Normal faults

Timing is a key data for understanding the origin of faulting. The Balcones fault system (BFS) extends ~600 km along the northern margin of the Gulf of Mexico oil basin and controls springs that supply the major cities in Texas, but its origin is unclear. We provide its first direct timing by applying U–Pb geochronology on seven calcite-mineralized fault surfaces. We have found that this extensional fault system formed during the Palaeocene-middle Eocene time (from 61.3 ± 2.7 to 45.4 ± 2.1 Ma), which is much earlier than previous estimates. We show that the formation of the BFS coincides with the largest clastic influx in the northern Gulf of Mexico basin that resulted from Laramide uplift and erosion. This timing and the location of the BFS along the Ouachita suture, support our interpretation of this fault system formed as the result of lithospheric flexure related to Paleogene sedimentary loading.

In the Tabernas Desert, a Neogene basin situated in SE Spain, major E-W folds coexist with large E-W dextral faults and abundant NNW-SSE faults. These faults affect the Plio-Quaternary sediments. The palaeostress ellipsoids indicate a roughly constant tension (3 axe) in the ENE-WSW direction, coexisting with a changeable direction of the main compression (1 axe) from the vertical to the NW-SE direction. The numerous NNW-SSE faults in southern areas, directly to the east of the city of Almería, tend to propagate northwards, forming a tectonic discontinuity of the Betic Internal Zone. The 2002 seismic swarm of the Gergal earthquake occurred in this NNW-SSE fault zone. In this setting, the E-W fractures act as transfer faults.

Fallas normales de alto ángulo en el Neógeno del margen Atlántico de la Cuenca del Golfo San Jorge (46ºS-67º 30´O, Patagonia Argentina) R.E. Giacosa (1)(2) , J.M. Paredes (2) , A.M. Nillni (2) , M. Ledesma (2)(3) y F. Colombo (4) (1) Servicio Geológico Minero Argentino. CC 38 B, Km 8 (A-9003).

Timing is a key data for understanding the origin of faulting. The Balcones fault system (BFS) extends ~600 km along the northern margin of the Gulf of Mexico oil basin and controls springs that supply the major cities in Texas, but its origin is unclear.  We provide its first direct timing by applying U–Pb geochronology on seven calcite-mineralized fault surfaces. We have found that this extensional fault system formed during the Palaeocene-middle Eocene time (from 61.3 ± 2.7 to 45.4 ± 2.1 Ma), which is much earlier than previous estimates. We show that the formation of the BFS coincides with the largest clastic influx in the northern Gulf of Mexico basin that resulted from Laramide uplift and erosion. This timing and the location of the BFS along the Ouachita suture, support our interpretation of this fault system formed as the result of lithospheric flexure related to Paleogene sedimentary loading.

A detailed study of normal faults in the calcarenites and chalks of the Mons Basin reveals that pressure-solution processes play a significant role in fault zones, which affects the determination of the amounts of extension. Drag folding associated with normal simple shear and rock dissolution concurred to produce layer flexuring in the hanging wall of the normal faults, whereas the footwall remained almost intact. Analysing the distribution of fossil belemnite rostra in and away from fault zones enabled us to evaluate these effects. The sense and amount of belemnite tilt constrain the minimum thickness of dissolved hanging wall fringe, revealing significant pressure-solution in addition to drag folding. We developed 2-D and 3-D analytical models of along-fault belemnite rotations induced by drag folding and dissolution, and we applied these models to our data from the Mons Basin. The results show that the pressure-solution effect is significant, implying a smaller ratio between horizontal and vertical components of motion than expected. It follows that the across-fault amount of extension is smaller than that predicted from the single fault geometry, suggesting that although the existence of Campanian-Maastrichtian-Danian extension in the Mons Basin area is beyond doubt, its amplitude should not be overestimated.

The Calabrian Arc is the epicentral region of one third of the strongest earthquakes of Italy (M w ≥ 7.0). These are confined within a narrow peninsula which is the emerging portion of a slab-related accretionary wedge, and all occurred in the past four centuries. Therefore, here more than anywhere in Italy, historical seismicity alone is not sufficient for seismic hazard assessment. We carried out geological and paleoseismological studies in southern Calabria that allowed characterizing the seismogenic behavior of the Cittanova fault which was responsible for one of the most catastrophic earthquakes to ever occur in Europe, the M w 7.0 5 February 1783 event. We have found out conclusive evidence for four Holocene earthquakes prior to 1783, with a recurrence time longer (~3.2 kyr) than the other Apennine faults (0.3-2.4 kyr). We have also estimated a robust slip rate for the late Upper Pleistocene (0.6 mm/yr) and an extension rate (0.4 mm/yr) that could reflect the residual back-arc opening of the Tyrrhenian Basin.

La depositación de la Formación Salamanca y del Grupo Río Chico (Paleoceno) en la Cuenca del Golfo San Jorge excedió el ámbito de la cuenca continental del Grupo Chubut (Cretácico). Estas unidades muestran cambios estratigráficos significativos en dirección norte-sur para el flanco norte de la cuenca. Por un lado, la acumulación de depósitos bioclásticos basales en la Formación Salamanca (Miembro Bustamante) sólo tuvo lugar en el borde norte de la cuenca, donde la sedimentación marina ocurrió sobre rocas volcánicas jurásicas del Complejo Marifil. Hacia el sur, la transgresión inicial está representada por los depósitos epiclásticos del Miembro Hansen ("Glauconítico"), apoyados sobre el Grupo Chubut (Cretácico). Así, los depósitos bioclásticos del Miembro Bustamante se interpretan como una evidencia del desarrollo de colonias de invertebrados marinos sustrato-controladas, ya que las rocas volcánicas constituyeron un sustrato más propicio para su colonización por organismos ...

We present new observations of the geometry and pattern of fault growth from the Thyolo fault, an 85 km long border fault in the southern Malawi Rift, from high-resolution topography and field observations. The rift has a polyphase tectonic history and the Thyolo fault is located towards the edge of the Proterozoic Unango Terrane. Recent activity is demonstrated by an 18.6 ± 7.7 m high fault scarp. Different patterns of segmentation are indicated by fault geometry and fault displacement profiles: two substantial reductions in scarp height do not coincide with surface geometry changes. The surface scarp is divided into two geometrically defined overlapping sections, which are joined by a ~5 km long, fault perpendicular scarp. The scarp height in this linking section is similar to the bounding sections, yet the river drainage network and sediment depocenter distribution is not typical of relay zones. Microstructural and compositional analyses show a 15-180 m thick damage zone with an ...

