deformation paths in the shallow levels of an accretionary prism the eocene shimanto belt of southwest japan detailed structural analysis of coherent turbidites in a part of the eocene shimanto belt on the muroto peninsula shikoku island japan reveals a structural evolution that is typical of the shallow levels of accretionary prisms in style and sequence of deformational fabrics but anomalous in that it includes both a marked rotation in shortening direction and a major out of sequence thrust deformation throughout a history of progressive lithification and burial was dominated by subhorizontal shortening and seaward vergent shear structures evolved from predominantly mesoscale thrusts and folds to larger more brittle faults and finally to kilometer scale amplified fault bend folds with an associated pressure solution cleavage this structural progression is consistent with increasing strength of the rocks decreasing fluid pressures and increasing temperature and pressure as the sediments were dewatered lithified and accreted to the prism in contrast to this straightforward structural evolution the prism scale deformation of the eocene shimanto belt is difficult to reconcile with simple models of prism developmentthat assume orthogonal subduction and seaward stepping imbrication a 50 counterclockwise rotation in the shortening direction and a major out of sequence or intraprism thrust are recorded by two distinct structural domains in both the earliest structures are clastic dikes and faults indicative of north south shortening one domain is characterized by east striking regional scale folds and cleavage that are transected by an east northeast trending cleavage however the other is characterized by northeast striking folds and cleavage that are transected by a slightly more north striking cleavage the different deformation histories of the two do mains require that they be separated by a major intraprism fault this fault carries tectonic melange containing blocks of pillow basalt and chert indicating that it rooted in the decollement zone beneath the prism the tectonostratigraphy and deformation histories of the two domains suggest that both before and after the out of sequence fault developed higher structural levels of the prism experienced significant subhorizontal shortening whereas lower structural levels bypassed this zone of deformation

paleontologic evidence for complex tectonic interlayering of mississippian to permian deep water rocks of the golconda allochthon in tobin range north central nevada a detailed paleontologic study in the hoffman canyon area was undertaken to determine the degree of tectonic interlayering in a thick succession of largely deep water chert argillite and silty limestone composing the havallah sequence of the mesozoic golconda allochthon of western and north central nevada the study focused on the type area of the havallah formation which as originally defined was considered to be largely a conformable sequence of permian rocks the results indicate that the havallah sequence in the hoffman canyon area consists of a complex tectonic accumulation characterized by repetition of mississippian pennsylvanian and permian rocks in at least 12 superposed thrust plates such a style of deformation may be characteristic of much of the golconda allochthon although paleontologic evidence of this style is clear in only a few areas and is probably best defined in the hoffman canyon area detailed stratigraphic and paleontologic studies of the havallah sequence at a few other localities in nevada indicate considerable lithologic variability of comparable age rocks in the sequence from area to area indicating a complex depositional setting that at present is only partly understood

offscraping accretion of jurassic chert clastic complexes in the mino tamba belt central japan abstract detailed structural and biostratigraphical analysis of the jurassic inuyama sequence a coherent chert clastic complex in the mino tamba belt central japan clarifies the evolution of accretionary processes at shallow structural levels the inuyama sequence is characterized by a series of stacked thrust sheets each sheet consists of an early triassic to middle jurassic oceanic plate stratigraphy composed of four lithologic units which are in ascending order siliceous claystone ribbon chert siliceous mudstone and clastic rocks the structural features of the inuyama sequence demonstrate a four stage progressive deformation 1 a decollement was initiated within the siliceous mudstone when this sequence was just seaward of the deformation front clastic dikes and sills formed at the horizon just above the decollement at this time 2 the stratigraphic section above the decollement was imbricated by in sequence thrusting in the frontal part of the wedge this initial stage of thrusting and imbrication was followed by 3 the formation of duplex structures with fault related folds within the lower stratigraphic section as the decollement stepped down section to the lowest siliceous claystone interval finally 4 these thrust packages were overprinted by secondary prism thickening in the form of out of sequence thrust faulting

