heart Mountain detachment fault block. Prevolcanic Paleozoic carbonate rocks, which comprise the western part of this block, are metamorphosed and strongly folded, in contrast to the rocks of other Heart Mountain fault blocks. This metamorphism ends abruptly downward at the Heart Mountain fault plane. Rounded fragments of metamorphosed carbonate rock in Heart Mountain fault breccia and in related clastic dikes show that the metamorphism occurred before the faulting, presumably as a result of the Cathedral Cliffs igneous activity. Igneous rock fragments in the Heart Mountain fault breccia at White Mountain seem to have been derived from allochthonous sills and dikes that predate the Heart Mountain fault. Therefore these fragments are not evidence for volcanic activity during Heart Mountain faulting, nor for introduction of volcanic gases along the fault plane as suggested by Hughes (1970a). A fault plate would have to be large and coherent to be buoyed up by volcanic gas or other fluids. The present widespread distribution of the remnants of the upper plate indicates that the upper plate broke up to form a number of fault blocks very soon after the Heart Mountain faulting began.