Geological evolution of two crustal scale shear zones. Part I. The Rand thrust complex, northwestern Mojave Desert, California. Part II. The Magdalena metamorphic core complex, north central Sonora, Mexico
Geological evolution of two crustal scale shear zones. Part I. The Rand thrust complex, northwestern Mojave Desert, California. Part II. The Magdalena metamorphic core complex, north central Sonora, Mexico
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两个地壳尺度剪切带的地质演化。
DOI：
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发表时间：
1989
期刊：
影响因子：
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通讯作者：
J. Nourse
中科院分区：
文献类型：
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作者：
J. Nourse
对两个地壳尺度剪切带的地质构造进行了研究
The geology and structure of two crustal scale shear zones were studied
to understand the partitioning of strain within intracontinental orogenic
belts. Movement histories and regional tectonic implications are deduced from
observational data. The two widely separated study areas bear the imprint of
intense Late Mesozoic through Middle Cenozoic tectonic activity. A regional
transition from Late Cretaceous-Early Tertiary plutonism, metamorphism, and
shortening strain to Middle Tertiary extension and magmatism is preserved in
each area, with contrasting environments and mechanisms. Compressional
phases of this tectonic history are better displayed in the Rand Mountains,
whereas younger extensional structures dominate rock fabrics in the
Magdalena area. In the northwestern Mojave desert, the Rand Thrust Complex reveals a
stack of four distinctive tectonic plates offset along the Garlock Fault. The
lowermost plate, Rand Schist, is composed of greenschist facies
metagraywacke, metachert, and metabasalt. Rand Schist is structurally
overlain by Johannesburg Gneiss (= garnet-amphibolite grade orthogneisses,
marbles and quartzites), which in turn is overlain by a Late Cretaceous
hornblende-biotite granodiorite. Biotite granite forms the fourth and highest
plate. Initial assembly of the tectonic stack involved a Late Cretaceous? south
or southwest vergent overthrusting event in which Johannesburg Gneiss was
imbricated and attenuated between Rand Schist and hornblende-biotite
granodiorite. Thrusting postdated metamorphism and deformation of the
lower two plates in separate environments. A post-kinematic stock, the Late
Cretaceous Randsburg Granodiorite, intrudes deep levels of the complex and
contains xenoliths of both Rand Schist and mylonitized Johannesburg? gneiss.
Minimum shortening implied by the map patterns is 20 kilometers. Some low angle faults of the Rand Thrust Complex formed or were
reactivated between Late Cretaceous and Early Miocene time. South-southwest
directed mylonites derived from Johannesburg Gneiss are commonly
overprinted by less penetrative north-northeast vergent structures.
Available kinematic information at shallower structural levels indicates that
late disturbance(s) culminated in northward transport of the uppermost plate.
Persistence of brittle fabrics along certain structural horizons suggests a
possible association of late movement(s) with regionally known detachment
faults. The four plates were juxtaposed and significant intraplate movements
had ceased prior to Early Miocene emplacement of rhyolite porphyry dikes. In the Magdalena region of north central Sonora, components of a pre-Middle
Cretaceous stratigraphy are used as strain markers in tracking the
evolution of a long lived orogenic belt. Important elements of the tectonic
history include: (1) Compression during the Late Cretaceous and Early
Tertiary, accompanied by plutonism, metamorphism, and ductile strain at
depth, and thrust driven? syntectonic sedimentation at the surface. (2) Middle
Tertiary transition to crustal extension, initially recorded by intrusion of
leucogranites, inflation of the previously shortened middle and upper crustal
section, and surface volcanism. (3) Gravity induced development of a normal
sense ductile shear zone at mid crustal levels, with eventual detachment and
southwestward displacement of the upper crustal stratigraphy by Early
Miocene time. Elucidation of the metamorphic core complex evolution just described
was facilitated by fortuitous preservation of a unique assemblage of rocks and
structures. The "type" stratigraphy utilized for regional correlation and strain
analysis includes a Jurassic volcanic arc assemblage overlain by an Upper
Jurassic-Lower Cretaceous quartz pebble conglomerate, in turn overlain by
marine strata with fossiliferous Aptian-Albian limestones. The Jurassic strata,
comprised of (a) rhyolite porphyries interstratified with quartz arenites, (b)
rhyolite cobble conglomerate, and (c) intrusive granite porphyries, are
known to rest on Precambrian basement north and east of the study area. The
quartz pebble conglomerate is correlated with the Glance Conglomerate of
southeastern Arizona and northeastern Sonora. The marine sequence
represents part of an isolated arm? of the Bisbee Basin. Crosscutting structural relationships between the pre-Middle Cretaceous
supracrustal section, younger plutons, and deformational fabrics allow the
tectonic sequence to be determined. Earliest phases of a Late Cretaceous-Early
Tertiary orogeny are marked by emplacement of the 78 ± 3 Ma Guacomea
Granodiorite (U/Pb zircon, Anderson et al., 1980) as a sill into deep levels of the
layered Jurassic series. Subsequent regional metamorphism and ductile strain
is recorded by a penetrative schistosity and lineation, and east-west trending
folds. These fabrics are intruded by post-kinematic Early Tertiary? two mica
granites. At shallower crustal levels, the orogeny is represented by north
directed thrust faulting, formation of a large intermontane basin, and
development of a pronounced unconformity. A second important phase of
ductile strain followed Middle Tertiary? emplacement of leucogranites as sills
and northwest trending dikes into intermediate levels of the deformed section
(surficial volcanism was also active during this transitional period to regional
extension). Gravitational instabilities resulting from crustal swelling via
intrusion and thermal expansion led to development of a ductile shear zone
within the stratigraphic horizon occupied by a laterally extensive
leucogranite sill. With continued extension, upper crustal brittle normal
faults (detachment faults) enhanced the uplift and tectonic denudation of this
mylonite zone, ultimately resulting in southwestward displacement of the
upper crustal stratigraphy. Strains associated with the two ductile deformation events have been
successfully partitioned through a multifaceted analysis. R_f/O measurements
on various markers from the "type" stratigraphy allow a gradient
representing cumulative strain since Middle Cretaceous time to be determined.
From this gradient, noncoaxial strains accrued since emplacement of the
leucogranites may be removed. Irrotational components of the postleucogranite
strain are measured from quartz grain shapes in deformed
granites; rotational components (shear strains) are determined from S-C
fabrics and from restoration of rotated dike and vein networks. Structural
observations and strain data are compatable with a deformation path of: (1)
coaxial strain (pure shear?), followed by (2) injection of leucogranites as
dikes (perpendicular to the minimum principle stress) and sills (parallel to the
minimum principle stress), then (3) southwest directed simple shear.
Modeling the late strain gradient as a simple shear zone permits a minimum
displacement of 10 kilometers on the Magdalena mylonite zone/detachment
fault system. Removal of the Middle Tertiary noncoaxial strains yields a
residual (or pre-existing) strain gradient representative of the Late
Cretaceous-Early Tertiary deformation. Several partially destrained cross
sections, restored to the time of leucogranite emplacement, illustrate the idea
that the upper plate of the core complex bas been detached from a region of
significant topographic relief. 50% to 100% bulk extension across a 50
kilometer wide corridor is demonstrated. Late Cenozoic tectonics of the Magdalena region are dominated by Basin
and Range style faulting. Northeast and north-northwest trending high angle
normal faults have interacted to extend the crust in an east-west direction. Net
extension for this period is minor (10% to 15%) in comparison to the Middle
Tertiary detachment related extensional episode.