The age of the Lac-Saint-Jean Anorthosite Complex and associated mafic rocks, Grenville Province, Canada

1992 ◽  
Vol 29 (7) ◽  
pp. 1412-1423 ◽  
Author(s):  
Michael D. Higgins ◽  
Otto van Breemen
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Strike Slip ◽  
Short History ◽  
Southwestern Part ◽  
Grenville Province ◽  
South Central ◽  
History Of ◽  
Emplacement Mechanism ◽  
Anorthosite Complex

U–Pb analyses of zircon and baddeleyite from the south-central and southeastern parts of the Lac-Saint-Jean Anorthosite Complex (LSJA) give an igneous crystallization age of 1157 ± 3 Ma. Parts of the anorthosite were deformed in the solid state and subsequently intruded by a diorite megadyke, which also gives a crystallization age of 1157 ± 3 Ma, indicating that crystallization and deformation of the anorthosite were essentially synchronous. The diorite megadyke was intruded into a north-northeast-trending shear zone and deformed by sinistral strike-slip movements. Emplacement was followed by intrusion of a subparallel leucotroctolite megadyke that again gives the same crystallization age and hence dates movement of the shear zone at 1157 ± 3 Ma. This short history of crystallization and synchronous deformation rules out slow diapiric rise as the emplacement mechanism for the anorthosite. Instead, anorthosite parental magmas probably rose up offsets in subvertical strike-slip shear zones to their present level.In the southwestern part of the LSJA an age of 1142 ± 3 Ma is interpreted to represent igneous crystallization. Contemporary thermal metamorphic effects recorded in the southeastern sector by growth of new zircon in granophyric segregations and zircon coronas on baddeleyite suggest this event was more widespread at slightly deeper levels. Evidence has not been found for a separate Grenville regional metamorphism.The emplacement into the LSJA at 1076 ± 3 Ma of two small leucogabbro intrusions was part of a widespread magmatic event similar to the main event at 1157–1142 Ma.

scholarly journals Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

2018 ◽  
Vol 55 (9) ◽  
pp. 1063-1078 ◽  
Author(s):  
Michelle J. Markley ◽  
Steven R. Dunn ◽  
Michael J. Jercinovic ◽  
William H. Peck ◽  
Michael L. Williams
Keyword(s):  
Shear Zone ◽  
Crustal Thickening ◽  
Boundary Zone ◽  
Grenville Province ◽  
Metamorphic History ◽  
Crustal Thinning ◽  
Tectonic Significance ◽  
History Of ◽  
Central Gneiss Belt ◽  
Scale Shear

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

scholarly journals The emplacement mode of Upper Cretaceous plutons from the southwestern part of the Sredna Gora Zone (Bulgaria): structural and AMS study

2009 ◽  
Vol 60 (1) ◽  
pp. 15-33 ◽  
Author(s):  
Neven Georgiev ◽  
Bernard Henry ◽  
Neli Jordanova ◽  
Nikolaus Froitzheim ◽  
Diana Jordanova ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Shear Zone ◽  
Upper Cretaceous ◽  
Plate Boundary ◽  
Shear Zones ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Southwestern Part ◽  
Structural Investigations ◽  
Plate Convergence ◽  
Microgranular Enclaves

