Structure of the Sarandí del Yí Shear Zone
errane (cross
5.5. Quartz CPO patterns
The easternmost outcrops, where both dykes and mylonitic foliation could be measured in the field, were used for the analysis (m^m′ = 75°, x^S1 = 17°, x^m′ = 35°). Therefore, values presented herein represent minimum estimates because deformation increases further to the east, where the mylonitic belt is present but no dykes were recognized (Fig. 5.2).
The obtained geometrical solution and the calculated values are presented in Fig. 5.9b. On the basis of these results, a significant deviation from simple shear is observed, with a 41 % volume decrease and a shear strain of 2.03. However, volume loss should be even higher considering the strain within the dykes themselves, whereas shear strain is heterogeneously distributed and even higher towards the shear zone.
5.4.2. S‐C´ shear bands
A quantitative strain analysis of the Sarandí del Yí mylonites was carried out on the basis of the method of Kurz and Northrup (2008), which allows to estimate vorticity values in thin sections on the basis of the angle between the mylonitic foliation (S) and the maximum inclined shear band orientation (C′). Within high‐strain rocks, the most steeply inclined shear band orientation provides the best estimate of the bulk kinematic vorticity Wk. Consequently, S‐C′ shear band measurements allow characterizing the coaxiality/non‐coaxiality of general shear.
Wk values range between 0.00 and 0.24, considering the maximum inclined shear band orientation for each sample (Fig. 5.10). Since these values indicate a proportion of simple shear lower than 15%, pure‐shear dominated deformation related to sinistral shearing of the shear zone is inferred.
5.5. Quartz CPO patterns
Samples of the mylonites were collected to analyse the CPO of quartz. All samples were obtained from the Sarandí del Yí Shear Zone, except AA‐12, which corresponds to the mylonites of the easternmost Piedra Alta Terrane. Sample locations are presented in Appendix 1.
Chapter
: Rose diagra p (2008). Freq shear band or
ams of synth quency is indi ientation (MIS illian et al.,
etic S‐C′ shea icated in perc SBO). Wk: bul
ar bands orie centage. The full line show orticity, n: nu
ording to the ws the orient
mber of meas
e method of ation of the surements.
Chapter
Fig. 5.11:
Shear Zon foliation distributi in Appen
5
: Quartz CPO ne, except for
(Z) and the on (m.u.d.). C dix 1.
pole figures f r AA‐12 that r lineation (X) Colour scaling
from samples epresents the are marked.
g is from blue
of the mylon e mylonites of Contour lev (minimum) t
Str
nites. All samp f the easternm els are at 0.5 o red (maxim
ructure Saran
ples correspon most Piedra A
5, 1, 2 and 3 um). Sample
ndí del Yí Sh
nd to the Sara Alta Terrane. P 3 multiples o locations are
ear Zone
68 andí del Yí Pole of the of uniform indicated
Chapter 5 Structure Sarandí del Yí Shear Zone
69
5.6. Discussion
5.6.1. Deformation of the eastern Piedra Alta Terrane
On the basis of detailed mapping in the Piedra Alta Terrane, a progressive strain increase towards the east can be identified in its eastern part. This fact is documented by the increase in both the magnitude of the rotation of the dykes and the strain within them and by the presence of mylonites further to the east (Fig. 5.2). The obliquity between the mylonitic foliation and the margins of the belt, S‐C′ shear bands and the clockwise rotation of the dykes point to dextral shearing.
Volume loss (>40%) was associated with the dextral shearing and could result either from metamorphic reactions and/or from vertical extension supported by the scarce subvertical lineations (Fig. 5.2b).
Reaction of pyroxene and plagioclase to hornblende, plagioclase, chlorite, epidote, clinozoisite, muscovite/sericite, biotite, quartz and opaque minerals in the strained dykes indicates maximum amphibolite facies metamorphism during the shearing event. Quartz microstructures observed in the wall‐rock account for grain boundary migration recrystallization, probably at temperatures above 600°C due to the presence of chessboard pattern extinction (Kruhl, 1996; Stipp et al., 2002). Recrystallization and formation of subgrains along the edges of feldspars indicate deformation conditions above ca. 550°C (Voll, 1976; Pryer, 1993). Furthermore, quartz microstructures in the mylonites indicating grain boundary migration recrystallization associated with prism <a> slip detected by CPO patterns point to deformation conditions between 500 and 650°C (Mainprice et al., 1986; Stipp et al., 2002). Therefore, the dextral shearing in the eastern margin of the Piedra Alta Terrane is interpreted to occur under middle to upper amphibolite facies conditions (ca. 600‐650°C).
5.6.2. Deformation of the Sarandí del Yí Shear Zone
Both macro‐ and microindicators are indicative of sinistral shear for the Sarandí del Yí Shear Zone. Likewise, the dominance of protomylonites to the west and ultramylonites to the east as well
Chapter 5 Structure Sarandí del Yí Shear Zone
70
as the concentration of tight to isoclinal folds in the easternmost shear zone indicates a strain increase towards the east.
