Magmatic strains and foliation triple points of the Merrimac plutons, northern Sierra Nevada, California: implications for pluton emplacement and timing of subduction

Guglielmo, Giovanni

doi:10.1016/0191-8141(93)90094-q

Cited by 18 publications

(4 citation statements)

References 55 publications

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“…S2 [footnote 1]), can be interpreted as a differentiation sequence from diorite at the bottom (or southern) part of the eastern pluton to tonalite at the top (northern part of eastern pluton), or perhaps as emplacement of compositionally diverse magmas adjacent to each other. Such an orientation of compositions suggests a regional top orientation to the northeast, as has been documented by Guglielmo (1993b) based on compositional asymmetry and a slight east to northeast plunge of the poles of magmatic foliation planes for the other two plutons.…”

Section: ■ Discussionsupporting

confidence: 61%

“…However, in light of the geochronologic data and refinement of contacts between the most mafic eastern pluton and slightly less mafic northern pluton, we posit instead that the southern pluton partially molded itself to the already crystallized contacts of the northern and eastern plutons. The absence of solid-state deformation within the plutons and along the contact between the three main plutons (Guglielmo, 1993a(Guglielmo, , 1993b, combined with wall rocks wrapping around the plutons (Hietanen, 1976) and brecciation and stoping along discordant contacts between the plutons and the wall rocks, still suggests forceful rather than passive emplacement. However, regional structure likely provided the pathway for that emplacement, particularly where dikes extend ~5 km into the country rock along and parallel to the westward projection of the contact between the northern and southern plutons (Hietanen, 1951, p. 593), which in turn is parallel to the structures in the country rock.…”

Section: ■ Discussionmentioning

confidence: 99%

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Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

et al. 2021

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In much of the western Cordillera of North America, the geologic framework of crustal structure generated in the Mesozoic leaves an imprint on later plutonic emplacement, subsequent structural setting, and present landscape morphology. The Merrimac plutons in the northern Sierra Nevada (California, USA) are a good example of the influence of pre-existing structure at a larger scale. This paper updates and refines earlier studies of the Merrimac plutons, with the addition of analysis of gravity and magnetic data and new 206Pb/238U zircon dates. The gravity and magnetic data not only confirm the presence of two different neighboring plutons, but also (1) support the presence of a third pluton, (2) refine the nature of the contact between the Merrimac plutons as being structurally controlled, and (3) estimate the depth extent of the plutons to be ~4–5 km. The zircon 206Pb/238U dates indicate that the two main plutons have statistically different crystallization ages nearly 4 m.y. apart. Geomorphic analyses, including estimates of relief, roughness and drainage density and generation of chi plots, indicate that the two main plutons are characterized by different elevations with large longitudinal channel knickpoints that we speculatively attribute to possible reactivation of pre-existing structure in addition to lithologic variations influencing relative erosion susceptibility in response to prior accelerated surface uplift.

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Section: ■ Discussionsupporting

confidence: 61%

Section: ■ Discussionmentioning

confidence: 99%

Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

et al. 2021

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“…1c). Formation of such a triple point requires that the wall rocks were still ductile at the time of pluton emplacement (Guglielmo 1993). Indeed, new Ar-Ar ages from the Angavo shear zones point to temperatures well above 500(C at that time (Gre ´goire et al 2008).…”

Section: Pluton-related Strain Vs Tectonic Strainmentioning

confidence: 99%

Four-stage building of the Cambrian Carion pluton (Madagascar)

