Basement-involved Deformation Research Articles

Evidence for a Late Cretaceous to Paleogene basement-involved retroarc wedge in the southern U.S. Cordillera: A case study from the northern Chiricahua Mountains, Arizona

Late Cretaceous to Paleogene contractional deformation in the southern U.S. Cordillera is commonly attributed to the Laramide Orogeny, in part because of the prevalence of moderate- to high-angle, basement-involved reverse faults. However, it is unclear if the tectonic models developed for the archetypal Laramide foreland belt in the U.S. Rocky Mountain region are applicable to the southern U.S. Cordillera. New geologic mapping of the northern Chiricahua Mountains in southeast Arizona, USA, indicates the presence of an originally sub-horizontal thrust fault, the Fort Bowie fault, and a thin-skinned ramp-flat thrust system that is offset by a younger thrust fault, the Apache Pass fault, that carries basement rocks. Cross-cutting relationships and new geochronologic data indicate deformation on both faults occurred between 60 Ma and 35 Ma. A biotite 40Ar/39Ar plateau age of 48 Ma from the hanging wall of the basement-involved Apache Pass fault is interpreted to record erosion related to reverse fault movement and rock uplift. The presence of thrust faults in southeast Arizona raises the possibility of a latest Cretaceous−Eocene retroarc orogenic wedge that linked the Sevier and Mexican thrust belts to the north and south, respectively. Basement-involved deformation does not rule out the presence of a retroarc wedge, and many Cordilleran orogenic systems include basement-involved thrusting.

Laramide crustal detachment in the Rockies: Cordilleran shortening of fluid-weakened foreland crust

ABSTRACTWhat causes previously stable continental crust in the forelands of Cordilleran orogenic systems to shorten during low-angle subduction? The National Science Foundation/EarthScope Bighorn Project combined seismic imaging of the crust and Moho with kinematic modeling of Laramide (Late Cretaceous–Paleogene) basement-involved deformation to address this question. In north-central Wyoming, asymmetrical ENE-verging upper-crustal folds are highly discordant with broader, N-trending warps in the Moho, indicating crustal detachment. Restorable cross sections of ENE-directed detachment at a depth of ~30 km, combined a smaller component of NNW–SSE shortening due to the east-narrowing shape of the crustal allochthon, can explain the anastomosing network of Laramide basement-cored arches without major deformation of the underlying mantle lithosphere.Thrust-related fold geometries and west-to-east initiation of deformation in the Laramide and Sevier thrust belts point to Cordilleran end-loading from the west. Differences between Laramide (~N65E) and plate (~N25E) convergence directions, along with the fanning of Laramide shortening directions from nearly E–W to the south to NE–SW to the north, indicate slip partitioning during end-loading west of the Rockies.Sub-horizontal detachment with a near-zero critical taper within cratonic crust suggests an extremely weak Laramide detachment zone during deformation. Analogous lower-crustal deformation in subduction forearcs is associated with slow earthquakes and slab dehydration. We hypothesize that low-angle subduction of the Farallon Plate suppressed fluid-consuming melting and corner-flow processes that characterize higher-angle subduction. This allowed subduction-generated fluids to escape upward into the overlying continental lithosphere, causing retrograde metamorphism and increased fluid pressure that facilitated crustal detachment. This hydration-based hypothesis predicts that crustal detachment will accompany major earthquakes in active analog orogens.

Basement-involved tectonism in the western Central Andes: Insights from the eastern Domeyko and Frontal cordilleras, and the Salar de Atacama Basin

