Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand
Virginia ToyR. SutherlandJohn TownendMichael J. AllenLeeza BecroftAustin BolesCarolyn BoultonB. M. CarpenterAlan F. CooperSimon C. CoxChristopher DaubeD. R. FaulknerAngela HalfpennyNaoki KatoStephen KeysMartina KirilovaYusuke KometaniTimothy A. LittleElisabetta MarianiBenjamin MeloshC.D. MenziesLuiz F. G. MoralesChance MorganHiroshi MoriAndré NiemeijerRichard J. NorrisDavid J. PriorKatrina SauerAnja M. SchleicherNorio ShigematsuD.A.H. TeagleHarold TobinRobert ValdezJack WilliamsSamantha YeoLaura‐May BaratinNicolas C. BarthAdrian BensonCarolin BoeseBernard CélérierC. J. ChamberlainRonald ConzeJamie CoussensLisa CrawMai‐Linh DoanJ. D. EcclesJason GrieveJulia GrochowskiAnton GulleyJamie HowarthKatrina JacobsLucie Janků-ČápováT.N. JeppsonRobert LangridgeDeirdre MallyonRay MarxCécile MassiotLoren MathewsonJo MooreOsamu NishikawaBrent PooleyAlex PyneM. K. SavageDoug SchmittSam Taylor‐OffordPhædra UptonKonrad WeaverThomas WiersbergMartin Zimmer
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Abstract:
During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.Keywords:
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Bedrock
In the paper,by the field profile observation and microscopic deformation structure study of north-east shear zone of Zhushanchong-Moshan gold ore field in Tianzhu county,it's thought that the founded rock mainly are primary cataclasite,cataclasite and ultracataclasite,the mylonite rock and the primary mylonitr of mylonite series is only founded in part areas,so the conclusion is this shear zone is a brittle shear zone,some is brittle-ductile shear zone,the ductile zone isn't founded.The known ore-bearing structure of the gold deposit is brittle-ductile shear zone structure.
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Abstract Late Gothian (c. 1.58 Ga) and Sveconorwegian (1.1–0.9 Ga) structures outline a 35 km long, NNE‐oriented, open gneiss synform in the Varberg‐Horred region of SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major shear zone and tectonic boundary, the Mylonite Zone, forms a branching shear zone system which converges in the eastern part of the synform. A subdivision between the Gothian and Sveconorwegian events is made by using the intervening anorogenic intrusions as structural markers. This, and the non‐recognition of a previously assumed orogenic event, results in a geodynamic model which is similar for the crustal segments on both sides of the largely N‐S trending Mylonite Zone, except for the higher grade Sveconorwegian metamorphism to the east. The evolution is characterised by one or more major Gothian gneiss‐forming events, followed by intermittent anorogenic magmatism and a later Sveconorwegian development that, outside discrete shear zones, gave rise to moderate fabric‐forming deformation and only localised formation of migmatitic leucosomes. The final Gothian orogenic episode at c. 1.58 Ga and three distinct anorogenic events between 1.51 and 1.20 Ga are correlated across the Mylonite Zone, thus supporting models where the Mylonite Zone constitutes an intracratonic Sveconorwegian shear zone. The Sveconorwegian development is interpreted to include eastward thrusting on the Mylonite Zone, followed by dominantly static metamorphism prior to 0.98 Ga, due to the thickened crust. Subsequent uplift and rapid cooling preserved granulite‐facies assemblages in the southern Eastern Segment. Late Sveconorwegian extensional movements occurred until c. 0.92 Ga along the largely west‐dipping Mylonite Zone system. Åhäll, K.‐L, 1995: Crustal units and role of the Mylonite Zone system in the Varberg‐Horred region, SW Sweden. GFF, Vol. 117 (Pt. 4, December), pp. 185–198. Stockholm. ISSN 1103–5897.