Razanatseheno

Nédélec²,

Rakotondrazafy

et al. 2009

Earth and Environmental Science Transactions of the Royal Socie

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The 532±5 Ma old Carion pluton is a dark, porphyritic ferro-potassic granitoid emplaced near the late Pan-African Angavo mega-shear zone. A rough normal zoning from tonalitic to granitic compositions can be recognised in the field. Steep magmatic foliations are evidenced by K-feldspar megacryst preferred orientations. Microstructures are either magmatic or typical of incipient solid-state deformation in near solidus conditions. Magnetic susceptibility magnitudes (K) range from 11 to 111×10−3 SI in the pluton and can be correlated to the petrography (highest K values in the tonalites; lowest K in the granites; granodiorites in between). The susceptibility magnitudes display a complex zoning pattern. Combined with the arrangement of magnetic foliation trajectories, it is possible to delineate four nested sub-units, regarded as four magmatic pulses successively emplaced from the west to the east of the pluton. The four pulses are characterised by very similar magma geochemistry, but variable magmatic differentiation. The highest degrees of magnetic susceptibility anisotropies (up to 1·6) are observed along internal contacts between sub-units and along the borders of the pluton. The magnetic lineations are also steeply plunging in some places in each sub-unit, possibly imaging the different feeder zones. Magma emplacement occurred at the end of the activity of the Angavo shear zone, hence avoiding re-orientation of the magmatic structures by the late Pan-African transcurrent tectonics. The diachronicity of the four magmatic pulses is consistent with previously determined palaeomagnetic data, because only the two older sub-units display a magnetic reversal sequence, whereas the two youngest sub-units lack any reversion. Emplacement of these four magmatic batches was responsible for a strain aureole and suggests a diapiric mode of ascent.

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“…Question marks, areas of no data. The three insets show the cleavage patterns and relationships of the Bear Mountains Fault system (BMFS) (equals BB + WC + BMFZ in a) near three plutons as follows: (A) 142 Ma Merrimac pluton showing magmatic foliation pattern in pluton, deflection of regional fabrics in aureole and associated fabric ellipsoids (after Guglielmo, 1993); (B) > 138 Ma (three ages = K/Ar cooling ages) Rocklyn‐Penryn pluton with magmatic foliation traces in pluton, syn‐emplacement cleavage patterns in host rock and outline of BMFS (after H. Yu & S. R. Paterson, unpublished mapping 1994); (C) 151 Ma Guadalupe Igneous Complex (GIC) showing internal contacts (after Best, 1963) and host rock bedding, cleavage traces and outline of BMFS (after Paterson et al ., 1991). Timing relationships around each pluton indicate that the regional cleavage post‐dates the GIC, and is syn‐emplacement around the other two.…”

Section: Introductionmentioning

confidence: 99%

Inclusion trail patterns in porphyroblasts from the Foothills Terrane, California: a record of orogenesis or local strain heterogeneity?

Paterson

Vernon

2001

Journal Metamorphic Geology

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A major problem with the current use of porphyroblast±matrix microstructural relationships to infer orogenic histories, such as multiple orthogonal orogenic events, is that other evidence for these events is typically lacking. For example, a comparison of regional relationships and local structures formed in and adjacent to porphyroblasts present in contact aureoles in the Foothills Terrane, Sierra Nevada, California, shows that: (1) except in shear zones, contact aureoles and local zones along lithological contacts, the Foothills Terrane has a single regional cleavage, although locally formed by multiple processes; (2) the regional cleavage and locally developed porphyroblast inclusion trails have variable orientations, and neither dataset supports the formation of dominantly subhorizontal and subvertical cleavages in this orogen; (3) structural and metamorphic heterogeneities occur at all scales and can markedly affect inclusion trail patterns in porphyroblasts; (4) complex porphyroblast growth features and internal inclusion trail patterns can form in porphyroblasts that grow during short time intervals in contact aureoles, indicating that local complexity in porphyroblasts does not imply regional complexity. Because of these conclusions, multiple datasets, rather than data acquired only from porphyroblasts, should be considered when attempting to understand the evolution of orogens. Furthermore, using microstructural information preserved only in porphyroblasts to infer orogenic processes and plate motions is generally unjusti®ed.

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Magmatic strains and foliation triple points of the Merrimac plutons, northern Sierra Nevada, California: implications for pluton emplacement and timing of subduction

Cited by 18 publications

References 55 publications

Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

Four-stage building of the Cambrian Carion pluton (Madagascar)

Inclusion trail patterns in porphyroblasts from the Foothills Terrane, California: a record of orogenesis or local strain heterogeneity?

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