low latitude and southern hemisphere origin of anisian triassic bedded chert in the inuyama area mino terrane central japan paleomagnetic and paleontologic studies of middle triassic to earliest jurassic radiolarian bedded chert in the inuyama area southern mino terrane of central japan have shown that the chert was deposited in an equatorial region with a paleolatitude between 10 n and 10 s and in particular the lower to middle anisian chert depositional basin was situated in the southern hemisphere the chert has four characteristic components of remanent magnetization the highest blocking temperature component carried by hematite shows positive fold and reversal tests and is regarded as the primary remanent magnetization acquired just after deposition the other three components are interpreted as a present field overprint and two postfolding remagnetizations the absolute tilt corrected inclinations of the primary component are less than 20 for all the samples of middle triassic to earliest jurassic age with the mean inclination of the lower to middle anisian chert being 11 0 4 3 paleomagnetic polarity reversals observed for the lower to middle anisian chert are well correlated with those of the european triassic sections under paleontologic age constraints using radiolarians and conodonts provided that the anisian chert in inuyama was deposited in the southern hemisphere this conclusion illuminates the paleogeographic evolution of the chert bearing accretionary complexes not only in japan but along the eastern margin of asia as well

development of actual water repellency in a grass covered dune sand during a dehydration experiment water repellency is a soil property which temporally changes in severity and is most pronounced when soils are dry in the past numerous researchers air or oven dried soil samples in the laboratory to determine the potential water repellency however measurement on air and oven dried samples can overestimate as well as underestimate the conditions occurring in the field under prolonged drought conditions to reveal and determine realistic and potentially the highest persistence we studied the influence of dehydration upon repellency in a dune sand grassland between 11 april and 5 september 2002 by artificially sheltering the soil the shelter was built to protect the soil from getting wetted by precipitation during this period the soil was sampled eight times in vertical transects over a distance of 75 cm to a depth of 33 cm during the dehydration period on each sampling date soil water contents were measured and the persistence or stability of actual water repellency was determined in 120 field moist samples collected at 8 depths at the start of the dehydration experiment the mean volumetric soil water contents in the transect varied between 5 and 10 5 at the end of the dehydration process the grass cover was wilted and yellow brown and the soil profile dried up to volumetric water contents between 1 and 2 5 during the dehydration period the average water storage in the upper 33 cm of the soil total of 8 layers decreased from 22 mm to less than 5 mm slightly water repellent and wettable dune sand layers at depths of 7 to 19 cm on 11 april were found to be altered into extremely water repellent soil within eight days of dehydration the most extreme soil water repellency with water drop penetration times of more than 6 h was detected in large parts of the five soil layers sampled between depths from 9 5 to 33 cm on 4 june 11 july and 5 september 2002 the maximal water repellency was found to be evidently less in the organic rich surface layer in comparison with the organic poor deeper layers for each soil layer the relation between soil water content and actual water repellency was determined this resulted into three distinguished zones a a water repellent zone b a transition zone and c a wettable zone the threshold values of the volumetric water content that describe the transition zone varied per depth

well documented travel history of mesozoic pelagic chert in japan from remote ocean to subduction zone the mino tanba belt in southwest japan a segment of the cordilleran type orogenic chain of jurassic east asia is composed mainly of a middle upper jurassic subduction accretion complex in which triassic and lower jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with middle upper jurassic clastic rocks high resolution microfossil conodont and radiolaria research has identified very low average sedimentation rates of about 0 5 g cm 1000 yr in the chert units similar to those of modern pelagic sediments accumulated in open ocean environments judging from the low average sedimentation rate high purity of biogenic silica long duration of continuous deposition 50 m y and wide along strike extent 1000 km the bedded radiolarian cherts in the mino tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments accordingly the alleged origin in smaller marginal basins is untenable upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench the tectonic interlayering of these cherts and coarse grained terrigenous elastics is a secondary feature that was added through duplexing underplating in the subduction zone on the basis of the primary stratigraphy and field occurrence of triassic bedded chert in the mino tanba belt newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a cordilleran type bedded chert from its birth at a mid oceanic ridge to its demise at a subduction zone

silica diagenesis ii general mechanisms abstract amorphous silica phases opal a precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions opal a dissolves and yields solutions of still relatively high silica content in pore waters containing abundant cations open framework polymers form which flocculate to yield opal ct opal ct becomes increasingly ordered primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase opal ct dissolves to yield pore waters of low silica concentration which allows slow growth of quartz crystals from monomeric solution the quartz crystals then slowly increase in size and crystallinity carbonates appear to enhance opal ct formation possibly due to the activity of positively charged hydroxyl complexes hence polymerization in relatively pure systems is involved in opal a formation and polymerization in impure systems is involved in opal ct formation and slow growth from monomeric low silica concentration solutions is involved in precipitation of quartz in sedimentary realms