Northern Chile is a natural laboratory to understand the relation between different basement-involved structural styles, mechanisms of basement uplift, and the interaction of Pre-Andean rift-related and Andean contractional structures. Field and geophysical data (two-dimensional seismic profiles) are key to understanding the geometry and kinematics of these structures in the subsurface. Herein, we analyzed the basement-involved tectonism in the easternmost part of the Domeyko Cordillera, the southern end of the Salar de Atacama Basin and the Frontal Cordillera in northern Chile based on an updated revision of the structures exposed on the surface and the other ones present in depth. Our results indicate that the Pre-Andean configuration of the region was the first-order factor controlling the basement-involved deformation. Late Permian to Jurassic rift-related, basement rooted, normal faults accommodated much of the shortening in the upper plate, thus triggering their partial and/or full reverse reactivation, situation that in some places was accompanied by purely reverse faulting, thus allowing the exhumation of large Paleozoic basement granitic blocks. This event occurred in a similar manner in the studied regions from Late Cretaceous to Paleocene in an arc-system geological context. The initial distribution of half-grabens, syn-rift and post-rift sequences governed the variations in the basement-involved structural styles ranging from basement-cored anticlines to tectonic wedges, among others. However, the flat-slab process has frequently been invoked as the main process responsible for the basement-involved tectonism in northern Chile. Our results suggest a strong control exerted by inherited stratigraphic and structural features more than variations in slab geometry.

The dichotomy in noble gas signatures linked to tectonic deformation in Wufeng-Longmaxi Shale, Sichuan Basin

Geochemical homogeneity in shale is often assumed when tracing subsurface fluids and characterizing sedimentary basins. This study presents measurements of the bulk gas composition, stable isotopes, and noble gas volume fraction and isotopes for shale gas samples collected from gas wells in the Wufeng-Longmaxi Shale, the southern Sichuan Basin, China. The dryness [C1/(C2 + C3)] ranging from 166.3 to 251.2, combined with δ13C1 and δDC1 that vary from −28.8 to −27.3‰ and − 153 to −145‰, respectively, point to a late mature thermogenic origin of hydrocarbon gas. 3He/4He ratios of gas samples are around 0.01 times the air value suggesting dominantly crust-derived He. 21Ne/22Ne and 40Ar/36Ar ratios of many gas samples are higher than the corresponding air values indicating the mixing of crustal and atmospheric noble gases. Multiple dichotomous patterns are observed in noble gas signatures of forelimb and backlimb samples, and depression and crest samples. 20Ne/22Ne ratios of some crest samples are higher than that of depression samples in the backlimb, pointing to the presence of diffusion-driven fractionation that is likely caused by the long-distance migration from depression to crest. Elemental ratios of air-derived noble gas isotopes – 22Ne/36Ar, 84Kr/36Ar, and 132Xe/36Ar are compared to the recharge water values, suggesting the interactions of oil, gas, and water phases in the shale over geologic time. Forelimb samples generally display older ages than backlimb samples, indicating a larger flux of external radiogenic 4He due to the higher density of deep faults in the forelimb area caused by the basement-involved deformation. The basement-involved deformation also causes pore collapse especially in the forelimb leading to a lower porosity that results in a more pristine noble gas signature in the forelimb due to the reduced impact of younger recharge water.

Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

The thermo-kinematic evolution of the eastern Aar Massif, Swiss Alps, was investigated using peak temperature data estimated from Raman spectroscopy of carbonaceous material and detailed field analyses. New and compiled temperature-time constraints along the deformed and exhumed basement-cover contact allow us to (i) establish the timing of metamorphism and deformation, (ii) track long-term horizontal and vertical orogenic movements and (iii) assess the influence of temperature and structural inheritance on the kinematic evolution. We present a new shear zone map, structural cross sections and a step-wise retrodeformation. From text{ca.;26,Ma} onwards, basement-involved deformation started with the formation of relatively discrete NNW-directed thrusts. Peak metamorphic isograds are weakly deformed by these thrusts, suggesting that they initiated before or during the metamorphic peak under ongoing burial in the footwall to the basal Helvetic roof thrust. Subsequent peak- to post-metamorphic deformation was dominated by steep, mostly NNW-vergent reverse faults (text{ca.} 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than 50^{circ} already at inception. This produced the massif-internal structural relief and was associated with large vertical displacements (7 km shortening vs. up to 11 km exhumation). From 14 Ma onwards, the eastern Aar massif exhumed “en bloc” (i.e., without significant differential massif-internal exhumation) in the hanging wall of frontal thrusts, which is consistent with the transition to strike-slip dominated deformation observed within the massif. Our results indicate 13 km shortening and 9 km exhumation between 14 Ma and present. Inherited normal faults were not significantly reactivated. Instead, new thrusts/reverse faults developed in the basement below syn-rift basins, and can be traced into overturned fold limbs in the overlying sediment, producing tight synclines and broad anticlines along the basement-cover contact. The sediments were not detached from their crystalline substratum and formed disharmonic folds. Our results highlight decreasing rheological contrasts between (i) relatively strong basement and (ii) relatively weak cover units and inherited faults at higher temperature conditions. Both the timing of basement-involved deformation and the structural style (shear zone dip) appear to be controlled by evolving temperature conditions.

