Microfabric diversity and grain shape analysis of fault rocks from the selected areas of the Western Tatra Mountains

Abstract

1 In sti tute of Geo log i cal Sci ences, Jagiellonian Uni ver sity, Oleandry 2a, 30-063 Kraków, Po land Kania M. (2014) Microfabric di ver sity and grain shape anal y sis of fault rocks from the se lected ar eas of the West ern Tatra Moun tains. Geo log i cal Quar terly, 58 (1): 3-18, doi: 10.7306/gq.1129 Fault rocks in brit tle and brit tle-duc tile shear zones played a key role in the evo lu tion of the West ern Tatra Moun tains crys tal-line rocks (Po land-Slovakia). Microfabrics of these rocks, in clud ing grain shape anal y ses, were in ves ti gated in the six ar eas of the West ern Tatra Moun tains. Based on stud ies of thin sec tions, 14 types of fault rock microfabric are dis tin guished, accord ing to the fol low ing cri te ria: (a) the pres ence and abun dance lev els of a cataclastic ma trix and (b) the pres ence and form of a pre ferred ori en ta tion fea tures. Gen eral ten den cies ob served in these ar eas in di cate south wards in creas ing non-co ax ial de for ma tion as well as the dom i na tion of ultracataclasites or ultramylonites to phyllonites in ar eas with neg a tive re lief (e.g., sedlo Zabrat’ Pass, Dziurawa Prze³êcz Pass). A model of shear zone evo lu tion em brac ing fol low ing three stages is proposed: (1) de for ma tion par ti tion ing and block-con trolled cataclastic flow, (2) ma trix-con trolled cataclastic flow, (3) se lec tive leach ing and de po si tion of sil ica, lead ing to the for ma tion of soft ened and hard ened de for ma tion do mains re spec tively. These microstructural ob ser va tions were sup ported by sta tis ti cal anal y ses of the grain shape in di ca tors (com pact ness, isometry, el lip tici ty, so lid ity, con vex ity). Two trends of re la tion ships be tween com pact ness and con vex ity were noted: the first, hor i zon tal on the cor re la tion di a grams, was in ter preted as an ef fect of rapid cataclasis and then sericitization, the second , with a strongly neg a tive cor re la tion co ef fi cient, was con sid ered as an ef fect of long-term cataclastic flow. The dif fer ent microfabric data and microstructural in ter pre ta tions de scribed in this pa per are con sis tent with a new model of the tec tonic his tory of the West ern Tatra Moun tains evo lu tion, with an im por tant role for a non-co ax ial de for ma tion dur ing Al pine orog eny in brit tle and brit tle-duc tile con di tions.

Microstructures of shear zones from selected domains of the Western Tatra Mountains

“…The duc tile shear zones were later re ac tivated, how ever, new brit tle and brit tle-duc tile shear zones were also cre ated form ing a com plex lithological struc ture. The rate of de for ma tion was dif fer ent in dif fer ent parts of the com plex (Kania, 2014). Brit tle re work ing of the older duc tile struc tures could lead to the for ma tion of tec tonic melange zones, how ever, in most cases it seems that the older dis con ti nu ities were the ba sis for sim i larly (but not ex actly the same) ori ented brit tle shear zones.…”

“…This was de ter mined dur ing im age anal y ses of the mo sa ics of thin sec tion pho tos, with no less than 50 quartz and feld spar grains mea sured in 13 sam ples rep re sent ing dif fer ent rock types (the same pho tos which were used for morphometry in Kania, 2014). Then, the sta tis ti cal con cen tra tion co ef fi cient was de ter mined for each of the sam ples with the fol low ing for mula (Krawczyk and S³omka, 1994 where: L -con cen tra tion co ef fi cient, j -grain rel a tive ori en ta tion, n -num ber of grains ana lysed.…”

“…Sim i lar co ef fi cients are ob served in most of cataclasites, however, the pres ence of monoclinic struc tures (S-C cataclasites) makes the co ef fi cient higher (L > 50%). The high est val ues are The tex tural types are de fined in Kania (2014) Ta ble 4…”

“…This pa per is sub jected to the meso-and, es pe cially micro struc tures. Ap ply ing these two meth ods: grain shape sta tis tics (Kania, 2014) and meso-and micro struc tures de scrip tions (this pa per), these two pa pers pro vide a new descrip tion of the shear ing-re lated rock fab ric in the West ern Tatra Mts., which was also pre sented in Kania's (2012) Ph.D. the sis. The shape-pre ferred ori en ta tion mea sure ments pub lished in this pa per form a link be tween a morphometric ap proach presented ear lier and struc ture de scrip tions pre sented now.…”

“…crys tal line core. This is a con tin u a tion and supplementation of the pa per by Kania (2014). That ear lier pa per was fo cused on tex tural fea tures (i.e.…”