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

2021 ◽  
Author(s):  
Sam Wimpenny
Keyword(s):  
Lower Crust ◽  
Free Water ◽  
Static Friction ◽  
Force Balance ◽  
Normal Faults ◽  
Mountain Building ◽  
Spatially Correlated ◽  
Seismogenic Layer ◽  
The Andes ◽  
Earthquake Focal Mechanism

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.

scholarly journals Observations and dynamical implications of active normal faulting in South Peru

2020 ◽  
Vol 222 (1) ◽  
pp. 27-53 ◽  
Author(s):  
Sam Wimpenny ◽  
Carlos Benavente ◽  
Alex Copley ◽  
Briant Garcia ◽  
Lorena Rosell ◽  
...  
Keyword(s):  
Late Quaternary ◽  
Normal Fault ◽  
Temporal Variations ◽  
Alluvial Fan ◽  
Force Balance ◽  
Fault System ◽  
Normal Faults ◽  
Shear Stresses ◽  
Total Force ◽  
The Andes

SUMMARY Orogenic plateaus can exist in a delicate balance in which the buoyancy forces due to gravity acting on the high topography and thick crust of the plateau interior are balanced by the compressional forces acting across their forelands. Any shortening or extension within a plateau can indicate a perturbation to this force balance. In this study, we present new observations of the kinematics, morphology and slip rates of active normal faults in the South Peruvian Altiplano obtained from field studies, high-resolution DEMs, Quaternary dating and remote sensing. We then investigate the implications of this faulting for the forces acting on the Andes. We find that the mountains are extending ∼NNE–SSW to ∼NE–SW along a normal fault system that cuts obliquely across the Altiplano plateau, which in many places reactivates Miocene-age reverse faults. Radiocarbon dating of offset late Quaternary moraines and alluvial fan surfaces indicates horizontal extension rates across the fault system of between 1 and 4 mm yr–1—equivalent to an extensional strain rate in the range of 0.5–2 × 10−8 1 yr–1 averaged across the plateau. We suggest the rate and pattern of extension implies there has been a change in the forces exerted between the foreland and the Andes mountains. A reduction in the average shear stresses on the sub-Andean foreland detachment of ≲4 MPa (20–25 per cent of the total force) can account for the rate of extension. These results show that, within a mountain belt, the pattern of faulting is sensitive to small spatial and temporal variations in the strength of faults along their margins.

scholarly journals Southward expanding plate coupling due to variation in sediment subduction as a cause of Andean growth

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Jiashun Hu ◽  
Lijun Liu ◽  
Michael Gurnis
Keyword(s):  
Sediment Transport ◽  
Three Dimensional ◽  
Mountain Building ◽  
Crustal Shortening ◽  
First Order ◽  
The Andes ◽  
Tectonic Processes ◽  
History Of ◽  
Plate Coupling ◽  
Trench Sediments

AbstractGrowth of the Andes has been attributed to Cenozoic subduction. Although climatic and tectonic processes have been proposed to be first-order mechanisms, their interaction and respective contributions remain largely unclear. Here, we apply three-dimensional, fully-dynamic subduction models to investigate the effect of trench-axial sediment transport and subduction on Andean growth, a mechanism that involves both climatic and tectonic processes. We find that the thickness of trench-fill sediments, a proxy of plate coupling (with less sediments causing stronger coupling), exerts an important influence on the pattern of crustal shortening along the Andes. The southward migrating Juan Fernandez Ridge acts as a barrier to the northward flowing trench sediments, thus expanding the zone of plate coupling southward through time. Consequently, the predicted history of Andean shortening is consistent with observations. Southward expanding crustal shortening matches the kinematic history of inferred compression. These results demonstrate the importance of climate-tectonic interaction on mountain building.

scholarly journals The relationship between sticky spots and radar reflectivity beneath an active West Antarctic ice stream