tectonics and hydrogeology of accretionary prisms role of the decollement zone abstract at convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony by changes in stress orientation and by a discontinuity in plate velocity the decollement zone initiates in a weak sediment layer typically a low permeability hemipelagic mud lying below a more rapidly deposited stronger more permeable trench turbidite sequence at the deformation front trench turbidites tend to be offscraped whereas the finer grained hemipelagic and pelagic sediments are more likely to be underthrust offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried undergo high pressure metamorphism and are more likely to be preserved in the stratigraphic record burial rates associated with underthrusting are high exceeding 20 km my with high rates of underthrusting sediment descending below the decollement zone is probably buried faster than it can dewater resulting in emplacement of relatively high porosity deposits at depth fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources the episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it the thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence

recognition of trench fill in orogenic flysch sequences ancient trench fill deposits within erogenic eugeosynclinal sequences are best recognized by their stratigraphic position above older hemipelagic and pelagic deposits such coarsening upward sequences reflect migration of an abyssal site toward and ultimately into the trench axial transport and continuity of turbidites may help to better define trench geometry and sedimentation patterns but alone are not strong evidence for trench sedimentation likewise sandstone petrography because it is influenced by such variables as tectonic and geologic peculiarities of the source area and transport of detritus from distant terrenes is not a useful tool for recognition of trench deposits melanges and broken formations are best considered as evidence for gravity transport of material onto the trench floor or tectonic deformation of trench fill when they are part of a coarsening upward sequence

a new type of active margin the convergent extensional margin as exemplified by the middle america trench off guatemala the results of dsdp legs 67 and 84 during which a transect of holes was drilled across the middle america trench off guatemala show no accretion at the front of that subduction zone since early eocene time the tectonic evolution of the trench includes extensional structure on the landward as well as the seaward trench slopes the guatemalan margin is proposed as the model of a new type of active margin the convergent extensional active margin ce margin

mid crustal horizontal shear zone in the forearc region of the mid cretaceous sw japan arc inferred from strain analysis of rocks within the ryoke metamorphic belt abstract the strain geometry of a cretaceous low p high t metamorphosed accretionary complex in sw japan is one of the key geological constraints in understanding the cretaceous tectonics of the forearc region at the eastern margin of eurasia we studied the strain geometry of upper greenschist to lower amphibolite facies metamorphic rocks from the mid cretaceous low p high t ryoke metamorphic belt which during the mid cretaceous were located at mid crustal depths close to the volcanic front strain analysis focused on deformed radiolarian fossils in metachert and sandstone chert clasts within metapelite the maximum stretching direction x direction of the strain ellipsoid of the schistosity forming deformation at the peak metamorphism is oriented e w and the xy plane of the strain ellipsoid developed parallel to schistosity the metachert and metapelite exhibit plane strain to general flattening strain that probably largely resulted from the schistosity forming deformation based on the results of the previous strain analyses and this study it reveals that the schistosity forming deformation occurred under bedding normal shortening resulting in general flattening strain throughout the ryoke metamorphic belt the x direction of the strain ellipsoid of the schistosity forming deformation is oriented parallel to the length of the ryoke metamorphic belt throughout the sw japan arc the schistosity formed parallel to bedding planes that developed horizontally in non metamorphosed accretionary complexes of sw japan indicating horizontal xy plane of the strain ellipsoid these observations indicate the occurrence of horizontal shear deformation within the middle crust 15 km depth at the time of the low p high t metamorphism the deep seated horizontal shear zone and the shallower incipient median tectonic line mtl fault system could result in a crustal scale detachment fault in the forearc region of the mid cretaceous sw japan arc after the formation of the detachment the left lateral pull apart forearc basins onogawa izumi group distributed on the north side of the mtl and the southern marginal mylonite zone might be formed by the sinistral movement along the detachment fault