Structural style and kinematics of the Taihang-Luliangshan fold belt, North China: Implications for the Yanshanian orogeny

AbstractThe Middle–Late Jurassic to earliest Cretaceous fold belts of the Yanshanian orogen in North China remain enigmatic with respect to their coeval deformation histories and possible relationship to the contemporaneous Cordilleran-style margin of eastern Asia. We present geological mapping, structural data, and a >400-km-long, strike-perpendicular balanced cross section for the Taihang-Luliangshan fold belt exposed in the late Cenozoic central Shanxi Rift. The northeast-southwest–trending Taihang-Luliangshan fold belt consists of long-wavelength folds (∼35–110 km) with ∼1–9 km of structural relief cored by Archean and Paleoproterozoic metamorphic and igneous basement rocks. The fold belt accommodated ≥11 km of northwest-southeast shortening between the Taihangshan fault, bounding the North China Plain, in the east and the Ordos Basin in the west. Geological mapping in the Xizhoushan, a northeast-southwest–oriented range within the larger Taihangshan mountain belt, reveals two major basement-cored folds: (1) the Xizhou syncline, with an axial trace that extends for ∼100 km and is characterized by a steep to overturned forelimb consistent with a southeast sense of vergence, and (2) the Hutuo River anticline, which exposes Archean–Paleoproterozoic rocks in its core that are unconformably overlain by shallowly dipping (<∼20°) Lower Paleozoic rocks. In the Luliangshan, Mesozoic structures include the Luliang anticline, the largest recognized anticline in the region, the Ningjing syncline, which preserves a complete section of Paleozoic to Upper Jurassic strata, and the Wuzhai anticline; together, these folds are characterized by a wavelength of ∼45–50 km. Shortening in the Taihang-Luliangshan fold belt is estimated to have occurred between ca. 160 Ma and 135 Ma, based on the age of the youngest deformed Upper Jurassic rocks in the Ningjing syncline, previously published low-temperature thermochronology, and regional correlations to better-studied Yanshanian fold belts. The timing of basement-involved deformation in the Taihang-Luliangshan fold belt, which formed >1000 km from the nearest plate margin, corresponds with the termination of arc magmatism along the eastern margin of Asia, implying a potential linkage to the kinematics of the westward-subducting Izanagi (paleo-Pacific) plate.

A sub-salt structural model of the Kelasu structure in the Kuqa foreland basin, northwest China

The Kuqa foreland basin, adjacent to the South Tianshan Mountains, is a major hydrocarbon accumulation basin in Western China. The Kelasu structural belt is the focus for hydrocarbon exploration in the basin due to the presence of ramp-related anticline traps and a thick salt seal. The model of the Kelasu sub-salt structure is still contentious because of the structural complexity and poor seismic imaging below the salt layer. The area–depth–strain (ADS) method is applied to the southern part of the Kelasu Fault, a regional fault that cuts basement rocks. The ADS results are consistent with the seismic data, which indicate that both thin-skinned thrusting and basement-involved deformation occur within the Kelasu structure, with the Kelasu Fault acting as the boundary between the two regions of contrasting deformation. The ADS results also suggest that the depth of the lower detachment of the thin-skinned thrust belt is 9.5–10 km, which may correspond to the base of the Triassic. The Kelasu structure has undergone approximately 8.15–10.76 km of horizontal shortening in the east and 16.34 km in the west of the structure.