2014 ◽  
Vol 55 (67) ◽  
pp. 29-38 ◽  
Author(s):  
David W. Ashmore ◽  
Robert G. Bingham ◽  
Richard C.A. Hindmarsh ◽  
Hugh F.J. Corr ◽  
Ian R. Joughin
Keyword(s):  
Free Water ◽  
Radar Data ◽  
Force Balance ◽  
Ice Streams ◽  
West Antarctic ◽  
Ice Stream ◽  
Surface Data ◽  
Fast Flow ◽  
The Relationship ◽  
Modelling Approach

AbstractIsolated areas of high basal drag, or ‘sticky spots’, are important and poorly understood features in the force balance and dynamics of West Antarctic ice streams. Characterizing sticky spots formed by thin or drying subglacial till using ice-penetrating radar is theoretically possible, as high radar bed-returned power (BRP) is commonly related to an abundance of free water at the ice/bed interface, provided losses from englacial attenuation can be estimated. In this study we use airborne radar data collected over Evans Ice Stream to extract BRP profiles and test the sensitivity of BRP to the adopted englacial attenuation correction. We analyse 11 ~ 2 0 km profiles in four fast-flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions. In the majority of profiles we note that the increase in basal drag is accompanied by a decrease in BRP and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence. A comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation, and our findings generally support previous studies that advocate a modelling approach.

Early exhumation of the Frontal Cordillera (southern Central Andes at ~33.5°S) and implications for Andean mountain-building

2020 ◽  
Author(s):  
Robin Lacassin ◽  
Magali Riesner ◽  
Martine Simoes ◽  
Tania Habel ◽  
Audrey Margirier ◽  
...  
Keyword(s):  
Central Andes ◽  
Crustal Thickening ◽  
Mountain Range ◽  
Mountain Building ◽  
Ongoing Debate ◽  
First Order ◽  
The Andes ◽  
Minimum Age ◽  
Southern Central ◽  
Basement Structures

&lt;p&gt;The Andes are the modern active example of a Cordilleran-type orogen, with mountain-building&amp;#8232; and crustal thickening within the upper plate of a subduction zone. Despite numerous studies of&amp;#8232; this emblematic mountain range, several primary traits of this orogeny remain unresolved or poorly documented. The timing of uplift and deformation of the Frontal Cordillera basement culmination of&amp;#8232; the Southern Central Andes is such an example, even though this structural unit appears as a first-order topographic and geological feature. Constraining this timing and in particular the onset of uplift is a key point in the ongoing debate about the initial vergence of the crustal-scale thrusts at the start of the Cenozoic Andean orogeny. To solve for this, new apatite and zircon (U-Th)/He ages from granitoids of the Frontal Cordillera at ~33.5&amp;#176;S are provided here. These data, interpreted as an age-elevation thermochronological profile, imply continuous exhumation initiating well before ~12&amp;#8211;14 Ma, and at most by ~22 Ma when considering the youngest zircon grain from the lowermost sample (Riesner et al. 2019). The inverse modeling of the thermochronological data using QTQt software confirms these conclusions and point to a continuous cooling rate since onset of cooling. The minimum age of exhumation onset is then refined to ~20 Ma by combining these results with data on sedimentary provenance from the nearby basins. Such continuous exhumation since ~20 Ma needs to have been sustained by tectonic uplift on an underlying crustal-scale thrust ramp. Such early exhumation and associated uplift of the Frontal Cordillera question the classically proposed east-vergent models of the Andes at this latitude. Additionally, this study provides further support to recent views on Andean mountain-building proposing that the Andes-Altiplano orogenic system grew firstly over west-vergent basement structures before shifting to dominantly east-vergent thrusts.&amp;#160;&lt;br&gt;Riesner M. et al. 2019, Scientific Reports, DOI: 10.1038/s41598-019-44320-1&lt;/p&gt;

Anomalously deep earthquakes in the March 2018 swarm along the Western Margin of Afar

