Younger Faults Research Articles

Evolution of biological dispersal corridors through a tectonically active mountain range in New Zealand

AbstractAim To assess the geological evolution and biogeographical implications of low mountain passes. In particular, we question the common biogeographical belief that major mountain belts form impervious physical barriers to biological dispersal, and that related taxa found on opposites sides of mountains are necessarily a result of vicariant tectonic processes.Location The Southern Alps of New Zealand form a long (500 km) narrow mountain belt at the oblique collisional Pacific–Australian tectonic plate boundary. High mountains were uplifted during the Pliocene (2–5 Ma) and uplift has continued to the present day.Methods We integrate previous work from several disciplines to obtain an overview of inter‐relationships between plate tectonic processes, geomorphology and biogeography along the main mountain barrier in New Zealand, and then extend this approach to other major mountain belts.Results The Southern Alps initially formed a barrier to at least some biological dispersal, including vicariant formation of separate species of freshwater non‐migratory galaxiid fish on either side. However, the high mountain barrier was breached in several places when passive transport of topography occurred, from the low‐erosion rain shadow on the eastern side towards the high‐erosion, high‐rainfall western side. This tectonic transport resulted in the capture of eastern rivers by west‐draining rivers, leaving low passes at the topographic divide. These low‐elevation corridors permitted biological dispersal across the mountains, although continued uplift raises these passes. A new set of passes has formed in the northern part of the mountains where younger faults are cutting across the older mountain topography. These potential dispersal corridors are becoming lower with continued erosion, and more common as the defining structures migrate southwards.Main conclusions Biological dispersal across the Southern Alps may be facilitated by numerous mountain passes, especially via the new passes formed by cross‐cutting faults. More low‐lying corridors existed than is readily apparent now, as old river capture‐related passes have been blocked by ongoing uplift. The dynamic mountain‐building and erosional environment typified by the Southern Alps occurs in all the world’s collisional mountain belts, such as the Andes, Himalayas, European Alps and North American Cordillera. Sister taxa occurring across mountain belts are not necessarily a result of vicariance driven by the rise of the mountains, as numerous passes may have permitted intermittent dispersal. The evolution of low passes may have been more prevalent than is currently appreciated, suggesting that topographically complex mountain ranges might be more effectively viewed as dynamic filters within a probability landscape rather than as static and impervious high‐altitude barriers to all but the rarest of biological dispersal events. In some cases, the biological disjunctions observed across mountains may more directly reflect habitat differentiation driven by orographic mountain development that has limited the probability of trans‐alpine dispersal success.

Miocene extension in the East Range, Nevada: A two-stage history of normal faulting in the northern Basin and Range

The East Range in northwestern Nevada is a large, east-tilted crustal block bounded by west-dipping normal faults. Detailed mapping of Tertiary stratigraphic units demonstrates a two-phase history of faulting and extension. The oldest sedimentary and volcanic rocks in the area record cumulative tilting of ~30°–45°E, whereas younger olivine basalt fl ows indicate only a 15°–20°E tilt since ca. 17–13 Ma. Cumulative fault slip during these two episodes caused a minimum of 40% extensional strain across the East Range, and Quaternary fault scarps and seismic activity indicate that fault motion has continued to the present day. Apatite fi ssion track and (U-Th)/ He data presented here show that faulting began in the East Range ca. 17–15 Ma, coeval with middle Miocene extension that occurred across much of the Basin and Range. This phase of extension occurred contemporaneously with middle Miocene volcanism related to the nearby northern Nevada rifts, suggesting a link between magmatism and extensional stresses in the crust that facilitated normal faulting in the East Range. Younger fault slip, although less well constrained, began after 10 Ma and is synchronous with the onset of low-magnitude extension in many parts of northwestern Nevada and eastern California. These fi ndings imply that, rather than migrating west across a discrete boundary, late Miocene extension in western Nevada is a distinct, younger period of faulting that is superimposed on the older, middle Miocene distribution of extended and unextended domains. The partitioning of such middle Miocene deformation may refl ect the infl uence of localized heterogeneities in crustal structure, whereas the more broadly distributed late Miocene extension may refl ect a stronger infl uence from regional plate boundary processes that began in the late Miocene.

