Meers Fault

The Meers Fault runs along the northern side of Meers Valley^[1] through Comanche County and Kiowa County and close to Caddo County^[2] in an east-southeast to north-northwest direction. Towns close to the fault are Cooperton, Meers, Apache, Fort Sill,^[3] Treasure Island and Elgin;^[4] Oklahoma State Highway 19, Oklahoma State Highway 115, Oklahoma State Highway 58 and U.S. Route 281 cross the fault^[3] and Oklahoma State Highway 44 might do so as well.^[5] The fault scarp is located on private land;^[6] the southeastern part runs through farmland and the northwestern part runs through hilly terrain.^[1]

The Meers fault is a reverse fault^[7] (at first it was interpreted to be a normal fault^[8]) with a straight path despite a variable topography;^[9] it probably does take the form of a broad dislocation rather than of a plane^[10] and its expression varies depending on the substrate rock.^[11] Underground, the fault may be over 100 kilometers (62 mi) long,^[9] and may be connected to another buried fault to the west (Willow Fault), potentially defining a 180 kilometers (112 mi) long fault system.^[12] It dips first northeastward^[13] but deeper it becomes either vertical or down-to-the-southwest,^[14] but certainly steep at depth.^[15] One interpretation is that the Meers fault is a "back-thrust" that dips northwards.^[16] Unlike many other faults, there is no evidence of segmentation in the Meers fault.^[17]

A conspicuous^[18] 5-meter (16 ft) high and 26-kilometer (16 mi)^[19]–22-kilometer (14 mi) long fault scarp just north of the Wichita Mountains is noticeable on Google Earth;^[18] it has formed on the Holocene part of the fault^[19] and continues southeastwards in the form of more subtle scarps^[13] although it may not exactly coincide with the path of the fault.^[20] Because the scarp is not present along the entire length of the fault, it is subdivided in a southeastern section in Comanche County and a northwestern section in Kiowa County, with only the southeastern section featuring a scarp.^[13][21] The scarp marks the Holocene section of the fault.^[19] The Meers fault is the only Mid-Continent fault scarp^[22] and has been called the "finest" such scarp east of the Rocky Mountains.^[1]

In low sun-angle photography additional scarps and splays can be observed.^[14] The erosion/sedimentation patterns^[23][24] and the path of drainages^[10] such as Canyon Creek may have been influenced by movement along the fault,^[25] and topographic ridges are offset.^[9] Finally ductile folding,^[26] vegetation and landform variations have also been recognized on the Meers fault.^[27] In some rock formations faulting has mainly led to warping, instead of brittle displacements^[28] and in several sites evidence of faulting appears to be concealed by floodplain sedimentation.^[29]

The fault separates Cambrian-Proterozoic^[30] igneous rocks from thick^[3] Cambrian-Ordovician age^[31] carbonates to the northeast.^[3] The igneous rocks belong to the Amarillo-Wichita uplift and are much more magnetic than the carbonates; this has been used to trace the fault with aeromagnetic techniques although the igneous rocks also reduce its visibility in reflection seismology studies.^[3] The nature of the surrounding rocks also influences the expression of the Meers fault, as it has a more pronounced scarp in erosion-resistant rock units.^[32]

The Meers fault is among the most prominent tectonic structures in the region.^[22] Other faults in the area are the Blue Creek Canyon fault which is connected to the Meers fault at its northwestern end, the Broxton Fault Complex northeast and the Mountain View fault to the north^[3] which intersects with^[33] and is also connected to the Meers fault.^[14] Additional faults in the region are the Cement, the Cordell and the Duncan-Criner faults.^[34]

All these faults lie in the area of the Frontal Wichita fault system, which is located between the Anadarko Basin to the north and the Amarillo-Wichita uplift to the south^[3] and separates the two.^[35] The fault system, which also includes the Meers fault^[36] as its southern margin,^[37] was active during the Mississippian to Permian, generating a total offset of about 12 kilometers (7.5 mi).^[31] The development of the Meers fault may have been influenced by the South Oklahoma aulacogen.^[13] Both the Wichita Mountains thrust^[18] and possibly a second fault are linked with the Meers fault,^[38] which is the only fault in the Wichita fault system with Holocene activity.^[39] A link to the Willow fault farther west has been proposed,^[40] which would prolong the fault system to 180 kilometres (110 mi) length and significantly increase its hazard.^[41]