The emplacement mode of Upper Cretaceous plutons from the southwestern part of the Sredna Gora Zone (Bulgaria): structural and AMS study Several plutons located in the southwestern part of the Sredna Gora Zone — Bulgaria are examples of the Apuseni-Banat-Timok-Sredna Gora type of granites emplaced during Late Cretaceous (86-75 Ma) times. The studied intrusive bodies are spatially related to and deformed by the dextral Iskar-Yavoritsa shear zone. The deformation along the shear zone ceased at the time of emplacement of the undeformed Upper Cretaceous Gutsal pluton, which has intruded the Iskar-Yavoritsa mylonites. A clear transition from magmatic foliation to high-, moderate- and low-temperature superimposed foliation and lineation in the vicinity of the Iskar-Yavoritsa and related shear zones gives evidence for simultaneous tectonics and plutonism. Away from the shear zones, the granitoids appear macroscopically isotropic and were investigated using measurements of anisotropy of magnetic susceptibility at 113 stations. The studied samples show magnetic lineation and foliation, in agreement with the magmatic structures observed at a few sites. Typical features of the internal structure of the plutons are several sheet-like mafic bodies accompanied by swarms of mafic microgranular enclaves. Field observations indicate spatial relationships between mafic bodies and shear zones as well as mingling processes in the magma chamber which suggest simultaneous shearing and magma emplacement. Structural investigations as well as anisotropy of magnetic susceptibility (AMS) data attest to the controlling role of the NWSE trending Iskar-Yavoritsa shear zone and to the syntectonic emplacement of the plutons with deformation in both igneous rocks and their hosts. The tectonic situation may be explained by partitioning of oblique plate convergence into plate-boundary-normal thrusting in the Rhodopes and plate-boundary-parallel transcurrent shearing in the hinterland (Sredna Gora).

Biostratigraphy of the Campanian-Maastrichtian, Mid-Maastrichtian, and Cretaceous-tertiary boundaries in the Raton Basin: Implications for models of early Laramide tectonism and for the origin of the enigmatic Disturbed Zone

2016 ◽  
Vol 53 (2) ◽  
pp. 75-91 ◽  
Author(s):  
Keith Berry
Keyword(s):  
New Mexico ◽  
High Resolution ◽  
South Dakota ◽  
Sedimentation Rates ◽  
Southwestern Part ◽  
Pierre Shale ◽  
South Central ◽  
Disturbed Zone ◽  
History Of ◽  
Differential Sedimentation

The Campanian-Maastrichtian stage boundary is placed within the upper part of the Pierre Shale about 100 m below the base of the Trinidad Sandstone at Berwind Canyon, Colorado. Correlation of ammonite biostratigraphy with palynostratigraphy places this boundary near the top of the Vermejo Formation in the southwestern part of the Raton Basin, northeastern New Mexico. The informal mid-Maastrichtian boundary should be placed at the level of the Trinidad Sandstone at Berwind Canyon, although it should be placed at the level of the Vermejo-Raton unconformity in northeastern New Mexico based upon palynostratigraphy. High-resolution megafloral biostratigraphy of the K-T boundary is also discussed. Chronostratigraphic and paleoenvironmental indicator species reveal differential sedimentation rates across the basin. The magnitude of the Vermejo-Raton unconformity varies primarily in a north-south direction across the Raton Basin and was greatest in northeastern New Mexico and least in south-central Colorado, which may affect interpretations of the early Laramide history of the Raton Basin. The results of this study preclude correlation of the basal part of the Vermejo Formation with the Disturbed Zone of southwestern South Dakota.

Complex kinematics in a major ductile shear zone, NW Shetland: Evidence of ductile extrusion during Caledonian transpression

2021 ◽  
Author(s):  
Timothy Armitage ◽  
Robert Holdsworth ◽  
Robin Strachan ◽  
Thomas Zach ◽  
Diana Alvarez-Ruiz ◽  
...  
Keyword(s):  
Shear Zone ◽  
Hanging Wall ◽  
Regional Scale ◽  
Shear Zones ◽  
White Mica ◽  
Strike Slip ◽  
Amphibolite Facies ◽  
Lateral Extrusion ◽  
Shear Sense ◽  
Ductile Shear

