Reverse Fault Type Research Articles

短スパンアレイ観測による奥羽脊梁山地周辺の地殻不均質構造のイメージング

Ou Backbone range, northeastern Japan, is bounded both at eastern and western edges by active faults with reverse-fault type, and seismic activity is quite high in this range. The known largest event is the Rikuu Earthquake with M7.2 that occurred along the western edge in 1896. Two small aperture seismic arrays were deployed in 1997 and 1998 in order to image spatial distribution of P wave scatterers in the crust beneath the range. Waveforms of 7 and 13 explosions observed by the respective arrays were analyzed in a frequency band of 6-20Hz, Hypocentral distances between explosions and arrays are too large to apply conventional analyses such as CMP stacking or migration to observed data. We developed a method to estimate P-wave scatterer distribution from array observation data. Observed waveforms were slant stacked along various directions from the arrays, and energy density of waveform was further processed by diffraction curve summation. Spatial distribution of P wave scatterers was imaged by the presently developed procedure. The results show that strong scatterers are distributed around the fault plane of the 1896 Rikuu Earthquake and in the mid- to lower-crust of the range. The distribution of scatters in the upper crust correlates to hypocenter distribution of micro-earthquakes.

近地で観測されたレーリー波から推定した1993年能登半島沖地震の震源深さ, 震源メカニズムと能登半島周辺の地下構造

The Noto-Hanto-Oki earthquake (MJMA=6.6) occurred NE off Noto Peninsula on February 7, 1993. According to JMA (Japan Meteorological Agency), the focal depth of this event was 25km, which was deeper than most inland shallow events in Japan. There were distinct differences among the focal mechanisms obtained from the data of the P-wave initial motions, the long-period seismic waves, the short-period strong ground motions, and the tsunami waves. However, most of them indicated that the earthquake fault of this event was of the reverse fault type. We evaluated the source depth and the focal mechanism of the event through parametric simulation studies in the period range from 4 to 20 seconds of long period strong ground motions observed at 4 local stations. These stations were located from 46 to 133km from the epicenter. First, the subsurface structure from the source to each station was estimated as a multi-layered model by an inversion analysis of the group velocities from the observed Rayleigh waves. The structure obtained for each station was consistent with the distribution of gravity anomaly. It was concluded from the best fit of the simulation and the location of aftershock region that the fault plane of the event dipped 45° toward N140°E with a slip angle of 70°. We found that the fault plane simultaneously dipped 45° toward N330°E with a slip angle of 70°. However, this case was inconsistent with the deformation of the sea bottom by active faults. The depth of the equivalent point source, whose location corresponds to the center of the fault plane, was 10km in the best case of the simulation. The depth was significantly shallower than the focal depth reported by JMA and was consistent with other inland shallow earthquakes in Japan. All the results were obtained from the simulation using the point source model. However, it was also found that the conclusion did not change for the results from the model of finite moving source of this event.

Interseismic uplift and strain accumulation in Nepal Himalaya

Savage (1983, 1995) and Thatcher and Rundle (1984) have postulated elastic half space and viscoelastic coupling models of strain accumulation and earthquake occurrences at the upper surface of the subducting lithospheric plate called detachment. These models infer the presence of a locked zone on the detachment during interseismic periods. The analysis based on interseismic uplift data of about two decades in Nepal Himalaya indicates absence of the postulated locked zone and instead suggests occurrence of thrust fault type (up dip) slip on the entire detachment (Jackson and Bilham, 1994). A question then arises as to whether the strains are accumulating at all in this region. Further, if strains are accumulating we may like to investigate the process responsible. The answer is provided by a plausible model visualising strain accumulation due to differential motion of the medium particles on the detachment and those above it. This relative down dip motion of the medium particles on the detachment may be occurring even if the subducting and the overriding plates are not fully coupled or locked. Sections with high coupling on the detachment would have accumulated larger strains compared to those with low coupling. A temporal decrease in up dip slip rate on the detachment, by virtue of increase in coupling or strain hardening, further represents interseismic accumulation of shear strains. On the other hand, a temporal increase in up dip slip rate, when activates unstable motion, may culminate into occurrence of a large earthquake. This model explains the observed elevation changes in the Himalaya and corresponding inference of up dip slip on the detachment. It also provides plausible justification to the occurrence of moderate and great earthquakes on a part of the detachment lying primarily below the Lesser-Higher Himalaya.