Detrital record of initial basement exhumation along the Laramide deformation front, southern Rocky Mountains

New geochronological constraints on upper crustal exhumation in the southern Rocky Mountains help delineate the latest Cretaceous–Paleogene history of drainage reorganization and landscape evolution during Laramide flat-slab subduction beneath western North America. Detrital zircon U-Pb data from the Raton basin of southern Colorado and northern New Mexico define the inception of coarse-grained siliciclastic sedimentation and a distinctive shift in provenance, from distal to proximal sources, that recorded shortening-related uplift and unroofing along the Laramide deformation front of the northern Sangre de Cristo mountains. This Maastrichtian–early Paleocene (~70-65 Ma) change—from distal foreland accumulation of siliciclastic sediment derived from the thin-skinned Cordilleran (Sevier) fold-thrust belt to deposition of coarse-grained sediment proximal to a Laramide basement block uplift—reflects eastward deformation advance and reorganization of drainage systems that supplied large-volume Paleocene–lower Eocene deposits to the Gulf of Mexico. The timing of unroofing along the eastern deformation front is synchronous with basement-involved deformation across the interior of the Laramide province, suggesting abrupt wholesale uplift rather than a systematic inboard advance of deformation. The growth and infilling of broken foreland basins within the Laramide interior and at its margins had a major impact on continental-scale drainage systems, as the ponded/axial Laramide basins trapped large volumes of sediment and reorganized major source-to-sink sediment pathways.

Basement-involved deformation overprinting thin-skinned deformation in the Pampean flat-slab segment of the southern Central Andes, Argentina

AbstractIn the southern Central Andes, the Andean foreland was deformed due to Neogene shallowing of the Nazca slab beneath the South America plate. In this 27–33ºS Pampean flat-slab segment, the N-trending Argentine Precordillera transpressional fold-and-thrust belt and the Sierras Pampeanas broken foreland developed as a consequence of inward migration of the orogenic front. At 28ºS, a NNE-trending westward-dipping, thick Neogene synorogenic sequence is exposed in the Sierra de los Colorados, which shares deformation features of the Precordillera and the Sierras Pampeanas. Integration of new structural and kinematic data and available structural, kinematic, geophysical and palaeomagnetic information allows consideration of the Sierra de los Colorados area as part of the northern sector of the Precordillera during the middle Neogene. Atc. 9 Ma, basement block exhumation started with the uplift of the Sierra de Umango-Espinal that was triggered by deformation along the NE-trending Tucumán oblique belt. This stage marked the beginning of compartmentalization of the incipiently deformed Vinchina foreland. Sincec. 6.8–6.1 Ma, basement block uplift linked to the Miranda–Chepes and Valle Fértil NNW-trending sinistral transpressional belts, as well as kinking of the Neogene sequence by localized WNW-striking cross-strike structures, resulted in multiple segmentation that produced a complex mosaic of basement-block pieces. The overprint of these regional, basement-involved, oblique, brittle–ductile transpressional and cross-strike megazones could be related to high interplate coupling. Localized mechanical and rheological changes introduced by magmatism favoured this thick-skinned deformation overprint.

Sedimentary record of regional deformation and dynamics of the thick-skinned southern Puna Plateau, central Andes (26–27°S)

