Younger Extension Research Articles

Regional Exhumation and Tectonic History of the Shanxi Rift and Taihangshan, North China

AbstractThis study presents a comprehensive low‐temperature thermochronometric data set from the Shanxi Rift, Taihangshan, and eastern Ordos block in North China, including new apatite fission track and apatite (U‐Th‐Sm)/He data and published apatite and zircon fission track and (U‐Th‐Sm)/He data. We use these data and new thermal history inversion models to reveal that the Shanxi Rift and Taihangshan experienced an increase in cooling rates between ca. 110–70 Ma and ca. 50–30 Ma. A preceding ca. 160–135 Ma cooling event is generally restricted to the western rift margin in the Lüliangshan and Hengshan. In contrast, the ca. 50–30 Ma cooling event was widespread and occurred coevally with the opening of the Bohai Basin and slip across the NNE‐striking Eastern Taihangshan fault. In the southern rift zone, however, exhumation beginning ca. 50 Ma was likely associated with fault block uplift across the ESE–striking Qinling and Huashan faults, which accompanied the extensional opening of the Weihe Graben. Coeval fault slip along the NNE–striking Eastern Taihangshan faults and ESE–striking Qinling and Huashan faults was associated with NW‐SE extension in North China related to oblique subduction of the Pacific plate under Eastern Asia and slow convergence rates. The Shanxi Rift is commonly attributed to Late Miocene and younger extension, but our new thermochronologic data do not precisely record the onset of rifting. However, our inversion models do suggest ≤∼50°C of Neogene–Quaternary cooling, consistent with ≤∼2 km of footwall uplift across most range‐bounding faults.

Reconstructing late Cenozoic deformation in central Panamint Valley, California: Evolution of slip partitioning in the Walker Lane

New geologic mapping and Ar-Ar geochronology of the late Cenozoic volcanic-sedimentary units in central and southern Panamint Valley, California, provide the first known Miocene palinspastic reconstruction vectors for Panamint Valley. Panamint Valley contains active faulting and potentially accommodates a significant percentage of the slip of the Walker Lane at this latitude. Volcanism in Panamint Valley occurred during two time intervals, one ca. 15–13.5 Ma ago and a second ca. 4.5–4 Ma ago. The reconstruction vectors are based on unique relationships of sedimentary source areas and the only known Miocene intrusive zones to determine the displacement across Panamint Valley since ca. 15 Ma ago. The Argus Range was displaced ~17 km to the west-northwest, and the southern Slate Range was displaced 10.5 km to the north-northwest relative to the Panamint Range. Our displacement vector for reconstructing the past ~15 Ma of slip across Panamint Valley is 14 km shorter than previously published reconstruction models. We interpret this smaller slip value to be a function of the previous studies using displacement vectors that included a component of pre–15 Ma ago slip. The Harrisburg fault of the Tucki Mountain detachment system is a likely candidate for an earlier slip, possibly during the regionally observed extension during Late Cretaceous and Eocene. We created a model of the ca. 0–15 Ma ago displacement history of Panamint Valley using our new slip vectors and the slip vector for the Hunter Mountain fault. The Miocene extension begins with or slightly before ca. 15 Ma ago volcanism and may have continued to <~13.5 Ma ago. We interpreted the slip during Miocene extension to have occurred on one master detachment fault. Pliocene and younger extension is oblique to the Miocene extension, and the detachment fault was then cut up into discrete segments, the Emigrant, Panamint, and Slate Range detachment faults. The Panamint detachment was reactivated in an oblique normal sense, while slip on the other two detachment faults ceased; slip now occurs on nearby steeper normal faults. The Panamint detachment ends to the north and south in triple junctions: at the north end, slip is partitioned onto the Hunter Mountain and Towne Pass faults, and at the south end, slip is partitioned onto the Manly Pass and Southern Panamint Valley faults. The southern triple junction has an unstable geometry and it must migrate northward, lengthening the Southern Panamint Valley fault at the expense of the Panamint detachment. The continued slip on the unfavorably oriented low-angle Panamint detachment may be explained by the presence of weak fault gouge along it or by a regional pattern of slip partitioning. Major regional strike-slip faults, the Northern Death Valley and Garlock faults, are proximal to the northern and southern triple junctions. These two large faults may drag the two ends of the Panamint detachment with them, creating the triple junctions. The modern complex geometries and kinematics of Panamint Valley may therefore be a function of older structures being reactivated and interference with nearby faults.

Thermochronologic constraints on deformation and cooling history of high‐ and ultrahigh‐pressure rocks in the Qinling‐Dabie orogen, eastern China

The Hong'an block is the best place to study the exhumation of high‐pressure (HP) and ultrahigh‐pressure (UHP) metamorphic rocks in the Qinling‐Dabie orogen of eastern China because it lacks the extensive Cretaceous tectonic and thermal overprint observed in the Dabie Shan. We measured timing of deformation and rate of cooling of the HP‐UHP rocks by 40Ar/39Ar analyses of synkinematic minerals from tectonites of key structural zones in the Hong'an and Tongbai Shan. Normal‐sense shear along the north dipping Huwan detachment at the northern edge of the Hong'an block occurred between 237 and 231 Ma; this detachment facilitated the bulk of the exhumation of the HP‐UHP rocks. Our new 40Ar/39Ar ages, combined with U/Pb zircon and Sm/Nd ages of 245–240 Ma, suggest that exhumation of UHP rocks from mantle depths occurred at rates of 5–25 mm/yr from ∼245 to 230 Ma. The mountain range is a warped extensional footwall, within which white mica cooled from 225 to 205 Ma. Locally, younger extension is recorded by white mica recrystallization at 198–194 Ma, after which the entire block had cooled to below 300°C. Early Cretaceous 40Ar/39Ar ages from the Tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral‐oblique slip along the Xiaotian‐Mozitan fault along the northern boundary. Coeval dextral and sinistral shear zones along the northern and southwestern margins of the Hong'an and Dabie Shan would have caused eastward lateral extrusion of these two blocks, perhaps driven by collision of the Lhasa block with Eurasia.

Assessing the nature of tectonic contacts using fission‐track thermochronology: an example from the Calabrian Arc, southern Italy

Fission‐track thermochronology applied to the nappe pile of the Calabrian Arc of southern Italy, particularly within the continental basement rocks, has provided important new constraints on the nature of some of the tectonic contacts. In southern Calabria an important phase of lower Miocene crustal extension is indicated. In northern Calabria no Oligocene or younger extension is seen. Here, the emplacement of continental basement rocks with Alpine metamorphism over ophiolitic rocks with little or no metamorphism is constrained as a thrust of lower to middle Miocene age related to collision of the Calabrian Arc with the Adria plate margin. It is proposed that reduction in the plate convergence velocity during collision of a retreating subduction zone with a continental margin is, at least partly, an explanation for the onset of extension in southern Calabria during the Miocene.

Geophysical evidence for crustal thickness variations between the Denali and Tintina Fault Systems in west‐central Yukon

Analyses of receiver functions recorded at two broadband seismic stations in the northern Cordillera indicate that the crust is thinner beneath Dawson (∼ 35 km) than Whitehorse (∼ 39 km). A simple two‐dimensional gravity model, constrained by the seismic results, shows that this change in crustal thickness occurs at about 63°N, in a zone approximately 35 km wide, where the Moho dips at ∼8° to the south. The Bouguer anomaly high associated with the thinner crust can be traced westward into east‐central Alaska where crustal thinning and extension are well documented; therefore we propose that west‐central Yukon (north of 63°N) has been extended also. On the basis of geological and other geophysical data we examine three likely time windows, in the Cretaceous, early Tertiary, and Present. We discuss data which indicate that early Tertiary and younger extension may be related to the transfer of motion from the Denali Fault System inboard to the Tintina Fault System. Earlier Cretaceous extension has been variously attributed to collision related back arc extension, syncollisional processes, or gravitational collapse of an overthickened crustal section. The relative importance of extension in these periods remains nebulus; however, the correspondence between the transition in crustal thickness determined in this study and a mapped boundary between “lower” plate and "upper" plate rocks suggests mid‐Cretaceous extension exerted a significant influence on current Moho depths.

Quantitative analysis of populations of small faults

Tectonic strain includes contributions from small faults, with length less than the thickness of the brittle upper crust. This study derives algebra to determine the strain associated with small faults that follow a power-law (i.e. fractal) distribution, independent of assumptions concerning how they relate to large faults in the same region, and discusses how to apply this algebra for a range of small fault sample types. The method is illustrated using as case studies the Jurassic extension in the North Sea, the Miocene and younger extension in the Gulf of Suez, and the Hercynian strike-slip faulting the Badajoz-Cordoba fault zone in southern Spain. My preferred estimates for the strain accommodated on small normal faults, with length <10 km, are ∼0.04 in the North Sea and ∼0.1 in the Gulf of Suez: ∼20 and ∼30% of the overall extensional strain in each region. This estimate makes the percentage of strain accommodated on small faults in the North Sea smaller than values in studies that assume that a single power law fits both small and large faults. My preferred estimate for the strain accommodated on small strike-slip faults in the Badajoz-Cordoba fault zone is ∼0.3, which may be ∼75% of the overall strain accommodated in this zone. These faults are more closely packed than the normal faults in the other localities.