Numerical Simulation of Displacement and Stress Fields Associated with the 1993 Kushiro-oki, Japan, Earthquake

—We constructed a three-dimensional finite element model to simulate coseismic and postseismic displacement and stress fields associated with the 1993 Kushiro-oki earthquake, which was a very large intermediate-depth earthquake that occurred within the subducted Pacific plate at a depth of 107 km beneath the southeastern part of Hokkaido, Japan. Taking the configuration of the subducted Pacific plate into account, we constructed a realistic model with lateral heterogeneity of viscoelastic structure. We assigned a variable slip distribution to the fault plane, which was obtained from inversion analysis of near-field seismic waveforms. The result shows that elastic deformation associated with the faulting reflects the assigned inhomogeneous slip distribution on the fault plane near the fault region, while it does not reflect the distribution on the free surface of the model. The calculated postseismic deformation does not reflect the slip distribution, but shows symmetric spatial patterns concerning the dipping direction of the fault both near the fault region and on the model surface. For the next 20 years following the earthquake, the amount of the calculated deformation is a fraction of the coseismic deformation. The calculated coeseismic deformation is large just above and below the fault plane, reaching 1 m, while the postseismic deformation is dominant near the upper and lower material boundaries between the subducted plate and the surrounding asthenosphere. The spatial distribution of maximum shear stress near the fault plane corresponds to the assigned slip distribution, amounting to 32 MPa. The directions of principal stress-change axes represent reverse fault type in the SSE region of the fault, whereas normal fault type is dominant in the NNW region with the exception of some asymmetrical spatial patterns of the principal stress-change axes on the fault due to the inhomogeneous slip distribution. Time variations both in the amount and the directions of stresses are minor, suggesting that the coseismic state of the stress would remain unchanged for two decades after the event.

Detailed Mapping of Focal Mechanisms in/around the 1995 Hyogo-ken Nanbu Earthquake Rupture Zone.

Earthquake focal mechanisms before and after the 1995 Hyogo-ken Nanbu earthquake have been investigated using seismic records from regional seismic networks. Before the mainshock, seismicity was very active at the Tamba Plateau, a neighboring area of the Hyogo-ken Nanbu earthquake rupture zone. In contrast, the seismicity along the Hyogo-ken Nanbu earthquake rupture zone was not so active. Most earthquakes in these regions had source mechanisms of E-W compression and were of the strike-slip or reverse-fault type. Most aftershocks along the Hyogo-ken Nanbu earthquake rupture zone have strike-slip solutions with P-axis in the E-W or ESE-WNW direction, which is compatible with the trend of aftershock distribution and the strike of active faults the same as the mainshock mechanism. Simultaneously, many other aftershocks were of the reverse-fault type with E-W compression. This area is still controlled by the regional stress field of E-W compression observed before the mainshock. Although, we could find various types of mechanisms in the aftershock sequence, some normal fault-type events were also observed in the mainshock rupture zone. We could find events of SE-NW compression, and this direction is nearly perpendicular to the trend of the mainshock rupture zone. Some aftershocks that occurred near the epicenter of the mainshock had solutions of N-S compression. The geometry of the active fault systems and/or local stress change induced by the mainshock may cause these complex features of focal mechanisms. After the mainshock, the focal mechanisms of earthquakes in the Tamba Plateau were approximately E-W compressional; the same as that before the mainshock.

１９９５年奄美大島近海（喜界島近海）地震の地震活動とテクトニクス １９９５年奄美大島近海地震 活動と被害の概況

A shallow earthquake of MJMA 6.7 on October 18, 1995, at about 50km southeast off Kikai-jima in the Ryukyu Islands caused slope landslides, subsidence of concrete slabs at fishing ports and collapses of walls piled up with coral ragged stones. Several fishing boats moored to wharves were damaged by tsunami; four boats were turned over at ports in Amami-Oshima.From May 19 to 30, 1995, earthquakes of MJMA≤4.1 had occurred in the focal area after a long seismic quiescence since the 1911 Kikai-jima Earthquake of M=8.0. Some of them were felt in JMA intensity 1 or 2 at Kikai-jima. Numerous felt shocks also struck the island immediately after the main shock; 62 and 125 shocks on October 18 and 19, respectively. Numbers of aftershocks decreased rapidly, though they lasted through the end of 1996.The damage distribution of the 1911 event being compared with that of the 1995 event, their focal areas were considered to overlap with each other. It was, however, possibly an interplate earthquake of a thrust fault type in contrast with the 1995 event of a normal fault type in the subducting plate, because the first motions of P wave at Naze and of tsunami wave at Kikai-jima were reversed between the two events.

