Chile Ridge

From approximately 14 to 3 million years ago, a series of trenches collided the Chile Trench, forming what is part of the Chile Ridge. In the 2010 Concepcion earthquake (magnitude 8.8) struck the ridge. == Regional geology ==

Geology of the Chile ridge indicated by black lines. 'FZ' means fracture zone. The pink arrows indicate the direction of the Nazca plate and Antarctic plate movements as well as their rate of migration. They show that the Nazca plate is moving in an ENE direction, which is oblique to the boundary with the South American plate, while the Antarctic plate is moving in an E-W direction, which is almost perpendicular to the plate boundary. In addition, the Nazca plate migrates over four times faster than the Antarctic plate. The dark purple circle shows the Taitao Peninsula where the Chile ridge collides to the South American plate. The yellow line shows the plate boundary. The geology of the Chile ridge is closely related to the geology of the Taitao Peninsula (East of the Chile ridge). This is because the Chile ridge subducts beneath the Taitao Peninsula, which give rise to unique lithologies there. The obduction and the thrusting causes low-pressure metamorphism and forms the ophiolite complex. This metamorphism indicates the onset of hydrothermal alteration in a spreading ridge environment. Characteristics of Taitao Ophiolite Taitao ophiolite lithosphere forms a special sequence from the top to bottom: pillow lavas, sheeted dike complex, gabbros and ultramafic rock units. For the ultramafic rock units, it proved that there are at least two melting events that happened before. Bathymetry Bathymetry of the Chile ridge is inspected, which is the submarine topography that studies the depths of landforms under the water level. It is discovered that there are large abyssal hills extend along two sides of the ridge. The abyssal hills grow cyclically which is caused by the cyclic fault growth. During faulting cycles, the extension of the Chile ridge brought about 'diffusion' tectonic deformation which forms numerous tiny faults. The continuous divergence of the ridge causes the extensional strain to concentrate, the tiny faults to link together to generate tall and long abyssal-hill-scale faults. The huge faults push the old and inactive faults away from the ridge axis by extensional force. This process would repeat again. Therefore, the further the abyssal hill to the ridge axis, the older the age it is. == The Chile Ridge movement ==

are deployed in the Chile triple junction (CTJ). Moderate to high offshore seismicities for magnitude higher than 4 is detected in the segmented Chile Ridge as well as the transform faults. It is predicted that the subduction of the spreading Chile Ridge under South America to the north of the Chile triple junction give rise to the seismic events. Furthermore, intraplate seismicity in the overriding South American plate is more likely resulted from the deformation of the Liquiñe-Ofqui fault system. Chiloe microplate This is a tiny plate between Nazca plate and South American plate, it locates east of the Chile ridge. It is proved that Chiloe microplate (Fig-5, 6) is migrated northwards relative to the South American plate which is rather immobile. The Golfo de Penas basin is formed because of the northward movement of Chiloe microplate. Seismicity of Liquiñe-Ofqui fault system in the Aysén Region The Liquiñe-Ofqui fault system is a right-lateral strike-slip fault separating Chiloe microplate and the South America plate. The northward migration of Chiloe microplate along the Liquiñe-Ofqui fault creates the Golfo de Penas basin in the late Miocene period. The Liquiñe-Ofqui fault is a fast-slipping fault (with a geodetic rate of 6.8–28 mm/yr). Intraplate seismicity has mainly been taken place in this fault system. Also, enormous stress from the Nazca plates and South American plate collision has accumulated along the fault system. Throughout history, only limited seismic studies have been conducted in the Aysén Region, southern Chile. There is only an event of seismic magnitude higher than 7 happening in 1927. This hinders the finding in seismicity near the Chile Ridge. Nevertheless, in 2007, the Liquiñe-Ofqui fault system releases the accumulated stress brought by the subduction of Nazca underneath the South America plate with seismicity magnitude reaching 7 in an earthquake. Recently, 274 seismic events have been detected in 2004–2005. Seismicity of the Patagonian slab window There is an intraplate seismicity gap between 47° and 50°S (area with abnormal high heat flow), which coincides with the Patagonian slab window, disrupting most seismic events. The local seismic data only reveals a low-magnitude (magnitude lower than 3.4) seismic event, which is not related to tectonic process. The reason behind this is that the Antarctic plate undergoes shallow subduction which causes very limited seismic deformation. (Fig-5) == Geological formation related to the Chile Ridge movement ==