The knowledge of spatial position and geologic characteristics of active faults is fundamental to locate these elements on geologic maps and better define the potential seismological hazard of an area. A Quaternary fault in the "Piano di Castelluccio" basin, in Central Italy, has been already studied through paleoseismological analysis, provided stratigraphic data on sedimentary units and highlighted evidences of "recent" faulting related to past strong earthquakes. This fault has been defined "silent", because events were not included in the current seismic catalogues, retaining uncertainties in the definition of the seismic hazard of that area. 2D/3D GPR surveys were done to image the fault zone and to provide new complementary data on the shallow sediments. The 3D data provides continuity of information over the investigation site, characterizing the structure in a total non-invasive way, whilst long 2D profiles were used to extend the study on a wider area. Final images show a characteristic GPR signature of a tectonic structure and faulted units: data reveal its position and continuity in space and highlight clear geometric features, providing useful qualitative and quantitative complementary data.

Fluid flow in fractures and host rocks has been investigated in shallow buried Miocene alluvial fan deposits. A structural, petrographical (optical, CL, SE microscopes and XRD) and geochemical (microprobe and d 18 O-d 13 C stable isotopes) study has been performed in normal faults affecting Serravalian-Tortonian siliciclastic rocks of the Vallès-Penedès basin. These faults formed during the development of the Vallès-Penedès fault-related syncline, which caused the rotation of the earliest fractures. Faulting occurred continuously before, during and after host rock cementation. Rocks affected by faulting are represented by clay-rich gouges, which formed thanks to the high phyllite clast content within the otherwise clean and mature sandstones and conglomerates. Despite the low permeability of these rocks, cross-fault and fault-parallel fluid flows occurred in most of the faults. Host rocks and veins were cemented by two generations of calcite, i.e. Cc1 and Cc2. Cc1 precipitated from meteoric waters at shallow burial conditions whereas Cc2 precipitated from meteoric waters in a confined aquifer. Palygorskite has been identified in shear zones within the gouges indicating their later formation by interaction of Mg-rich fluids with previous smectites. These fluids probably derived from Miocene seawater expelled from the underlying TransitionaleMarine Complex "TMC" by compaction. Sedimentation, fracturing and cementation occurred in a very short lapse time of about 6e7 Ma, between the Serravalian-Tortonian age of the sediments and the end of the extensional tectonics in the Vallès-Penedès fault (Pliocene).

Topography, bathymetry, regional structural observations, and fault slip measurements support the idea that the Mona rift is an active, offshore extensional structure separating a colliding area (eastern Hispaniola) from a subducting area (northwestern Puerto Rico). Near the city of Aguadilla in northwestern Puerto Rico, paleostress reconstruction through fault slip analysis demonstrates that the Mona rift is opening in an E-W direction. Fault slip analysis also indicates that this opening is oblique in the southern part of the rift. We propose that oblique rifting results from accommodation of E-W extension by oblique right-lateral reactivation of previously mapped, northwest-trending Eocene basement convergent structures (Aguadilla faults, Cerro-Goden fault). The evolution of the stress field during the Miocene and the present E-W opening of the Mona rift support the assumption that the Miocene 25° counterclockwise rotation of Puerto Rico has stopped and that this island is presently moving to the east relative to the colliding Hispaniola.

High rates of Holocene sedimentation in a lake spanning the Thingvellir rift zone of western Iceland provide an unusual opportunity to study the interaction of tectonic and magmatic processes on timescales of thousands of years. Lake Thingvallavatn is oriented SW-NE, parallel to the trend of normal faults and fissures which extend northeast from Hengill, a central volcano system. Thingvallavatn's lake sediments provide a continuous high-fidelity record of tectono-magmatic processes. "Chirp" sub-bottom profiler and sidescan records, together with sediment core information, enabled the lake stratigraphy to be constrained well since the emplacement of a post-glacial lava at 9.1 ± 0.3 ka. This lava, together with three younger horizons, enabled detailed study of the main tectonic and magmatic events. A major Hengill volcanic event which controlled the development of the present-day lake morphology occurred at 1.9 ka with the eruption of a scoria cone within the lake (Sandey). During this event, the Nesjahraun lava was erupted into the southern part of the lake, the Sandey scoria cone was formed, and major faulting and subsidence occurred in the northern part of the lake, resulting in the formation of an asymmetrical rift. Within the southern part of the lake, a deformed sequence of sediments of age 2.9-1.5 ka, between undeformed younger and older sediments, indicate that liquefaction phenomena are associated with the emplacement of the Nesjahraun lava. Analysis of fault displacement reveals that the total throw summed over all faults across the width of the rift zone is approximately constant (110-130 m) along the long axis of the rift. We estimate an extension rate on the faults of 3.3-8.2 mm yr-1 since 9.1 ka, assuming fault dips of 60-75°, which represents 17-43 % of the total plate boundary extension estimated from global plate motion inversion. We speculate that the remaining extension must either be taken up elsewhere in Iceland, for example in the Eastern Rift Zone or along the South Island Seismic Zone, or that extension estimated over the last 9kyr underestimates the long-term extension rate due to incomplete sampling of the episodic magmatic component.

The stratigraphy and structural evolution of the western Florida margin is much better understood than its northern Yucatan conjugate because previous hydrocarbon exploration has been more extensive on the Florida margin. In the deep-water northeastern GOM, the post-salt section records late Jurassic-Cretaceous gravity sliding of rafted blocks along a basinward-dipping layer of Callovian salt. This study uses approximately 31,000 line-km of 2D seismic data to describe a coeval area of widespread, gravity sliding along the less-studied, northern Yucatan margin. We define three structural domains based on their distinctive salt structures and associated deformation: 1) a region in the northeastern study area consists of relatively undeformed, late Jurassic-Cretaceous section underlain by 0-300 m-thick salt; 2) areas in the central and southwestern study area contain late Mesozoic gravity slides defined by 300-600 m-thick salt rollers and normal faults rooted onto a 1-4°b asinward-dipping salt detachment; and 3) the distal margin of the western and central study area exhibits large salt diapirs up to 6 km tall that deform Quaternary stratigraphic units. In the central Yucatan study area containing late Mesozoic gravity slides, a stratigraphic unit equivalent to the productive Oxfordian Norphlet sandstone of the deep-water northeastern GOM is identified based on its similar seismic character. Reconstructing the extent of this Norphlet-equivalent unit along the Yucatan margin to its location during Oxfordian deposition places it adjacent to an extensive area of Norphlet sandstone mapped in the deep-water northeastern GOM by previous authors. Our reconstruction suggests a 48,000 km 2 fairway of potential aeolian, late Jurassic reservoirs on the Yucatan and Campeche margins that includes productive reservoirs in the southeastern Bay of Campeche, which previous authors correlated with the Norphlet Formation.