Evidence from Earthquake Fault Plane Solutions on Upper Crustal Stresses in the Garhwahl Himalaya

Abstract The individual thrust-fault type fault plane solution of the 1991 Uttarkashi earthquake is considered along with a reverse fault type and a strike-slip type composite fault plane solutions for the Garhwal Himalaya. The two latter solutions are based on data from 49 small and micro-earthquakes. All 50 earthquakes occured in the vicinityof Main Central Thrust over a distance of about 150km from Yamuna to Alaknanda valleys. The data are consistent with the view that a thrust-fault type stress environment prevails throughout the uppercrust in the region. The maximum effective principal stress (s1) is horizontal to sub-horizontal with azimuth along N42° everywhere in the upper crust except in the hypocental region of the Utrarkashi earthquake, where it is N26°. lntermediate effective principal stress (s2)is horizontal in the NW quadrant and the minimum effective principal stress (s3) is vertical to sub-vertical correspondingly. Reactivation of the strike-slip faults occurs under the influence of s1 and s2. This is within the theory of fault reactivation under general three dimensional stresses. Pore pressures could be relatively high and may approach lithostatic levels over most parts of upper crust in the region.

Spatial distribution of earthquakes off Sanriku, northeastern Japan, in 1989 determined by ocean-bottom and land-based observation.

In October-November, 1989, five M≥6 earthquakes, including an M7.1 earthquake, sequentially took place off Sanriku, northeastern Japan, and the earthquake activity became very high in this region. Since the seismically active zone was far from the land observation network, it was difficult to obtain accurate hypocenter locations. We carried out ocean bottom seismographic observation in order to locate the hypocenters precisely and investigate characteristics of the activity. Combining the OBS and land station data for hypocenter determination, we obtained two main conclusions. (1) Almost all epicenters of the present activity distribute landward from the 4 km isobath. Some earthquake clusters were observed including an M5.6 earthquake and its aftershocks. (2) Focal depths of the earthquakes are shallower than 20 km and the activity is interpreted to take place around the boundary between the subducting Pacific plate and the landward plate. Taking account of the mechanisms of the larger earthquakes in this active period, this activity is a thrust fault type and is different from the 1933 Sanriku earthquake which was a normal fault type and might have broken the entire oceanic lithosphere.

Microearthquakes and tectonics around an outer rise: the Zenisu ridge, Japan

To investigate the activity and tectonic implications of intraplate microearthquakes occurring within the deflecting oceanic plate along the Nankai-Suruga trough, Japan, we deployed eight ocean bottom seismometers (OBS's) on the Zenisu ridge, for a period of 25 days in 1985. Hypocenters of approximately 40 microearthquakes were identified around the Zenisu ridge, using the OBS data. The OBS-located hypocenters were distributed mainly on the southern half flank of the ridge. There are no earthquakes located by the OBS data within the upper layer of the Philippine Sea plate. The focal depths range from 18 to 35 km, indicating that the earthquakes occurred within the lower layer of the mechanically strong part of the lithosphere. The composite focal plane solution of the microearthquakes that occurred on the Zenisu ridge was a strike-slip or reverse fault type, having P-axis in the WNW. The tectonic origin of the earthquakes located on the ridge was demonstrated to be found in the bending, with a regional compression, of the Philippine Sea plate along the eastern Nankai-Suruga through.

Shear structures in anhydrite at the base of thrust sheets (Antalya, Southern Turkey)

Minor-scale structures in anhydrite layers and pods within one thrust contact of the Antalya Thrust System are described. Folds whose size range from some centimeters to several meters display a wide variety of attitudes. Their axes are curvilinear and vary from orthogonal to subparallel to the stretching lineation, leading to sheath folds and eyed type sections. Shear bands are frequent. In alternating white (pure) and brown (clay rich) anhydrite, regularly spaced extensional shear bands form typical tilted block patterns. In homogeneously foliated anhydrite they form isolated meter long extensional shear bands with brecciation along their central planes. Reverse-fault type shear bands are associated with meter scale reclined folds with axes subperpendicular to the stretching lineation. Normal and reverse fault type shear bands are self-exclusive, and both indicate a sense of shear toward the south at the scale of thrust system.A model of progressive deformation of the anhydrite layers is presented. Anhydrite layers which have undergone a strong unstable shearing deformation along a thrust, have been boudinaged and now form elongate asymmetric pods. In thinned layers, normal extensional shear bands developed and recumbent fold axes are strongly reoriented toward the direction of the stretching lineation. In the thickened layers, reverse-fault type shear bands developed, particularly within the inverted limbs of asymmetric reclined folds whose axes are at a high angle to the stretching lineation.

