Variations in the Seismogenic Thickness of East Africa

Craig, T. J.; Jackson, James

doi:10.1029/2020jb020754

Cited by 28 publications

(27 citation statements)

References 73 publications

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“…It has been proposed that the depth at which rocks transition from a preferentially brittle to a preferentially ductile mode of deformation (''brittle-ductile transition'' or BDT, provides a conservative estimate of the downdip extent of the seismogenic zone (e.g., Scholz, 1988;Sibson, 1982). Furthermore, compilations of crustal seismicity in different regions show that old or mafic crustal sectors are characterized by deep-seated seismicity, which supports the notion of a rheological control inherited from paleotectonic processes (e.g., Ammirati et al, 2013Ammirati et al, , 2016Craig & Jackson, 2021;Jackson et al, 2008;Maggi et al, 2000;Pérez Luján et al, 2015).…”

supporting

confidence: 66%

Long‐Term Lithospheric Strength and Upper‐Plate Seismicity in the Southern Central Andes, 29°–39°S

Piceda

Scheck‐Wenderoth

Cacace

et al. 2022

Geochem Geophys Geosyst

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We examined the relationship between the mechanical strength of the lithosphere and the distribution of seismicity within the overriding continental plate of the southern Central Andes (SCA, 29°–39°S), where the oceanic Nazca Plate changes its subduction angle between 33°S and 35°S, from subhorizontal in the north (<5°) to steep in the south (∼30°). We computed the long‐term lithospheric strength based on an existing 3D model describing variations in thickness, density, and temperature of the main geological units forming the lithosphere of the SCA and adjacent forearc and foreland regions. The comparison between our results and seismicity within the overriding plate (upper‐plate seismicity) shows that most of the events occur within the modeled brittle domain of the lithosphere. The depth where the deformation mode switches from brittle frictional to thermally activated ductile creep provides a conservative lower bound to the seismogenic zone in the overriding plate of the study area. We also found that the majority of upper‐plate earthquakes occurs within the realm of first‐order contrasts in integrated strength (12.7–13.3 log Pam in the Andean orogen vs. 13.5–13.9 log Pam in the forearc and the foreland). Specific conditions characterize the mechanically strong northern foreland of the Andes, where seismicity is likely explained by the effects of slab steepening.

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supporting

confidence: 66%

Long‐Term Lithospheric Strength and Upper‐Plate Seismicity in the Southern Central Andes, 29°–39°S

Piceda

Scheck‐Wenderoth

Cacace

et al. 2022

Geochem Geophys Geosyst

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“…These field observations are consistent with laboratory experiments that show the creep strength of feldspars and pyroxenes is drastically reduced by a few hundredths of a weight percent of structurally-bound water at lowercrustal temperatures [Mackwell et al, 1998;Rybacki and Dresen, 2004]. Therefore, a seismogenic and mechanically strong lower crust, like that beneath the Andean forelands, is often considered to be a proxy for a dry, granulitic lower crust [Sloan et al, 2011;Craig and Jackson, 2021].…”

Section: A Dry Metastable Lower Crust Beneath the Andean Forelandssupporting

confidence: 85%

Weak, Seismogenic Faults Inherited From Mesozoic Rifts Control Mountain Building in the Andean Foreland

Wimpenny¹

2022

Preprint

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New earthquake focal mechanism and centroid depth estimates show that the deformation style in the forelands of the Andes is spatially correlated with rift systems that stretched the South American lithosphere in the Mesozoic. Where the rifts trend sub-parallel to the Andean range front, normal faults inherited from the rifts are being reactivated as reverse faults, causing the 30--45 km thick seismogenic layer to break up. Where the rift systems are absent from beneath the range front, the seismogenic layer is bending and being thrust beneath the Andes like a rigid plate. Force-balance calculations show that the faults inerhited from former rift zones have an effective coefficient of static friction < 0.2. In order for these frictionally-weak faults to remain seismogenic in the lower crust, their wall rocks are likely to be formed of dry granulite. Xenolith data support this view, and suggest that parts of the lower crust are now mostly metastable, having experienced temperatures at least 75--250 degrees hotter than present. The conditions in the lower crust make it unlikely that highly-pressurised free water, or networks of intrinsically-weak phyllosilicate minerals, are the cause of their low effective friction, as, at such high temperatures, both mechanisms would cause the faults to deform through viscous creep and not frictional slip. Therefore pre-existing faults in the Andean forelands have remained weak and seismogenic after reactivation, and have influenced the style of mountain building in South America. However, the controls on their mechanical properties in the lower crust remain unclear.

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“…Our observations join a growing body of evidence that earthquake depths in East Africa generally increase southwards, mirroring increases in plate thickness and strength (Craig et al., 2011). Most recently, abrupt changes in earthquake depths near major terrain boundaries have been cited as evidence for sharp changes in crustal composition (Craig & Jackson, 2021). To this end, that earthquake depths are the shallowest near Duguna volcano perhaps suggests that short length‐scale variations in brittle layer thickness may result from variations in geothermal gradient associated with rift valley silicic magmatism.…”

Section: Discussionmentioning

confidence: 99%

Seismicity and Crustal Structure of the Southern Main Ethiopian Rift: New Evidence From Lake Abaya

Ogden

Keir

Bastow

et al. 2021

Geochem Geophys Geosyst

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The Main Ethiopian Rift (MER) captures the transition from embryonic fault-controlled continental rifting in the south to more evolved magma-rich continental rifting in the north (e.g., .In the relatively mature central and northern MER, the locus of strain has largely shifted from 60 kmlong Miocene border faults to narrower (20 km-wide), Quaternary magmatic zones of short length-scale (1 km), small-offset faults (the Wonji Fault Belt, WFB:

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Variations in the Seismogenic Thickness of East Africa

Cited by 28 publications

References 73 publications

Long‐Term Lithospheric Strength and Upper‐Plate Seismicity in the Southern Central Andes, 29°–39°S

Long‐Term Lithospheric Strength and Upper‐Plate Seismicity in the Southern Central Andes, 29°–39°S

Weak, Seismogenic Faults Inherited From Mesozoic Rifts Control Mountain Building in the Andean Foreland

Seismicity and Crustal Structure of the Southern Main Ethiopian Rift: New Evidence From Lake Abaya

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