THE 1959 HEBGEN LAKE, MONTANA, EARTHQUAKE: TWO GEOLOGIC POINTS OF VIEW by Irving J. Witkind U.S. Geological Survey Denver, CO 80225 Some 16 years ago a major earth- quake, centered near Hebgen Lake, Montana, shook Yellowstone National Park, and abruptly brought to an end the bustling 1959 tourist season. That earthquake, formally known as "The Hebgen Lake, Montana, earthquake of August 17, 1959", was widely publi- cized, and as a result of the public's keen interest in the geologic features formed during the earthquake, the Forest Service set aside the area of major damage as an educational and the northeast edge of the Henrys Lake recreational area. Now, attractive ex- basin to end against the reactivated hibits and skilled naturalists explain the northwest-trending Hebgen and Red various geologic phenomena to the Canyon faults. They assume that the many tourists who annually pass Madison Range subsided along with through the area. the flanking basins, but not as much. Immediately after the earthquake Dual-basin concept.—A second the Geological Survey began a series of group of geologists concluded that the intensive geologic investigations of the deformational pattern involved two ba- epicentral area. It soon became ap- sins of subsidence, one on each side of a parent that geologists viewing the same tectonically stable Madison Range. phenomena interpreted them differ- Each of these basins slopes gently ently. Two major concepts evolved each northeastward to end against reacti- with its own adherents, and as the vated faults; the West Yellowstone Ba- field work continued it became obvious sin (east of the Range) ends against that a vexatious controversy had begun. the reactivated Hebgen and Red Can- Neither side could uncover convincing yon faults, and the Missouri Flats evidence that would resolve the con- (west of the Range) end against the flict. Finally, as the time came to pre- reactivated Madison Range fault. In pare a formal report on the completed this interpretation the Madison Range studies, the geologists concerned agreed did not subside. to disagree. STRUCTURAL SETTING Now, some 16 years later after ad- Underlying this controversy is the ditional geologic and geophysical work, fact that the advocates of each con- where do we stand? It would be pleas- cept interpret the regional structure dif- ant indeed to indicate that these ad- ferently. This part of southwestern ditional studies have resolved the prob- Montana and southeastern Idaho is lem. In fact, the results are inconclusive marked by northwest-trending moun- and seemingly favor neither concept. tain ranges separated by narrow valleys —typical basin-and-range structures THE TWO CONCEPTS that result from extensional block fault- As geologists studied the epicentral ing (see map). Most of these mountains area shortly after the earthquake it are bordered on one flank by a high- quickly became apparent that large angle range-front normal fault in which blocks of the Earth's crust had sub- the valley is downthrown relative to the sided, but there were sharply different range. Thus, for example, the north- opinions as to which blocks had sub- west-trending Madison Range is sepa- sided and the exact pattern of deforma- rated from the Madison Valley by the tion impressed on these blocks. high-angle Madison Range normal Single-basin concept.—One group fault. Similarly, the other northwest- of geologists concluded that the defor- trending ranges are separated from mation pattern was best satisfied by the their adjacent valleys by high-angle uneven subsidence of a single crustal normal faults. This persistent northwest block deformed into a basin. In their trend of these structural mountain and interpretation, the axis of this basin valley pairs is interrupted by the Cen- slopes gently northeastward from near tennial Mountains—Centennial Valley

4 AISAROKA

45° Index map of southwestern Montana and southeastern Idaho showing the basin-and- range pattern in this part of the northern VIAL VALLEY NE Rocky Mountains. The general northwest FAULT CENTENNIAL structural grain of the region is disrupted CENTENNIAL MTs by the eastward-trending Centennial Moun- TONE tains-Centennial Valley structural pair. AREA OF MAP BEL ATEAU Heavy lines indicate high-angle normal SNAKE RIVER PLAIN 0 50 KM faults. Bar and ball are on downthrown side. 44°

113° 112°

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fo Wade Lake 0 10 20 MILES ISSOURI ch FLATS 0 10 20 KILOMETRES II. Lake 0 Hebgen Lake 1•

\-Th TARHEE LAKE Mrs \PASS Elk Lake

Lima Reservoir Red River West Yellows HovIt4rd Lower Red Red Pock Lake ALLEY / Mud Lake. Rock Lake CENTENNIAL. o Henrys lake Lakeview HENRYS ido ONTANA LAKE BASIN ° /...—ACENTENNIAL , MOUNTAIrrS" 5 -1 —IDAHO I \ -- 44 °30'

I WYOMING

112'30' 112'15 112 °00 III ° 45' 111 30 111

Part of southwestern Montana and southeastern Idaho showing general relations between the Centennial and Madison Range faults. The Centennial fault can be traced eastward to the western edge of the Henrys Lake basin; it is uncertain whether it extends still further eastward. structural pair, which trends eastward The adherents of the single-basin directly across the structural grain of concept believe that the eastward-trend- the region. Like the northwest-trending ing Centennial structures represent a structures, the Centennial Mountains fundamental change in a long-estab- also are separated from the Centennial lished structural pattern. In their view Valley by a high-angle normal fault— these are younger structures that are the Centennial fault. Here too, the val- being extended eastward across the ley is downthrown relative to the moun- older northwest-trending structures rep- tains. Fresh scarps along the Madison resented by the Madison Range—Madi- Range and Centennial faults, as well son Valley structural pair. Cohsequently, as along most of the other high-angle they consider the earthquake to have faults, indicate that segments of vir- been caused by movement on an east- tually all the faults have been active ward-trending deep-seated fault related within recent geologic time. to these Centennial structures. They

