Maria Fold and Thrust Belt
Geologic History

Geologic History - 40Ar/39Ar step-heating - References

Introduction: What is the Maria Fold and Thrust Belt, and why is it important?

The Maria Fold and Thrust Belt (MFTB) is a thick-skinned fold and thrust belt of Mesozoic-age crustal shortening that stretches West-East from southeastern California to west-central Arizona (Spencer and Reynolds, 1990). It is characterized by generally south-vergent folds and top-to-the-south thrusts that place Proterozoic basement rocks over highly-deformed Paleozoic and Mesozoic sedimentary and volcanic strata (Spencer and Reynolds, 1990). This makes it differ from other parts of the Andean-style Cordilleran Orogen, in which deformation generally occurred via East-West shortening in North-South-trending belts.

Regional setting of the MFTBMFTB_loc.gif
Left: The Maria Fold and Thrust Belt is oriented orthogonally to the structures of the rest of the Sevier Belt. From Knapp and Heizler (1990). Right: The Maria fold and Thrust Belt is located in Southeastern California and western Arizona, as indicated by the box with the lines running diagonally through it. Figure from Reynolds et al. (1989).

Present-day Geography

Rocks deformed in the Maria Fold and Thrust Belt can be found in around a dozen present-day ranges across the Mojave and Sonoran deserts of western Arizona and southeastern California. According to Spencer and Reynolds, 1990 (Figure 1). These include the:
Maria Fold and Thrust Belt map
Generalized geologic map of the Maria Fold and Thrust Belt, adapted from Spencer and Reynolds (1990). Click for a larger version.
In addition to these, some additional ranges are sometimes included in the Maria Fold and Thrust Belt.
The south-dipping Mule Mountains thrust, oriented opposite many of the north-dipping MFTB thrusts, is sometimes considered its own entity, as are the Buckskin and Rawhide Mountains, which saw a large amount of Tertiary extensional deformation. The McCoy Mountains are the type locality for the McCoy Mountains Formation, which is associated with MFTB deformation, but is often not considered to be part of the MFTB proper.

Geologic Setting

The region of the present-day Maria Fold and Thrust Belt was first formed in Proterozoic time. After its formation, its story was one of a quiet portion of the stable North American continental lithosphere, steadily accumulating sediments. This lasted until a Late Triassic through Early Triassic uplift. Later in the Mesozoic, the Cordilleran orogeny formed the complex patterns of deformation associated with the MFTB.

Proterozoic:
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The Mojave and Yavapai Provinces are the Proterozoic basement rocks of the Maria Fold and Thrust Belt. (Modified from USGS.)

The Maria Fold and Thrust Belt lies both in the Mojave Provence and in the transition zone between the Mojave Provence and the Yavapai Provence (Duebendorfer et al., 2006, Figure 1: after Wooden and DeWitt, 1991). The Mojave and Yavapai Provinces are tectonostratigraphic terranes: that is, they contain consistent and related suites of rocks. They, along with the Mazatzal Province of southeastern Arizona and New Mexico, constitute the major tectonostratigraphic terranes of the American Southwest.

The Mojave Provence is the oldest of these three tectonostratigraphic terranes. It is poorly-known because (1) it is discontinuously exposed in the cores of mountain ranges, (2) its rocks are often out of place because they lie in the upper plates of Mesozoic thrust faults with tens to hundreds of kilometers of displacement, and (3) many of these rocks are highly deformed and metamorphosed, resulting in the destruction of their original lithologic and geometric relationships (Baldridge, 2004).

In spite of these difficulties, geologists have deciphered a history for the southern part of the Mojave Provence, which is where the Maria Fold and Thrust Belt lies. Samarium-neodymium work shows that the earliest portions of the Mojave Provence were extracted from the mantle around 2.6 Ga. These dates, now found in magmatic rocks, is likely from sedimentary detritus that was subducted and re-erupted in a volcanic arc (Baldridge, 2004). The oldest zircon ages here are 2.3. Ga. These supracrustal rocks were intruded by granodioritic to granitic magmas between 1.76 and 1.64 Ga. During  this period of magmatism, between 1.74 and 1.72 Ga, the Mojave Provence accreted to the Yavapai Provence. Between 1.71 and 1.70 Ga, the rocks of the Mojave province were transformed into complex assemblages of gneiss and migmatite during high-pressure low-temperature metamorphism associated with the Ivanpah Orogeny (Baldridge, 2004). It was during this time that the transition zone formed between the Mojave and Yavapai tectonostratigraphic terranes.