Fault growth and propagation during incipient continental rifting: Insights from a combined aeromagnetic and Shuttle Radar Topography Mission digital elevation model investigation of the Okavango Rift Zone, northwest Botswana

Digital Elevation Models (DEM) extracted from the Shuttle Radar Topography Mission (SRTM) data and high‐resolution aeromagnetic data are used to characterize the growth and propagation of faults associated with the early stages of continental extension in the Okavango Rift Zone (ORZ), northwest Botswana. Significant differences in the height of fault scarps and the throws across the faults in the basement indicate extended fault histories accompanied by sediment accumulation within the rift graben. Faults in the center of the rift either lack topographic expressions or are interpreted to have become inactive, or have large throws and small scarp heights indicating waning activity. Faults on the outer margins of the rift exhibit either (1) large throws or significant scarp heights and are considered older and active or (2) throws and scarp heights that are in closer agreement and are considered young and active. Fault linkages between major fault systems through a process of “fault piracy” have combined to establish an immature border fault for the ORZ. Thus, in addition to growing in length (by along‐axis linkage of segments), the rift is also growing in width (by transferring motion to younger faults along the outer margins while abandoning older faults in the middle). Finally, utilization of preexisting zones of weakness allowed the development of very long faults (>100 km) at a very early stage of continental rifting, explaining the apparent paradox between the fault length versus throw for this young rift. This study clearly demonstrates that the integration of the SRTM DEM and aeromagnetic data provides a 3‐D view of the faults and fault systems, providing new insight into fault growth and propagation during the nascent stages of continental rifting.

The Rachaya‐Serghaya fault system (Lebanon): Evidence of coseismic ruptures, and the AD 1759 earthquake sequence

The Rachaya and Serghaya faults are the easternmost fault branches of the Dead Sea Transform Fault within the Lebanese restraining bend. They lie east of the Yammouneh fault (the main strand of the Dead Sea Transform Fault within the restraining bend), extend along the western and eastern flanks of the Anti‐Lebanon range, respectively, and show left‐lateral strike‐slip movement manifested as offset drainage. We studied both faults through combined field investigations in geomorphology and paleoseismology. Young fault scarps, mole tracks, pressure ridges and offset streams detected along the faults' traces attest to recent coseismic ruptures. Two paleoseismic investigations highlight their seismogenic potential and indicate earthquake recurrence along them: the Rachaya and Serghaya faults are active and the sources of recent historical earthquakes, the last of which might be the 30 October–25 November 1759 (Ms 6.6 and 7.4) earthquake sequence that caused severe damage in the eastern Mediterranean region. Such a possible correlation suggests that the two faults are probably structurally interconnected, as movement on one fault may stimulate movement on the other fault. In addition, both faults may define together an active seismogenic fault system that accommodates some of the regional displacement that takes place within the Lebanese restraining bend. Our results highlight that the seismogenic potential of the Rachaya and Serghaya faults must be included in any seismic hazard assessment of the region.

Morphometric and geomorphic approaches for assessment of tectonic activity, Dead Sea Rift (Israel)