<p>Ductile shear zones are heterogeneous areas of strain localisation which often display variation in strain geometry and combinations of coaxial and non-coaxial deformation. One such heterogeneous shear zone is the c. 2 km thick Uyea Shear Zone (USZ) in northwest Mainland Shetland (UK), which separates variably deformed Neoarchaean orthogneisses in its footwall from Neoproterozoic metasediments in its hanging wall (Fig. a). The USZ is characterised by decimetre-scale layers of dip-slip thrusting and extension, strike-slip sinistral and dextral shear senses and interleaved ultramylonitic coaxially deformed horizons. Within the zones of transition between shear sense layers, mineral lineations swing from foliation down-dip to foliation-parallel in kinematically compatible, anticlockwise/clockwise-rotations on a local and regional scale (Fig. b). Rb-Sr dating of white mica grains via laser ablation indicates a c. 440-425 Ma Caledonian age for dip-slip and strike-slip layers and an 800 Ma Neoproterozoic age for coaxial layers. Quartz opening angles and microstructures suggest an upper-greenschist to lower-amphibolite facies temperature for deformation. We propose that a Neoproterozoic, coaxial event is overprinted by Caledonian sinistral transpression under upper greenschist/lower amphibolite facies conditions. Interleaved kinematics and mineral lineation swings are attributed to result from differential flow rates resulting in vertical and lateral extrusion and indicate regional-scale sinistral transpression during the Caledonian orogeny in NW Shetland. This study highlights the importance of linking geochronology to microstructures in a poly-deformed terrane and is a rare example of a highly heterogeneous shear zone in which both vertical and lateral extrusion occurred during transpression.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.0cf6ef44e5ff57820599061/sdaolpUECMynit/12UGE&app=m&a=0&c=d96bb6db75eed0739f2a6ee90c9ad8fd&ct=x&pn=gepj.elif&d=1" alt=""></p>

Exhumation of the crustal-scale Gaoligong strike-slip shear belt in Southeast Asia

2021 ◽  
pp. jgs2021-038
Author(s):  
Yanlong Dong ◽  
Shuyun Cao ◽  
Franz Neubauer ◽  
Haobo Wang ◽  
Wenyuan Li ◽  
...  
Keyword(s):  
Shear Zone ◽  
Early Cretaceous ◽  
Vertical Motion ◽  
Shear Zones ◽  
Strike Slip ◽  
Western Boundary ◽  
The Tibetan Plateau ◽  
Content Type ◽  
Lateral Extrusion ◽  
Supplementary Material

Lateral extrusion of blocks is a well-known geological process during continent–continent collision, which always expresses by either brittle strike-slip faults or ductile shear zones. However, vertical motion along such fault systems remains poorly constrained. The Gaoligong shear zone (GLG-SZ) formed the western boundary of the Indochina block during the India–Eurasia collision, resulting in the exhumation of deep crustal rocks, including a large volume of syntectonic granites. Combined zircon U-Pb dating and 40Ar/39Ar thermochronology revealed that both the unfoliated and foliated granitic intrusions were emplaced during the Early Cretaceous (112–125 Ma), post-magmatic melting occurred from the Early Oligocene (ca. 35 Ma), and subsequent cooling during the Middle Miocene (ca. 13 Ma). The average emplacement depth of Early Cretaceous samples revealed that at least 15 km of hangingwall of the GLG-SZ must have been removed by vertical motion during shearing. Syn-shearing exhumation underlines the role of the lateral motion of the shear zone initiation by magma-assisted rheological weakening and exhumation at high ambient temperatures within the shear zone. A new model links magmatic channel flow underneath the Tibetan Plateau with magma intrusions and the high geothermal gradients along the shear belts, such as the GLG-SZ.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5598365

Kinematic data on major Variscan strike-slip faults and shear zones in the Polish Sudetes, northeast Bohemian Massif

1997 ◽  
Vol 134 (5) ◽  
pp. 727-739 ◽  
Author(s):  
P. ALEKSANDROWSKI ◽  
R. KRYZA ◽  
S. MAZUR ◽  
J. ŻABA
Keyword(s):  
Shear Zone ◽  
Fault Zone ◽  
Bohemian Massif ◽  
Early Carboniferous ◽  
Shear Zones ◽  
Strike Slip ◽  
Late Carboniferous ◽  
Late Devonian ◽  
The West ◽  
West Sudetes