Beginning about 1.6 Ga, the Mojave Provence and the rest of Laurentia entered a period of tectonic quiescence that lasted approximately 100 million years. Although activity later resumed in the Yavapai and Mazatzal Provinces to the South and East, the Mojave Provence did not see major deformation for the remainder of the Proterozoic. (Baldridge, 2004)

Paleozoic:

paleozoic_strat.png
Left two columns are from the Grand Canyon, and right two columns are from the Big Maria Mountains. Thicknesses in the Big Maria Mountains are modern, not original depositional. Figure compiled and constructed by Salem (2009) based on data collected by Beus and Morales (2003) and Salem (2005). Stratigraphic correlations were described by Hamilton (1982).
At the dawn of the Paleozoic, the region that would become the Maria Fold and Thrust Belt was a quiescent shallow sea underlain by the North American craton. The Paleozoic stratigraphy of the Maria Fold and Thrust Belt is the same as that which is found in the Grand Canyon, as they were in the same region and depositional environment: In fact, these rocks are found as far from the Grand Canyon as southwesternmost Arizona, USA, and Sonora, Mexico. At the bottom is the Upper Cambrian Tapeats Quartzite. On top of this, from bottom to top, are the Bright Angel Schist, the Muav Marble, undifferentiated dolostones, the Temple Butte Formation, the Redwall Marble (which may include a broader age range of metamorphosed limestones than does the Redwall in the Grand Canyon proper), and the Pennsylvanian-Permian Supai Group. Above the Supai Group are the Hermit Schist, Coconino Quartzite, Toroweap Marble, and Kaibab Marble; the Toroweap and Kaibab are typically undifferentiable and simply are called the "Kaibab" in the MFTB. (Hamilton, 1987; Salem, 2009.)

The continental margin of Late Paleozoic (Mississippian to Permian) North America tells a more active story. From the start of the Paleozoic until the Mississippian, the continental margin of the Western US was a passive margin that underwent shallow marine sedimentation. In early Mississippian time, this passive margin became an active margin. The Antler (Early Mississippian), Havallah, and Sonoma (Late Permian to Early Triassic) terranes accreted in that order to the western margin of North America, and overthrust one another in sequence (Burchfiel and Davis, 1972, 1975; Dickinson et al., 1983). This added a large amount of material to the western margin of North America.

At some point between and including the Pennsylvanian and the Triassic, the southwestern margin of North America was truncated. This truncation removed terranes that had accreted prior to it, and prevented terranes from accreting after its initiation. Burchfiel and Davis (1975) suggested that this was due to either rifting or transform faulting around the Permo-Triassic boundary to early Triassic time, following the Sonoma Orogeny. This truncation event would have removed the accreted terranes and exposed the North American craton on the continental margin. Stone and Stevens (1988), Stevens et al. (1997), and Stevens et al. (2005) argue that this truncation event was caused by the left-lateral Mojave-Sonoran Megashear, which translated the southwestern edge of North America southeastward to the Caborca-Hermosillo region of Mexico. They also place the event much earlier, during Pennsylvanian time. Though neither Stone and Stevens (1988) nor Stevens et al. (1997) state how this would relate to the Permo-Triassic Sonoma Orogeny, the change of this margin from a convergent boundary to a transform one presumably would have prevented the Sonoma Orogeny from reaching the Southwest.



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The Great Unconformity in the Big Maria Mountains. Cambrian Tapeats Quartzite (Єt) lies unconformably above Proterozoic Granite (Xg). Photo by Anthony Salem (Salem, 2009).