2020 ◽  
Author(s):  
Alessandro La Rosa ◽  
Cecile Doubre ◽  
Carolina Pagli ◽  
Federico Sani ◽  
Giacomo Corti ◽  
...  
Keyword(s):  
Lower Crust ◽  
Surface Deformation ◽  
Receiver Function ◽  
Velocity Model ◽  
Normal Faults ◽  
Active System ◽  
Western Margin ◽  
Afar Depression ◽  
Crustal Earthquakes ◽  
Using Data

&lt;p&gt;During the evolution of continental rift systems, extension focuses along on-axis magmatic segments while extensional structures along the rift margins seem to progressively become inactive. However, how strain is partitioned between rift axes and rift margins is still poorly understood. The Afar Rift is the locus of extension between Nubia, Arabia and Somalia and is believed to record the latest stages of rifting and incipient continental break-up. The Afar rift axis is bounded at its western margin by a seismically active system of normal faults separating the Afar depression from the Ethiopian Plateau through a series of large bounding faults and marginal grabens. Although most of the extension in Afar is currently accommodated on-axis, several earthquakes with Mw &gt; 5.0 occurred in the past decades on the Western Afar Margin (WAM). Here we analysed the most recent M&lt;sub&gt;w&lt;/sub&gt; 5.2 earthquake on the WAM on 24 March 2018 and the following seismic sequence using data recorded by a temporary seismic network, set up between 2017 and 2018. We located 800 events from the 20 March to the 30 April 2018 using twenty-three local seismic stations and a new velocity model for the WAM based on a new receiver function study. Preliminary results show that seismicity during the 2018 event focused at mid-to-low crustal depths (from ~15 km to ~35 km) along west-dipping fault planes. Shallower upper crustal earthquakes also occurred on west-dipping fault planes.&lt;/p&gt;&lt;p&gt;The hypocentral location of the mainshock has also been investigated using InSAR. We processed four independent interferograms using Sentinel-1 data acquired from a descending track. None of them shows any significant surface deformation, confirming the large depth of the hypocenters. Furthermore, we tested possible ranges of depth by producing a series of forward models assuming fault located at progressively increasing depths and corresponding to a Mw 5.2 earthquake. Our models show that surface deformations are &lt; 1 cm at depths greater than 15 km, in agreement with our hypocentral depth of 18 km for the main shock estimated from seismic data.&amp;#160;&lt;/p&gt;&lt;p&gt;Our seismicity observations of slip along west-dipping faults show that deformation across the WAM is currently accommodated by antithetic faulting, as suggested by structural geology studies. Lower crustal earthquakes might occur in a strong lower crust due to the presence of mafic lower crust and/or be induced by migrating fluids such as magma or CO&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;

scholarly journals Rifted margins: Ductile deformation, boudinage, continentward-dipping normal faults and the role of the weak lower crust

2018 ◽  
Vol 53 ◽  
pp. 20-40 ◽  
Author(s):  
Camille Clerc ◽  
Jean-Claude Ringenbach ◽  
Laurent Jolivet ◽  
Jean-François Ballard
Keyword(s):  
Lower Crust ◽  
Ductile Deformation ◽  
Normal Faults ◽  
Rifted Margins

Modeling and friction estimation for wheeled omnidirectional mobile robots

Robotica ◽  
2015 ◽  
Vol 34 (9) ◽  
pp. 2140-2150 ◽  
Author(s):  
Andre G. S. Conceicao ◽  
Mariane D. Correia ◽  
Luciana Martinez
Keyword(s):  
Mobile Robots ◽  
Least Squares Method ◽  
Center Of Mass ◽  
Static Friction ◽  
Friction Model ◽  
Force Balance ◽  
Indoor Environments ◽  
Wheeled Mobile Robots ◽  
Friction Estimation ◽  
Simulation Results

SUMMARYIn this study, a model for wheeled mobile robots that includes a static friction model in the force balance at the robot's center of mass is presented. Additionally, a least-squares method to linearly combine functions is proposed to estimate the friction coefficients. The experimental and simulation results are discussed to demonstrate the effectiveness of this approach in indoor environments for two floor types.

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