The Puna Plateau, adjacent Eastern Cordillera and the Sierras Pampeanas of the central Andes are largely characterized by thick-skinned, basement-involved deformation. The Puna Plateau hosts ∼N–S trending bedrock ranges bounded by deep-seated reverse faults and sedimentary basins. We contribute to the understanding of thick-skinned dynamics in the Puna Plateau by constraining regional kinematics of the poorly understood southern Puna Plateau through a multidisciplinary approach. On the southeastern plateau, sandstone modal composition and detrital zircon U–Pb and apatite fission-track data from Cenozoic strata indicate basin accumulation during the late Eocene to early Oligocene (∼38–28 Ma). Provenance analysis reveals the existence of a regional-scale basin covering the southern Puna Plateau during late Eocene to early Oligocene time (∼38–28 Ma) that was sourced from both the western plateau and the eastern plateau margin and had a depocenter located to the west. Petrographic and detrital zircon U–Pb data reveal erosion of proximal western and eastern sources after ∼12 Ma, in mid-late Miocene time. This indicates that the regional basin was compartmentalized into small-scale depocenters by the growth of basement-cored ranges continuing into the late Miocene (∼12–8 Ma). We suggest that the Cenozoic history of the southern Puna Plateau records the formation of a regional basin that was possibly driven by lithospheric flexure during the late Eocene to early Oligocene, before the growth of distributed basement-cored ranges starting as early as the late Oligocene.

The persistence and role of basin structures on the 3D architecture of the Marañón Fold-Thrust Belt, Peru

The 3D architecture of fold-thrust belts commonly involves thin-skinned and thick-skinned deformation. Both thick- and thin-skinned deformation styles have been suggested to occur in the Marañón Fold-Thrust Belt (MFTB) in Peru, but the relative timing and strain partitioning associated with them are not well understood. We demonstrate that inherited basement structures along the Peruvian convergent margin reactivated during the evolution of the MFTB. We present results from field mapping, interpretation of remote sensing imagery, and cross section construction and restoration. The results show that the Chonta Fault, a median pre-folding basin normal fault, was inverted and acted as a mechanical buttress during initial east-vergent contraction of the fold-thrust belt. This fault separates the belt into two domains of distinctly different structural styles. During the Eocene, units to the west of the Chonta Fault deformed by folding, using the fault as a buttress, and subsequently propagated eastward by thin-skinned thrusting. This was followed in the Miocene by west-vergent, basement-involved deformation, which overprinted the earlier east-vergent, thin-skinned structures. The proposed tectonic model of the MFTB highlights the role of basement-fault reactivation during orogenesis and the involvement of deep structures in partitioning deformation styles.

Arcabouço estrutural da Bacia do São Francisco nos arredores da Serra da Água Fria (MG), a partir da integração de dados de superfície e subsuperfície

Structural framework of the Sao Francisco Basin around Agua Fria Range (Minas Gerais, Brazil), from the integration of surface and subsurface data. The Agua Fria Range, located in the eastern portion of the Sao Francisco Basin, central Brazil, has an essential unique seismic profile (240-RL- 300). This section allows the interpretation of the three main structural styles recognizable in this area, which belongs to a tectonic compartment named Pirapora Salient, which is characterized by basement-involved deformation syngenetic to a tectonic inversion of an intracratonic basin. Integration of both seismic and field data made possible to establish three structural domains: Eastern, Central and Western. They are defined in the shallower tectonic level respectively as: a fault-related folds predominance; an extensive monoclinal; un- dulated subhorizontal strata. The deformation rate decreases westward. The eastern domain shows a dramatic structural relief of about 2500m resulting from a strong uplift of this portion related to a regional datum. Deep- er structural investigation around Agua Fria Range shows an inverted half-graben (Mild Inversion), which is limited by a basement high in its western border (Boqueirao High). While the easternmost normal fault system of Boqueirao High experimented weak structural inverson, the associated covering strata underwent significa- tive shortening as a result of folding related to horizontal displacement. The Boqueirao High formed a rigid buttress that controlled the internal folding/shortening of the synrift sequence. The generation of the Boqueirao Monoclinal was governed by the uplift of the Boqueirao High.The total shortening throughout the area is a result of two mechanisms: the inversion of originally normal faults asssociated to uplift of the basement blocks, together with the folding of covering strata enlarged by horizontal forces.

