The El Tigre Fault is a 120 km long, roughly north-south trending, major

strike-slip fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

located in the Western

Precordillera Precordillera is a Spanish geographical term for hills and mountains lying before a greater range, foothills. The term is derived from ''cordillera'' (mountain range)—literally "pre-mountain range"—and applied usually to the Andes. Some places ...

in Argentina. The

lies just to the east of the Andes mountain range in South America. The northern boundary of the fault is the

Jáchal River The Jáchal River is a river in the province of San Juan, Argentina. It is part of the Desaguadero River basin, and one of the most important permanent watercourses in the province, with an average flow of . It is born from the confluence of th ...

and its southern boundary is the San Juan River. The fault is divided into three sections based on fault trace geometry, Northern extending between 41–46 km in length, Central extending between 48–53 km in length, and Southern extending 26 km in length. The fault displays a right-lateral (horizontal) motion and has formed in response to stresses from the

Nazca Plate The Nazca Plate or Nasca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the Na ...

subducting under the

South American Plate The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-A ...

. It is a major fault with crustal significance. The Andes Mountain belt trends with respect to the Nazca Plate/South American Plate convergence zone, and deformation is divided between the Precordilleran thrust faults and the El Tigre strike-slip motion. The El Tigre Fault is currently seismically active. Strikeslip-fault

Geology

El Tigre Fault is a right-lateral N10°E trending fault, known for its good grade of exposures and markers of horizontal

displacement Displacement may refer to: Physical sciences Mathematics and Physics *Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...

. Its linear traces are apparent throughout the length of the fault. Morphology of El Tigre strike-slip fault is visible on the western slope of the Precordillera fold and thrust belt. With evidence of activity during the Middle Pleistocene to present day, it is considered a

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

fault. Geomorphic and ¹⁰Be ( Beryllium) exposure ages have been used in some studies to estimate the

age and slip rate. Slip rate is estimated to be approximately 1 mm/year and offsets range from 60 to 180 m. The

Nazca Nazca (; sometimes spelled Nasca; qu, Naska) is a city and system of valleys on the southern coast of Peru. It is also the name of the largest existing town in the Nazca Province. The name is derived from the Nazca culture, which flourished in ...

/South American oblique convergence zone off Chile is N76° and El Tigre releases the north-south stress component of continental plate motion at about 30°-31°. In the San Juan Province, it is part of an east-verging thin-skinned belt, and is located in a major active seismic area. Moment magnitude estimates reveal that a 7 ± 0.5 scale earthquake could be produced.

Fault zones

Transpression-Transtension Final Original Figure

Northern

The northern subdivision is approximately 41–46 km long. One estimation shows the segment begins where the fault bends to the northeast and is 41 km long. Another estimation places the distinction 5.5 km south of this bend resulting in the northern segment 46 km long. This section is more structurally complex than the central and southern sections, due to the segment's northern edge ending in a horse tail termination. This faulted area can be interpreted from the 1 km to 5 km separation of several disperse rupture strands.

Central

The central subdivision is approximately 48–53 km long. This area exhibits transpressive and transtensive

geomorphological Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or n ...

features.

Sag pond A sag pond is a body of fresh water collected in the lowest parts of a depression (geology), depression formed between two sides of an active geologic fault, strike-slip, transtensional or normal fault zone. Formation A sag pond is formed along ...

s ( releasing basins) form when the right lateral fault bends to the left causing the crust to extend ( transtensive).

Pressure ridge A pressure ridge is a topographic ridge produced by compression. Depending on the affected material, "pressure ridge" may refer to: * Pressure ridge (ice), between ice floes * Pressure ridge (lava), in a lava flow * Pressure ridge (seismic), in ...

s form when the right-lateral fault bends to the right causing the crust to compress ( transpressive). A bedrock

scarp Scarp may refer to: Landforms and geology * Cliff, a significant vertical, or near vertical, rock exposure * Escarpment, a steep slope or long rock that occurs from erosion or faulting and separates two relatively level areas of differing elevatio ...

with an east-facing slope shows vertical displacement along this part of the fault. The scarp has a slope of 18-24° and maximum height of 85 m. Tectonic shortening appears to have changed direction from WSW-ENE to W-E during the Pleistocene, altering the kinematics to the present transpressive/transtensive system from a mainly transcurrent one.

Southern

The southern subdivision is approximately 26 km long. This segment is characterized by the right-lateral offset of drainage networks. It exhibits an uninterrupted linear trace and strike-slip component that are useful in determining offset. The termination point for El Tigre in the south is recognized by a merging within the Precordilleran Paleozoic strata, as well as its extremely disturbed surface deformation. By dating the alluvial fans in this segment, some studies conclude a horizontal displacement rate of approximately 1 mm/yr. The southern segment along with the central segment are crossed by several oblique and transverse faults almost perpendicular to the El Tigre Fault. These faults are inferred due to the long linear strands of stream channels, as the faults are not visible on the surface.

Discrepancies

The faults location in a seismically active zone and a low erosional environment makes it a good study area. Although many characteristics of geomorphology have been preserved, the area has not been extensively studied using the new methodologies currently available. The fault has sparked new interest in its

geometrical Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ca ...

and

kinematic Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move. Kinematics, as a fie ...

characteristics within recent years. Previous studies on the El Tigre Fault have a range of inconstancies. Information obtained on the fault can vary from a reactivated fault with a normal component in Jurassic and

Palaeocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''palai� ...

, an Eocene strike-slip fault, an

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

northwest-verging thrust fault, and a south-east dipping normal fault inverted in the

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

. Research models in the 1980s describe the fault as system anywhere from 800 km up to 1000 km in length. The kinematics, geometry, extension, and deformation have not been widely agreed upon, therefore the new interest in the El Tigre Fault should lead to further studies using modern technology. These future studies should shed light on the discrepancies that have resulted from lack of in depth information in the past.

References

{{Major South American geological formations Geology of San Juan Province, Argentina Strike-slip faults Seismic faults of Argentina