Mesozoic:

Lying above the well-known Paleozoic section is the Mesozoic sequence of the Maria Fold and Thrust Belt. Lying above this is the Triassic Buckskin Formation, which correlates with the Moenkopi Formation further East, and the Triassic-Jurassic Vampire Formation, correlative to the Chinle. The Jurassic Aztec Quartzite lies atop the Vampire Formation; it has been correlated with and named after the aeolian Jurassic Aztec Sandstone of southern Nevada, which in turn is correlative with the Navajo Sandstone of the Colorado Plateau. However, it is more likely correlative with the Page and Carmel Sandstones. Above these sedimentary strata lie a sequence of Jurassic volcanic rocks of the Dome Rock Group; these volcanics are associated with widespread Jurassic arc magmatism that extended from British Columbia, Canada, to northern Sonora, Mexico. Jurassic plutonism also occurred, but these plutons are unlikely to be correlated with the Jurassic volcanism, and some may be Cretaceous in age. Later in time, the Jurassic-Cretaceous McCoy Mountains Formation was deposited as >7 km thick siliciclastic wedge to the south of the MFTB. (Salem, 2009)

The Triassic Buckskin Formation is the stratigraphically lowest Mesozoic unit in the Maria Fold and Thrust Belt (Salem, 2009). It comprises anhydrite-bearing schists and calcareous sandstones, and is named for the Buckskin Mountains of western Arizona (Reynolds, 1987). Its maximum thickness is approximately 600 meters at Palen Pass (Salem, 2009). Its basal unit is a green gypsiferous schist and sandstone, and its upper unit is a green to orange calcareous sandstone. Reynolds et al. (1989) correlated it with the Triassic Moenkopi Formation of the Colorado Plateau and elsewhere in the western United States, which contains similar (though unmetamorphosed) lithology and unconformably overlies the Permian Kaibab Limestone (Salem, 2009). Pelka (1973) and Stone and Kelly (1989) likewise correlate it with the lower two members of the Palen Formation.

The Triassic-Jurassic Vampire Formation comprises a lower conglomerate unit and an upper unit of volcaniclastic sandstones (Reynolds, 1987). The conglomerate consists of angular to rounded clasts of heterogeneous composition, which indicates sediment input from a nearby source that had a diverse lithology (Reynolds, 1989). The gray-green sandstone unit has volcanically derived clasts. Together, these units constitute the 400 meter thick Vampire Formation (Salem, 2009). It is correlative to the Chinle Group of the Colorado Plateau, and unconformably overlies units that range in age from Proterozoic through Triassic. This unconformity, coupled with the basal conglomerate, is evidence for Late Triassic-Early Jurassic uplift. This is possibly due to the onset of active-margin tectonics further west, as the Cordilleran magmatic arc initiated during Late Triassic time (Reynolds et al., 1989; Salem, 2009; Stewart et al., 1972; Asmerom, 1988). Further evidence of uplift and volcanism exists in an influx of volcanic ash into the Chinle Group, Triassic-age volcanic rocks in southeastern Arizona, and Triassic-age plutonism in southwestern Arizona and southeastern California (Busby-Spera, 1988; Asmerom et al, 1988; Salem, 2009).

The Jurassic Aztec Quartzite is an erg with a maximum thickness of 300 meters in the Palen mountains. It indicates a return to stable cratonal conditions. Though it is typically correlated with the massive Navajo/Aztec Sandstone erg that blanketed much of the western US during Early Jurassic time, U-Pb dates on interbedded volcanic rocks at Palen Pass give an age of 1748 Ma (Middle Jurassic). This indicates that it is more likely correlative to the Page or Carmel Sandstones of the Colorado Plateau (Fackler-Adams et. al., 1997; Salem, 2009).