Consequence of the subduction of the Chile Ridge Patagonia slab window The most obvious impact of the subduction of the Chile ridge is the formation of slab window. It is formed when the segments of separating Chile Ridge subducts under the southern South America plate. The trailing edge of the Nazca plate is completely melted in the subduction zone, and the leading edge of the Antarctic plate diverges, a widening gap is created between the two plates as very little crust is melted after subduction. In this case, only a very little amount of magma is produced underneath the slab window. The ridge segment between Taitao and Darwin transform faults are currently located near the Chile Trench and collide with the South American plate. Ridge axes The ridge axes are the middle part of the ridge where newer crusts are formed. The central ridge axis of Chile Ridge is trending in the direction of north-northwest (NNE). Ridge axes are also known as topographic axial rift valleys. With the help of satellite altimetry data and magnetic data, gravity lows are discovered near the ridge axes. Fracture zones File:Chile ridge fz segments.png|thumb|upright=1.6|Fig-7 This picture shows the several segments of Chile ridge which is divided by numerous transform fault zones. The segment numbers are shown in red words next to the ridge segments. The Chiloe microplate is located at the east of the Chile ridge and the Liquiñe-Ofqui fault zone is located between the Chiloe microplate and the main South American plate. Figure made with GeoMapApp (www.geomapapp.org) It is also named as fault zones. They are the transform faults and separate the Chile Ridge into segments, causing the entire ridge axis to trend southeastward. Fracture zones are trending east-northeast (ENE). The total length of the Chile ridge axis offset is 1380 km caused by the 18 fault zones, among the fault zones, there are also 2 complex fault systems. The longest fault zones are Chiloe fault with 234 km long, and Guafo fault being the shortest (39 km). Through various research on the magnetic and bathymetry data, fracture zones' locations are located. While major fault zones are surveyed by the bathymetry method and defined as troughs. Same bathymetry data also discovered the Fault zones in East Pacific Rise as well as the low-velocity-spreading Mid-Atlantic ridge. Segmentation of Chile Ridge Chile Ridge is divided into a wide range of several short spreading segments which have different lengths and offset distances, in the following section, 7 segments will be discussed. From the table below, it reveals that the spreading ridge segments range in length from about 20 to 200 km, the offsets within segments are about 10 to 1100 km. There are actually a total of 10 first-order ridge segments in the northern ridge (N1-N10), 5 first-order ridge segments (V1-V5) in Valdivia fracture zone, 5 first-order ridge segments (S1-S5) are in the southern ridge. Moreover, both segments N9 and S5 are divided into two parts by non-transform offsets. The table above summarized the longer, more regular and less complicated faults: N1, N5, N8, N9N, N9S, N10, V4, S5N, and S5S. Hourglass morphology Deep contours are located along the segment ends while shallow contours are located at the segment center. The segment center is narrower as the while the axial valley located at the segment ends are wider. This forms an hourglass morphology. (Fig-8) Valdivia fracture zone It is located in the middle of the Chile ridge (Fig-1, 2, 7), and separates the ridge into northern and southern sections, discovered by the bathymetry and magnetic profiles study, as well as the gravity anomaly detection. The Valdivia Fault Zone has caused the offset of the north and south Chile ridge for more than 600 km in the E-W direction. There are six fault zones between the Valdivia Fault Zone. == Interaction between Chile Ridge and Chile Trench ==

Geophysical and geothermal analysis in the southern Chile triple junction has been examined. Magnetic and bathymetric data have been recorded across the Chile Ridge which recognizes a slight transformation in the configuration of the spreading ridge when the ridge converges with the trench. The overriding South America plate is dominantly impacted by the ridge collision. The Chile-Peru Trench becomes steeper and narrower when the Chile Ridge is subducting. Chile Ridge segment within the Taitao fracture zone collides with the southern end of the trench. The collision of the ridge may also be associated with the obduction process onto the landward trench slope. Geothermal data along the southern triple junction are measured. The heat flow analysis in the collision zone of the trench indicated a high value of heat pulse (345 mW/m2) related to the Chile ridge subduction in the lower part of the trench. Furthermore, by the application of bottom simulating reflectors (BSR), more convincing evidence of the existence of high heat flow underneath the trench slope, as a wider range of heat flow observations grid is shown from the north to the south of the triple junction. Also, the hypothesized conductive heat flow is consistent with the heat flow data from BSR. == Importance of the spreading ridge subduction ==

Understanding the spreading ridge subduction is crucial as it controls the evolution of continental crust. The subduction of the Chile Ridge beneath the Chile Trench provides a suitable analog for the initiation of the Archean continental crust via the melting of deep oceanic crust. Other example of spreading ridge subduction The Kula-Farallon/Resurrection ridge subduction The subduction of Kula-Farallon/Resurrection ridge started during Late Cretaceous-Paleocene, this is currently located at the Chugach complex, Alaska where mafic-ultramafic high grade metamorphism is found nowadays. The ridge subduction controls the magmatism of the North American boundary. == See also ==