Topography, bathymetry, regional structural observations, and fault slip measurements support the idea that the Mona rift is an active, offshore extensional structure separating a colliding area (eastern Hispaniola) from a subducting area (northwestern Puerto Rico). Near the city of Aguadilla in northwestern Puerto Rico, paleostress reconstruction through fault slip analysis demonstrates that the Mona rift is opening in an E-W direction. This fault slip analysis also indicates that the opening is oblique in the southern part of the Mona rift. We propose that oblique rifting results from the accommodation of E-W extension by oblique right-lateral reactivation of
previously mapped, NW-trending Eocene basement convergent structures (Aguadilla faults, Cerro Goden fault). The evolution of the stress field during the Miocene and the present E-W opening of the Mona rift supports the assumption that the Miocene 25° counterclockwise rotation of Puerto Rico has stopped and that this island is presently moving to the east relative to the colliding Hispaniola.

The D'Entrecasteaux Island (DEI) gneiss domes are fault-bounded domes with 2.5 km of relief exposing ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic gneisses and migmatites exhumed in an Oligocene-Miocene arc-continent collision and subduction zone subject to Late Miocene to Recent continental extension. To study the style of continental extension accompanying exhumation of the DEI gneiss domes, a grid of 1518 km of 2-D multichannel seismic (MCS) reflection data and well data is interpreted from the offshore areas surrounding the DEI, including the Trobriand basin and the Goodenough basin. The offshore study is combined with onshore geologic information to constrain the Oligocene to Recent tectonic evolution of the basins. MCS and well data are consistent with the Trobriand basin forming as a forearc basin caused by southward Miocene subduction at the Trobriand trench. At 8 Ma, the margin transitioned to an extensional tectonic environment. Since then, the Trobriand basin has subsided 1-2.5 km with few normal faults deforming the basin fill. South of the DEI, the Goodenough rift basin developed after extension began (8 Ma) with the hanging wall of the northdipping Owen-Stanley normal fault bounding the southern margin of the basin. The lack of evidence of upper crustal extension accompanying subsidence in the Trobriand and Goodenough basins suggests depth-dependent lithospheric extension from 8 to 0 Ma has accompanied uplift of the DEI gneiss domes and supports schematic model of uplift of the DEI domes involving vertical exhumation of buoyant, postorogenic lower crust, far-field extension from slab rollback, and an inverted two-layer crustal density structure.

The article outlines the main findings and conclusions of the QUAKER research project and other related studies on the behaviour of foundations built on top of a rupturing dip-slip fault. Although emphasis is placed on normal faults, the derived conclusions are valid for reverse faults, as well. A key conclusion is that it is quite feasible to design a foundation to withstand an underneath rupturing fault. Practical design recommendations suitable for developing future Code requirements on the subject, are developed on the basis of the presented conclusions.

Triggered by reactivation of the strike-slip North Anatolian Fault, the disastrous M w 7.4 Kocaeli (Turkey) earthquake also produced normal faulting in the pull-apart basin of Gölcük. Surface scarps from such faulting reached almost 2.5 m in height. Several structures were crossed by the surface rupture. As expected, many of them either collapsed or were severely damaged. But, surprisingly, several structures survived the outcropping dislocation essentially unscathed. In fact, in some cases the surface rupture path seemed to have deviated, as if to "avoid" rupturing directly underneath a structure. In other cases damage was substantial even though the fault rupture was "masked" by the near-surface soil and did not create a scarp. The rigidity and continuity of the foundation appears to have been one of the crucial factors affecting structural performance. Interestingly, the examined structures were supported on a variety of foundation types, ranging from isolated footings, to rigid box-type foundations, to piled foundations. The paper outlines a reconnaissance of the area, providing a documented description of the observed interplay between the rupturing fault, the soil, and the structure, along with the results of soil exploration and geological trenching. In the companion paper, Part II, each system is analysed numerically to confirm the conclusions of the present paper, reveal the main aspects of Fault Rupture-Soil-Foundation-Structure Interaction (FR-SFSI), and help develop deeper insights into the mechanics of successful performance of structures built on top of such faults.

The D'Entrecasteaux Island (DEI) gneiss domes are fault-bounded domes with~2.5 km of relief exposing ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic gneisses and migmatites exhumed in an Oligocene-Miocene arc-continent collision and subduction zone subject to late Miocene to recent continental extension. Multichannel seismic reflection data and well data show the Trobriand basin formed as a fore-arc basin caused by southward Miocene subduction at the Trobriand trench. Subduction slowed at~8 Ma as the margin transitioned to an extensional tectonic environment. Since then, the Trobriand basin has subsided 1-2.5 km as a broad sag basin with few normal faults deforming the basin fill. South of the DEI, the Goodenough rift basin developed after extension began (~8 Ma) as the hanging wall of the north-dipping Owen-Stanley normal fault that bounds the basin's southern margin. The lack of upper crustal extension accompanying subsidence in the Trobriand and Goodenough basins suggests depth-dependent lithospheric extension since 8 Ma has accompanied uplift of the DEI gneiss domes. Structural reconstructions of seismic profiles show 2.3-13.4 km of basin extension in the upper crust, while syn-rift basin subsidence values indicate at least 20.7-23.6 km of extension occurred in the entire crust since~8 Ma. Results indicating thinning is preferentially accommodated in the lower crust surrounding the DEI are used to constrain a schematic model of uplift of the DEI domes involving vertical exhumation of buoyant, postorogenic lower crust, far-field extension from slab rollback, and an inverted two-layer crustal density structure.

The D'Entrecasteaux Island (DEI) gneiss domes are fault-bounded domes with 2.5 km of relief exposing ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic gneisses and migmatites exhumed in an Oligocene-Miocene arc-continent collision and subduction zone subject to Late Miocene to Recent continental extension. To study the style of continental extension accompanying exhumation of the DEI gneiss domes, a grid of 1518 km of 2-D multichannel seismic (MCS) reflection data and well data is interpreted from the offshore areas surrounding the DEI, including the Trobriand basin and the Goodenough basin. The offshore study is combined with onshore geologic information to constrain the Oligocene to Recent tectonic evolution of the basins. MCS and well data are consistent with the Trobriand basin forming as a forearc basin caused by southward Miocene subduction at the Trobriand trench. At 8 Ma, the margin transitioned to an extensional tectonic environment. Since then, the Trobriand basin has subsided 1-2.5 km with few normal faults deforming the basin fill. South of the DEI, the Goodenough rift basin developed after extension began (8 Ma) with the hanging wall of the northdipping Owen-Stanley normal fault bounding the southern margin of the basin. The lack of evidence of upper crustal extension accompanying subsidence in the Trobriand and Goodenough basins suggests depth-dependent lithospheric extension from 8 to 0 Ma has accompanied uplift of the DEI gneiss domes and supports schematic model of uplift of the DEI domes involving vertical exhumation of buoyant, postorogenic lower crust, far-field extension from slab rollback, and an inverted two-layer crustal density structure.