harding_coney_mccoy_fm.gif
Generalized stratigraphic column of the Late JurassicCretaceous McCoy Mountains Formation and underlying Jurassic Rhyodacites. From Harding and Coney (1985).
Arc magmatism during Jurassic time resulted in the explosive eruption of rhyodacitic felsic volcanics (Harding and Coney, 1985; Barth et al., 2004). These units include the Planet Volcanics of the Buckskin Mountains (Reynolds et al., 1987) and the Dome Rock Group of the Palen Mountains and vicinity (Tosdal et al., 1989), which are correlative with one another. They are 1260 meters thick in the northern Palen Mountains. They comprise two members: a lower greenish-gray micaceous schist that is likely correlative with rhyodacite tuffs and tuffaceous sedimentary rocks in the Palen Mountains, and a upper unit correlative with rhyolitic ignimbrite, lava, and a hypabyssal quartz porphyry in the Palen Mountains (Salem, 2009). The presence of these volcanics indicates a resurgence of tectonic and magmatic activity in the area, and foreshadows Cretaceous deformation in the Maria Fold and Thrust Belt. Indeed, Tosdal et al. (1989) note that they are part of the Cordilleran magmatic arc that extends from British Columbia in the North to Sonora in the South. U-Pb ages for the Dome Rock Group are 1748 Ma at their base, and are 1623 Ma and 1558 Ma nearer to their top, placing them in Middle to Late Jurassic time (Salem, 2009).

Aerially extensive Jurassic-age plutonism sweeps through southwestern North America and date to ~160 Ma (Tosdal et al., 1989). Felsic plutons comprise three major members in the MFTB: a  dark green hornblende-bearing diorite, a light gray granodiorite with centimeter-scale euhedral lavender-color feldspars, and a leucocratic granite. In addition to this is a set of fine-grained mafic intrusive rocks that are presumed to be intruded at the same time (Salem, 2009).

Above the Jurassic volcanics, the 7.3-km thick Jurassic-Cretaceous McCoy Mountains Formation was deposited. The McCoy Mountains Formation is siliciclastic, and consists of six distinct members. It is often weakly metamorphosed, and its top is never exposed. This formation is local to the Maria Fold and Thrust Belt, and therefore is very different from the other Formations in Utah, Colorado, Arizona, and elsewhere in the southwestern United States. It is likewise dissimilar to the exotic terranes to the West, and is relatively less-well known. Its depocenter is the McCoy Basin, to the South of the MFTB. (Barth et al., 2004; Harding and Coney, 1985; Salem, 2009).

harding_coney_mccoy_pic.gif
Contact between the Jurassic arc rhyodacites and the Cretaceous (perhaps late Jurassic at the base) McCoy Mountains Formation. View is to the East, from the Palen Mountains, so North is to the left. Contact dips ~40 to the south. From Harding and Coney (1985).

Detrital zircon data place the McCoy Mountains Formation at a maximum depositional age of 116 Ma near the bottom of the formation, and 84 Ma near its top. It is possible that its basal sandstone is Jurassic in age, but at least 90% of the Formation was deposited during the Cretaceous (Barth et al., 2004).

The McCoy Mountains Formation was deposited in the McCoy–Bisbee Basin. This basin was an extension of the opening of the Gulf of Mexico, and its formation was aided by thermally-softened lithosphere due to the arc magmatism occurring in the Southwest at that time (e.g., Salem, 2009). In the Southwest, it is interpreted by Salem (2009) to be a Late Jurassic–Cretaceous rift system. The Jurassic and earlier Cretaceous portions of the McCoy Mountains Formation deposited in this extensional setting.

The later Cretaceous portions of the McCoy Mountains Formation are interpreted to be deposited in the foreland basin of the actively-deforming Maria Fold and Thrust Belt to the north and behind active magmatic arc of the Cordilleran orogen to the West. This deposition was also synchronous with East-West-shortening Sevier tectonism North of the MFTB (Barth et al., 2004).

salem_fullstrat.jpg
Generalized stratigraphic column of the pre-Tertiary rocks of the Maria Fold and Thrust Belt and McCoy Basin. Numbers in yellow hexagons are published detrital zircon ages from Barth et al. (2004). Figure from Salem (2009), Figure 2.4.


Tectonic Setting

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Left: Aptian tectonic reconstruction by Scotese (1991). Note the east-dipping subduction zone on the western margins of North and South America that runs nearly from pole to pole. Right: North American paleogeographic reconstruction by Ron Blakey (NAU Geology) also from Aptian time (115 Ma).

During Mesozoic time and extending into the Cenozoic, a broad east-dipping subduction zone bounded the western margin of the Americas and extended nearly from pole to pole. (This same subduction zone existed earlier, but its geometry was different due to the changing positions of the continents.) Subduction along this margin caused volcanism, and the accretion of island arcs and exotic terranes via this tectonic conveyor belt deformed both North and South America.