ocean plate stratigraphy in east and southeast asia abstract ancient accretionary wedges have been recognised by the presence of glaucophane schist radiolarian chert and melange recent techniques for the reconstruction of disrupted fragments of such wedges by means of radiolarian biostratigraphy provide a more comprehensive history of ocean plate subduction and successive accretion of ocean floor materials from the oceanic plate through offscraping and underplating reconstructed ocean floor sequences found in ancient accretionary complexes in japan comprise from oldest to youngest pillow basalt limestone radiolarian chert siliceous shale and shale and sandstone similar lithologies also occur in the melange complexes of the philippines indonesia thailand and other regions this succession is called ocean plate stratigraphy ops and it represents the following sequence of processes birth of the oceanic plate at the oceanic ridge formation of volcanic islands near the ridge covered by calcareous reefs sedimentation of calcilutite on the flanks of the volcanic islands where radiolarian chert is also deposited deposition of radiolarian skeletons on the oceanic plate in a pelagic setting and sedimentary mixing of radiolarian remains and detrital grains to form siliceous shale in a hemipelagic setting and sedimentation of coarse grained sandstone and shale at or near the trench of the convergent margin radiolarian biostratigraphy of detrital sedimentary rocks provides information on the time and duration of ocean plate subduction the ages of detrital sediments becomes younger oceanward as younger packages of ops are scraped off the downgoing plate ops reconstructed from ancient accretionary complexes give us the age of subduction and accretion direction of subduction and ancient tectonic environments and is an important key to understanding the paleoenvironment and history of the paleo oceans now represented only in suture zones and orogenic belts

structural analysis of the shimanto accretionary complex kyushu japan based on foraminiferal biostratigraphy the paleogene hyuga group in the shimanto supergroup southern kyushu japan is lithologically divided into four formations the aradani tashiro unama and ouchibaru these formations are characterized by imbricate structure and each formation is repeatedly sliced and distributed by thrusts dipping to the north landward the mudstone unit of the tashiro formation is composed mainly of massive mudstones with thin sandstone interbeds it occurs in seven belts of which the shiitani nagano and unama belts have larger widths than others based on a biostratigraphic study of abundant planktonic foraminiferal fossils from the tashiro formation five planktonic foraminiferal zones were recognized in the strata of these three belts they are morozovella spinulosa zone i globigerinatheka index zone ii globorotalia cerroazulensis cocoaensis zone iii pseudohastigerina barbadoensis zone iv and chiloguembelina cubensis zone v in ascending order they are firmly correlated with the well calibrated zones known from low latitude regions and are assigned to latest middle eocene earliest late eocene latest eocene earliest oligocene and early oligocene age respectively on the basis of the distribution of the fossil zones in the three belts the stratigraphy and geologic structure of mudstone unit of the tashiro formation are summarized as follows 1 the strata of each belt are cut into many sheets mainly by thrusts dipping to the north landward forming an imbricate structure and the same strata appear repeatedly many times with a disordered array within a single belt 2 many of the sheets show a normal sequence but a few sheets are overturned 3 the thickness of each sheet is less than 200 m 4 in each belt the older planktonic foraminiferal zones occur widely in the southern part and the younger ones in the northern part therefore the strata of each belt show a tendency of northward landward or inward younging 5 if the distribution of the foraminiferal zones of the three sections is viewed collectively the upper zones iv v are more widely distributed in the shiitani belt in the southernmost part of the investigated area than the nagano belt that is in the central part 6 to date the youngest zone v has been found only in the shiitani belt therefore it is concluded that each belt shows a roughly northward landward younging internally but the arrangement of the overall foraminiferal zones among the three belts analyzed shows a southward oceanward younging the stratigraphic arrangement and structure of the three mudstone belts of the tashiro formation may be characteristic of accretionary prisms