Determination of earthquake focal mechanisms and crustal structure in eastern Australia using surface waves

The surface waves from two medium sized ( M ∼ 5.5) earthquakes in southeast Australia (1966 Mt. Hotham and 1982 Wonnangatta) were examined to determine average S-wave crustal structure for the region and the focal mechanisms of the two earthquakes. The S-wave models were determined from an analysis of the group velocities of the Rayleigh waves. For all paths a two-layer crust overlying a Moho at about 40 km is required to fit the observations. In the upper layer, down to about 25 km, the velocity gradients are very small and the S-wave velocities are in the range 3.4–3.6 km/s. In the lower crust the S-wave velocities increase gradually to about 4.6 km/s at the Moho where the velocity gradients are again very small. The region where the S-wave velocities increase significantly with depth corresponds to the region of the lower crust which generates strong seismic reflections (Mathur, 1983). The thin crust postulated beneath Bass Strait and Tasmania (Johnson, 1973) is not substantiated by these data and a crustal thickness of about 40 km is also required for this region. For the longer profiles (~ 1800 km) to the north and northwest of the earthquakes, the velocity step at the Moho is smaller than for the shorter profiles (~ 700 km). However, because the surface waves are less dispersed for the shorter profiles, the resolution is poorer and the models less reliable. For the 1982 Wonnangatta earthquake, the surface-wave synthetics from a focal mechanism similar to that obtained by Denham et al. (1985) fit the observations at five regional stations very well, and confirm the NW-SE compressive stress regime found by other workers. However, for the 1966 Mt. Hotham earthquake, the surface wave synthetics generated from the normal faulting solution, which was obtained from an analysis of P-wave first motions (Denham et al., 1985), do not agree with the Rayleigh wave-trains observed at Australian stations. Rather, the surface wave observations support a thrust fault type mechanism with the pressure axis having a similar direction to that obtained from the 1982 Wonnangatta earthquake and other earthquakes in the region.

Source mechanisms of subcrustal and upper mantle earthquakes around the northeastern Kyushu region, southwestern Japan, and their tectonic implications.

The source mechanisms of subcrustal and upper mantle earthquakes with magnitudes from 6.0 to 6.8 that occurred around the northeastern Kyushu region have been closely investigated to clarify its tectonic features in relation to the subducting Philippine Sea plate with a laterally bending configuration.Two subcrustal earthquakes that occurred in Suonada and the Bungo channel, which are located close to the leading edge of the subducting Philippine Sea plate, show the mechanism of normal faulting type. These events may have been generated by lateral bending of the oceanic plate. The Kunisaki peninsula earthquake of August 26, 1983 (M=6.8, h=116 km), which was the largest upper-mantle earthquake in this region, shows a reverse fault type mechanism. Comparisons between the observed and synthetic seismograms suggest that the southeastward dipping nodal plane may be the fault plane, and that the rupture propagated northwest-upwards. The seismic moment was estimated to be MO=-1.13×1026 dyn·cm. The focal mechanism of three upper mantle earthquakes inland of Kyushu, including the Kunisaki peninsula earthquake, suggests that they may have been caused under the stress regime of down-dip extension. The tensional stress working southwest-downwards at intermediate depths in the Suonada and inland Kyushu regions may be explained by the gravitational pull acting on the Philippine Sea plate subducting deeper southwestwards.