A series of 31 short, parallel, steep channels, mostly of first or second order, occur at intervals of 300–500 m along 16 km of the western margin of the northern Dead Sea Rift (Israel). The analysis of tectonic and geometric activity along the marginal faults between the various segments is based on morphometric parameters, such as sinuosity of the mountain front, channel gradients, drainage basin elongation ratio, planimetric ratio, facet areas, hypsometric curves, and integrals. Longitudinal profiles of the channels were studied in relation to the underlying lithology and the presence of marginal faults. Despite the great differences in climatic and geomorphologic settings, comparison with other studies worldwide showed morphometric values similar to other tectonically active regions. The southern segments of the study area, which face the Hula deep depression, indicate enhanced geomorphic and tectonic activity. The northern segment proved dissimilar to the others, and is explained by its different tectonic setting. Six sedimentary, fan-like units composed of polymictic conglomerates were deposited along the mountain piedmont at the channel mouths. The units were dated by OSL, K–Ar, and archaeology to < 1.1 Ma (Q 1 and Q 1–1), < 0.56 Ma (Unit Q 2), ~ 120 ka (Unit Q 3), and < 10 ka (Units Q 4 and Q 5). The morphology and structure of the sedimentary units indicate pulses of depositional activity, followed by periods of quiescence and calcic-soil development. No major faulting occurred along the Naftali escarpment during the Upper Pleistocene, despite the high tectonic morphometric values obtained for the study area. The marginal faults displaced Unit Q 2 during the Mid-Pleistocene, and since then tectonic activity has shifted from the western marginal faults eastward to younger faults deeply buried in the center of the graben. The field data calibrate the morphometric analysis: carbonate (limestone and dolomite) hillslopes developed in the Mediterranean climate have maintained ‘active tectonic class’ morphology since the last major faulting events during the Mid-Pleistocene.

Fabric anisotropy controls faulting in the continental crust

The influence of pre-existing structural anisotropy on faulting in the continents is best tested in recently exhumed crust (e.g. Nanga Parbat Massif, NW Himalayas), where earlier brittle structures have been annealed. The kinematics of young faults, formed in a single, continuing tectonic regime (NNW compression), are distinctly different, depending upon the orientation of the early ductile foliations around them. Faulting is subparallel and statistically simple where foliation is moderately dipping but highly complex where foliation is steeply dipping. Thus structural anisotropy does control faulting in the continental crust, a result with important implications for seismogenesis, fluid flow and basin evolution.

Clay quantification and Ar–Ar dating of synthetic and natural gouge: Application to the Miocene Sierra Mazatán detachment fault, Sonora, Mexico

Direct dating of brittle fault rocks has been predicated on the assumption that illite in fault gouge is a mixture of two populations of clays: one detrital, derived from the wall rock, and the other authigenic, forming in the brittle fault zone during faulting. Owing to complex diagenetic histories of wallrock shales in previous fault-dating studies, this assumption has remained largely untested. We demonstrate the validity of our clay quantification technique using calculated WILDFIRE© patterns to accurately model both artificial mixtures of 2M 1 and 1M d illite, and natural mixtures in young fault gouges. Using our quantification approach, we obtain well-defined detrital and authigenic ages from the Sierra Mazatán metamorphic core complex in Sonora, Mexico. Because the age population of the wall rock is well constrained by cooling ages, we confirm that the detrital component in the gouge is the age of the footwall granite, while the age of the authigenic component is the time at which faulting ceased, which is in agreement with field constraints. Based on the Ar–Ar age and the thermal conditions for the growth of the authigenic illite we also conclude that the Sierra Mazatán detachment continued slipping in the brittle regime at strain rates similar to those for the plastic regime (on the order of 10 −14 s −1).

Disruption of the Hellenic arc: Late Miocene extensional detachment faults and steep Pliocene‐Quaternary normal faults—Or what happened at Corinth?

Extensional faults exposed in the Peloponnesus and mainland Greece, most of which are described here for the first time, record a transition from regional extension of the Aegean domain to the modern tectonic system. The East Peloponnesus Detachment System trends north‐northwest from the southern Peloponnesus to ∼30 km north of the Gulf of Corinth, dips gently northeast, and is late Miocene–early Pliocene in age. It has a minimum displacement of 25–30 km and appears to be the youngest of the regional‐scale extensional systems with significant displacement that formed parallel to the Hellenic arc. The partially coeval East Sterea Extensional System, which extends from the Gulf of Corinth to the Aegean Sea, contains low‐angle normal faults that both crosscut and trend parallel to older structures of the Hellenic arc. Late Miocene to early Pliocene displacement within this zone disrupted the arc‐parallel structures of the Hellenides. Upper Pliocene‐Quaternary normal faults, which trend approximately east‐west and generally dip steeply at the surface, continue the disruption of the Hellenic arc. Much of the subsidence within the Gulf of Corinth appears to be unrelated to the younger faults and is instead related to the motion on the East Peloponnesus Detachment, which crosscuts the modern graben.

