scholarly journals Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

2017 ◽  
Author(s):  
Giancarlo Molli ◽  
Luca Menegon ◽  
Alessandro Malasoma
Keyword(s):  
Shear Zone ◽  
Continental Crust ◽  
Fluid Pressure ◽  
Deformation Mode ◽  
Shear Zones ◽  
Small Scale ◽  
Plastic Shear ◽  
Stick Slip ◽  
Brittle Ductile Transition

Abstract. The switching in deformation mode (from distributed to localized) and mechanisms (viscous versus frictional) represent a relevant issue in the frame of crustal deformation, being also connected with the concept of the brittle-ductile transition and seismogenesis. In subduction environment, switching in deformation mode and mechanisms may be inferred along the subduction interface, in a transition zone between the highly coupled (seismogenic zone) and decoupled deeper aseismic domain (stable slip). On the other hand, the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as fundamental in the localization of deformation and shear zone development, thus representing a case in which switching deformation mode and mechanisms interact and relate to each other. This contribution analyzes an example of a crystal plastic shear zone localized by brittle precursor formed within a host granitic-protomylonite during deformation in subduction-related environment. The studied structures, possibly formed by transient instability associated with fluctuations of pore fluid pressure and episodic strain rate variations may be considered as a small scale example of fault behaviour associated with a cycle of interseismic creep and coseismic rupture or a new analogue for episodic tremors and slow slip structures. Our case-study represents, therefore, a fossil example of association of fault structures related with stick-slip strain accomodation during subduction of continental crust.

scholarly journals Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

Solid Earth ◽  
2017 ◽  
Vol 8 (4) ◽  
pp. 767-788 ◽  
Author(s):  
Giancarlo Molli ◽  
Luca Menegon ◽  
Alessandro Malasoma
Keyword(s):  
Shear Zone ◽  
Continental Crust ◽  
Ambient Pressure ◽  
Deformation Mode ◽  
Shear Zones ◽  
Small Scale ◽  
Low Grade ◽  
Stick Slip ◽  
Brittle Ductile Transition

Abstract. The switching in deformation mode (from distributed to localized) and mechanisms (viscous versus frictional) represent a relevant issue in the frame of crustal deformation, being also connected with the concept of the brittle–ductile transition and seismogenesis. In a subduction environment, switching in deformation mode and mechanisms and scale of localization may be inferred along the subduction interface, in a transition zone between the highly coupled (seismogenic zone) and decoupled deeper aseismic domain (stable slip). However, the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as fundamental in some cases for the localization of deformation and shear zone development, thus representing a case in which switching deformation mechanisms and scale and style of localization (deformation mode) interact and relate to each other. This contribution analyses an example of a millimetre-scale shear zone localized by brittle precursor formed within a host granitic protomylonite. The studied structures, developed in ambient pressure–temperature (P–T) conditions of low-grade blueschist facies (temperature T of ca. 300 °C and pressure P ≥ 0. 70 GPa) during involvement of Corsican continental crust in the Alpine subduction. We used a multidisciplinary approach by combining detailed microstructural and petrographic analyses, crystallographic preferred orientation by electron backscatter diffraction (EBSD), and palaeopiezometric studies on a selected sample to support an evolutionary model and deformation path for subducted continental crust. We infer that the studied structures, possibly formed by transient instability associated with fluctuations of pore fluid pressure and episodic strain rate variations, may be considered as a small-scale example of fault behaviour associated with a cycle of interseismic creep and coseismic rupture or a new analogue for episodic tremors and slow-slip structures. Our case study represents, therefore, a fossil example of association of fault structures related to stick-slip strain accommodation during subduction of continental crust.

Regional setting and kinematic features of the Needle Falls Shear Zone, Trans-Hudson Orogen

1993 ◽  
Vol 30 (7) ◽  
pp. 1338-1354 ◽  
Author(s):  
Mel R. Stauffer ◽  
John F. Lewry
Keyword(s):  
Shear Zone ◽  
Simple Shear ◽  
Country Rock ◽  
Shear Zones ◽  
Small Scale ◽  
Lake Zone ◽  
Shear Sense ◽  
Regional Tectonic ◽  
Ductile Shear ◽  
Shear Sense Indicators