A shallow-dipping ductile mylonitic shear zone and concordant brittle detachment fault (Mai’iu fault) together make up the dominant geological structure that controls the orientation of dip slopes on the flanks of Mount Dayman, eastern Papuan Peninsula, Papua New Guinea. The dip slopes dip in all directions from the peak of Mount Dayman and forms a domed landform that is much less dissected by streams compared to the adjacent Mount
Suckling domed landform. The orientation of mega-corrugations on the domed surface of Mount Dayman (footwall) is consistent with NNE-directed transport of the hangingwall block, which is composed of low-grade undifferentiated volcanic and sedimentary rocks and minor ultramafic rocks. Though previously documented as a thrust surface, the geometry and style of structures and map relations presented in this study indicate an extensional origin for the domed mylonitic foliation (S1) and mineral elongation lineation (L1). The field relationships are consistent with the domed landform comprising the core of a metamorphic core complex. Observations of dominantly NNE-trending regional lineaments in aerial photography and Shuttle Radar Topography Mission (SRTM) data correlate with detailed fi eld analysis of mineral elongation lineations (L1) in the main metamorphic core complex–bounding shear zone. Field relationships show a crosscutting sequence of structures that includes: (1) ductile S2 folia with ESE-plunging blue sodic-calcic amphibole mineral elongation lineations;; (2) narrow, steeply dipping ductile D2 shear zones; and (3) semi-brittle to brittle fault zones. S-C′ fabrics, asymmetric strain shadows around porphyroclasts, and fault drag indicate a top-down-to-the-NNE sense of shear for most structures. Kinematic vorticity analysis of the highest-grade ductile deformation indicates a kinematic vorticity number (W k) between 0.34 and 0.56, suggesting general shear for the early stage of deformation (D1). The NNE-directed lineaments and L1 mineral elongation lineations are consistent with the Australia-Woodlark Eulerian pole for periods between the early Pliocene (3.6 Ma) and Pleistocene (0.52 Ma). This observation is consistent with ca. 3.3 Ma granite and monzonite intrusions that cut the mylonitic fabrics and limit the age of the mylonitic fabrics to older than 3.3 Ma on Mount Suckling. A SE-dipping sedimentary sequence (Gwoira Conglomerate) characterizes part of the hanging wall of the metamorphic core complex. Petrography of the clasts within the sedimentary rocks indicates that metabasite rocks were the dominant source. The unit is in fault contact with the metabasite footwall across prehnite-bearing D3 brittle fault zones.

The 750 km2 Dayman dome of the Late Cretaceous Suckling-Dayman massif, eastern Papua New Guinea, is a domed landform that rises to an elevation of 2850 m. The northern edge of the dome is a fault scarp >1000 m high that is now part of an active microplate boundary separating the continental crust of the New Guinea highlands from the continental and oceanic crust of the Woodlark microplate. Previous work has shown that a parallel belt of eclogite-bearing core complexes northeast of the Dayman dome was exhumed from up to 24-28 kbar in the last few million years. The remarkably fresh and lightly eroded scarp of the Dayman dome exposes shallowly dipping mylonitic (S1) metabasite rocks (500 m thick) on the northern flank of Mount Dayman. Field relationships near the base of this scarp show a cross-cutting suite of ductile and brittle mesostructures that includes: (i) rare ductile S2 folia with a shallowly ESE-plunging mineral elongation lineation defined by sodic-calcic blue amphibole; (ii) narrow steeply-dipping ductile D2 shear zones; and (iii) semi-brittle to brittle fault zones. Pumpellyite-actinolite facies assemblages reported by previous workers to contain local aragonite, lawsonite and/or glaucophane are found in the core of the complex at elevations greater than 2000 m. These assemblages indicate peak metamorphic pressures of 6-9.5 kbar, demonstrating exhumation of the core of the Dayman dome from depths of 20-30 km. The S1 metamorphic mineral assemblage in metabasite includes actinolite-chlorite-epidote-albite-quartz-calcite-titanite, indicative of greenschist facies conditions for the main deformation. New mineral equilibria modeling suggests that this S1 assemblage evolved at 5.9-7.2 kbar at 425°C. Modeling variable Fe 3+ indicates that the sodic-calcic blue amphibole (D2) formed under a higher oxidation state compared with the S1 assemblage, probably at <4.5 kbar. A SE-dipping, Mio-Pliocene sedimentary sequence (Gwoira Conglomerate) forms a hangingwall block juxtaposed by low-angle fault contact with the metabasite footwall. Prehnite-bearing D3 brittle fault zones separate the two blocks and likely accommodated the final exhumation of the S1 greenschist facies assemblage in the footwall. These results indicate that the extensive Mt Dayman fault surface coincides with a domed S1 greenschist facies foliation that was last active at >20 km depth. Exhumation of this foliation must therefore be controlled by brittle faults of the active microplate boundary that are largely not observed in the study area. The structural record of the final exhumation of the Dayman dome to the surface was likely lost as a result of erosion, poor exposure or wide spacing of semi-brittle to brittle fault zones.

Fluid flow in fractures and host rocks has been investigated in shallow buried Miocene alluvial fan deposits. A structural, petrographical (optical, CL, SE microscopes and XRD) and geochemical (microprobe and d 18 O-d 13 C stable isotopes) study has been performed in normal faults affecting Serravalian-Tortonian siliciclastic rocks of the Vallès-Penedès basin. These faults formed during the development of the Vallès-Penedès fault-related syncline, which caused the rotation of the earliest fractures. Faulting occurred continuously before, during and after host rock cementation. Rocks affected by faulting are represented by clay-rich gouges, which formed thanks to the high phyllite clast content within the otherwise clean and mature sandstones and conglomerates. Despite the low permeability of these rocks, cross-fault and fault-parallel fluid flows occurred in most of the faults. Host rocks and veins were cemented by two generations of calcite, i.e. Cc1 and Cc2. Cc1 precipitated from meteoric waters at shallow burial conditions whereas Cc2 precipitated from meteoric waters in a confined aquifer. Palygorskite has been identified in shear zones within the gouges indicating their later formation by interaction of Mg-rich fluids with previous smectites. These fluids probably derived from Miocene seawater expelled from the underlying TransitionaleMarine Complex "TMC" by compaction. Sedimentation, fracturing and cementation occurred in a very short lapse time of about 6e7 Ma, between the Serravalian-Tortonian age of the sediments and the end of the extensional tectonics in the Vallès-Penedès fault (Pliocene).