Lineaments in the central part of the Moscow Syneclise and their relations to faults in the basement

It is well known that very few young faults have been recorded within the vast territories of ancient platforms that are overlain by a virtually undeformed sedimentary cover of Phanerozoic sediments. At the same time, interpretation of topographic maps of various scales, aerial photographs, and satellite images reveals predominantly linear and occasionally arcuate zones (lineaments), some of which coincide with large fracture zones related to the Riphean rifts and grabens in the platform basement. Elevated emanations of hydrogen, radon, helium, and other gases were detected over some of the lineaments, thus indicating anomalous permeability of these zones in comparison with the adjacent areas. An opinion was stated that the elevated permeability of these fault zones is provided by block vibrations as a response to lunisolar tide effects. The lineaments are of a great environmental significance because they are conduits of groundwater migration. The origin of lineaments in the platform sedimentary cover and their relations to structural elements in the basement, to evolution, and to causes of activity have provoked great interest.

Thermal maturity of a fold–thrust belt based on vitrinite reflectance analysis in the Western Foothills complex, western Taiwan

Burial depth, cumulative displacement, and peak temperature of frictional heat of a fault system are estimated by thermal analysis in the fold–thrust belt of the Western Foothills complex, western Taiwan based on the vitrinite reflectance technique. The regional thermal structure across the complex reveals that the rocks were exposed to maximum temperatures ranging from 100 °C to 180 °C, which corresponds to a burial depth of 3.7–6.7 km. A large thermal difference of 90 °C were observed at the Shuilikeng fault which make the eastern boundary of the fold–thrust belt where it is in contact with metamorphic rock of Hsuehshan Range. The large thermal difference corresponds to cumulative displacements on the Shuilikeng fault estimated to be in the range of 5.2–6.9 km. However, thermal differences in across the Shuangtung and Chelungpu faults cannot be determined apparently due to small vertical offsets. The large displacement observed across the Shuilikeng fault is absent at the other faults which are interpreted to be younger faults within the piggyback thrust system. Localized high temperatures adjacent to fault zones were observed in core samples penetrating the Chelungpu fault. Three major fracture zones were observed at core lengths of 225 m, 330 m, and 405 m and the two lower zones which comprise dark gray narrow shear zones. A value of vitrinite reflectance of 1.8%, higher than the background value of 0.8%, is limited at a narrow shear zone of 1 cm thickness at the fracture zone at 330 m. The estimated peak temperature in the range of 550–680 °C in the shear zone is far higher than the background temperature of 130 °C, and it is interpreted as due to frictional heating during seismic faulting.

Crustal controls on magmatic-hydrothermal systems: A geophysical comparison of White River, Washington, with Goldfield, Nevada