late jurassic to early early cretaceous tectonic nature on the ne asian continental margin constraints from mesozoic accretionary complexes abstract it is widely accepted that subduction of the paleo pacific plate beneath eurasia began in the early mesozoic however the tectonic nature and processes of the ne asian continental margin in the late jurassic to early early cretaceous remain unclear in this contribution lithological provenance and metamorphic deformation data are used to constrain the formation of mesozoic accretionary complexes ac along the ne asian continental margin and determine the tectonic relationship between the ne asian continental margin and the paleo pacific plate in the late jurassic to early early cretaceous the mesozoic ac formed during the late jurassic including the raohe ac in ne china the khabarovsk samarka and taukha acs in the russian far east and the mino tamba ac in japan and early cretaceous including the shimanto ac in japan and the kiselevka manoma ac in the russian far east the jurassic ac underwent a complex evolution including the formation of seamounts middle late jurassic terrigenous clastic sedimentation adjacent to the trench collision of seamounts with eurasia northward migration of the ac early early cretaceous terrigenous clastic sedimentation a second period of accretion and final emplacement the earliest mesozoic ac arrived at the trench at ca 195 ma combined with the occurrence of early mesozoic calc alkaline igneous rocks along the ne asian continental margin we suggest that subduction of the paleo pacific plate beneath eurasia began in the early jurassic jurassic ac in japan was affected by a regional high p t metamorphic event at 160 ma which might have been related to subduction collision of seamounts with eurasia at the trench ages and hf isotopic compositions of detrital zircon grains from the middle upper jurassic and cretaceous ac sandstones together with paleontological and paleomagnetic data indicate that jurassic clastic sediments were sourced mainly from the south china block scb whereas the lower cretaceous sandstones were sourced from the eastern central asian orogenic belt caob the inferred northward migration of jurassic ac in the late jurassic to early early cretaceous indicates that the boundary between the ne asian continental margin and the paleo pacific plate was characterized by strike slip tectonics at this time likely due to a small angle of obliquity in the subduction of the paleo pacific plate beneath eurasia in the late early cretaceous westward subduction of the paleo pacific plate beneath eurasia resulted in the emplacement of the raohe and khabarovsk acs at 137 130 ma and the formation of acs in the russian far east and japan

structural fabric of the paleozoic golconda allochthon antler peak quadrangle nevada progressive deformation of an oceanic sedimentary assemblage the golconda allochthon in nevada consists of structurally complex mid to late paleozoic basinal sedimentary and volcanic rocks deformed under low temperature and pressure conditions and emplaced onto the north american shelf in the permo triassic above the basal thrust in the antler peak area the allochthon consists in ascending structural order of late early to early late permian thick bedded chert pumpernickel subunit 1 pennsylvanian and early permian thin bedded chert argillite and siltstone pumpernickel subunit 2 mississippian siliciclastic turbidites jory unit and undated chert and argillite of the trenton unit open to tight folds in subunit 1 trend north south and are east vergent folds in thinner bedded subunit 2 vary considerably in style fold axes in this unit are distributed within the mean axial plane oriented n s 60 w but exhibit a west northwest trending data gap boudin axes measured in zones of highly disrupted layering interpreted as zones of movement or shear exhibit similar patterns without data gaps these girdled distributions in part reflect rotation of these elements upon greater strain towards the east west trending direction of tectonic transport or towards parallelism with the x axis of the strain ellipsoid asymmetry of folds indicate movement was west to east locally developed late stage folds in subunit 2 are subhorizontal trend north south and are compatible with fold fault solutions that indicate east directed thrusting striations measured within fault zones yield bidirectional east west slip lines our fold and fault data indicate that east west shortening of these basinal sediments was accomplished by folding and east directed thrust faulting during a single progressive deformational event this conclusion is supported by the presence of a major west dipping thrust fault in the map area that places mississippian over permian rocks the allochthon formed as an east vergent submarine fold and thrust belt that was subsequently thrust on the continental shelf autochthonous rocks beneath the golconda thrust are virtually undeformed suggesting that most of the structure in the allochthon formed prior to its ultimate emplacement

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

chemical and isotopic variations along the superfast spreading east pacific rise from 6 to 30 s chemical data of 39 fresh basaltic glasses from the east pacific rise epr between 6 and 30 s and pb sr and nd isotopic compositions of 12 basalt glasses are presented major and trace element data indicate a wide compositional range including primitive basalts mg 0 67 and highly evolved feti basalts mg 0 34 molmg mg fe2 the compositional range can be attributed to low pressure fractional crystallization fractionation corrected major element concentrations provide evidence for varying mantle melting conditions calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar at temperatures between 1430 and 1280 c and total degrees of partial melting between 17 and 20 by weight leached and hand picked basalt glasses display large variations in 87sr 86sr 0 70235 0 70270 143nd 144nd 0 51312 0 51323 and 206pb 204pb 18 064 18 665 but are similar to other n type morb from the epr the isotopic ratios of basalts from 13 to 23 s show strong correlations and delineate two systematic trends from 23 to 17 s 87sr 86sr and pb isotope ratios increase and 143nd 144nd decrease in agreement with previous results mahoney et al 1989 a reverse trend is indicated by basalts from 17 to 13 s however k ti and la sm n continuously increase from 23 to 13 s this opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17 s north of 13 5 s garrett fracture zone isotopic data show no systematic variation with ridge location and display an overall weaker covariation the results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions p t f f degrees of melting there is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30 s we interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components in contrast variations in trace elements are attributed to young mantle differentiation processes