Triggered by reactivation of the strike-slip North Anatolian Fault, the disastrous M w 7.4 Kocaeli (Turkey) earthquake also produced normal faulting in the pull-apart basin of Gölcük. Surface scarps from such faulting reached almost 2.5 m in height. Several structures were crossed by the surface rupture. As expected, many of them either collapsed or were severely damaged. But, surprisingly, several structures survived the outcropping dislocation essentially unscathed. In fact, in some cases the surface rupture path seemed to have deviated, as if to "avoid" rupturing directly underneath a structure. In other cases damage was substantial even though the fault rupture was "masked" by the near-surface soil and did not create a scarp. The rigidity and continuity of the foundation appears to have been one of the crucial factors affecting structural performance. Interestingly, the examined structures were supported on a variety of foundation types, ranging from isolated footings, to rigid box-type foundations, to piled foundations. The paper outlines a reconnaissance of the area, providing a documented description of the observed interplay between the rupturing fault, the soil, and the structure, along with the results of soil exploration and geological trenching. In the companion paper, Part II, each system is analysed numerically to confirm the conclusions of the present paper, reveal the main aspects of Fault Rupture-Soil-Foundation-Structure Interaction (FR-SFSI), and help develop deeper insights into the mechanics of successful performance of structures built on top of such faults.

The Calabrian Arc is the epicentral region of one third of the strongest earthquakes of Italy (M w ≥ 7.0). These are confined within a narrow peninsula which is the emerging portion of a slab-related accretionary wedge, and all occurred in the past four centuries. Therefore, here more than anywhere in Italy, historical seismicity alone is not sufficient for seismic hazard assessment. We carried out geological and paleoseismological studies in southern Calabria that allowed characterizing the seismogenic behavior of the Cittanova fault which was responsible for one of the most catastrophic earthquakes to ever occur in Europe, the M w 7.0 5 February 1783 event. We have found out conclusive evidence for four Holocene earthquakes prior to 1783, with a recurrence time longer (~3.2 kyr) than the other Apennine faults (0.3-2.4 kyr). We have also estimated a robust slip rate for the late Upper Pleistocene (0.6 mm/yr) and an extension rate (0.4 mm/yr) that could reflect the residual back-arc opening of the Tyrrhenian Basin.

Puerto Rico is located within a zone of tectonic transition between mainly east-west, North America-Caribbean strike-slip motion to the west in Hispaniola and east-northeast-oriented underthrusting to the east beneath the Lesser Antilles island arc. Various models and tectonic mechanisms have been proposed for the Neogene to present-day deformation of southern Puerto Rico, its island margin, and the Muertos trench by previous workers that include normal, thrust, and strike-slip faulting accompanied by large-scale rotations. In this study, we present the results of a regional study integrating onland mapping of striated fault surfaces in rocks ranging in age from Oligocene to possibly as young as earliest Pliocene, and offshore mapping of faults deforming the uppermost sediments beneath the seafl oor. The tectonic geomorphology and distribution of late Quaternary marine terraces and beach ridges in south-central Puerto Rico suggest either stability or slow late Quaternary uplift along the south-central part of the coast. In contrast, the coastline of southwestern Puerto Rico exhibits no late Quaternary coastal sediments and a pattern of long-term drowning of coastal features. Fault striation studies of three formations composing the Puerto Rico-Virgin Islands carbonate platform of south-central Puerto Rico (Juana Diaz Formation basal clastic unit, Juana Diaz Formation upper carbonate unit, Ponce Formation) indicate two distinct extensional phases affecting the youngest formation (Ponce Formation of middle Miocene-early Pliocene age). The fi rst event, a north-northeast-directed extensional event is accommodated by normal faults striking mainly to the west-northwest. A second, southeast-directed extensional event crosscut and reactivated faults formed during the fi rst event and produced at least one northeast-trending Quaternary rift bounded by northeast-striking normal faults (Ponce basin). Offshore seismic profi ling by previous workers and reported in this study support the presence of late Holocene seafl oor-rupturing, northeast-striking normal faults that

The Cerro Goden fault zone is associated with a curvilinear, continuous, and prominent topographic lineament in western Puerto Rico. The fault varies in strike from northwest to west. In its westernmost section, the fault is ~500 m south of an abrupt, curvilinear mountain front separating the 270-to 361-m-high La Cadena de San Francisco range from the Rio Añasco alluvial valley. The Quaternary fault of the Añasco Valley is in alignment with the bedrock fault mapped by D. McIntyre (1971) in the Central La Plata quadrangle sheet east of Añasco Valley. Previous workers have postulated that the Cerro Goden fault zone continues southeast from the Añasco Valley and merges with the Great Southern Puerto Rico fault zone of south-central Puerto Rico. West of the Añasco Valley, the fault continues offshore into the Mona Passage (Caribbean Sea) where it is characterized by offsets of seafl oor sediments estimated to be of late Quaternary age. Using both 1:18,500 scale air photographs taken in 1936 and 1:40,000 scale photographs taken by the U.S. Department of Agriculture in 1986, we identifi ed geomorphic features suggestive of Quaternary fault movement in the Añasco Valley, including aligned and defl ected drainages, apparently offset terrace risers, and mountain-facing scarps. Many of these features suggest right-lateral displacement. Mapping of Paleogene bedrock units in the uplifted La Cadena range adjacent to the Cerro Goden fault zone reveals the main tectonic events that have culminated

Topography, bathymetry, regional structural observations, and fault slip measurements support the idea that the Mona rift is an active, offshore extensional structure separating a colliding area (eastern Hispaniola) from a subducting area (northwestern Puerto Rico). Near the city of Aguadilla in northwestern Puerto Rico, paleostress reconstruction through fault slip analysis demonstrates that the Mona rift is opening in an E-W direction. This fault slip analysis also indicates that the opening is oblique in the southern part of the Mona rift. We propose that oblique rifting results from accommodation of E-W extension by oblique right-lateral reactivation of previously mapped, NW-trending Eocene basement convergent structures (Aguadilla faults, Cerro Goden fault). The evolution of the stress fi eld during the Miocene and the present E-W opening of the Mona rift support the assumption that the Miocene 25° counterclockwise rotation of Puerto Rico has stopped and that this island is presently moving to the east relative to the colliding Hispaniola.

The D'Entrecasteaux Island (DEI) gneiss domes are fault-bounded domes with~2.5 km of relief exposing ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic gneisses and migmatites exhumed in an Oligocene-Miocene arc-continent collision and subduction zone subject to late Miocene to recent continental extension. Multichannel seismic reflection data and well data show the Trobriand basin formed as a fore-arc basin caused by southward Miocene subduction at the Trobriand trench. Subduction slowed at~8 Ma as the margin transitioned to an extensional tectonic environment. Since then, the Trobriand basin has subsided 1-2.5 km as a broad sag basin with few normal faults deforming the basin fill. South of the DEI, the Goodenough rift basin developed after extension began (~8 Ma) as the hanging wall of the north-dipping Owen-Stanley normal fault that bounds the basin's southern margin. The lack of upper crustal extension accompanying subsidence in the Trobriand and Goodenough basins suggests depth-dependent lithospheric extension since 8 Ma has accompanied uplift of the DEI gneiss domes. Structural reconstructions of seismic profiles show 2.3-13.4 km of basin extension in the upper crust, while syn-rift basin subsidence values indicate at least 20.7-23.6 km of extension occurred in the entire crust since~8 Ma. Results indicating thinning is preferentially accommodated in the lower crust surrounding the DEI are used to constrain a schematic model of uplift of the DEI domes involving vertical exhumation of buoyant, postorogenic lower crust, far-field extension from slab rollback, and an inverted two-layer crustal density structure.