Needle Falls Shear Zone is the southern part of a major northeast-trending ductile shear system within the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan. Throughout its exposed length of ~400 km, the shear zone separates reworked Archean continental crust and infolded Paleoproterozoic supracrustals of the Cree Lake Zone, to the northwest, from mainly juvenile Paleoproterozoic arc terrains and granitoid plutons of the Reindeer Zone, to the southeast. It also defines the northwest margin of the ca. 1855 Ma Wathaman Batholith, which forms the main protolith to shear zone mylonites. Although not precisely dated, available age constraints suggest that the shear zone formed between ca. 1855 and 1800 Ma, toward the end of peak thermotectonism in this part of the orogen.In the Needle Falls study area, shear zone mylonites exhibit varied, sequentially developed, ductile to brittle fabric features, including C–S fabrics, winged porphyroclasts (especially delta type), small-scale compressional and extensional microfaults ranging from thin ductile shear zones to late brittle faults, early isoclinal and sheath folds, later asymmetric folds related to compressional microfaults, and variably rotated and (or) folded quartz veins. All ductile shear-sense indicators suggest dextral displacement, as do most later ductile–brittle transition and brittle features. In conjunction with a gently north–northeast-plunging extension lineation, such data indicate oblique east-side-up dextral movement across the shear zone. However, preexisting structures in country rock protoliths rotate into the shear zone in a sense contrary to that predicted by ideal dextral simple shear, a feature thought to reflect significant flattening across the shear zone. Other ductile to brittle fabric elements in the mylonites are consistent with general noncoaxial strain, rather than ideal simple shear. Amount of displacement cannot be measured but indirect estimates suggest approximately 40 ± 20 km.The Needle Falls Shear Zone is too small and has developed too late in regional tectonic history to be considered a crustal suture. Rather, it is interpreted as either a late-tectonic oblique collisional structure or as the result of counterclockwise oroclinal rotation of the southern part of the orogen.

scholarly journals Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 489-511 ◽  
Author(s):  
Francesca Prando ◽  
Luca Menegon ◽  
Mark Anderson ◽  
Barbara Marchesini ◽  
Jussi Mattila ◽  
...  
Keyword(s):  
Shear Zone ◽  
Nuclear Waste ◽  
Strain Localization ◽  
Microstructural Analysis ◽  
Fluid Pressure ◽  
Rheological Parameters ◽  
Nuclear Waste Repository ◽  
Brittle Deformation ◽  
Differential Stress ◽  
Brittle Ductile Transition

Abstract. The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (<20 cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 ∘C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.

scholarly journals Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan)

2021 ◽  
Author(s):  
Yan Lavallée ◽  
Takahiro Miwa ◽  
James D. Ashworth ◽  
Paul A. Wallace ◽  
Jackie E. Kendrick ◽  
...  
Keyword(s):  
Fluid Flow ◽  
Shear Zone ◽  
Damage Zone ◽  
Volcanic Eruptions ◽  
Shear Zones ◽  
Stick Slip ◽  
Anisotropic Permeability ◽  
Unzen Volcano ◽  
Porosity And Permeability ◽  
Low Porosity

Abstract. The permeability of magma in shallow volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2-m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ~1-m wide, highly-sheared region with relatively low porosity and permeability, and an outer 20-cm wide cataclastic fault zone. The low porosity, highly-sheared rock further exhibits an anisotropic permeability network with slightly higher permeability along the shear plane (parallel to the conduit margin) and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3-m long by ~9-cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with an increased permeability of ~3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e., ductile) shear during ascent up to the point of rupture, estimated by Umakoshi et al. (2008), at ~500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into, and along, the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the spine core with slight displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the upward shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

Geochronological evidence for repeated brittle reactivations of a pre-existing plastic shear zone: The Himdalen–Ørje deformation zone, Southern Norway

2021 ◽  
Author(s):  
Espen Torgersen ◽  
Roy Gabrielsen ◽  
Johan Petter Nystuen ◽  
Roelant van der Lelij ◽  
Morgan Ganerød ◽  
...  
Keyword(s):  
Shear Zone ◽  
Deformation Zone ◽  
Shear Zones ◽  
White Mica ◽  
Fault Gouge ◽  
Plastic Shear ◽  
Brittle Deformation ◽  
Sveconorwegian Orogen ◽  
Fault Gouges ◽  
Southern Norway

&lt;p&gt;It is well known that faults, once formed, become permanent weaknesses in the crust, localizing subsequent brittle strain increments. The case of repeated brittle reactivations localized along pre-existing plastic shear zones is less recognized, although this situation is frequently observed in many geologically old terranes.&lt;/p&gt;&lt;p&gt;We have studied the prolonged deformation history of the Himdalen&amp;#8211;&amp;#216;rje Deformation Zone (H&amp;#216;DZ) in SE Norway by combining K&amp;#8211;Ar and &lt;sup&gt;40&lt;/sup&gt;Ar&amp;#8211;&lt;sup&gt;39&lt;/sup&gt;Ar geochronology with structural analysis. The H&amp;#216;DZ consists of a large variation of deformation products from mylonites and cataclasites to pseudotachylites and fault gouge. Several generations of mylonites make up the ductile part of H&amp;#216;DZ, called the &amp;#216;rje shear zone, a km-think SW-dipping shear zone within the late Mesoproterozoic Sveconorwegian orogen. &lt;sup&gt;40&lt;/sup&gt;Ar&amp;#8211;&lt;sup&gt;39&lt;/sup&gt;Ar dating of white mica from one of these mylonites give a plateau age of c. 908 Ma, interpreted to constrain the timing of late-Sveconorwegian extensionial reactivation of the &amp;#216;rje shear zone.&lt;/p&gt;&lt;p&gt;This mylonitic fabric is extensively reworked in a brittle fashion along the SW-dipping Himdalen fault, a 10&amp;#8211;25 m thick fault zone of cataclasite, breccia, fault gouge and, in places, abundant pseduotachylite veins. &lt;sup&gt;40&lt;/sup&gt;Ar&amp;#8211;&lt;sup&gt;39&lt;/sup&gt;Ar dating of pseduotachylite material gives several small plateaus between c. 375 and 300 Ma, whereas K-feldspar clasts from the cataclasitically deformed host rock carry a Caledonian signal (plateau at c. 435 Ma). K&amp;#8211;Ar dating of three fault gouges constrain the timing of gouge development at c. 270 and 200 Ma. Two of the fault gouges also contain protolithic K-bearing mineral phases that overlap in age with the c. 375 Ma pseudotachylite &lt;sup&gt;40&lt;/sup&gt;Ar&amp;#8211;&lt;sup&gt;39&lt;/sup&gt;Ar plateau age, consistent with field observations of the former reworking the latter.&lt;/p&gt;&lt;p&gt;In sum, the H&amp;#216;DZ records multiple Paleozoic and Mesozoic brittle reactivations of the early Neoproterozoic (and older) mylonitic &amp;#216;rje shear zone. Most of the brittle deformation is interpreted to have accumulated during development of the Permian Oslo rift and its subsequent latest Triassic evolution. The suggested late Devonian (c. 375 Ma) initiation of brittle deformation does not have a clear tectonic association, but we speculate that it relates to strike-slip displacements caused by the Variscan orogen, as also suggested for the sub-parallel Tornquist zone to the south.&lt;/p&gt;

The role of regional-scale shear zones in paleogeographic reconstructions: the case study of the Variscan belt in the Mediterranean area

2021 ◽  
Author(s):  
Matteo Simonetti ◽  
Rodolfo Carosi ◽  
Chiara Montomoli ◽  
Salvatore Iaccarino
Keyword(s):  
Earth Sciences ◽  
Shear Zone ◽  
Regional Scale ◽  
Shear Zones ◽  
Mediterranean Area ◽  
Variscan Belt ◽  
Metamorphic Evolution ◽  
The Mediterranean ◽  
Scale Shear