The still highly disputable terrane boundaries in the Sudetic segment of the Variscan belt mostly seem to follow major strike-slip faults and shear zones. Their kinematics, expected to place important constraints on the regional structural models, is discussed in some detail. The most conspicuous is the WNW–ESE Intra-Sudetic Fault Zone, separating several different structural units of the West Sudetes. It showed ductile dextral activity and, probably, displacement magnitude of the order of tens to hundreds kilometres, during late Devonian(?) to early Carboniferous times. In the late Carboniferous (to early Permian?), the sense of motion on the Intra-Sudetic Fault was reversed in a semi-brittle to brittle regime, with the left-lateral offset on the fault amounting to single kilometres. The north–south trending Niemcza and north-east–southwest Skrzynka shear zones are left-lateral, ductile features in the eastern part of the West Sudetes. Similarly oriented (northeast–southwest to NNE–SSW) regional size shear zones of as yet undetermined kinematics were discovered in boreholes under Cenozoic cover in the eastern part of the Sudetic foreland (the Niedźwiedź and Nysa-Brzeg shear zones). One of these is expected to represent the northern continuation of the major Stare Mesto Shear Zone in the Czech Republic, separating the geologically different units of the West and East Sudetes. The Rudawy Janowickie Metamorphic Unit, assumed in some reconstructions to comprise a mostly strike-slip terrane boundary, is characterized by ductile fabric developed in a thrusting regime, modified by a superimposed normal-slip extensional deformation. Thrusting-related deformational fabric was locally reoriented prior to the extensional event and shows present-day strike-slip kinematics in one of the sub-units. The Sudetic Boundary Fault, although prominent in the recent structure and topography of the region, was not active as a Variscan strike-slip fault zone. The reported data emphasize the importance of syn-orogenic strike-slip tectonics in the Sudetes. The recognized shear sense is compatible with a strike-slip model of the northeast margin of the Bohemian Massif, in which the Kaczawa and Góry Sowie Units underwent late Devonian–early Carboniferous southeastward long-distance displacement along the Intra-Sudetic Fault Zone from their hypothetical original position within the Northern Phyllite Zone and the Mid-German Crystalline High of the German Variscides, respectively, and were juxtaposed with units of different provenance southwest of the fault. The Intra-Sudetic Fault Zone, together with the Elbe Fault Zone further south, were subsequently cut in the east and their eastern segments were displaced and removed by the younger, early to late Carboniferous, NNE–SSW trending, transpressional Moldanubian–Stare Mesto Shear Zone.

Striding-Athabasca mylonite zone: Complex Archean deep-crustal deformation in the East Athabasca mylonite triangle, northern Saskatchewan

1994 ◽  
Vol 31 (8) ◽  
pp. 1287-1300 ◽  
Author(s):  
Simon Hanmer ◽  
Randy Parrish ◽  
Michael Williams ◽  
Chris Kopf
Keyword(s):  
Flow Pattern ◽  
Shear Zone ◽  
Crustal Deformation ◽  
Principal Axis ◽  
Shear Zones ◽  
Granulite Facies ◽  
Strike Slip ◽  
Tectonic Zone ◽  
Early Proterozoic ◽  
Lower Deck

The geophysically defined Snowbird tectonic zone is manifested in northernmost Saskatchewan as a deep-crustal, multistage mylonitic structure, the East Athabasca mylonite triangle. The triangle, located at the northeastern apex of a stiff, crustal-scale "lozenge," is composed of mid-Archean annealed mylonites and late Archean ribbon mylonites, formed during two granulite facies events (850–1000 °C, 1.0 GPa). The flow pattern in the mylonites is geometrically and kinematically complex, and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis. The late Archean mylonites are divided into an upper structural deck, entirely occupied by a dip-slip shear zone, and an underlying lower deck. The latter is divided into two upright conjugate strike-slip shear zones, separated by a low-strain septum, which deformed by progressive coaxial flow. The flow pattern in the mid-Archean mylonites is compatible with that of the late Archean mylonites, and suggests that the crustal-scale lozenge influenced deformation since the mid-Archean. In the interval ca. 2.62–2.60 Ga, deformation in the upper and lower decks evolved from a granulite facies pervasive regime to a more localized amphibolite facies regime. With further cooling, deformation was localized within very narrow greenschist mylonitic faults at the lateral limits of the lower deck. By the late Archean, the East Athabasca mylonite triangle was part of a deep-crustal, intracontinental shear zone. This segment of the Snowbird tectonic zone was not the site of an Early Proterozoic suture or orogen.