The White River altered area, Washington, and the Goldfield mining district, Nevada, are nearly contemporaneous Tertiary (ca. 20 Ma) calc-alkaline igneous centers with large exposures of shallow (<1 km depth) magmatic-hydrothermal, acid-sulfate alteration. Goldfield is the largest known high-sulfidation gold deposit in North America. At White River, silica is the only commodity exploited to date, but, based on its similarities with Goldfield, White River may have potential for concealed precious and/or base metal deposits at shallow depth. Both areas are products of the ancestral Cascade arc. Goldfield lies within the Great Basin physiographic province in an area of middle Miocene and younger Basin and Range and Walker Lane faulting, whereas White River is largely unaffected by young faults. However, west-northwest–striking magnetic anomalies at White River do correspond with mapped faults synchronous with magmatism, and other linear anomalies may reflect contemporaneous concealed faults. The White River altered area lies immediately south of the west-northwest–striking White River fault zone and north of a postulated fault with similar orientation. Structural data from the White River altered area indicate that alteration developed synchronously with an anomalous stress field conducive to left-lateral, strike-slip displacement on west-northwest–striking faults. Thus, the White River alteration may have developed in a transient transtensional region between the two strike-slip faults, analogous to models proposed for Goldfield and other mineral deposits in transverse deformational zones. Gravity and magnetic anomalies provide evidence for a pluton beneath the White River altered area that may have provided heat and fluids to overlying volcanic rocks. East– to east-northeast–striking extensional faults and/or fracture zones in the step-over region, also expressed in magnetic anomalies, may have tapped this intrusion and provided vertical and lateral transport of fluids to now silicified areas. By analogy to Goldfield, geophysical anomalies at the White River altered area may serve as proxies for geologic mapping in identifying faults, fractures, and intrusions relevant to hydrothermal alteration and ore formation in areas of poor exposure.

PETROLEUM GEOLOGY OF MIDDLE–LATE TRIASSIC AND EARLY JURASSIC SEQUENCES IN THE SIMPSON BASIN AND NORTHERN EROMANGA BASIN OF CENTRAL AUSTRALIA

Prospective Middle–Late Triassic and Early Jurassic petroleum systems are widespread in central Australia where they have only been sparsely explored. These systems are important targets in the Simpson/Eromanga basins (Poolowanna Trough and surrounds), but the petroleum systems also extend into the northern and eastern Cooper Basin.Regional deposition of Early–Middle Triassic red-beds, which provide regional seal to the Permian petroleum system, are variously named the Walkandi Formation in the Simpson Basin, and the Arrabury Formation in the northern and eastern Cooper Basin. A pervasive, transgressive lacustrine sequence (Middle–Late Triassic Peera Peera Formation) disconformably overlies the red-beds and can be correlated over a distance of 500 km from the Poolowanna Trough into western Queensland, thus providing the key to unravelling Triassic stratigraphic architecture in the region. The equivalent sequence in the northern Cooper Basin is the Tinchoo Formation. These correlations allow considerable simplification of Triassic stratigraphy in this region, and demonstrate the wide lateral extent of lacustrine source rocks that also provide regional seal. Sheet-like, fluvial-alluvial sands at the base of the Peera Peera/Tinchoo sequence are prime reservoir targets and have produced oil at James–1, with widespread hydrocarbon shows occurring elsewhere including Poolowanna–1, Colson–1, Walkandi–1, Potiron–1 and Mackillop–1.The Early Jurassic Poolowanna Formation disconformably overlies the Peera Peera Formation and can be subdivided into two transgressive, fluvial-lacustrine cycles, which formed on a regional scale in response to distal sea level oscillations. Early Jurassic stratigraphic architecture in the Poolowanna Trough is defined by a lacustrine shale capping the basal transgressive cycle (Cycle 1). This shale partitions the Early Jurassic aquifer in some areas and significant hydrocarbon shows and oil recoveries are largely restricted to sandstones below this seal. Structural closure into the depositional edge of Cycle 1 is an important oil play.The Poolowanna and Peera Peera formations, which have produced minor oil and gas/condensate on test respectively in Poolowanna–1, include lacustrine source rocks with distinct coal maceral compositions. Significantly, the oil-bearing Early Jurassic sequence in Cuttapirrie–1 in the Cooper Basin correlates directly with the Cycle–1 oil pool in Poolowanna–1. Basin modelling in the latter indicates hydrocarbon expulsion occurred in the late Cretaceous (90–100 Ma) with migration into a subtle Jurassic age closure. Robust Miocene structural reactivation breached the trap leaving only minor remnants of water-washed oil. Other large Miocene structures, bound by reverse faults and some reflecting major inversion, have failed to encounter commercial hydrocarbons. Future exploration should target subtle Triassic to Jurassic–Early Cretaceous age structural and combination stratigraphic traps largely free of younger fault dislocation.

