Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

<p>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.</p

Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone

International audienceFault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging‐wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP‐2). We present observational evidence for extensive fracturing and high hanging‐wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP‐2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging‐wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off‐fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation

Os plut?ns Fel?cio e Merc?s: registros da orog?nese Riaciana-Orosiriana no bloco Guanh?es

?rea de concentra??o: Geologia Regional.Texto da obra em outro idioma: (Ingl?s) p. 17-64.Palavras-chave retiradas da Ficha Catalogr?fica.As rochas gran?ticas que ocorrem na regi?o norte do bloco Guanh?es - um dos blocos do embasamento presentes no Or?geno Ara?ua? - foram pioneiramente investigadas no in?cio do s?culo XX, e desde ent?o eram consideradas como parte de um ?nico corpo ?gneo, identificado como Bat?lito Rio Itangu? nos mapas geol?gicos regionais. No ?mbito deste projeto de mestrado, investiga??es de campo e an?lises petrogr?ficas e geof?sicas subsidiaram a subdivis?o dessas rochas em dois diferentes pl?tons - nomeados como Fel?cio e Merc?s - bem como uma revis?o de sua ?rea de ocorr?ncia. O Pl?ton Fel?cio cont?m granodioritos mesocr?ticos de granula??o fina, com a mineralogia principal composta por biotita, plagiocl?sio, K-feldspato, epidoto e quartzo, e allanita, apatita, titanita e zirc?o como minerais acess?rios. Essas rochas s?o metaluminosas, de baixo pot?ssio, magnesianas e c?lcicas a calc-alcalinas, para as quais foi obtida uma idade de conc?rdia U-Pb em zirc?es de 2151 ? 12 Ma. O Pluton Merc?s, por sua vez, cont?m monzogranitos e sienogranitos de granula??o fina a m?dia, leucocr?ticos e com variedades porfir?ticas e equigranulares. Os principais minerais constituintes dos monzogranitos s?o biotita, muscovita, plagiocl?sio, K-feldspato e quartzo, e dos sienogranitos s?o quartzo, K-feldspato, hornblenda e plagiocl?sio. Essas rochas s?o ricas em pot?ssio, peraluminosas, ferroanas e alc?licas a ?lcali-c?lcicas, cujos dados apresentaram uma disc?rdia com interceptos superior de 2014 ? 11 Ma e inferior de 539 ? 13 Ma, interpretados como idade de cristaliza??o e metamorfismo, respectivamente. Apesar de terem assinaturas geoqu?micas e geocronol?gicas distintas, os pl?tons Fel?cio e Merc?s apresentam algumas caracter?sticas comuns, como o enriquecimento em LILE, ETR leves e elementos HFS altamente incompat?veis, al?m de anomalias negativas de Ti, Ta, Nb, e P, o que pode indicar uma fonte comum para os magmas que deram origem aos dois pl?tons. As idades de cristaliza??o dos plutons s?o compat?veis com o evento orog?nico Paleoproteroz?ico que amalgamou o paleocontinente S?o Francisco-Congo, que apresenta registros dentro do cr?ton S?o Francisco e em blocos do embasamento contidos no Or?geno Ara?ua?. O Pl?ton Fel?cio ? atribu?do ao est?gio acrescion?rio do evento, apresentando poss?vel correla??o com o Arco Mantiqueira. Por sua vez, o Pl?ton Merc?s est? relacionado ao est?gio tardi a p?s-colisional da orogenia, contempor?neo ao processo de colapso gravitacional da cadeia de montanhas. Sugere-se que a cunha mant?lica metassomatizada por fluidos provenientes da desidrata??o da litosfera subductada tenha atuado como fonte para ambos os pl?tons, ao passo que a espessa crosta continental resultante da colis?o parece ter atuado como fonte secund?ria do magma que originou o Pl?ton Merc?s. Pela primeira vez, registros da orogenia Paleoproteroz?ica s?o relatados no bloco Guanh?es. Essas informa??es contribuem para o aumento do nosso conhecimento de processos tect?nicos que levaram ? agrega??o do bloco continental S?o Francisco-Congo, uma pe?a chave no cen?rio Pr?-Cambriano global.Universidade Federal dos Vales do Jequitinhonha e Mucuri (UFVJM)Disserta??o (Mestrado) ? Programa de P?s-gradua??o em Geologia, Universidade Federal dos Vales do Jequitinhonha e Mucuri, 2019.The granitic rocks that occur in the northern region of the Guanh?es block, one of the basement inliers present in the Ara?ua? orogen, were first studied in the beginning of the 20th century, and, since then, are considered part of a single igneous body mapped as Rio Itangu? batholith in the regional geologic maps. In the present study, field investigations, petrographic and geophysical analyses subsidized the subdivision of the rocks in two different plutons, named Fel?cio and Merc?s, and a review on their occurrence area. Fel?cio pluton is composed of mesocratic fine-grained granodiorites with the main mineralogy consisting of biotite, plagioclase, K-feldspar, epidote and quartz, and allanite, apatite, titanite and zircon as accessory minerals. These rocks are low-K, magnesian, calcic to calc-alkalic and metaluminous, and yielded a U-Pb zircon crystallization age of 2151 ? 12 Ma. Merc?s pluton is composed of equigranular and porphyritic mediumgrained leucocratic monzogranites and syenogranites. The monzogranites have biotite, muscovite, plagioclase, K-feldspar, quartz as their main mineralogy, and the same accessories as the granodiorites from Fel?cio pluton, and the syenogranites are composed of quartz, k-feldspar, hornblende and plagioclase. Merc?s pluton rocks are rich in K, ferroan, alkali-calcic to alkalic and peraluminous, and yielded a discordia upper intercept age of 2014 ? 11 Ma and a lower intercept at 539 ? 13 Ma, interpreted as the crystallization age and the metamorphic overprint, respectively. Although the plutons have different geochemical and geochronological signatures, they also have common points such as the enrichment in LILE, LREE and highly incompatible HFSE and negative anomalies of Ti, Ta, Nb, and P, which indicates a common source for the magmas that originated the two plutons. The emplacement ages of the plutons are coeval with the Paleoproterozoic orogenic event that assembled the S?o Francisco-Congo paleocontinent, which left many records both inside the S?o Francisco craton and in basement inliers within the Ara?ua? orogen. Fel?cio pluton is correlated to the accretionary stage of the event, presenting a possible correlation with Mantiqueira arc. In turn, Merc?s pluton is related to the late- to post-collisional stage of the orogeny, coeval with the gravitational collapse of the mountain belt. It is suggested that the mantle wedge metasomatized by fluids from slab dehydration acted as a source for the magmas that generated both plutons, whereas the continental crust thickened by the collision of the blocks seem to have been a secondary source for the generation of Merc?s pluton. For the first time, records of the Paleoproterozoic orogeny are reported in the Guanh?es block. These findings help to enhance our knowledge on the tectonic processes that led to the assembly of the S?o Francisco-Congo paleocontinent, a key piece in the world?s Precambrian scenario

Extreme hydrothermal conditions at an active plate-bounding fault

International audienceTemperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes1. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre2, 3. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades4, 5. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

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

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

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

Extreme hydrothermal conditions at an active plate-bounding fault

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300-450 degrees Celsius, usually found at depths greater than 10-15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional-mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults