scholarly journals Overview of the new 1:25,000-scale geologic mapping of the McCallum-Slate Creek fault system, Eastern Alaska Range, Alaska

2019 ◽  
Author(s):  
R.J. Gillis ◽  
P.G. Fitzgerald ◽  
K.D. Ridgway ◽  
B.M. Keough ◽  
J.A. Benowitz ◽  
...  
Keyword(s):  
Fault System ◽  
Geologic Mapping ◽  
Alaska Range

scholarly journals Northward migration of the Oregon forearc on the Gales Creek fault

Geosphere ◽  
2020 ◽  
Vol 16 (2) ◽  
pp. 660-684 ◽  
Author(s):  
Ray E. Wells ◽  
Richard J. Blakely ◽  
Sean Bemis
Keyword(s):  
Gravity Data ◽  
Magnetic Anomalies ◽  
Fault System ◽  
Coast Range ◽  
Geologic Mapping ◽  
Willamette Valley ◽  
Potential Field Data ◽  
Columbia River Basalt ◽  
Geophysical Surveys ◽  
Step Over

Abstract The Gales Creek fault (GCF) is a 60-km-long, northwest-striking dextral fault system (west of Portland, Oregon) that accommodates northward motion and uplift of the Oregon Coast Range. New geologic mapping and geophysical models confirm inferred offsets from earlier geophysical surveys and document ∼12 km of right-lateral offset of a basement high in Eocene Siletz River Volcanics since ca. 35 Ma and ∼8.8 km of right-lateral separation of Miocene Columbia River Basalt at Newberg, Oregon, since 15 Ma (∼0.62 ± 0.12 mm/yr, average long-term rate). Relative uplift of Eocene Coast Range basalt basement west of the fault zone is at least 5 km based on depth to basement under the Tualatin Basin from a recent inversion of gravity data. West of the city of Forest Grove, the fault consists of two subparallel strands ∼7 km apart. The westernmost, Parsons Creek strand, forms a linear valley southward to Henry Hagg Lake, where it continues southward to Newberg as a series of en echelon strands forming both extensional and compressive step-overs. Compressive step-overs in the GCF occur at intersections with ESE-striking sinistral faults crossing the Coast Range, suggesting the GCF is the eastern boundary of an R′ Riedel shear domain that could accommodate up to half of the ∼45° of post–40 Ma clockwise rotation of the Coast Range documented by paleomagnetic studies. Gravity and magnetic anomalies suggest the western strands of the GCF extend southward beneath Newberg into the Northern Willamette Valley, where colinear magnetic anomalies have been correlated with the Mount Angel fault, the proposed source of the 1993 M 5.7 Scotts Mills earthquake. The potential-field data and water-well data also indicate the eastern, Gales Creek strand of the fault may link to the NNW-striking Canby fault through the E-W Beaverton fault to form a 30-km-wide compressive step-over along the south side of the Tualatin Basin. LiDAR data reveal right-lateral stream offsets of as much as 1.5 km, shutter ridges, and other youthful geomorphic features for 60 km along the geophysical and geologic trace of the GCF north of Newberg, Oregon. Paleoseismic trenches document Eocene bedrock thrust over 250 ka surficial deposits along a reverse splay of the fault system near Yamhill, Oregon, and Holocene motion has been recently documented on the GCF along Scoggins Creek and Parsons Creek. The GCF could produce earthquakes in excess of Mw 7, if the entire 60 km segment ruptured in one earthquake. The apparent subsurface links of the GCF to other faults in the Northern Willamette Valley suggest that other faults in the system may also be active.

scholarly journals Tectonostratigraphy and major structures of the Georgian Greater Caucasus: Implications for structural architecture, along-strike continuity, and orogen evolution

Geosphere ◽  
2022 ◽  
Author(s):  
Charles C. Trexler ◽  
Eric Cowgill ◽  
Nathan A. Niemi ◽  
Dylan A. Vasey ◽  
Tea Godoladze
Keyword(s):  
Fault System ◽  
Thrust Faults ◽  
Geologic Mapping ◽  
Greater Caucasus ◽  
The Core ◽  
The North ◽  
Thrust Sheets ◽  
Southern Half ◽  
Caucasus Mountains ◽  
Structural Architecture