We measure throw distributions for graben-bounding normal faults from two areas on Mars to investigate fault growth, displacement-length (D max-L) scaling, and extensional strain using a complementary suite of techniques. Faults in the northern plains are inferred to be restricted at 2-3 km depth, as shown by a transition from linear scaling, with D max-L ratios of w1 Â 10 À3 , to nonlinear scaling for faults >50 km long. On the Alba Patera volcano, faults conform to linear D max-L scaling, with a D max-L ratio of w6 Â 10 À3 , consistent with more deeply penetrating faults that are not restricted at depth. These grabens accommodate larger extensional strains (w0.84%) than the faults in the northern plains (w0.23%), with a temporal change from regionally distributed to localized deformation and associated increases in D max-L ratio, extensional strain, and perhaps down-dip fault height. The results suggest that both spatial and temporal variations in extensional strain and displacement-length scaling relations, along with fault restriction, are recorded by Martian fault populations.

Fallas normales de alto ángulo en el Neógeno del margen Atlántico de la Cuenca del Golfo San Jorge (46ºS-67º 30´O, Patagonia Argentina) R.E. Giacosa (1)(2) , J.M. Paredes (2) , A.M. Nillni (2) , M. Ledesma (2)(3) y F. Colombo (4) (1) Servicio Geológico Minero Argentino. CC 38 B, Km 8 (A-9003).

In accretionary prisms formed in subduction zones, accreted sediments deform due to the drag of the subducting plate, resulting in arc-normal compression. In the Cascadia and eastern Nankai subduction zones, however, arc-parallel normal faults, which indicate the occurrence of arc-normal extension, have been observed at the rear of the accretionary prisms. In this study, the deformation caused by the subducting plate in the accretionary prism is calculated using a simple fluid model that considers temperature-dependent rheology. The qualitative results show that a circulating flow is induced by the subducting plate under a lid formed at the rear of the accretionary prisms at higher temperatures, while, in lower-temperature prisms, the flow pattern is analogous to a simple corner flow, and the circulating flow is not induced. Prism materials flow trenchward just under the lid, and this return flow may be the cause of the arc-normal extension observed in the accretionary prisms. Most of the subduction velocity is accommodated by ductile deformation at the rear of the accretionary prisms, resulting in lower seismic coupling at the deeper interplate megathrusts. In the frontal part of the accretionary prisms, where imbricate thrusts develop, low rigidity due to high porosity and pore fluid pressure in the compacting sediment, together with higher concentrations of clay minerals with low frictional coefficients, probably prevents seismic slip at the interplate megathrusts. Locked zones develop between the extensional and imbricate thrust regions in the subduction zones. For low-temperature prisms, locked zones are located arcside of the imbricate thrust region. The fore-arc basin is thus formed above the locked zone in accretionary prisms. In the eastern Nankai subduction zone, another locked zone is estimated to be located in the lower crust. The location of the arc side of the imbricate arc-normal shortening (Booth-Rea et al., 2008; McNeill et al., 1997). In the middle of the prism, there is an active out-of-sequence thrust named the Cascadia main thrust (CMT), which is the seaward limit of the upper slope and shows displacements larger than that of the underlying thrusts (Booth-Rea et al., 2008). The lower slope between the CMT and the deformation front has very small tilts (<1°). The lower slope can mainly be divided into two domains, which are characterized by landward-vergent imbricate thrusts and folds without faults, arcward from the deformation front (Booth-Rea et al., 2008). The normal faults in the extensional region have arc-parallel strikes, indicating arcnormal extension. In the extensional region, landward-vergent, listric normal faults are dominant (Booth-Rea et al., 2008). Reflection seismic profiles show that the well-stratified layer in which the normal faults can be traced has a seismically opaque layer beneath. Stratigraphy results using outcrops and borehole data show that a chaotic and highly sheared formation from the Eocene to middle Miocene that is named the Mélange and Broken Formation (MBF) underlies younger stratified sediments; the opaque layer probably corresponds to the highly shared MBF (McNeill et al., 2000). The seaward stretch of the MBF is suggested to be the cause of the arc-normal extension in this region (McNeill et al., 2000). In this extensional region, the accretionary prism is suggested to be composed of accreted sediments from the middle Eocene and younger, which were thrust under the Crescend/Siletz mafic terrane in the Paleocene and early Eocene (Tréhu et al., 1994). In this region, the estimated western border of the mafic terrane warps around the Olympic Mountains and is located landward of the coast, as shown in Fig. 1 (McNeill et al., 1997;

Entre 1908 y 1922 Stappenbeck, Schiller, Keidel, Wichmann y Windhausen trabajaron en la zona por cuenta de la Dirección de Minas y Geología. En 1922 se creó Yacimientos Petrolíferos Fiscales, YPF, y se nombró al frente de la División Geología al italiano Guido Bonarelli. En 1925 llegó el italiano Egidio Feruglio a la recién creada Sección Geología de Comodoro Rivadavia. En 1926 se incorporó el ruso Vladimiro Vinda, y en 1927 se creó la Comisión Geológica del Golfo San Jorge, a cargo del italiano Enrico FossaMancini. Se integró con un equipo de geólogos italianos, rusos y argentinos. Feruglio, Piatnitzky, Stessin, Serghiescu, Ramaccioni, Chahnazaroff, Franceschi, Tarragona, Conci, Brandmayr, Wellhoefer, Biondi, Casanova, etc, son algunos de los protagonista entre 1927 y 1930. El primer trabajo encarado por la Comisión fue ajustar la estructura del yacimiento a partir de la nivelación de las capas terciarias y generar un modelo estructural de entrampamiento. Siguiendo los lineamientos...

The deposition of the Salamanca Formation and Rio Chico Group (Paleocene) in the Golfo San Jorge Basin exceeds the basin boundary of the underlying continental units of the Chubut Group (Cretaceous). These Paleocene units show major north-to-south stratigraphic changes for the north flank of the basin. On one hand, the accumulation of bioclastic deposits in the base of Salamanca Formation (Bustamante Member) only took place in the northern basin boundary, where the marine sedimentation occurred overlying Jurassic volcanic rocks of the Marifil Complex. In southward direction, the initial Paleocene transgression is represented by the epiclastic deposits of the Hansen Member (“Glauconitico”), overlying the Chubut Group (Cretaceous). In this way, the bioclastic deposits of the Bustamante Member are interpreted as an evidence of development of substrate-controlled marine invertebrate colonies, in which the volcanic rocks constituted a more favorable substrate to colonization of encrustin...

