Abstract Recent earthquakes have demonstrated that rupture may propagate through geometrically complex networks of faults. Ancient exhumed faults have the potential to reveal the details of complex rupture at seismogenic depths. We present a new set of field observational criteria for determining which of a population of pseudotachylyte fault veins formed in the same earthquake and apply it to map rupture networks representing single earthquakes. An exceptional exposure of an exhumed ancient strand of the Norumbega Shear Zone preserves evidence of multistranded earthquake rupture in the deep seismogenic zone of a continental transform fault. Individual fault strands slipped at least 2–18 cm, so significant slip is represented by each rupture network. Our data show that synchronously slipped faults intersect at angles of 0 to ∼55°, with the opening angles of fault intersections directed toward the dilational quadrants for dextral slip. Multistranded rupture on a fault network instead of rupture of a single fault may result in greater and/or more variable slip and cause slip rake to vary spatially and temporally. Slip on intersecting faults unequivocally means that there will be motion perpendicular to the average fault plane. Modern earthquakes displaying non‐double‐couple components to focal mechanism solutions and spatially varying rake, slip, and anomalous stress drop may be explained by rupture across fault networks that are too close (spatially and temporally) to be resolved seismically as separate events.
Research Article| May 01, 1986 Preexisting fault control for Mesozoic basin formation in eastern North America Mark T. Swanson Mark T. Swanson 1Department of Geosciences, University of Southern Maine, Gorham, Maine 04038 Search for other works by this author on: GSW Google Scholar Author and Article Information Mark T. Swanson 1Department of Geosciences, University of Southern Maine, Gorham, Maine 04038 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (5): 419–422. https://doi.org/10.1130/0091-7613(1986)14<419:PFCFMB>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Mark T. Swanson; Preexisting fault control for Mesozoic basin formation in eastern North America. Geology 1986;; 14 (5): 419–422. doi: https://doi.org/10.1130/0091-7613(1986)14<419:PFCFMB>2.0.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 SocietyGeology Search Advanced Search Abstract A specific near-universal structural control for Mesozoic basin formation is apparent within the Appalachians as a preexisting system of high-angle reverse and dextral strike-slip faults of late Paleozoic and older age. These discrete crustal discontinuities, rather than just the general structural grain, were reactivated during extension associated with Late Triassic-Early Jurassic rifting between eastern North America and western Africa. Selective reactivation of high-angle fault exposures, as well as the subsurface reactivation of horizontal thrust surfaces, has played an important role in localizing rift basin formation in eastern North America. 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.
Research Article| November 01, 1986 Comment and Reply on “Preexisting fault control for Mesozoic basin formation in eastern North America”: REPLY Mark T. Swanson Mark T. Swanson 1Department of Geosciences, University of Southern Maine, Gorham, Maine 04038 Search for other works by this author on: GSW Google Scholar Geology (1986) 14 (11): 974. https://doi.org/10.1130/0091-7613(1986)14<974:CAROPF>2.0.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 Mark T. Swanson; Comment and Reply on “Preexisting fault control for Mesozoic basin formation in eastern North America”: REPLY. Geology 1986;; 14 (11): 974. doi: https://doi.org/10.1130/0091-7613(1986)14<974:CAROPF>2.0.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 SocietyGeology Search Advanced Search Abstract No Abstract Available. 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.
The southwestern end of the Norumbega fault zone forms a 14° restraining bend in a prominent orogen-parallel dextral strike-slip system within the northern Appalachians. Late Paleozoic transpression at the Casco Bay restraining bend was controlled by the geometry of dextral strike-slip faults within an otherwise regional transtensional dextral strike-slip system. Regional shearing within this system may have begun in the Early Devonian and persisted until the latest Paleozoic with late high strain localization and syntectonic to post-tectonic granitic intrusion. Deformation related to this restraining bend involved distributed dextral shear strain and crustal shortening in a complex zone, ~30 km wide, referred to as the Casco Bay shear zone system. Regional upright F2 folds in this area are oblique to subparallel to the fault trace and developed strong hinge-parallel elongation. This is reflected in initially orthogonal boudin partings and veins within upright fold limbs. As these F2 folds were tightened and reoriented toward the fault trace, the limbs developed pervasive dextral shear fabrics and asymmetric kinematic indicators. Higher ductile shear strains were accommodated along lithologic contacts and within less-competent units. The cross-section geometry of the Casco Bay shear zones is interpreted as a positive flower structure that cored a transpressional zone of shortening, thickening, and dextral shear accompanied by strain-localized syntectonic intrusion. Oblique-slip lineations in a number of zones suggest vertical components to dextral shear including an early phase of dextral overthrusting to the northwest and a later phase of dextral oblique extension, down to the southeast, along the main Flying Point fault zone. Late structures including small normal faults, normal kink-bands to several meters in width, chevron folds, and crenulations with horizontal axial planes suggest late vertical shortening of the transpressional uplift. Swanson, M. T., 1999, Dextral transpression at the Casco Bay restraining bend, Norumbega fault zone, coastal Maine, in Ludman, A., and West, D. P., Jr., eds., Norumbega Fault System of the Northern Appalachians: Boulder, Colorado, Geological Society of America Special Paper 331.
Stretching lineations (L2) throughout the high-grade metamorphic rocks of the Casco Bay area are defined by the alignment of grain aggregates and elongate minerals generally parallel to subhorizontal upright F2 fold hinges. L2 lineations were developed due to regional layer-parallel shear related to dextral transpression along the Flying Point segment of the Norumbega Fault Zone during the later Paleozoic. The reorientation of boudin partings, quartz veins and pegmatite intrusions, the asymmetry of boudin pods, late vein folds and crenulations as well as a range of microscopic kinematic indicators within these rocks clearly indicate an overall dextral shear sense and a variable dip-slip component with local transport directions parallel to L2 during deformation. The distribution of L2 lineations about the trace of the NE- trending Flying Point Fault Zone shows: (a) E-plunging L2 in a broad zone on the NW side within SE-dipping, locally, pegmatite-injected, porphyroclastic schists and gneisses and; (c) sub-horizontal L2 within subordinate fault slices of folded Casco Bay Group lithologies to the SE. The Flying Point Fault zone itself consists of the straight planar gneisses and related rocks as a 2 km wide corridor of high shear strain reflected in the development of quartz-vein sheath folds parallel tomore » L2. Variably-deformed mafic and felsic intrusions preserved as asymmetric pods and lenses within the flanking lithologies have been obliterated within this zone of high shear strain. This kinematic pattern and distribution of lineations is interpreted as an asymmetric transpressional uplift dominated by a broad NW front suffering oblique escape toward the west under dextral reverse motions and a major near-vertical zone of decoupling that developed at a restraining bend at the southwest end of the Norumbega Fault Zone.« less