No switching in the orientation of the lineation with progressive deformation is observed, suggesting flattening with both vertical and lateral extension (Sengupta and Ghosh, 2004). Pure‐
shear dominated deformation with flattening is also indicated by the orientation of folds axes and planes (Jones et al., 2004) and supported by the Wk values obtained by the strain analysis of the S‐C′
shear bands as well as the quartz CPO fabrics (Schmid and Casey, 1986). This is further constrained by strain data from Oyhantçabal et al. (2001), which suggest pure‐shear dominated deformation under magmatic conditions during the emplacement of the synkinematic Solís de Mataojo Granitic Complex. Therefore, pure‐shear dominated deformation with a subordinated sinistral strike‐slip component is interpreted for the Sarandí del Yí Shear Zone.
The abundance of core and mantle structures in feldspars with new grains sometimes also within internal microshear zones accounts for dislocation climb and recrystallization of feldspars at temperatures of 450‐550°C (Passchier and Trouw, 2005, and references therein). Quartz microstructures denote dominant diffusion processes and recrystallization due to grain boundary migration, also indicated by quartz CPO data. However, local evidence of subgrain rotation recrystallization, particularly in folds, may support a late activity of this mechanism; hence, deformation conditions probably occurred at grain boundary migration recrystallization conditions near the transition with the subgrain rotation regime (450‐550°C; Stipp et al., 2002). On the other hand, formation of synkinematic white mica is interpreted to have taken place due to the breakdown of feldspar, which points to temperatures below 500°C (Wiberley, 1999, and references therein).
Deformation conditions during sinistral shearing can thus be constrained to lower amphibolite to upper greenschist facies conditions (ca. 550‐450°C). Lastly, a significant cataclastic reworking is observed in the Sarandí del Yí Shear Zone. The mineral association indicates greenschist to subgreenschist conditions during this event.
Chapter 5 Structure Sarandí del Yí Shear Zone
71
5.6.3. Structural evolution of the Sarandí del Yí Shear Zone
Heterogeneous shear zones are characterized by lateral variations of strain parameters, generally with strain increase towards the core of the shear zone. However, long‐lived shear zones may show not only spatial but also temporal variations of strain. Shear zones can thus be classified based on thickness variation with time (Means, 1984, 1995; Hull, 1988; Horsman and Tikoff, 2007;
Vitale and Mazzoli, 2008). Increasing and decreasing thickness may be interpreted in terms of strain hardening and softening, respectively, that in turn can result from the combination of different deformation conditions and processes.
Since deformation of the eastern margin of the Piedra Alta Terrane seems to be related to the evolution of the Sarandí del Yí Shear Zone, a polyphase evolution can be interpreted (Fig. 5.12).
Cross‐cutting relationships indicate retrograde conditions (e.g., mylonitic protoliths in the cataclasites) associated with progressive strain localization towards the east. In this context, a first phase of dextral shearing took place under middle to upper amphibolite facies conditions (Fig.
5.12a). A subsequent sinistral event under lower amphibolite to upper greenschist facies conditions reworked the eastern margin of the shear zone giving rise to the development of the Sarandí del Yí mylonitic belt (Fig. 5.12b). During this stage, more localized strain in the easternmost part of the shear zone gave rise to the development of late folded mylonitic foliation. This high strain zone was then overprinted by cataclasis (Fig. 5.12c).
This evolutionary model is thus comparable with the conceptual model of Sibson (1977, 1983) and natural examples such as the Nordre Strømfjord Shear Zone (Bak et al., 1975), the Great Slave Lake Shear Zone (Hanmer, 1988) and the Imbricate Zone and the Stortrømmen Shear Zone (Smith et al., 2007), among others. Consequently, the Sarandí del Yí Shear Zone represents an example of shear zone with thickness decreasing in time that evolved from upper amphibolite to subgreenschist facies conditions and records a kinematic switch from dextral to sinistral shear at ca.
600‐550°C.
Chapter ed to progre on of the sh . Schematic s
ation of defo
he Sarandí del strain distribu ution is indica
the Sarandí the zone wit c that was o
Str
ne related to p ated with the
del Yí Shear
th the highe only preserve
ructure Saran
progressive st grey scale (d
r Zone under
Chapter 5 Structure Sarandí del Yí Shear Zone
73
domains may have experienced only partial annealing but no significant deformation during retrograde conditions, as schematized by Herwegh et al. (2008).
5.7. Conclusions
Integration of macro‐ to microstructural data indicates that the Sarandí del Yí Shear Zone represents a crustal‐scale shear zone that underwent complex long‐term deformation. During juxtaposition of the Piedra Alta (Río de la Plata Craton) and Nico Pérez terranes, deformation in the Sarandí del Yí Shear Zone started under upper to middle amphibolite facies conditions with dextral shearing in the easternmost Piedra Alta Terrane. Subsequent lower amphibolite‐upper greenschist facies metamorphism was related to pure‐shear dominated sinistral shearing, which was accompanied by contemporaneous plutonism reflected by the Solís de Mataojo Granitic Complex.
Post‐mylonitization magmatism is indicated by the Cerro Caperuza granite. Afterwards, a late cataclastic event took place and reworked the easternmost border of the shear zone.
The evolution of the shear zone reflects progressive strain localization under retrograde metamorphic conditions during crustal exhumation. Furthermore, strain localization also resulted in decrease in the shear zone thickness.
Chapter 6 Geochronology Sarandí del Yí Shear Zone
74