&lt;p&gt;Paleogeographic reconstruction and recognition of the tectono-metamorphic evolution of ancient orogenic belt is often complex. The combination of an adequate amount of paleomagnetic, metamorphic, structural and geochronological data is necessary. Fundamental data derive from the study of regional-scale shear zones, that can be directly observed, by combining detailed field work with structural analysis, microstructural analysis and petrochronology. The Southern European Variscan Belt in the Mediterranean area was partially overprinted by the Alpine cycle (Stampfli and Kozur, 2006) and correlations are mainly based on lithological similarities. Little attention has been paid to the compatibility of structures in the dispersed fragments. A main debate is the connection among the Corsica-Sardinia Block (CSB), the Maures-Tanneron Massif (MTM) and the future Alpine External Crystalline Massifs (ECM) (Stampfli et al., 2002; Advokaat et al., 2014) and if these sectors were connected by a network of shear zones of regional extent, known as the East Variscan Shear Zone (EVSZ).&lt;/p&gt;&lt;p&gt;We present a multidisciplinary study of shear zones cropping out in the CSB (the Posada-Asinara shear zone; Carosi et al., 2020), in the MTM (the Cavalaire Fault; Simonetti et al., 2020a) and in the ECM (the Ferriere-Molli&amp;#232;res and the Emosson-Berard shear zones; Simonetti et al., 2018; 2020b).&lt;/p&gt;&lt;p&gt;Kinematic and finite strain analysis allowed to recognize a transpressional deformation, with a major component of pure shear and a variable component of simple shear, coupled with general flattening deformation. Syn-kinematic paragenesis, microstructures and quartz c-axis fabrics revealed that shear deformation, in all the studied sectors, occurred under decreasing temperature starting from amphibolite-facies up to greenschist-facies. A systematic petrochronological study (U-Th-Pb on monazite collected in the sheared rocks) was conducted in order to constrain the timing of deformation. We obtained ages ranging between ~340 Ma and ~320 Ma. Ages of ~340-330 Ma can be interpreted as the beginning of the activity of the EVSZ along its older branches while ages of ~320 Ma, obtained in all the shear zones, demonstrate that they were all active in the same time span.&lt;/p&gt;&lt;p&gt;The multidisciplinary approach revealed a similar kinematics and tectono-metamorphic evolution of the studied shear zones contributing to better constrain the extension and timing the EVSZ and to strength the paleogeographic reconstructions of the Southern Variscan belt during Late Carboniferous time, with important implications on the evolution of the Mediterranean area after the Late Paleozoic. This case study demonstrates how paleogeographic reconstructions could benefit from datasets obtained from large-scale structures (i.e., shear zones) that can be directly investigated.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Advokaat et al. (2014). Earth and Planetary Science Letters 401, 183&amp;#8211;195&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Carosi et al. (2012). Terra Nova 24, 42&amp;#8211;51&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Carosi and Palmeri (2002). Geological Magazine 139.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Carosi et al. (2020). Geosciences 10, 288.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Simonetti et al (2020a). International Journal of Earth Sciences 109, 2261&amp;#8211;2285&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Simonetti et al. (2020b). Tectonics 39&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Simonetti et al. (2018). International Journal of Earth Sciences. 107, 2163&amp;#8211;2189&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Stampfli and Kozur (2006). Geological Society, London, Memoirs 32, 57&amp;#8211;82&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Stampfli et al. (2002). Journal of the Virtual Explorer 8, 77&lt;/p&gt;

Using apatite to resolve the age and protoliths of mid-crustal shear zones: A case study from the Taxaquara Shear Zone, SE Brazil

Lithos ◽  
2020 ◽  
Vol 378-379 ◽  
pp. 105817
Author(s):  
B.V. Ribeiro ◽  
J.A. Mulder ◽  
F.M. Faleiros ◽  
C.L. Kirkland ◽  
P.A. Cawood ◽  
...  
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Se Brazil ◽  
Crustal Shear Zones

scholarly journals Tectonic pressure gradients during viscous creep drive fluid flow and brittle failure at the base of the seismogenic zone

Geology ◽  
2021 ◽  
Author(s):  
Luca Menegon ◽  
Åke Fagereng
Keyword(s):  
Shear Zone ◽  
Brittle Failure ◽  
Failure Modes ◽  
Fluid Pressure ◽  
Shear Zones ◽  
Elevated Pressure ◽  
Hydraulic Gradient ◽  
Seismogenic Zone ◽  
Viscous Shear ◽  
Host Rocks

Fluid-pressure cycles are commonly invoked to explain alternating frictional and viscous deformation at the base of the seismogenic crust. However, the stress conditions and geological environment of fluid-pressure cycling are unclear. We address this problem by detailed structural investigation of a vein-bearing shear zone at Sagelvvatn, northern Norwegian Caledonides. In this dominantly viscous shear zone, synkinematic quartz veins locally crosscut mylonitic fabric at a high angle and are rotated and folded with the same sense of shear as the mylonite. Chlorite thermometry indicates that both veining and mylonitization occurred at ~315–400 °C. The vein-filled fractures are interpreted as episodically triggered by viscous creep in the mylonite, where quartz piezometry and brittle failure modes are consistent with low (18–44 MPa) differential stress. The Sagelvvatn shear zone is a stretching shear zone, where elevated pressure drives a hydraulic gradient that expels fluids from the shear zone to the host rocks. In low-permeability shear zones, this hydraulic gradient facilitates buildup of pore-fluid pressure until the hydrofracture criterion is reached and tensile fractures open. We propose that hydraulic gradients established by local and cyclic pressure variations during viscous creep can drive episodic fluid escape and result in brittle-viscous fault slip at the base of the seismogenic crust.