structure of the havallah sequence golconda allochthon nevada evidence for prolonged evolution in an accretionary prism the golconda allochthon of northern and central nevada contains the havallah sequence and correlative units of latest devonian to early late permian age the havallah sequence is dominated by radiolarian ribbon chert and argillite associated with variable but generally subordinate siliciclastic cal carenitic and volcaniclastic turbidites and slump deposits cherts of all ages except pennsylvanian locally rest depositionally on tholeiitic basalts some were altered and mineralized by ridge type hydrothermal systems suggesting deposition in an ocean basin containing active spreading center s for much of the upper paleozoic the havallah sequence was thrust eastward along the basal golconda thrust onto coeval sediments of the western north american shelf during the latest paleozoic earliest mesozoic sonoma orogeny detailed studies of the havallah sequence in the sonoma and tobin ranges and in battle mountain indicate the presence of complex diagenetic and structural fabrics that developed prior to the obduction of the allochthon a large number of thrusts composite thrusts and shear zones slice the havallah into numerous tectonic packets of contrasting lithology and or internal structural style internal structures in chert packets include bedding parallel and bedding normal solution cleavage solution boudins three or more sets of east verging folds of variable geometry and features associated with high pore fluid pressures such as crack seal fractures dilation breccias clastic dikes and sills and clastic intrusions along thrust surfaces the fabrics suggest a general progressive evolution from deformation characterized by bedding normal compression and slight east west extension d1 to deformation dominated by bedding parallel east directed shear d2 local chert packets may have suffered alternating episodes of high and low pore fluid pressure the degree of development and the styles and orientations of various structural elements vary sometimes radically from packet to packet the pre golconda thrust fabrics can be modeled as the result of imbrication and deformation of successive batches of ocean floor sediments into the toe of an accretionary prism in front of an east facing arc relatively undeformed permian calcarenite units interpreted as being trench slope deposits suggest that this prism was well developed by permian time coarse slump deposits containing chert clasts with pre depositional structural fabrics suggest a prism that was active long enough to recycle previously tectonized cherts if we are correct the classical sonoma orogeny d3 marked the culmination of a protracted structural evolution that may have spanned much of the upper paleozoic

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

diagenetic controls on the structural evolution of siliceous sediments in the golconda allochthon nevada u s a abstract a model is proposed that links the diagenesis of siliceous sedimentary rocks with deformation to explain the heterogeneous structural fabric of radiolarian cherts in the upper paleozoic golconda allochthon of nevada u s a numerous thrust faults slice the cherts into packets each with a unique set of internal structures fold geometries boudin profiles pressure solution features etc vary from packet to packet and layer to layer analogous variations in the diagenetically mixed siliceous sediments of the miocene monterey formation california suggest the cherts of the golconda allochthon were similarly mixed when they were deformed radiolarian sediments composed of biogenic silica opal a developed ductile structures and pressure solution features because of their high porosity weak lithification and the high solubility of disordered silica lowporosity strongly lithified relatively insoluble quartz cherts the final product of silica diagenesis deformed in a brittle fashion and developed fewer pressure solution features diagenetically intermediate ct cherts and ct porcelanites exhibited transitional behavior mixed diagenetic zones produced structures with layer by layer changes in structural style the dehydration of opal a and opal ct helped create the excess pore fluid pressures responsible for thrust faults hydraulic fractures dilation breccias and elastic intrusions the presence of ductile structures and numerous pressure solution features in the oldest cherts of the havallah sequence suggests that these cherts were deformed while diagenetically immature possibly within a long lived upper paleozoic accretionary prism