Lunar activity from recent gas release

Samples of material returned from the Moon have established that widespread lunar volcanism ceased about 3.2 Gyr ago. Crater statistics and degradation models indicate that last-gasp eruptions of thin basalt flows continued until less than 1.0 Gyr ago, but the Moon is now considered to be unaffected by internal processes today, other than weak tidally driven moonquakes and young fault systems. It is therefore widely assumed that only impact craters have reshaped the lunar landscape over the past billion years. Here we report that patches of the lunar regolith in the Ina structure were recently removed. The preservation state of relief, the number of superimposed small craters, and the 'freshness' (spectral maturity) of the regolith together indicate that features within this structure must be as young as 10 Myr, and perhaps are still forming today. We propose that these features result from recent, episodic out-gassing from deep within the Moon. Such out-gassing probably contributed to the radiogenic gases detected during past lunar missions. Future monitoring (including Earth-based observations) should reveal the composition of the gas, yielding important clues to volatiles archived at great depth over the past 4-4.5 Gyr.

Failure modes of the lineaments on Jupiter's moon, Europa: Implications for the evolution of its icy crust

Lineaments referred to as ridges, troughs, bands, and faults on the icy surface of Jupiter's moon, Europa, have long been interpreted as extensional structures due to brittle fracturing of ice and intrusion of mobile materials from the interior of the satellite. Based on detailed mapping and possibly analogous structures present on Earth, we propose that the kinematics and failure mechanisms of these structures are variable and more complex than previously thought. A dense network of structures of multiple generations, forming the background on the surface of the planet, is here interpreted as localized zones of volumetric strain, likely compaction and/or dilation bands. The next class of linear failure structures is shear bands with significant offset of pre-existing markers. A few additional phases of less pervasive but more prominent volumetric deformation bands overprint the shear zones and background network. The mode of younger features can be characterized as sharp, dilational, brittle fracturing and subsequent shearing, thereby producing comminution and fragmentation in various sizes, leading to a series of younger faults with detectable lateral, as well as vertical, offset. This rich variability in the nature of the distribution, localization, kinematics, and formation mechanisms, if true, suggests that the conditions prevailing within the crust of Europa must have changed dramatically over time. The implication of this conclusion is that structures interpreted to be compaction/dilation bands and shear bands on Europa are composed of deformed materials similar to the surrounding ice, whereas only the younger faults, developed by brittle fracturing and fragmentation, may be conduits for mobile substrate to reach the surface and thus offer the highest potential for recovering evidence for life in the satellite.

Reactivation of pressure-solution seams by a strike-slip fault-sequential, dilational jog formation and fluid flow

This work examines how older pressure-solution seams become sheared or reactivated by slip because of movement on a younger fault segment. The reactivation then leads to the creation of secondary structures. These are followed by further changes in the local stress-strain field that result in slip reactivation on the secondary structures and in the creation of third-order structures. This sequence of deformation reflects and reveals transient changes in the stress-strain field along the margins of the fault during active slip on the fault. The reactivation may lead to enhanced rock permeability and/or porosity that allow for temporary periods of fluid movement. Thus, we believe that this serves as an important model to contribute to the understanding of movement of fluids such as oil and gas around active faults.

MALPAS, J., FLETCHER, C. J. N., ALI, J. R. &amp; AITCHISON, J. C. (eds) 2004. Aspects of the Tectonic Evolution of China. Geological Society Special Publication no. 226. vi+362 pp. London, Bath: Geological Society of London. Price £80.00, US $134.00; GSL members' price £40.00, US $67.00; AAPG/SEPM/GSA/RAS/EFG/PESGB members' price £48.00, US $80.00 (hard covers). ISBN 1 86239 156 4

MALPAS, J., FLETCHER, C. J. N., ALI, J. R. &amp; AITCHISON, J. C. (eds) 2004. Aspects of the Tectonic Evolution of China. Geological Society Special Publication no. 226. vi+362 pp. London, Bath: Geological Society of London. Price £80.00, US $80.00 (hard covers). ISBN 1 86239 156 4 - Volume 142 Issue 5