A test of detailed Nd isotope mapping in the Grenville Province: delineating a duplex thrust sheet in the Kipawa–Mattawa region

2006 ◽  
Vol 43 (4) ◽  
pp. 421-432 ◽  
Author(s):  
M K Herrell ◽  
A P Dickin ◽  
W A Morris
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Data Sets ◽  
Aeromagnetic Data ◽  
Thrust Sheet ◽  
Grenville Province ◽  
Nd Isotope ◽  
Tectonic Model ◽  
Thrust Sheets ◽  
Archean Crust

Over sixty new neodymium model ages were determined on orthogneisses from the Kipawa–Mattawa region of the Grenville Province to refine previous Nd isotope mapping work in this area. The combined Nd data sets support a tectonic model involving three major thrust sheets in the Kipawa area, separated by major shear zones. The uppermost sheet is correlated with the Allochthonous Polycyclic Belt, represented in the study area by the Lac Watson nappe, along with two allochthonous klippen. These have Nd model ages < 1.8 Ga, consistent with previous work. Within the underlying Parautochthonous Belt, previous workers identified a second major shear zone, separating rocks with Archean and Proterozoic crystallization ages, respectively. These two thrust sheets also have distinct Nd isotope signatures. The lowermost sheet consists of metamorphosed but otherwise relatively pristine Archean crust with Nd model ages > 2.6 Ga, whereas the overlying sheet consists of magmatically reworked Archean parautochthon with model ages from 1.8–2.6 Ga. A residual magnetic-field map developed from aeromagnetic data was compared with the terrane boundaries determined from isotopic data. The aeromagnetic data accurately reflect the margin of relatively pristine Archean crust in the study area, but this boundary does not correspond to the Allochthon Boundary Thrust. Instead, this boundary resulted from downcutting of the basal shear zone of the allochthon. This caused décollement of the strongly reworked Archean parautochthon to generate a duplex thrust sheet that was transported northwestwards over pristine Archean crust.

scholarly journals The Timing of Strike-Slip Deformation Along the Storstrømmen Shear Zone, Greenland Caledonides: U–Pb Zircon and Titanite Geochronology

2014 ◽  
Vol 41 (1) ◽  
pp. 19 ◽  
Author(s):  
Benjamin W. Hallett ◽  
William C. McClelland ◽  
Jane A. Gilotti
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Strike Slip ◽  
Fault System ◽  
Oblique Collision ◽  
Plate Convergence ◽  
Ion Probe ◽  
Slip Displacement ◽  
Land Deformation ◽  
Lateral Escape

The Storstrømmen shear zone (SSZ) in the Greenland Caledonides is widely interpreted to have formed in a transpressional regime during sinistral, oblique collision between Baltica and Laurentia in the Silurian to Devonian. New mapping of the SSZ at Sanddal documents a 100 m thick, greenschistfacies mylonite zone cutting the eclogite to amphibolite-facies gneiss complex. We present U–Pb ion probe geochronology on zircon and titanite from a variety of lithologies that shows the SSZ was active from late Devonian to the Carboniferous (at least until 350 Ma). The age of thrusting in the foreland is not well known, but must be younger than the age of eclogite-facies metamorphism at ~400 Ma. It is, therefore, possible that contraction is the same age as strike-slip motion, and that transpression is a viable model. The timing of the SSZ is synchronous with dextral strike-slip displacement on the Germania Land deformation zone. Simultaneous displacement on sinistral and dextral, conjugate shear zones suggests that the SSZ is part of a strikeslip fault system that led to lateral escape of material northward (present day coordinates) during the waning stages of plate convergence between Laurentia and Baltica.SOMMAIRELa zone de cisaillement de Storstrømmen (SSZ) dans les Calédonides du Groenland est généralement comprise comme ayant été formée durant un régime de transpression sénestre lors de la collision oblique entre Baltica et Laurentie, du Silurien au Dévonien.  Une nouvelle cartographie de la SSZ à Sanddal décrit une zone de 100 m d’épaisseur de mylonite au faciès des schistes verts qui recoupe un complexe de gneiss au faciès éclogite à amphibolite.  Notre analyse géochronologique par sonde ionique U-Pb sur zircon et titanite sur diverses lithologies, montre que la SSZ a été active de la fin du Dévonien jusqu’au Carbonifère (au moins jusqu’à 350 Ma).  L’âge du chevauchement dans l’avant-pays n’est pas bien connue, mais il doit être plus jeune que le métamorphisme au faciès d’éclogite à ~400 Ma.  Il est donc possible que la contraction soit du même âge que le mouvement de coulissage, et que la transpression soit un modèle viable.  La chronologie de la SSZ est synchrone au mouvement de coulissage dextre de la zone de déformation de Germania Land.  Les déplacements simultanés, sénestre et dextre, sur des zones de cisaillement conjuguées permettent de penser que la SSZ fait partie d’un système de décrochement qui a engendré une éjection latérale de matériau vers le nord (selon les coordonnées actuelles) durant les stades de convergence des plaques Laurentie et Baltica.