北海道および東北地方における稍深発地震の発震機構

Precise focal mechanism study of earthquakes occurring in Hokkaido and northern Honshu is made by using 35 stations of microearthquake observation networks of Tohoku University and of Central Research Institute of Electric Power Industry.In the northern Honshu and at the junction between the Northeastern Japan Arc and the Kurile Arc, the upper seismic plane of the double-planed deep seismic zone is characterized by down dip compression and the lower seismic plane by down dip extension. The events occurring in the shallower part of the upper seismic plane (shallower than 60km in the northern Honshu and shallower than 70km at the junction) have low-angle thrust fault type.The upper seismic plane is not down dip compression but has various types of focal mechanisms beneath Hokkaido, the southwestern end of the Kurile Arc, although the shallower part of it has low-angle thrust fault type as the case of northern Honshu. This upper seismic plane disappears at the depths greater than about 120km. The lower seismic plane is characterized by down dip exension, which is the same as northern Honshu. In Hokkaido, the lower seismic plane with down dip extension type extends to the deeper part down to about 300km depth. On the contrary, in nortern Honshu, it is the upper seismic plane with down dip compression type that extends to the deeper part down to at least 200km depth.The characteristics of the double-planed deep seismic zone in the southwestern end of the Kurile Arc may suggest relatively strong slab pull force superimposed on the stress system generating the double-planed deep seismic zone.

Study of Microearthquake Activity in Four Geothermal Areas of Taiwan: ABSTRACT

Detailed microearthquake surveys were conducted in four geothermal areas of Taiwan, namely, Tatun volcanic region, Chingshui- Tuchang geothermal area, Lushan hot spring area, and Hungyeh hot spring area, during the past few years. It is found that most of them have microearthquake activity. Based on the microearthquake data, fracture zones permitting deep circulation of water are inferred. Microearthquakes in the Tatun volcanic region are concentrated in a 1.9 mi (3 km) wide and 4.3 mi (7 km) long, northeast-striking elongated zone, where conspicuous geothermal manifestations are present. The focal depths of these events are mostly less than 0.6 mi (1 km). The occurrence of these microearthquakes may be related to the minor normal faults in that region. In the Chingshui- uchang geothermal area, microearthquakes are located in a northeast-striking zone, 25 mi (40 km) long and 4.3 mi (7 km) wide. The foci clearly define a northwest-dipping fracture zone with a thickness of about 1.9 mi (3 km). Three composite fault plane solutions are all of the normal fault type. No microearthquake activity was found within a 3 mi (5 km) radial range around the Lushan hot spring area, during a 34-day recording period. A highly active microearthquake zone was found in a region 1.25 mi (2 km) to the west of the Hungyeh hot spring area and the foci define an east dipping fracture zone. However, the causative faults of these events are of the thrust fault type. End_of_Article - Last_Page 987------------

1976年10月31日日高山脈地震

On October 31, 1976 an earthquake of magnitude 5.0 occurred under the west slope of the Hidaka range, southern Hokkaido.It is the first remarkable event since a seismic observation through a telemetering system at the Research Center for Earthquake Prediction (RCEP), Hokkaido University started in July, 1976. Using data obtained from three stations of RCEP near the epicentral region, a seismic investigation was made on the above event.This earthquake occurred at a depth of about 55km in the uppermost mantle, however, its aftershock activity is very similar to that of earthquake in the crust elsewhere.Background seismicity around the focal region of this shock was estimated as 0.04 shocks (M≥0.5) per day.Frequency of aftershocks per day has decreased to this level about 140 days after the main shock. Aftershocks are confined in a region of about 6×6×4km3 including the main shock. No distinct seismic activity change is found prior to the main shock.Focal mechanisms of the main shock and of the largest aftershock are of normal fault type. Maximum tension axes lie in nearly horizontal plane and their directions are perpendicular to the strike of the Hidaka range. A large earthquake (M=6.7) which occurred under the east slope of the Hidaka range on January 21, 1970, showed focal mechanism of reverse fault type. But all of B-axes of above three events are parallel to the strike of the Hidaka range.These imply some complexity of focal process beneath both sides of the Hidaka range.

1970年秋田県南東部地震の余震の発生様式について

A study is made on the spatial and temporal changes in aftershock activity of the southeastern Akita earthquake of 1970, in relation to the characteristics of earthquake faulting in the upper crust. Aftershocks are found to have occurred almost exclusively in the so-called granitic layer of the crust. For the first 50 hours after the main shock occurrence the hypocenters of aftershocks were concentrated nearly on a plane dipping by 52° towards N64°E, which is in accordance with one of P-wave nodal planes in the focal mechanism solution of the main shock. Afterwards, the aftershock activity had a general tendency of upward migration, and spread in the northeast direction to constitute a group of shallow aftershocks which we call the eastern group. While shallow aftershocks in the western group were located approximately on the focal plane found for events in the first 50 hours, those in the eastern group are interpreted to have occurred along the other nodal plane in the mechanism solution of the main shock, indicating the existence of a conjugate fault system. Composite mechanism solutions are obtained from P-wave data for shallow aftershock groups. They are almost the same mechanism of reverse fault type for the main shock. Aftershocks deeper than 15km scattered around the horizontal boundary between the granitic and the basaltic layers. The composite mechanism solution for this group differs in its larger strike slip component from the other solutions, though it is of reverse fault type. If the nodal plane with a small dip angle is assumed to be the fault plane, the direction of slip motion agrees well with that of relative motion between the Eurasia and the Pacific plates.