Although the Greater Caucasus Mountains have played a central role in absorbing late Cenozoic convergence between the Arabian and Eurasian plates, the orogenic architecture and the ways in which it accommodates modern shortening remain debated. Here, we addressed this problem using geologic mapping along two transects across the southern half of the western Greater Caucasus to reveal a suite of regionally coherent stratigraphic packages that are juxtaposed across a series of thrust faults, which we call the North Georgia fault system. From south to north within this system, stratigraphically repeated ~5–10-km-thick thrust sheets show systematically increasing bedding dip angles (<30° in the south to subvertical in the core of the range). Likewise, exhumation depth increases toward the core of the range, based on low-temperature thermochronologic data and metamorphic grade of exposed rocks. In contrast, active shortening in the modern system is accommodated, at least in part, by thrust faults along the southern margin of the orogen. Facilitated by the North Georgia fault system, the western Greater Caucasus Mountains broadly behave as an in-sequence, southward-propagating imbricate thrust fan, with older faults within the range progressively abandoned and new structures forming to accommodate shortening as the thrust propagates southward. We suggest that the single-fault-centric “Main Caucasus thrust” paradigm is no longer appropriate, as it is a system of faults, the North Georgia fault system, that dominates the architecture of the western Greater Caucasus Mountains.

scholarly journals Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary

Geosphere ◽  
2019 ◽  
Vol 15 (4) ◽  
pp. 1066-1101 ◽  
Author(s):  
Jeffrey M. Trop ◽  
Jeff Benowitz ◽  
Ronald B. Cole ◽  
Paul O’Sullivan
Keyword(s):  
Late Cretaceous ◽  
Detrital Zircon ◽  
Volcanic Rocks ◽  
Strike Slip ◽  
Suture Zone ◽  
Fault System ◽  
Arc Magmatism ◽  
Alaska Range ◽  
Denali Fault ◽  
Dike Swarms

AbstractThe Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks with geochemical compositions transitional between arc and intraplate affinity. New detrital-zircon data from the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma, which is also reflected in zircon Pb loss in Cantwell Formation bentonites. Teklanika Formation volcanism may reflect hypothesized slab break-off and a Paleocene–Eocene period of a transform margin configuration. Mafic dike swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized by strike-slip extension following a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma. Diking represents the last recorded episode of significant magmatism in the central and eastern Alaska Range, including along the Denali fault. Two tectonic models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene arc root and/or thickened suture zone lithosphere, or a slab window created during possible Paleocene slab break-off. Fluvial strata exposed just south of the Denali fault in the central Alaska Range record synorogenic sedimentation coeval with diking and inferred strike-slip displacement. Deposition occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons. U-Pb detrital-zircon geochronology and clast compositional data indicate the fluvial strata were derived from sedimentary and igneous bedrock presently exposed within the Alaska Range, including Cretaceous sources presently exposed on the opposite (north) side of the fault. The provenance data may indicate ∼150 km or more of dextral offset of the ca. 29 Ma strata from inferred sediment sources, but different amounts of slip are feasible.Together, the dike swarms and fluvial strata are interpreted to record Oligocene strike-slip movement along the Denali fault system, coeval with strike-slip basin development along other segments of the fault. Diking and sedimentation occurred just prior to the onset of rapid and persistent exhumation ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture zone is interpreted to reflect the translation along and convergence of southern Alaska across the Denali fault driven by highly coupled flat-slab subduction of the Yakutat microplate, which continues to accrete to the southern margin of Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma created a more convergent regime along the apex of the Denali fault curve, likely contributing to the shutting off of near-fault extension-facilitated arc magmatism along this section of the fault system and increased exhumation rates.

Laramide Uplift near the Ray and Resolution Porphyry Copper Deposits, Southeastern Arizona: Insights into Regional Shortening Style, Magnitude of Uplift, and Implications for Exploration

2020 ◽  
Vol 115 (1) ◽  
pp. 153-175
Author(s):  
Daniel A. Favorito ◽  
Eric Seedorff
Keyword(s):  
Porphyry Copper ◽  
Black Hills ◽  
Fault System ◽  
Normal Faults ◽  
Reverse Fault ◽  
Geologic Mapping ◽  
Porphyry Copper Deposits ◽  
Kinematic Modeling ◽  
Copper Deposits ◽  
Strike Direction