Earthquake cycles and shear zones: interplay between earthquakes, aseismic fault slip, and bulk viscous deformation

2021 ◽  
Author(s):  
Kali Allison ◽  
Laurent Montesi ◽  
Eric Dunham
Keyword(s):  
Grain Size ◽  
Viscous Flow ◽  
Shear Zone ◽  
Shear Zones ◽  
Strike Slip ◽  
Fault Slip ◽  
Coseismic Slip ◽  
Bulk Viscous ◽  
Shear Heating ◽  
Seismic Cycles

&lt;p&gt;The interaction between the seismogenic portion of faults and their ductile roots is central to understanding the mechanics of seismic cycles. It is well established that faults are highly localized within the cold and brittle upper crust, but less is known about fault and shear zone structure in the warmer, more ductile, lower crust and in the upper mantle. Increasing temperature with depth causes two transitions in behavior: a frictional transition from seismic to aseismic fault behavior and a transition from brittle to ductile off-fault deformation (BDT). To explore the effects of these two transitions on seismic cycle characteristics (e.g., recurrence interval, nucleation depth, and down-dip limit of coseismic rupture), we simulate seismic cycles on a 2D strike-slip fault. All phases of the earthquake cycle are simulated, allowing the model to spontaneously generate earthquakes and to capture aseismic fault slip and off-fault viscous flow in the interseismic period. The fault is represented with rate-and-state friction. In the off-fault material, distributed viscous flow occurs through dislocation creep. We also consider two possible weakening mechanisms that may be active in lower crustal shear zones: shear heating and grain size reduction, which changes the ductile rheology from dislocation to diffusion creep. This model makes it possible to self-consistently simulate the variations of stress, strain rate, and grain size in the vicinity of a strike-slip fault.&lt;/p&gt;&lt;p&gt;We find that the viscous shear zone beneath the fault (defined as the region of elevated viscous strain rate) is roughly elliptically shaped, extending up to 10 km below the fault and with a width of 1 to 3 km. When weakening mechanisms are neglected, the BDT occurs below the depth of the transition from seismic to aseismic fault slip. In these cases, seismic cycle characteristics are similar to those of a traditional elastic cycle simulation that neglects viscoelastic deformation. However, the inclusion of shear heating, which produces a thermal anomaly relative to the background geotherm, shallows the BDT enough to limit the down-dip propagation of coseismic slip in some cases. In these cases, earthquakes penetrate 1-2 km into the shear zone, consistent with observations of zones in which both viscous flow and coseismic slip occur. Also, in these simulations, very little aseismic fault slip occurs. Instead, tectonic plate motion is accommodated primarily through coseismic slip and bulk viscous flow. Preliminary simulations that include the effects of grain size reduction within the shear zone show similar effects. Both weakening mechanisms narrow the shear zone by up to 20%, suggesting that the fault also plays a large role in controlling shear zone localization.&lt;/p&gt;

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