scholarly journals The Formation of Small-Scale Atmospheric Vortices via Horizontal Shearing Instability

2016 ◽  
Vol 73 (5) ◽  
pp. 2061-2084 ◽  
Author(s):  
Michael S. Buban ◽  
Conrad L. Ziegler
Keyword(s):  
Shear Zone ◽  
Linear Theory ◽  
Potential Temperature ◽  
Maximum Growth ◽  
Shear Zones ◽  
Small Scale ◽  
Near Surface ◽  
Zone Width ◽  
Constant Vorticity ◽  
Atmospheric Vortices

Abstract Motivated by high-resolution observations of small-scale atmospheric vortices along near-surface boundaries, this study presents a series of idealized simulations that attempt to replicate shear zones typical of drylines and other near-surface boundaries. The series of dry, constant potential temperature simulations are initialized with a north–south-oriented constant-vorticity shear zone and north–south periodic boundary conditions. In all simulations, the shear zones develop wavelike perturbations that eventually roll up into discrete vortices. These vortices have features resembling those observed in many laboratory and numerical studies (i.e., instabilities developed into elliptical cores connected by vorticity braids that precess and contain pressure minima in their centers). To assess the instability mechanism, the results are compared to linear theory. Excellent agreement is found between predictions from linear theory for the wavenumber of maximum growth as a function of shear zone width and growth rate as a function of shear zone vorticity, suggesting to a very good first approximation, horizontal shearing instability (HSI) is responsible for the growth of initial small perturbations. It is also found that predictions of linear theory tend to extend well into the nonlinear regime. Finally, preferred regions of cumulus formation are assessed by including moisture in four simulations. Maximum updrafts and simulated cumuli tend to form along the periphery of cores and/or along the braided regions adjacent to the cores. Because of the important modulating effect of misocyclone development via HSI and subsequent moisture transport, cumulus spacing and size/depth are also dependent on the shear zone width and vorticity.

scholarly journals The Formation of Small-Scale Atmospheric Vortices via Baroclinic Horizontal Shearing Instability

2016 ◽  
Vol 73 (5) ◽  
pp. 2085-2104 ◽  
Author(s):  
Michael S. Buban ◽  
Conrad L. Ziegler
Keyword(s):  
Shear Zone ◽  
Potential Temperature ◽  
Shear Zones ◽  
Parametric Model ◽  
Small Scale ◽  
Surface Boundary ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Constant Vorticity ◽  
Atmospheric Vortices

Abstract This study presents a series of idealized simulations that attempt to replicate shear zones typical of drylines and other near-surface boundaries in the presence of horizontal virtual density gradients. The series of dry simulations are initialized to contain a north–south-oriented potential temperature gradient collocated with a constant-vorticity shear zone and employ north–south periodic boundary conditions. In all simulations, the shear zones frontogenetically collapse as wavelike perturbations develop that eventually roll up into discrete vortices. Convergence associated with the developing solenoidally forced secondary vertical circulation induces an accumulative shear zone contraction, which in turn increases the vertical vorticity of both the shear zone and the intensifying vortices, owing primarily to stretching that is partially offset by tilting of the vertical vorticity into the horizontal by the secondary circulation. The simulated vortices bear strong morphological resemblance to vortices reported in many earlier laboratory and numerical studies. To assess hypothesized baroclinic effects on the instability mechanism, the present results are compared to a previous study of barotropic horizontal shearing instability (HSI). Linear theory has been modified for the baroclinic cases by introducing a parametric model of frontal contraction, according to which the growth rate expressions incorporate model-prescribed, continuously varying shear zone widths. This modified parametric model is found to provide excellent agreement with the growth rates computed from the present simulations, suggesting that HSI can be extended to the baroclinic shear zone cases to a very good approximation over a range of near-surface boundary types.

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