PHANEROZOIC BRITTLE TECTONICS IN THE SOUTH AMERICAN CONTINENTAL PLATFORM, SOUTHEAST BRAZIL: NEW INSIGHTS FROM FISSION TRACK STUDIES ON APATITE IN REACTIVATED FAULT ZONES

Apatite Fission Track Method (FTM) studies were performed on samples coming from two different fault domains in the Precambrian basement of southeast Brazil in order to evaluate subsequent Phanerozoic tectonic movements in that part of the South American Platform. The samples studied were collected along brittle faults in the Mantiqueira mountain range and in the Jundiai upland plain (Braganca Paulista and Extrema), approximately 100 km northeast of Sao Paulo, Brazil. The results of paleostress analysis, as well as the presence of as pseudotachylyte material in the reactivated fault zones, indicates a rapid strain rate and high frictional temperature along these faults. The recognition of deformation related to this brittle tectonic regime is of key importance for the reconstruction of Paleozoic and Mesozoic tectonic evolution of the South American Platform. The causal effects of these tectonic readjustments of cratonic rocks during the breakup of West-Gondwana during the Cretaceous and also in younger geological history, are some of the key parameters for understanding thePhanerozoic evolution of the Mantiqueira mountain range. Our FTM data shows the oldest regional thermal histories are recorded in the Jundiai upland plain beginning in the Upper Triassic (~190 Ma) at 50 o C, indicating a rapid cooling that is coincident with the tectonic subsidence of the Parana Basin and, probably, the uplift and preservation of Gondwana surface. These data also show slow linear heating (between 50 to 90 o C) of the southeastern Brazilian margin up to the Lower Cretaceous (~120 Ma). This phenomenon could either be related to migration of the Trindade plume, or extensional/compressional movements. At ~120 Ma a structural inversion occurred and the previous slow linear heating was replaced by slow linear cooling (from 90 to 25 o C) that has persisted up to the present time. Locally, in the younger fault domain, in the Mantiqueira Range near Extrema, fission tracks began to be recorded at ~ 110 Ma ago and were, since then, submitted to a slow linear cooling very similar to that observed for two country rock samples and the samples collected in the older faulted rocks (Braganca Paulista). Then considering the thermal histories of all the samples studied here, there is an agreement about a pervasive thermal event taking place ~110 Ma ago followed by a linear cooling up to the present, related to regional erosion. These processes are considered to be related to the South-Atlantic opening, with widespread extensional tectonic movements along the southeastern Brazilian coast.

PHANEROZOIC BRITTLE TECTONICS IN THE SOUTH AMERICAN CONTINENTAL PLATFORM, SOUTHEAST BRAZIL: NEW INSIGHTS FROM FISSION TRACK STUDIES ON APATITE IN REACTIVATED FAULT ZONES