Reservoir rocks from the Amposta oil field have been investigated to understand the fracture and vug porosity and to reconstruct the complete fluid flow history of the basin. Four main types of fracture systems (A, B, C and D) and five types of calcite cements have been clearly identified and characterised petrologically. Fractures type A and B, totally filled by calcite cement 1 (CC1) and 2 (CC2), probably correspond to early fractures developed during the Alpine compression. Fractures type C are vertical fractures developed during the Tertiary extension. Fractures type C are filled by: (a) reddish microspar calcite sediment (CS3), probably meteoric-karstic in origin, and (b) blocky calcite cement type 4 (CC4), associated with kaolinite, pyrite, barite formation and main oil migration. Fractures type D, filled with the greenish calcite sediment 5 (CS5), probably developed during a local compression, affecting and deforming the previous cements and the host rock.

Fluid flow in fractures and host rocks has been investigated in shallow buried Miocene alluvial fan deposits. A structural, petrographical (optical, CL, SE microscopes and XRD) and geochemical (microprobe and d 18 O-d 13 C stable isotopes) study has been performed in normal faults affecting Serravalian-Tortonian siliciclastic rocks of the Vallès-Penedès basin. These faults formed during the development of the Vallès-Penedès fault-related syncline, which caused the rotation of the earliest fractures. Faulting occurred continuously before, during and after host rock cementation. Rocks affected by faulting are represented by clay-rich gouges, which formed thanks to the high phyllite clast content within the otherwise clean and mature sandstones and conglomerates. Despite the low permeability of these rocks, cross-fault and fault-parallel fluid flows occurred in most of the faults. Host rocks and veins were cemented by two generations of calcite, i.e. Cc1 and Cc2. Cc1 precipitated from meteoric waters at shallow burial conditions whereas Cc2 precipitated from meteoric waters in a confined aquifer. Palygorskite has been identified in shear zones within the gouges indicating their later formation by interaction of Mg-rich fluids with previous smectites. These fluids probably derived from Miocene seawater expelled from the underlying TransitionaleMarine Complex "TMC" by compaction. Sedimentation, fracturing and cementation occurred in a very short lapse time of about 6e7 Ma, between the Serravalian-Tortonian age of the sediments and the end of the extensional tectonics in the Vallès-Penedès fault (Pliocene).

La Patagonia Central está caracterizada por un desarrollo anormal del sistema orogénico Andino. Particularmente, La faja plegada y corrida de San Bernardo constituye un rasgo recientemente interpretado como un sistema de antepaís fragmentado de orientación NNW que se destaca por su ubicación lejana respecto de la trinchera (550 Km) y una gran extensión longitudinal (600 Km) (Folguera y Ramos 2011, Bilmes et al. 2013). Si bien existe cierto consenso respecto al principal mecanismo que construyó este sistema montañoso (inversión tectónica), a través del cual las sedimentitas del Grupo Chubut fueron exhumadas, la edad del levantamiento es aún discutida. Mientras que algunos autores proponen un levantamiento durante el Mioceno medio (Peroni et al. 1995, entre otros), otros favorecen un inicio del levantamiento más antiguo, en el Cretácico tardío (Lesta 1980) ó en el Paleoceno (Paredes et al. 2006). Hacia el este, aproximadamente a una distancia de 1200 Km desde la trinchera, deformación...

Curved thrust systems are commonly associated to linked frontal, oblique and lateral thrust ramps, which are oriented normal, oblique and parallel, respectively, to the regional compressive tectonic transport direction. Abrupt lateral variations of the thrust architecture usually occur in thrust belts across lateral/oblique ramps and their identification is crucial since they may mark the transition from different structural styles, kinematics of the thrust sheets and stratigraphic domains. Arcuate thrusts systems composed by linkage of frontal, oblique and lateral thrust ramps characterizing curved thrust belts likely indicate that the thrust system architecture has been strongly influenced by the sub-thrust inherited extensional fault pattern. Indeed, in fold-and-thrust belts that suffered an extensional fault heritage the localisation of lateral/oblique thrust ramps is strictly controlled by deep-rooted pre-orogenic normal faults that are transpressively reactivated when favourab...

Extensional fault system characterization using displacement profiles and maps: Examples from the Golfo San Jorge Basin. Displacement profiles and maps were used in the subsurface of the Golfo San Jorge Basin in the Manantiales Behr region to analyze the factors affecting fault growth and propagation, the kinematic evolution of the extensional system and the migration and trapping of hydrocarbons. Two types of faults were recognized: faults that deform the basement and the sedimentary cover, and faults that involve only the sedimentary cover. The vertical growth of the faults within the sedimentary cover was restricted by lithologic variations and the intersection with the free surface. Faults involving the basement are syn-sedimentary and show Dmax/L relationships and aspect ratios that allow the distinction between this type of faults and shallower fault planes. The geometric coherence shown in the aggregated displacement profile and map suggests a synchronous kinematic evolution of the faults in the sedimentary cover. The relationship between the observed fault sizes could be indicative of an early linkage of fault segments and a later increase of throw. Reactivation of the pre-existent basement fabric had little influence on the development of faulting in the sedimentary cover because the Pozo D-129 Formation acted as a structural decoupling level. The low degree of connection between the shallow and deep fault zones disregards the hypothesis of hydrocarbon charge from a local deep ktichen. The mapping of fault segments with higher values of displacement and the recognition of transfer zones may help to locate and delineate hydrocarbon traps in the footwall blocks.

El área analizada en este trabajo se localiza en el sector occidental del bloque Manantiales Behr, Flanco Norte de la cuenca del Golfo San Jorge. En subsuelo esta zona se conoce como depocentro Granson y está limitada hacia el norte y sur por los sistemas de fallas de El Alba y Escalante respectivamente; mientras que en dirección oeste limita con el Alto de Voster (Vergés et al., 2001).

A comienzos del siglo XX y como resultado de un trabajo científico y técnico de exploración del subsuelo de la provincia de Chubut, se descubría un importante yacimiento de petróleo en la zona de Comodoro Rivadavia. Tal descubrimiento, que curiosamente en el relato histórico suele ser asignado al azar, marcó el inicio de la extracción de petróleo comercialmente rentable en la Argentina.

En el año 1910 se creó la Dirección General de Explotación del Petróleo de Comodoro Rivadavia, que continuó explorando la región e inició la evaluación de las dificultades técnicas involucradas en una explotación formal de los yacimientos. Walter Schiller, prestigioso geólogo alemán que en ese momento se encontraba a cargo de dos departamentos científicos del Museo de Ciencias Naturales de La Plata -Geología y Geografía Física y Mineralogía y Petrografía- fue el encargado de llevar adelante el estudio geológico detallado del área a explotar.