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ABSTRACT This guide begins with an overview of the internal structure and petrology of the Catalina Schist terrane as exposed on Santa Catalina Island, California, followed by a discussion of the tectonic setting and exhumational history of the terrane, and the Cenozoic tectonic and geological evolution of the Inner Borderland, within which it lies. The guide then presents an itinerary for a three-day field trip from 9–11 May 2020. Next, we present a tectonic model for the formation of the Catalina Schist, followed by a discussion of its relationship to the Pelona, Orocopia, Rand, and related schists in southern California. This field trip generally follows the GSA guide published in GSA Field Guide 59 (available at https://pubs.geoscienceworld.org/gsa): Platt, J.P., Grove, M., Kimbrough, D.L., and Jacobson, C.E., 2020, Structure, metamorphism, and geodynamic significance of the Catalina Schist terrane, in Heermance, R.V., and Schwartz, J.J., eds., From the Islands to the Mountains: A 2020 View of Geologic Excursions in Southern California: Geological Society of America Field Guide 59, p. 165–195, https://doi.org/10.1130/2020.0059(05).
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Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island.
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The density contrast between a deeply weathered schist and a marble which contains a small percentage less than 5% of clay-filled solution channels allows differentiation between the two bedrock units with a gravity survey. Gravity highs lie over the marble which, even with the solution channels, is more dense than the schist. Because of the solution channels, the marble presents a less suitable bedrock foundation for large bearing loads. The bedrock on the campus of the University of California at Santa Cruz consists of marble and schist in complex contact with each other. Application of the gravity method to a building site on the campus indicates that in special cases a gravity survey is an economic and accurate first step in providing information regarding the foundation suitability of bedrock at a building site while in the planning stages.
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ABSTRACT This guide begins with an overview of the internal structure and petrology of the Catalina Schist terrane as exposed on Santa Catalina Island, California, followed by a discussion of the tectonic setting and exhumational history of the terrane, and the Cenozoic tectonic and geological evolution of the Inner Borderland, within which it lies. The guide then presents an itinerary for a three-day field trip from 9–11 May 2020. Next, we present a tectonic model for the formation of the Catalina Schist, followed by a discussion of its relationship to the Pelona, Orocopia, Rand, and related schists in southern California.
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Since the Dingjie Area lies in the middle of Himalayan Orogen Belt in the south of Qinghai Tibet Plateau, it is the key place to study the development and uplifting of Himalaya. In the area, there form many kinds of ductile shear zones defined by their orientation, scale and stage. Within the deformed rocks in these ductile shear zones, there develop so abundant deformational textures such as larsen lineation,rotational porphyroclast,S C fabrics, domino structure, and so on, which shows the deformation of rocks in shear zones mainly resulted from the ductile deformation mechanism. And the deformed rocks are granite mylonites, feldspar quartz mylonites and super mylonite which were formed in the ductile shear zone. Temperature for deformation of ductile shear zone ranges from 208℃ to 559℃.
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This paper describes microstructures of the mylonite developed in Xilinhe region, discusses gold mineralizing with respect to NW Ductile Shear Zone. The author indicates that NW Ductile Shear Zone controls the distribution, occurrence and magnitude of gold mineralizing in Xilinhe region.
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The near NS ductile shear zone was first discovered in Proterozoic Bendong granodiorite pluton. It dips toward 240°～280° at the dip angle of 40°～63°. The lineations, which represent the shear direction, plunge to 216°～226° at the plunge angle of 39°～46°. The kinematics of Bendong ductile shear zone is characterized by sinistral-normal shearing, and shows a sliding from NE to SW. Typical granitic mylonites were developed in the ductile shear zone and clearly show a zoning from phyllonite through mylonite to initial mylonite from center to both walls. The discovery of Bendong ductile shear zone and mylonite indicates an important structural event superimposed on Proterozoic Bendong pluton.
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The eastern margin of the Chitradurga schist belt is marked by a NNW-SSE trending sub vertical crustal scale ductile shear zone. The kinematic indicators indicate a predominant sinistral sense of strike-lip movement along the shear zone. Syntectonically emplaced granitic rocks are converted to mylonites and ultramylonites as a result of crystalplastic deformation in the shear zone. In contrast, there are localized zones of brittle failure with attendant functional heat generation exemplified by the development of thin but conspicuous bands and veins of pseudotachylytes, which are emplaced either subparallel with or transgressing the C-planes of the mylonites. From our field and petrographic studies it is interpreted that these two coexisting rock types, namely the mylonite and pseudotachylyte, which are the results of contrasting deformational mechanisms, have generated near synchronously in a progressively developed ductile shear zone. The pseudotachylytes represent the brief interlude of sudden increase in strain rate in an overall ductile regime.
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