5 suggest that primary movement on this on, the Survey also conducted a gravity deep-seated fault triggered secondary survey of the area surrounding the movement on the northwest-trending Henrys Lake basin in an attempt to Hebgen and Red Canyon faults now determine whether the Centennial struc- marked by spectacular scarps. To sup- tures—chiefly the Centennial fault— port this view they suggest that the could be traced eastward across the deformational pattern impressed on the basin and through the Madison Range. downthrown crustal blocks is best de- Results of that survey have recently scribed as a single basin of subsidence been published in the Geological Sur- whose axis is alined with the eastward- vey's Journal of Research, volume 3, trending Centennial structures. number 2. By contrast, proponents of the The Centennial fault can be traced dual-basin concept believe that the eastward from near Mud Lake to the causative faults of the earthquake are western edge of the Henry's Lake the newly reactivated northwest-trend- basin. Wherever exposed, it dips north- ing Hebgen and Red Canyon faults. ward (valleyward) with the north block They point to the geologic pattern near downthrown relative to the south one. these faults as evidence that the re- Eastward beyond the western edge of newed movement on these faults was the basin the extent of the fault is in merely one more episode in a sequence doubt. A series of northeast-trending that began in the middle to late Ter- linear features have been interpreted tiary time, and that has continued un- as the fault trace. Of these, the most broken to the present. They see no evi- striking are two or three straight north- dence that the Centennial structures, west-facing embankments (implying some 40 km west of the epicentral down-throw of the north block) which area, have played any role in this par- point directly toward the mouth of ticular earthquake. the valley of Howard Creek. The fault is thought to extend into the mouth RECENT GEOLOGIC AND of the valley and more or less follow GRAVITY STUDIES Highway 20-191 through and across Because the Centennial and Madi- Targhee Pass into the West Yellowstone son Range structures seemingly inter- Basin. The Pass is attributed to move- sect in the Henrys Lake basin, the ment on the fault. The proposed course Geological Survey, in 1968, decided to of the fault across the West Yellow- study the area to determine whether stone Basin is presumably shown by a the Centennial structures, do indeed ex- series of eastward-trending small scarp- tend across the southern ends of the lets which face north and were formed Madison Valley and Madison Range. during the 1959 earthquake. Consequently, the 15-minute Henrys These features, however, can also Lake quadrangle (which includes the be interpreted as relicit valley walls Henrys Lake basin) was mapped at a formed when torrential melt waters, scale of 1:24,000, and subsequently produced from the waste of an ice mass published at a scale of 1:62,500 in the which lay in the West Yellowstone Miscellaneous Investigations map series Basin, escaped southwestward through of the Geological Survey as Map I- Targhee Pass and the valley of Howard 781—A. As the geologic work was going Creek and removed the north half of

6 an ancestral alluvial fan. The minor this gravity high is due to more dense scarplets that break the floor of the Precambrian crystalline rocks which West Yellowstone Basin may be the are exposed north of the highway, but result of slump and compaction of are concealed beneath about 1460 basin fill resulting from the shaking that metres of less dense sedimentary rocks occurred during the 1959 earthquake. south of the highway. Does the Centennial fault extend through Targhee Pass? Regrettably, CONCLUSIONS critical details are masked by surficial Despite the additional geologic and deposits, chiefly of glacial origin, and geophysical work done since 1959, it by volcanic ash-flow tuffs. seems reasonable to conclude that defi- The results of the gravity survey nitive data which would either prove or showed a low with about 10 milligals of disprove the single basin or dual-basin closure coinciding with the Henrys concepts have not been found. The Lake basin. The gravity data in the present data still permit differing and basin are too few to determine whether conflicting interpretations. Two major the Centennial fault does extend east- questions still remain to be answered: ward across the basin. The data do in- Does the Centennial fault extend east- dicate a southwest-dipping gradient ward across both the Madison Range along the front of the Madison Range fault and the southern end of the that reflects the Madison Range fault. Madison Range? And, did the Madison Seemingly there is no break in the Range subside along with the flanking trend of the isogals where they cross basins during the earthquake—a pre- the mouth of the valley of Howard cept inherent in the single-basin con- Creek—the point where the Centennial cept. fault is thought to cross the Madison The first question may be answered Range fault and extend into the Madi- by additional geophysical work. The son Range. Northeastward beyond this second question—subsidence of the point, however, there is a sharp change Madison Range—may be answered by in intensity in the gravity field near redetermination of altitudes of selected Targhee Pass where the field is about points along the crest of the range. The 12 milligals higher north of the high- pre-earthquake altitudes of these points way than to the south. One interpreta- are known and the data would help us tion of this high is that the block north evaluate whether the range did indeed of the projected fault has been raised, subside. Until these questions are but if so this is the only place along the answered, the final details of the Heb- Centennial fault where the north block gen Lake Earthquake continue to elude is upthrown. It seems more likely that us.

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