Schiller, quien había viajado anteriormente a la región, no solo confirmó la existencia allí de importantes yacimientos de petróleo y gas, sino que también elaboró una detallada estratigrafía del área, algo completamente novedoso para la geología local. Además, durante su trabajo en el terreno explorado para la apertura de pozos, identificó la presencia de restos arqueológicos que colectó y trasladó al Museo, donde fueron ingresados como parte de la colección que lleva su nombre. La misma consta de una serie de artefactos líticos tales puntas de flechas, puntas de lanza y bolas de boleadoras, así como también restos de sedimentos y tierra cocida recolectados en las cercanías de los pozos División, San Jorge, Chubut y El Jefe.

Pasados más de 100 años desde su recolección, dicho conjunto arqueológico sumó una dimensión histórica a su valor como evidencia del pasado prehistórico de la región, al funcionar también como testigo del descubrimiento del primer yacimiento de petróleo de la Argentina. Es su presencia en la institución la que da cuenta de las circunstancias y el paisaje en el que ocurrió tal hecho histórico.

Curved thrust systems are commonly associated to linked frontal, oblique and lateral thrust ramps, which are oriented normal, oblique and parallel, respectively, to the regional compressive tectonic transport direction. Abrupt lateral variations of the thrust architecture usually occur in thrust belts across lateral/oblique ramps and their identification is crucial since they may mark the transition from different structural styles, kinematics of the thrust sheets and stratigraphic domains. Arcuate thrusts systems composed by linkage of frontal, oblique and lateral thrust ramps characterizing curved thrust belts likely indicate that the thrust system architecture has been strongly influenced by the sub-thrust inherited extensional fault pattern. Indeed, in fold-and-thrust belts that suffered an extensional fault heritage the localisation of lateral/oblique thrust ramps is strictly controlled by deep-rooted pre-orogenic normal faults that are transpressively reactivated when favourably oriented. The identification of inversion structures in thrust belts has a direct impact on both the unravelling of the structural style and the hydrocarbon prospectivity of a region, because the inversion processes control the petroleum system elements from the initial extension up through the later compressional deformation. The study of tectonic inversion-related folds in regions where the mechanical stratigraphy is well known is valuable for the development of geometric and kinematic models that may add useful information for unravelling the structural style of thrust belts.
In this contribution, we document the geometrical characteristics of inversion structures (i.e., push-up inversion structures and fault-bend reactivation anticlines) occurring along oblique thrust ramps, by examining some examples of buried Apulian structures in the Central-Southern Apennines (i.e., Setteporte and Mt. Taburno push-ups) and outcropping thrust-related folds in the Central-Northern Apennines of Italy (i.e., Sabini Mts. push-up and OAS hanging-wall fault-bend macro-anticlines). The different geometrical features between push-up inversion structures and fault-bend reactivation anticlines are functionally controlled by: (i) the mechanical characteristics of the involved sedimentary multilayer, (ii) the original architecture of the former extensional basin, and (iii) the amount of thrust displacement. The recognition of such inversion structures is important in understanding the anatomy and structural style of foreland fold-and-thrust belts; in addition, this recognition is critical in interpreting the geometry and types of structural traps that may be expected along oblique thrust ramps. This study reveals that push-up inversion structures and fault-bend reactivation anticlines could coexist along oblique thrust ramps within a unique curved thrust system and may be detected among analogous foreland thrust belts that experienced structural inheritance of extensional basins.

High-angle pre-thrusting normal faults have been largely described in the Central-Northern Apennines foreland fold-and-thrust belt, mostly basing on cross-cutting relationships and thickness/facies variations across hanging-wall and footwall blocks. In the Central Apennines foreland fold-and-thrust belt of Italy, hinterland-dipping low-angle faults juxtaposing younger rocks over older ones rest on the summits of the major thrust-related anticlines within their forelimbs. These interesting structures are differently interpreted by various authors and they have been largely considered as younger-on-older thrusts within out-of-sequence thrust systems, post-orogenic normal faults, or gravity-driven slides. Within this interesting and open debate, the reconstruction of the structural assemblages is strictly dependent on the interpretation of such younger-on-older low-angle faults as well as the unraveling of the tectonic style characterizing the thrust belt.
In this contribution, we attempt to unravel the genesis and tectonic significance of the low-angle faults juxtaposing younger rocks over older ones in the Central Apennines thrust belt, by analyzing some stunning examples (i.e., Mt. Camicia, Mt. Cafornia, and Vallepietra-Filettino-Mt.Ortara low-angle faults). A detailed geological and structural characterization, combined with balanced geological cross-sections validated through section restoration, focuses on clarifying the tectonic significance of such younger-on-older low-angle faults because they are strongly related to the tectonic setting of the Central Apennines fold-and-thrust belt. The structural and kinematic analyses corroborated by stratigraphic timing constraints allowed us to define their kinematic and timing because of the following: normal sense kinematic indicators on the low-angle fault surfaces, synthetic R-type Riedel shear planes, drag folds related to extensional shear, rotated conjugate sets of normal faults, cut-off angle relationships, and thrust-top basin deposits sealing the low-angle faults. All of the examples described in this study have these structural characteristics in common and the structural-geological data reveal that these younger-on-older low-angle faults are Miocene pre-thrusting normal faults that have been rotated assuming a low-angle of dip during the Neogene thrust related shortcut anticline growth. The interpretation proposed in this study is consistent with fault-bend and fault-propagation folding models associated with break-forward in sequence thrust propagation implying more conservative degrees of shortening (approximately one order of magnitude less) for the Central Apennines thrust system, compared to the previous out-of-sequence models.

Structural studies of synorogenic basins in Timor using field and remote sensing techniques provide new structural and geomorphic evidence for syn-collisional extension in the converging plate boundary zone between the Australian Plate and Banda Arc. Fault mapping and kinematic analysis at scales ranging from outcrop (<1 m2) to the dimensions of the active orogen in East Timor (~100 km2) identify a predominance of NW-SE oriented dextral-normal faults and NE-SW oriented sinistral-normal faults that collectively bound large (5-20 km2) bedrock massifs throughout the island. These fault systems intersect at non-Andersonian conjugate angles of approximately 120° and accommodate an estimated 20 km of NE-directed extension across the Timor orogen based on reconstructions of fault-dismembered massifs. Major orogen-parallel ENE-oriented faults on the northern and southern sides of Timor exhibit normal-sinistral and normal-dextral kinematics, respectively. The overall pattern of deformation is one of lateral crustal extrusion sub-parallel to the Banda Arc. Stratigraphic relationships suggest that extrusion began prior to 5.5 Ma, before pronounced rapid uplift of the orogen. We link this to progressive coupling of the arc to an underthrust plateau on the Australian Plate, and subduction of its ocean crust. Our results enable us to track the structural evolution of the upper crust during dramatic plate-boundary reorganizations accompanying the transition from subduction to collision. The deformation structures that we document suggest that both upper and lower plate deformation during incipient island arc-continent collision was largely controlled by the geometry and topography of the lower plate.