Tilt and rotation of the footwall of a major normal fault system: Paleomagnetism of the Black Mountains, Death Valley extended terrane, California

Research Article| October 01, 1993 Tilt and rotation of the footwall of a major normal fault system: Paleomagnetism of the Black Mountains, Death Valley extended terrane, California DANIEL K. HOLM; DANIEL K. HOLM 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar JOHN W. GEISSMAN; JOHN W. GEISSMAN 2Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131-1116 Search for other works by this author on: GSW Google Scholar BRIAN WERNICKE BRIAN WERNICKE 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar Author and Article Information DANIEL K. HOLM 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 JOHN W. GEISSMAN 2Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131-1116 BRIAN WERNICKE 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (10): 1373–1387. https://doi.org/10.1130/0016-7606(1993)105<1373:TAROTF>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DANIEL K. HOLM, JOHN W. GEISSMAN, BRIAN WERNICKE; Tilt and rotation of the footwall of a major normal fault system: Paleomagnetism of the Black Mountains, Death Valley extended terrane, California. GSA Bulletin 1993;; 105 (10): 1373–1387. doi: https://doi.org/10.1130/0016-7606(1993)105<1373:TAROTF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Paleomagnetic data have been obtained from Miocene intrusions, including crosscutting mafic and felsic dikes, and Proterozoic crystalline rocks to evaluate deformation during Miocene and younger extension and unroofing of the Black Mountains, Death Valley, California. Synrift intrusions contain a well-defined and, at the site level, well-grouped magnetization, interpreted to be of dual polarity, the in situ direction of which is discordant in declination and inclination with an expected late Cenozoic reference direction. The oldest of these intrusions (11.6 ± 0.2 Ma) gives a magnetite-dominated remanent magnetization of high coercivity and high laboratory unblocking temperatures (about 550 to 580 °C) that is interpreted to predate unroofing from midcrustal depths. In situ site mean directions of this magnetization are directed toward the west and west-northwest with moderate to shallow positive (down) and negative (up) inclinations. The variation in direction of magnetization, particularly inclination, with site locality around the turtleback structures along the western flank of the Black Mountains, is interpreted to result from folding of the intrusion after remanence acquisition.Younger intrusions (≤8.7 Ma) generally give magnetizations with inclinations similar to expected Miocene values. Two populations of in situ site means are identified: one with southwest declination and negative inclination; the other with northward declination and positive inclination.A preferred interpretation for footwall deformation involves, from oldest to youngest: (1) southwest-side down tilting of the entire range block of some 20° to 40° to possibly 70° and, at least locally, folding of the crystalline rocks, on a trend parallel to the Death Valley turtlebacks, between 11.6 and 8.7 Ma; (2) progressive east-to-west footwall unroofing between 8.7 and ca. 6.5 Ma; and (3) clockwise rotation (50° to 80°) of much of the Black Mountains after the core detached from stable terrane to the west. We interpret late rotation of the Black Mountains as oroflexure related to right-lateral shear along the Death Valley fault zone. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Large-scale longitudinal extension in the southern Apennines contractional belt, Italy

During late Cenozoic thrusting, the interior of the peri-Tyrrhenian orogenic belt in the southern Apennines underwent two episodes of nearly orthogonal extension. Early extension was oriented subparallel to the axis of the tectonic belt and formed in response to progressive thrust-belt arcuation. The length of the tectonic belt increased by ∼50%, and the longitudinal strain was accommodated by low-angle normal faults concentrated in tectonic domains recording up to 150%-200% extension. Younger extension, oriented at a high angle to the orogen, was accompanied by Pliocene-Pleistocene uplift and by southeasterly migration of Tyrrhenian Sea rifting.

Paleomagnetic evidence for en echelon crustal extension and crustal rotations in western Montana and Idaho

The Bitterroot metamorphic core complex is one of many Eocene core complexes in the northern Cordillera. A middle Eocene dike swarm cuts the Bitterroot metamorphic core complex and its hanging wall, the Skalkaho slab. A 26° change in strike of the dikes across the core complex suggests either a refracted stress field or rotation during Eocene or younger extension. The nearby Bitterroot Valley and Big Hole Basin also indicate tectonic rotation. To distinguish between the hypotheses of refracted stress and tectonic rotation, we collected paleomagnetic data from 28 dikes. Nineteen sites provide reliable results. Eight sites from the hanging wall east of the mylonite zone give Dec = 46°, Inc = 69°, α95 = 13°; 11 dikes to the west yield Dec = 334°, Inc = 64°, α95 = 6°. The declinations diverge from the expected direction; the inclinations do not. The hanging wall rocks are rotated 58° ± 32° clockwise at the 95 % confidence level. The footwall shows 14° ± 11° of counterclockwise rotation. Both normal and reversed polarity dikes are present in each area, indicating that the dike swarms may adequately average paleosecular variation. Vertical contacts reveal that the dikes have not tilted since emplacement. These paleomagnetic results and available geochronologic data show that the Bitterroot dome was pulled from beneath the Skalkaho slab in the Eocene. As the dome slid toward the northwest, it experienced a slight counterclockwise rotation, while the upper plate moved clockwise around a vertical axis near its northern edge. Rotation of the Skalkaho slab occurs because the slab lies between en echelon extensional detachments of the core complex and the next basin to the east, the Big Hole Basin. The Boehls Butte area, north central Idaho, occupies a step in the Lewis and Clark fault zone and is probably a core complex related to right‐lateral motion on the fault system. Similarities in both the style and timing of extension suggest that Eocene ductile/brittle extension occupies a swath from British Columbia, past the Bitterroot metamorphic core complex, to the Pioneer core complex in south central Idaho.