Arcabouço estrutural da Bacia do São Francisco nos arredores da Serra da Água Fria (MG), a partir da integração de dados de superfície e subsuperfície

The Água Fria Range, located in the eastern portion of the São Francisco Basin, central Brazil, has an essential unique seismic profile (240-RL- 300). This section allows the interpretation of the three main structural styles recognizable in this area, which belongs to a tectonic compartment named “Pirapora Salient”, which is characterized by basement-involved deformation syngenetic to a tectonic inversion of an “intracratonic” basin. Integration of both seismic and field data made possible to establish three structural domains: Eastern, Central and Western. They are defined in the shallower tectonic level respectively as: a fault-related folds predominance; an extensive monoclinal; undulated subhorizontal strata. The deformation rate decreases westward. The eastern domain shows a dramatic structural relief of about 2500m resulting from a strong uplift of this portion related to a regional datum. Deeper structural investigation around Água Fria Range shows an inverted half-graben (Mild Inversion), which is limited by a basement high in its western border (Boqueirão High). While the easternmost normal fault system of Boqueirão High experimented weak structural inverson, the associated covering strata underwent significative shortening as a result of folding related to horizontal displacement. The Boqueirão High formed a rigid buttress that controlled the internal folding/shortening of the synrift sequence. The generation of the Boqueirão Monoclinal was governed by the uplift of the Boqueirão High.The total shortening throughout the area is a result of two mechanisms: the inversion of originally normal faults asssociated to uplift of the basement blocks, together with the folding of covering strata enlarged by horizontal forces.

Holocene Faulting, San Bernardino Valley Area, San Bernardino County, California

Aerial photographic, seismologic, and trenching investigations in the San Bernardino, California, region have revealed several previously unrecognized faults that displace the ground surface. Geomorphic and soil-stratigraphic analyses indicate the displaced strata are of Holocene age, possibly as young as about 5,000–7,000 years old. Stratigraphic offsets measured in the trenches indicate vertical separations on the order of 0.3 to 1 m (1 to 3 ft) on several faults and horizontal separations of up to about 3 m (10 ft) on another. Individual observed faults are short and discontinuous, but they are associated with clusters of lineaments that extend for several kilometers. Analyses of 1930s vintage aerial photographs indicate that the trenched faults and associated lineaments form alignments with faults in Sycamore Wash and Cajon Wash (Glen Helen fault) in the mountains to the northwest and with subtle lineaments to the southeast within the City of San Bernardino and within the Santa Ana River flood plain. Geophysical investigations by others indicate offsets of basement and valley-fill sediments coinciding with the southern part of the alignment. The area of greatest lineament and fault concentration coincides with a dense cluster of small earthquakes that have occurred during the past few decades. These faults and lineaments and their association with ongoing earthquake activity and basement-involved deformation indicate a significant active fault or fault zone extending from the San Gabriel Mountains into San Bernardino Valley, subparallel to and between the San Jacinto fault and the San Andreas fault. Empirical fault-length/earthquake-magnitude relationships and displacement-magnitude relationships indicate that such a fault could be capable of generating a magnitude 6.0 to 6.75 earthquake. Displacement relationships, although poorly constrained, indicate that the slip rate is very slow, perhaps on the order of 1 mm/yr (0.04 in./yr), and the recurrence interval is long, perhaps on the order of several thousand years.

Out-of-sequence, basement-involved structures in the Sadlerochit Mountains region of the Arctic National Wildlife Refuge, Alaska: Evidence and implications from fission-track thermochronology