Abstract This study integrates new geologic mapping and structural analysis with previous work near Walnut Canyon and Telegraph Canyon to address the style and magnitude of shortening and the relationship between contractional structures and porphyry preservation and localization between the Ray and Resolution porphyry copper deposits. Cenozoic extensional structures were superimposed on earlier contractional structures formed during the Laramide orogeny, which dates from ~80 to 50 Ma. This superposition requires that Cenozoic normal faults be restored prior to analysis of Laramide contractional structures and their relationship to nearby porphyry copper deposits. Five distinct sets of normal faults within the study area progressively tilted the region 65° east. The amount of extension was 10.3 km or 276%. Using key constraints such as offset strata, cutoff angles between faults and various units, and Laramide fault geometries, the study area was structurally reconstructed and verified using 2-D kinematic modeling of reverse fault offset and related folding. Total shortening is 7.2 km or 98%. Laramide reverse faults are interpreted as thick-skinned basement-cored uplifts, because they restore to moderate angles, have related fault-propagation folds, and involve significant crystalline basement rock. The Telegraph Canyon reverse fault has at least 5.3 km of offset, and the Walnut Canyon reverse fault has 3.2 km. The preferred estimate of the total vertical uplift for the fault system is 5.2 km but could be several kilometers greater. The restored strike direction of these faults combined with mid-Cenozoic erosion surfaces throughout the region suggests that this fault system may be responsible for the Laramide uplift of the Tortilla Mountains and Black Hills. In addition, most major porphyry centers appear to have been intruded into the footwall of this large uplift, with local examples including Ray and Resolution, suggesting that topography generated from this uplift may have been critical to preservation of these ore systems. Though definitive crosscutting relationships do not exist in the immediate map area, geologic relationships in a broader area suggest that shortening here began after 74 Ma and, in the Ray area, had ended by ~69 Ma and that porphyry formation postdated reverse faulting by as much as 5 m.y. to as little as <1 m.y.

scholarly journals Cretaceous to Oligocene magmatic and tectonic evolution of the western Alaska Range: Insights from U-Pb and 40Ar/39Ar geochronology

Geosphere ◽  
2020 ◽  
Author(s):  
James V. Jones ◽  
Erin Todd ◽  
Stephen E. Box ◽  
Peter J. Haeussler ◽  
Christopher S. Holm-Denoma ◽  
...  
Keyword(s):  
Tectonic Setting ◽  
Volcanic Rocks ◽  
Geologic Mapping ◽  
Localized Deformation ◽  
Alaska Range ◽  
South Central ◽  
Wrangellia Terrane ◽  
Terrane Accretion ◽  
Latest Eocene ◽  
Slab Window

New U-Pb and 40Ar/39Ar ages integrated with geologic mapping and observations across the western Alaska Range constrain the distribution and tectonic setting of Cretaceous to Oligocene magmatism along an evolving accretionary plate margin in south-central Alaska. These rocks were emplaced across basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Triassic and Jurassic plutonic and volcanic rocks of the Peninsular terrane, and Jurassic and Cretaceous siliciclastic strata of the Kahiltna assemblage. Plutonic rocks of different ages also host economic mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related rare-earth elements. The oldest intrusive suites were emplaced ca. 104–80 Ma into the Peninsular terrane only prior to final accretion. Deformation of the northern Kahiltna succession and underlying Farewell terrane occurred at ca. 97 Ma, and more widespread deformation ca. 80 Ma involved south-ver­gent folding and thrusting of the Kahiltna assemblage that records collisional accretion of the Peninsular-Wrangellia terrane and juxtaposition of sediment wedges formed on the inboard and outboard terranes. More widespread mag­matism ca. 75–55 Ma occurred in two general pulses, each having distinct styles of localized deformation. Circa 75–65 Ma plutons were emplaced in a transpressional setting and stitch the accreted Peninsular and Wrangellia terranes to the Farewell terrane. Circa 65–55 Ma magmatism occurred across the entire range and extends for more than 200 km inboard from the inferred position of the continental margin. The Paleocene plutonic suite generally reflects shallower emplacement depths relative to older suites and is associ­ated with more abundant andesitic to rhyolitic volcanic rocks. Deformation ca. 58–56 Ma was concentrated along two high-strain zones, the most prominent of which is 1 km wide, strikes east-northeast, and accommodated dextral oblique motion. Emplacement of widespread intermediate to mafic dikes ca. 59–51 Ma occurred before a notable magmatic lull from ca. 51–44 Ma reflect­ing a late Paleocene to early Eocene slab window. Magmatism resumed ca. 44 Ma, recording the transition from slab window to renewed subduction that formed the Aleutian-Meshik arc to the southwest. In the western Alaska Range, Eocene magmatism included emplacement of the elongate north-south Mer­rill Pass pluton and large volumes of ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, a latest Eocene to Oligocene magmatic pulse involved emplacement of a compositionally variable but spatially concentrated suite of magmas ranging from gabbro to peralkaline granite ca. 35–26 Ma, followed by waning magmatism that coincided with initiation of Yakutat shallow-slab subduction. Cretaceous to Oligocene magmatism throughout the western Alaska Range collectively records terrane accretion, translation, and integration together with evolving subduction dynamics that have shaped the southern Alaska margin since the middle Mesozoic.

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