Apatite Fission Track Method (FTM) studies were performed on samples coming from two different fault domains in the Precambrian basement of southeast Brazil in order to evaluate subsequent Phanerozoic tectonic movements in that part of the South American Platform. The samples studied were collected along brittle faults in the Mantiqueira mountain range and in the Jundiaí upland plain (Bragança Paulista and Extrema), approximately 100 km northeast of São Paulo, Brazil. The results of paleostress analysis, as well as the presence of as pseudotachylyte material in the reactivated fault zones, indicates a rapid strain rate and high frictional temperature along these faults. The recognition of deformation related to this brittle tectonic regime is of key importance for the reconstruction of Paleozoic and Mesozoic tectonic evolution of the South American Platform. The causal effects of these tectonic readjustments of cratonic rocks during the breakup of West-Gondwana during the Cretaceous and also in younger geological history, are some of the key parameters for understanding thePhanerozoic evolution of the Mantiqueira mountain range. Our FTM data shows the oldest regional thermal histories are recorded in the Jundiaí upland plain beginning in the Upper Triassic (~190 Ma) at 50ºC, indicating a rapid cooling that is coincident with the tectonic subsidence of the Paraná Basin and, probably, the uplift and preservation of Gondwana surface. These data also show slow linear heating (between 50 to 90oC) of the southeastern Brazilian margin up to the Lower Cretaceous (~120 Ma). This phenomenon could either be related to migration of the Trindade plume, or extensional/compressional movements. At ~120 Ma a structural inversion occurred and the previous slow linear heating was replaced by slow linear cooling (from 90 to 25ºC) that has persisted up to the present time. Locally, in the younger fault domain, in the Mantiqueira Range near Extrema, fission tracks began to be recorded at ~ 110 Ma ago and were, since then, submitted to a slow linear cooling very similar to that observed for two country rock samples and the samples collected in the older faulted rocks (Bragança Paulista). Then considering the thermal histories of all the samples studied here, there is an agreement about a pervasive thermal event taking place ~110 Ma ago followed by a linear cooling up to the present, related to regional erosion. These processes are considered to be related to the South-Atlantic opening, with widespread extensional tectonic movements along the southeastern Brazilian coast.

Fault reactivation control on normal fault growth: an experimental study

Field studies frequently emphasize how fault reactivation is involved in the deformation of the upper crust. However, this phenomenon is generally neglected (except in inversion models) in analogue and numerical models performed to study fault network growth. Using sand/silicon analogue models, we show how pre-existing discontinuities can control the geometry and evolution of a younger fault network. The models show that the reactivation of pre-existing discontinuities and their orientation control: (i) the evolution of the main fault orientation distribution through time, (ii) the geometry of relay fault zones, (iii) the geometry of small scale faulting, and (iv) the geometry and location of fault-controlled basins and depocenters. These results are in good agreement with natural fault networks observed in both the Gulf of Suez and Lake Tanganyika. They demonstrate that heterogeneities such as pre-existing faults should be included in models designed to understand the behavior and the tectonic evolution of sedimentary basins.

An autochthonous geological model for the eastern Andes of Ecuador

We describe a traverse across the Cordillera Real and sub-Andean Zone of Ecuador, poorly known areas with very little detailed mapping and very little age control. The spine of the Cordillera comprises deeply eroded Triassic and Jurassic plutons, the roots of a major arc, emplaced into probable Palaeozoic pelites and metamorphosed volcanic rocks. The W flank comprises a Jurassic (?) submarine basaltic–andesitic volcanic sequence, which grades up into mixed Jurassic/Cretaceous volcanic and sedimentary rocks of the Inter-Andean Valley. The sub-Andean Zone, on the E flank of the Cordillera, comprises a newly recognized Cretaceous basin of cleaved mudrocks, quartz arenites and limestones. East of the syndepositional Cosanga Fault, the Cretaceous basin thins into a condensed sequence that is indistinguishable from the rocks of the adjacent hydrocarbon-bearing Oriente Basin. The principal penetrative deformation of the Cordillera Real was probably latest Cretaceous/Palaeocene. It telescoped the magmatic belts, but shortening was largely partitioned into the pelites between plutons. The plutons suffered inhomogenous deformation; some portions completely escaped tectonism. The pelites conserve two foliations. The earliest comprises slaty cleavage formed under low- or sub-greenschist conditions. The later is a strong schistosity defined by new mica growth. It largely transposed and obliterated the first. Both foliations may have developed during a single progressive deformation. We find inappropriate recent terrane models for the Cordillera Real and sub-Andean Zone of Ecuador. Instead we find remarkable similarities from one side of the Cordillera to the other, including a common structural history. In place of sutures, we find mostly intrusive contacts between major plutons and pelites. Triassic to Cretaceous events occurred on the autochthonous western edge of the Archaean Guyana Shield. The latest Cretaceous–Paleocene deformation is interpreted as the progressive collision of an oceanic terrane(s) with the South American continent. Young fault movements have subsequently juxtaposed different structural levels through the Cordillera Real orogen.