Fission-track thermochronology and structural analysis set limits on the timing and nature of structural development of the Sadlerochit Mountains, along the southern edge of the coastal plain in the Arctic National Wildlife Refuge (ANWR) of northeastern Alaska. The Sadlerochit Mountains are the northernmost part of the north-vergent Brooks Range fold-and-thrust belt and lie close to the Arctic continental margin. Thermochronology results indicate that sedimentary rocks exposed within Ignek Valley, south of the Sadlerochit Mountains, were subjected to two episodes of rapid cooling from elevated paleotemperatures at ca. 45 Ma and at some time since ca. 31 Ma, whereas similar-aged rocks exposed along the northern flank of the Sadlerochit Mountains cooled rapidly at ca. 45 Ma and ca. 27 Ma. Combined with five additional analyses from the Beli Unit #1 well, located northwest of the Sadlerochit Mountains, the thermochronology results indicate that the Sadlerochit Mountains region was progressively heated during Late Cretaceous through middle Eocene time, after which two major episodes of rapid cooling occurred in the middle Eocene at ca. 45 ± 3 Ma (±2σ) and in the late Oligocene at ca. 27 ± 2 Ma (±2σ). These episodes of rapid cooling are interpreted to have occurred in response to kilometer-scale erosional denudation resulting from rapid (≤5 m.y.) uplift due to structural thickening during the emplacement of thrust sheets in a basement-involved duplex. Initially, at least one thrust sheet was probably emplaced to the north of the Sadlerochit Mountains at ca. 45 Ma. Subsequently, at ca. 27 Ma, (1) the Sadlerochit Mountains thrust sheet was probably emplaced out of sequence behind the earlier-emplaced thrust sheet(s), and (2) basement-involved deformation formed structures beneath the coastal plain to the north. Both of these events occurred far within the continent, >1200 km from the southern Alaska convergent plate boundary. These results indicate that maximum burial, and hence peak hydrocarbon generation, occurred prior to middle Eocene time, before the formation of potential traps in and immediately north of the Sadlerochit Mountains. However, (1) hydrocarbons generated from these rocks could have migrated updip into existing stratigraphic traps prior to structural deformation, and (2) hydrocarbons generated later in more distal parts of the basin could have migrated updip into subsurface structures formed in middle Eocene and late Oligocene time north of the Sadlerochit Mountains and along strike to the east and west.

Seismic refraction data in the Gulf of Saint Lawrence: implications for the lower-crustal blocks

A refined model for the wide-angle reflection-refraction profile 88-3 that crosses the foreland basin of the Appalachian orogen is presented. The two major layers of the cratonic crust have velocities of 6.2 and 6.7 km·s-1 with low gradients. Near the exposed North American craton, the 6.7 km·s-1 velocity appears at the shallow depth of 13 km and the M discontinuity at greater than 40 km depth. Towards the orogen, a southeastward-dipping ramp is developed at mid-crustal levels at the top of the 6.7 km·s-1 layer. Near the Appalachian Front lower-crustal velocities typical of the Grenville are replaced with a velocity of 7.2 km·s-1. The collinear reflection profile confirms the southward-dipping ramp, the shallowing M discontinuity, and the lateral changes in the lower crust. Refraction profiles combined with reflection and gravity data are used to distinguish the Grenville lower-crustal block. The Grenville lower-crustal block is defined as a pattern of lower-crustal and upper-mantle reflectivity, with associated velocities, and a more negative gravity anomaly than lower-crustal blocks of the Appalachian terranes. The Grenville block terminates northwest of the coast of Newfoundland near the Appalachian Front. Thus, this lower-crustal province does not underlie the Humber tectono-stratigraphic zone. This is different than the relationship of the edge of the Grenville lower-crustal block to surface zones in New England. A thick-skinned tectonic style with basement-involved deformation appears to be typical of the Newfoundland Appalachians; in contrast, a thin-skinned structural style is observed in New England. The change in structural styles occurs in the bend of the Appalachian orogen across the Gulf of St. Lawrence.

Mid- to Late Cretaceous ductile deformation and thermal evolution of the crust in the northern Dome Rock Mountains, Arizona

