Chile Ridge

The Chile Ridge, also known as the Chile Rise, is a submarine

The Chile Ridge involves spreading ridge subduction which is worth studying because it explains how the Archean continental crust initiation formed from deep oceanic crust.^[4]

From approximately 14 to 3 million years ago, a series of trenches collided the Chile Trench, forming what is part of the Chile Ridge.^{[citation needed]}

In the

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.^[4][5] The lithological units would be discussed from youngest to oldest, and Taitao Granites and Taitao Ophiolite would be our main focus.

Adakite is a felsic to intermediate rock and are usually calc-alkaline in composition. It is also silica-rich.^[2] The partial melting causes the alteration of the subducted basalts into eclogite and amphibolite which contains garnet.^[4]

Along the axis in the Chile ridge, magmatic rocks which are mafic to ultramafic are emplaced.

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.^[2][9]

The thermal configuration and the structure of the subduction zone affects the interactions of the

The Chile Ridge is formed by the divergence of the Nazca and Antarctic plates.^[4] It is spreading actively at the rate of about 6.4 – 7.0 cm/year since 5 Ma to present.^[4] The Late Miocene Nazca-Antarctic spreading ridge formation creates about 550 km-long Chile Ridge as there are differences in the convergence rates between Nazca and Antarctic plates.^[2] According to the results from space geodetic observations, Nazca-South America converges four times faster than that of Antarctica-South America.^[1][9]

In addition, the direction of the Nazca plate migration is different from the Antarctic plate migration since 3 Ma. The direction that Nazca plate moves is ENE, while the Antarctic plate is ESE. The net diverging movement of the two plates contributes to the spreading of the Chile Ridge.^[4]

The subduction of the ridge started is an oblique subduction with 10° – 12° oblique to the Chile trench since 14 Ma,^[4] which subducts beneath the southeastern Southern Patagonia.^[1][4] Thus it is found that both the Nazca-South American plate collision and Antarctic-South American plate collision have been taken place at the same time when the Chile ridge is separating, i.e. segments of Chile Ridge have been subducting beneath the South American plate.^[1] Due to the difference in the convergence rate, the formation of a slab window is favoured.^[1] Slab window is a gap underneath the South America plate, where the overriding South America plate has only little lithospheric mantle supporting it and is directly exposed to the hot asthenospheric mantle.^[1]

The experimental results from the magnetic anomalies within the oceanic crust suggest that about in 14–10 Ma (late-Miocene), some of the Chile Ridge segments were subducted beneath the Southern Patagonian Peninsula (located between 48° and 54°S) subsequently.^[2] From 10 Ma to the present, Chile Ridge was separated into several short segments by the fracture zones, and the segments of the ridge are subducted between 46° and 48° S.^[2][1] The above findings have proven that Chile Ridge has been encountered a northward migration.^[2][9][4] Thus it has been found that the spreading rate of Chile Ridge from 23 Ma to the present has slowed down. While the spreading rate of the ridge is correlated to time of the collisions of ridge and trench.^[1] Some studies have different discoveries in the rate of spreading which shows that the ridge may have spread uniformly for about 31 km/Myr half spreading rate starting from 5.9 Ma.^[9]

In the Chile Ridge Subduction Project (CRSP), seismic stations are deployed in the Chile triple junction (CTJ).^[12] The tectonic activity and seismicity are mainly driven by the subduction of Chile Ridge.^[13] A slab window is formed as the Nazca and Antarctic plates continue to diverge when colliding with Chile trench, a gap is created as new lithosphere production is becomes very slow.^[14][3][15] Moderate to high offshore seismicities for magnitude higher than 4 is detected in the segmented Chile Ridge as well as the transform faults.^[12] 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.^[14][13][16]

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

The Liquiñe-Ofqui fault system is a right-lateral strike-slip fault separating Chiloe microplate and the South America plate.^[13] The northward migration of Chiloe microplate along the Liquiñe-Ofqui fault creates the Golfo de Penas basin in the late Miocene period.^[16]

The Liquiñe-Ofqui fault is a fast-slipping fault (with a geodetic rate of 6.8–28 mm/yr).^[16] 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.^[16][13] 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.^[13] 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.^[16] Recently, 274 seismic events have been detected in 2004–2005.^[16]

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.^[16][14] (Fig-5)

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.^[3] The mantle in the slab window is rather hotter than the mantle that melts from the lithospheric crust, and the generation of magma is very slow. This is due to low-extent of hydration to the subduction zone, decreasing mantle convection velocity, as the production of magma in the subduction zone is mainly driven by the hydration that lowers the partial melting of the crust. A volcanic arc gap is formed above the slab window as the magma melted from the crust convects slowly which hampers the volcanism.^{[15][1][2][17]} The ridge segment between Taitao and Darwin transform faults are currently located near the Chile Trench and collide with the South American plate.^[1][3]

The presence of slab window underneath southern South America plate has been proven by the research which aims at determining the lithosphere and upper mantle structure proximate to the Chile Ridge.^[3] An intraplate seismic gap is recorded which coincides with the Patagonian slab window location.^[14][8] The experimental results of the P wave travel-time tomography show there is low-velocity zone in the predicted slab window location, migrating eastward with increasing depth.^[3]

Other than the generation of the slab window, the Chile Ridge subduction into the

The Chile triple junction is the intersection of Nazca, Antarctica and South American plate. The position of the junction shifts over time, and depends whether the spreading ridge subducts or the transform fault subducts beneath the South American plate. When the spreading ridge subducts, the triple junction shifts northwards; but if the fracture zone subducts, the triple junction shifts southwards.^[1] The junction has shifted to the north starting from the onset of Chile Ridge subduction since 17 Ma after the rupture of the Nazca-Antarctic-Phoenix triple junction.^[2] Since then, the Chile triple junction has arrived to its current position in the western Taitao Peninsula.^[14] Prior to 10 Ma, Chile triple junction reaches the southern Taitao peninsula. Currently, the temperature of Chile triple junction below the depth of 10 – 20 km is predicted to be 800 – 900 °C.^[18][13]

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.^[1]

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.^[9][1] 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).^[9] 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.^[1][8][9]

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.^[9][1] 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.

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)^[9]

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.^[4] 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.^[1]

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.^[13][8][14]

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.

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.[4] This is because the Chile Ridge subduction is the only example in the world that the overriding plate is a continental one. The correlations between the rocks in the past can also be examined. The ridge trench interaction can also be studied.^[4]

In addition, due to the presence of the Patagonian slab window and the obduction of the Nazca plate, the geological process that happened historically are not the same.^[4] Therefore, the Chile Ridge subduction is not conformable with the uniformitarian principle (geological process happened now is the same with that in the past).^[19]

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.^[4] The ridge subduction controls the magmatism of the North American boundary.^[4]

• Liquiñe-Ofqui fault
• Peru–Chile Trench
• Subduction
• Taitao Peninsula