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The tectonic setting of Northwestern South America is very complex due to interaction between the Nazca, Caribbean, and South America plates and several oceanic terranes. Of particular interest is the Nazca plate slab geometry change since the Mid-Miocene, and its tectonomagmatic effects on the overlying South American Plate. Several studies indicate that the modern Nazca slab is torn into two segments at 5.5oN in Colombia, the so-called "Caldas tear", where the northern segment dips at a shallow angle while the southern segment dips at a steep angle. This slab geometry difference is manifested in magmatic activity where only the region with steep slab shows typical subduction zone volcanism, while the one with flat slab lacks such activity. However, due to the limited number of geochronological data, the timing of this slab shallowing and tearing remains poorly understood. We conducted an extensive 40Ar/39Ar geochronology study on post mid-Miocene volcanic rocks in Colombia to study the temporal evolution of Nazca plate geometry and its effects on magmatism in Colombia. We find evidence of continuous magmatism north and south of the Caldas tear from 10.5 Ma to 6.4 Ma, with peak activity between 9-8 Ma. This is followed by a ~4 million year magmatic hiatus, until the resurgence of magmatism south of the Caldas tear as monogenetic domes at 2.1 Ma, and continuous volcanic activity in modern composite volcanoes since 1.1 Ma. Results of this study support the presence of a complex subduction system beneath Colombia where the northern segment slab has been flat since ~6.4 Ma ago, and the southern segment has re-steepened at ~2.1 Ma. This study showcases that the Nazca slab geometry was the most important factor driving magmatism in Colombia since the mid-Miocene.
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Receiver Function Derived Structural Constraints on Dynamic Processes Associated with the Young Nazca Lithosphere Subducting beneath Colombia
Subduction of the very young (<15 Myr old) oceanic lithosphere of the Nazca plate in central to southern Colombia is observationally related to an unusually high and unusually variable amount of intermediate (>50 km) depth seismicity. From 2010 through 2019 89% of central and southern Colombia’s 11,466 intermediate depth events occurred between 3.5°N and 5.5°N, highlighting these unusual characteristics of the young slab. In addition, morphologic complexity and possible tears characterize the Nazca slab in Colombia and complicate mantle flow in the region. Prior SKS-phase shear-wave splitting results indicate sub-slab anisotropy is dominated by plate motion parallel-to-subparallel orientations in the region, suggesting the young slab has entrained a relatively thick portion of the sub-slab mantle. These observations suggest the subduction of young lithosphere has significant effects on both the overlying and underlying asthenosphere in the Colombia subduction zone. Here we use more than 10 years of data to calculate receiver functions for the Red Sismológica Nacional de Colombia’s network of broadband seismometers. These receiver functions allow us to tie these prior observations of the Colombia subduction zone to distinct, structural features of the slab. We find that the region of high seismicity corresponds to a low seismic velocity feature along the top of the subducting plate between 3.5°N and 5.5°N that is not present to the south. Moderately elevated P-wave velocity to S-wave velocity ratios are also observed within the slab in the north. This feature likely represents hydrated slab mantle and/or uneclogitized oceanic crust extending to a deeper depth in the north of the region which may provide fluids to drive slab seismicity. We further find evidence for a thick layer of material along the slab’s lithosphere-asthenosphere boundary characterized by spatially variable anisotropy. This feature likely represents entrained asthenosphere at the base of the plate sheared by both the overlying plate and complex flow related to proposed slab tears just north and south of the study region. These observations highlight how structural observations provide key contextual constraints on short-term (seismogenic) and long-term (anisotropic fabric) dynamic processes in the Colombia subduction zone. Plain-language Summary The Nazca oceanic plate is very young (<15 million years old) where it is pulled or subducted beneath the South America plate in central and southern Colombia. Earthquakes occurring in the subducted Nazca plate at depths greater than 50 km are nearly 9x more common in central Colombia than in southern Colombia. The subducted Nazca plate also has a complex shape in this region and may have been torn both in northern Colombia and to the south near the Colombia-Ecuador border. The slow flow of mantle rock beneath the subducted plate is believed to be affected by this and earlier studies have inferred this flow is mostly in the same direction as the subducting plate's motion. We have used 10+ years of data to calculate receiver functions, which can detect changes in the velocity of seismic waves at the top and bottom of the subducted plate to investigate these features. We found that the Nazca plate is either hydrated or has rocks with lower seismic velocities at its top in the central part of Colombia where earthquakes are common. We also find that a thick layer of mantle rock at the base of the subducted plate has been sheared.
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- Award ID(s):
- 2041631
- PAR ID:
- 10487638
- Publisher / Repository:
- AGU Fall Meeting 2023
- Date Published:
- Journal Name:
- AGU Fall Meeting 2023
- Subject(s) / Keyword(s):
- subduction Nazca plate South America geology geophysics seismology receiver functions
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The tectonic setting of Northwestern South America is very complex due to interaction between the Nazca, Caribbean, and South America plates and several oceanic terranes. Of particular interest is the Nazca plate slab geometry change since the Mid-Miocene, and its tectonomagmatic effects on the overlying South American Plate. Several studies indicate that the modern Nazca slab is torn into two segments at 5.5oN in Colombia, the so-called "Caldas tear", where the northern segment dips at a shallow angle while the southern segment dips at a steep angle. This slab geometry difference is manifested in magmatic activity where only the region with steep slab shows typical subduction zone volcanism, while the one with flat slab lacks such activity. However, due to the limited number of geochronological data, the timing of this slab shallowing and tearing remains poorly understood. We conducted an extensive 40Ar/39Ar geochronology study on post mid-Miocene volcanic rocks in Colombia to study the temporal evolution of Nazca plate geometry and its effects on magmatism in Colombia. We find evidence of continuous magmatism north and south of the Caldas tear from 10.5 Ma to 6.4 Ma, with peak activity between 9-8 Ma. This is followed by a ~4 million year magmatic hiatus, until the resurgence of magmatism south of the Caldas tear as monogenetic domes at 2.1 Ma, and continuous volcanic activity in modern composite volcanoes since 1.1 Ma. Results of this study support the presence of a complex subduction system beneath Colombia where the northern segment slab has been flat since ~6.4 Ma ago, and the southern segment has re-steepened at ~2.1 Ma. This study showcases that the Nazca slab geometry was the most important factor driving magmatism in Colombia since the mid- Miocene.more » « less
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