Surface wave study of earthquakes near northwestern Hudson Bay, Canada

Source parameters of an earthquake in northwestern Hudson Bay that occurred on October 2, 1971, with magnitude mb = 5.0 were determined primarily by using surface waves. An estimate of focal depth of 21 km, which in this particular case was insensitive to minor variation in azimuth and focal mechanism, was determined mainly by the Rayleigh wave amplitude spectrums at periods between 7 and 20 s. The focal mechanism solution determined from Love and Rayleigh wave amplitude and phase spectrums in conjunction with P initial motions suggests a thrust fault type with maximum compressional direction nearly NE-SE. The maximum possible fault length was estimated to be 7 km by examining the Rayleigh amplitude spectral variation with azimuth at periods less than 20 s. Another earthquake in the same region, mb = 4.5, on May 14, 1964, was estimated to have a similar focal mechanism with a focal depth of possibly about 17 km. An implied tectonic map of eastern North America is given in Figure 15. High Q values for periods between 7 and 20 s for Rayleigh waves were found also from Hudson Bay earthquakes, in agreement with a similar result for Love waves from earthquakes on Baffin Island.

The focal mechanisms in Iran and their relations to tectonics

Data from focal mechanism solutions obtained by different authors and those of 8 fault-plane solutions found in this study have been used to search for the distribution of the main stress axes in Iran. For this purpose, the area has been divided into three regions as southern, central and northern Iran. The results indicate that the characteristics of the motion at the foci are different in each of the three regions. — By examining the B axes in south Iranian earthquakes, direction of tectonic motion has been obtained as N 66°E. Since the maximum and intermediate stress axes are nearly horizontal, it is concluded that focal movements in this region are of reverse fault type. Thus, there is a similarity between recent crustal movements and those occurring during Alpine orogeny which is in the form of an overthrusting to the southwest. — In central Iran earthquakes however, tension is predominant, and, therefore, in this region faultings are dip-slip normal or strike-slip, and the horizontal components of displacements are dextral. The mean direction of maximum tension axes is nearly perpendicular to the central Iranian complexes. — It is deduced from north Iranian shocks that, in this region, the earthquakes studied are of nearly almost pressural type, and horizontal components of the oblique displacements in foci are sinistral.

Geology of Rocky Mountain Front Ranges Near Bow River, Alberta

The front ranges of the Rocky Mountains near the Bow River, Alberta, are a fine example of the thrust-fault type of structure which characterizes the eastern Canadian Rockies. The ranges consist largely of repeated sequences of Paleozoic strata dipping 40° W. and separated by subsequent valleys eroded in Mesozoic or Upper Paleozoic beds. Cambrian to Upper Cretaceous strata are exposed. The map area is traversed by the Calgary-Banff highway and the main line of the Canadian Pacific Railway and, although visited by many geologists since the 1880's, it has not heretofore been mapped. The map area, which consists of three fault blocks that repeat most of the Paleozoic section, is bounded on the east by a major folded thrust fault which throws Middle Cambrian rocks over Upper Cretaceous strata. This fault marks the boundary between the Rocky Mountains and the Foothill belt. The Cascade Coal Basin is the westward limit of mapping. Middle Cambrian strata along the mountain front are overlain unconformably by the Ghost River formation of questionable Devonian age. This is overlain conformably by Upper Devonian dolomites and limestones and these are followed by Lower, Middle, and Upper Mississippian shales and limestones, Pennsylvanian quartzitic and dolomitic sandstones, Permian (?) chert and quartzite Triassic shaly siltstone, Jurassic black shale, and Lower Cretaceou or Jurassic sandstones, shales, and coal. No igneous rocks are exposed in this part of the Rocky Mountains. The great thrust faults, displacements on which are measured in miles, in some places develop drag folds, but the fault planes are clean breaks with little or no gouge, and the strata on both sides approach parallelism with the fault planes.