Mid- to Late Cretaceous basement-involved deformation in the Maria Tectonic Belt of southeastern California and west-central Arizona varies in style from low-angle overthrusts to km-scale fold nappes. To establish the relationship between tectonite fabrics and thermal evolution of the rocks, structural and petrographic analysis was conducted in the northern Dome Rock Mountains, where the two different styles converge. A penetrative northeast-dipping foliation forms the most conspicuous fabric in an ∼5 km wide zone of distributed ductile deformation. Within the broader zone of ductile deformation, northeast-dipping shear zones contain evidence for both southwest- and northeast-directed deformation. Textural analysis of mylonitic rocks reveals that southwest-directed deformation occurred under prograde conditions, whereas northeast-directed deformation occurred under retrograde conditions. Southwest-directed deformation resulted in tubular (extreme sheath) fold development on a kilometer scale in the footwall of a reverse-sense shear zone. Northeast-directed deformation cuts the map-scale tubular fold and is inferred to have developed as a manifestation of gravitational collapse of rocks mechanically weakened by fluid infiltration and upper greenschist- to lower amphibolite-facies metamorphism. We propose that the mode and degree of fluid infiltration, rather than temperature or structural level alone, controlled the style of deformation in the Maria Belt.

Basement-cover relations, Sevier orogenic belt, northern Utah

Precambrian crystalline basement and sedimentary cover rocks were deformed within a regional anticline in the Wasatch Range of northern Utah during the Cretaceous to early Tertiary Sevier orogeny. This regional fold had a complex uplift and deformation history and is an imbricated ramp anticline that marks the transition to basement-involved deformation. Large-scale thrusting and folding within the basement-cored anticline produced about 60% shortening, and most of the slip on thrusts within the anticline was transferred into slip on frontal thrusts of the Idaho-Utah-Wyoming thrust belt. A network of ductile deformation zones (DDZ9s) accommodated heterogeneous internal deformation of the basement, and cleavage, minor folds, and vein arrays accommodated internal deformation of the sedimentary cover. Kinematics of internal deformation are consistent between basement and cover rocks in most areas and vary systematically with structural position, recording early layer-parallel shortening, localized layer-parallel shear and minor detachment of the cover, and later fanning of structures during large-scale folding. DDZ sets with opposite shear senses accommodated bulk coaxial flattening perpendicular to cleavage away from major thrust faults, and DDZ9s with consistent shear sense produced simple shear near major thrusts. Cleavage intensifies and refracts and minor folds tighten in the cover near major thrusts and glide horizons. Vein arrays record principal extension down the dip of cleavage, minor extension parallel to the regional fold axis, and differential displacements at high angles to fold axes. Slip on basement DDZ9s produced about 5% to 20% bulk shortening subparallel to the basement-cover contact. Shortening in the cover ranges from about 5% to 30% in most areas but reaches 65% near imbricate thrusts and along glide horizons, reflecting heterogeneous simple shear. Principal extension is down the dip of cleavage, and minor extension parallels local fold axes, consistent with the geometry of mesoscopic structures.

Analysis of Laramide Basement-Involved Deformation, Fra Cristobal Range, New Mexico: ABSTRACT

In the Fra Cristobal Range near Truth or Consequences, New Mexico, 250 to 500 m(820 to 1640 ft) of relative basement uplift is present. The contact between Precambrian crystalline basement and cover strata (Cambrian and younger) is folded along a north-south deformation front defining the margin of the Laramide uplift. The style of deformation changes along strike from an overturned fold to a central domain of imbricate thrusts of Precambrian granite, overlying a gentle anticline of cover strata, and then back to an overturned fold. These changes in style occur across narrow transition zones characterized by transverse folds and tear faults, suggesting some control from basement structure. West-dipping, high-angle reverse faults and south-plunging S-folds on the overturned eastern limb of the south-plunging, basement-involved anticline suggest eastward vergence with the basement, in part forcing the fold in the cover strata. Closely spaced fractures are present in the granite hinge of the main overturned, basement-cored fold. Abundant microfaults with grains visibly offset are present in thin section. These features indicate a cataclastic flow mechanism to accommodate shortening strain in the core of the fold. The structural styles of the basement-involved Laramide deformation are similar to Berg's fold-thrust model; shortening in the basementmore » is accommodated by imbricate thrusting in the central domain and cataclastic flow on a small scale in the basement-cored folds. Preexisting basement structure may have controlled the location of structural transitions within the belt of crustal shortening and uplift, and movement of basement blocks may have in part forced the observed folding.« less