The Meers fault has been in existence for most of the Phanerozoic. It may have begun as a rift margin fault in the Proterozoic-Cambrian^[14] associated with the Southern Oklahoma Aulacogen^[42] but its maximum activity took place during the Mississippian and Permian when the Wichita Mountains and the Slick Hills were offset along it by about 2 kilometers (1.2 mi)^[14] and the Meers Valley formed along the fault.^[42] More fault movements occurred in the Permian and the Pleistocene^[14] although there are no post-Permian rock formations in the area that could allow an estimation of post-Paleozoic movements. However, Pleistocene sediments and Holocene alluvium have been offset, indicating fault movement during this time.^[36] Recent research has suggested that the whole fault may be of Quaternary age with little activity during the Pennsylvanian.^[43] During the history of the fault considerable uplift took place on its southern side^[32] while the recent movement has generated an opposite movement.^[13]

Two to four earthquakes occurred during the Holocene in the last 6,000 years.^[18] One of which occurred 1,100–1,300 years ago and the other 2,000–2,900 years ago;^[13] two others were dated to 4,700–3,110 and 5,960–4,740 years before present.^[44] The dates have been obtained through radiocarbon dating on soil in trenches dug in the fault scarp^[14] and of offset alluvium deposits.^[36] Additional faulting appears to have occurred over 12,000 years ago, but evidence thereof was partly eroded away during a time of a wetter climate^[45] and prior to the Holocene the fault might have been inactive for 100,000–130,000 years.^[46][13] The slip rate has been estimated to be 0.02 millimeters per year (0.00079 in/year), which is typical for intraplate faults.^[47]

The Meers fault is the only fault on Oklahoma which has generated a rupture on the surface,^[48] resulting in about 5 meters (16 ft) of vertical offset^[35] over a 43 kilometers (27 mi) long distance.^[18] It is possible that the fault rupture was limited by geological structures that occur at the northwestern end of the Meers fault^[49] where it splays out.^[36] The possibility that faulting continued for another 30 kilometers (19 mi) along its northwestern end is equivocal^[14] with some evidence indicating that recent faulting was limited to Comanche County;^[50] research published in 2019 indicates that the northwestern segment did not move during the 1,200 BP earthquake but was active in the 3,400–2,900 BP event.^[51] The underground rupture on the other hand might reach a length of 70 kilometers (43 mi).^[36]

Reconstructions of the intensity of the Holocene earthquakes indicates magnitudes of M_w 6.75–7.25^[19] with possibly similar intensities,^[52] with the terrain north of the fault being shifted upwards and leftwards relative to the terrain to its south;^[19] this latter movement^[36] and the position of the Meers fault are consistent with the tectonic stress pattern of North America^[7] which favours movement along the Meers fault^[9] unless one assumes a rotation of stress patterns in the South Oklahoma Aulacogen.^[53] The ratio of the horizontal to vertical motion is about 1.3–1.5^[15] or about 2:1^[13] although the amount of horizontal movement on the fault is controversial.^[54]

Alternatively, the fault movement could have occurred through aseismic creep as there is little evidence of strong ground shaking in the area^[35] as well as evidence against strong ground motion^[55] although the soils found close to the fault trace show evidence of fast movement.^[56] In general, the movement direction of the Meers fault is contentious.^[57]

The Meers fault presently is largely^[15][58] aseismic, with no earthquakes recorded along its Holocene trace^[19] or any evidence of aseismic creep^[59] although minor seismicity has been recorded^[60] and a M4.2 earthquake close to Lawton in 1998 is close to the southeastern end of the Meers fault.^[61] Likewise, seismicity is scarce in the wider region^[32] and photographic analysis has shown little evidence of recent fault movement in other faults of the Wichita fault system.^[62]