In Aptian time (Heller et al., 1986), subduction along the west coast of North America initiated the Andean-style Sevier Orogeny. The Sevier Belt formed in the foreland of the Sierra Nevada arc, and consists of large-scale east-vergent thrusts (Heller et al., 1986) and west-dipping, east-vergent folds. Its hinterland records high-grade metamorphism and plutonism associated with synconvergent extension; in this, it is unlike the Andes. (Salem, 2009)

Between approximately 80 and 40 Ma, the Laramide Orogeny deformed much of western North America (Salem, 2009). Laramide-age deformation reached as far inland as Colorado, Wyoming, and western South Dakota (the Black Hills). This is attributed to the shallowing of the angle of Farallon Plate subduction into a "flat slab". The end of the Sevier and the onset of the Laramide flat slab subduction likewise changed the style of convergence around the MFTB.

Evolution of the Maria Fold and Thrust Belt
barth_MFTB_McCoy.jpg
Theorized depositional tectonics of the McCoy Mountains Formation. From Barth et al. (2004). Click for larger image.

Prior to deformation, the rocks that constitute the Maria Fold and Thrust Belt were part of the stable North American craton (Spencer and Reynolds, 1990; above sections). Between 90 and 80 Ma (middle Late Cretaceous time), deformation in thick-skinned fold and thrust belts caused regional heating of the crust (Knapp and Heizler, 1990). These structures strike generally east-west and accommodate north-south-directed shortening.

This deformation is hypothesized to be related to the Sevier Orogeny (Salem, 2009), which lies in North-South-trending bands of East-West crustal shortening in a classic wedge shape (e.g., Dahlen and Suppe, 1988) that thickens to the west. This orogenic wedge consists of Paleozoic and Upper Proterozoic continental margin sedimentary rocks that have been displaced eastward onto the North American continent. This Sevier Belt shortening parallels the north-south plate boundary of east-west convergence between the North American and Farallon Plates. This makes logical sense: east-west compression along a north-south boundary creates north-south bands of east-west shortening.

However, MFTB structures trend orthogonally to the rest of the Sevier Orogeny. They lie east-west and accommodate north-south crustal shortening. Even considering the bend in the Farallon-North American plate boundary around southern California, MFTB structures are still at least 45 away from the angle of convergence. Salem (2009) and earlier researchers cited therein hypothesize that the McCoy Mountains Formation was deposited in a roughly east-west-oriented rift basin as an extension of the opening of the Gulf of Mexico, and that this rift basin created a pervasive lithospheric weakness that strongly influenced the orientation of the MFTB.

The MFTB is also unique in that it experienced basement involved ("thick skinned") deformation (Burchfiel and Davis, 1975). A likely explanation for this lies in the truncation of the margin of the American Southwest. Whether due to a Pennsylvanian Mojave-Sonoran Megashear (Stone and Stevens, 1988; Stevens et al., 1997; Stevens et al., 2005) or to a Permo-Triassic to Triassic event (Burchfiel and Davis, 1975, p. 373), the zone of thickened crust where the Antler, Havallah, and Sonoma terranes accreted was removed (or not allowed to form, in the case of the Sonoma Orogeny and Stone-Stevens hypothesis). This hypothesized truncation likely stretched from Central California to at least the Trans-Mexico volcanic belt (Burchfiel and Davis, 1975), and left cratonal rocks exposed at the continental margin. These then deformed as the Farallon Plate subducted under western North America. Because of how thin this cratonal sedimentary cover was, deformation involved the Precambrian basement and resulted in the exhumation of multiple crystalline nappes in the cores of many of the small mountain ranges in which the MFTB is exposed (Knapp and Heizler, 1987).

40Ar/39Ar step-heating analyses show two distinct phases in the thermal evolution of the MFTB (Knapp and Heizler, 1987). In the middle Late Cretaceous, 90–80 Ma, thick-skinned folding and thrust faulting caused a regional heating event. Although this heating signal is not apparent until ~90 Ma, Salem push the timing of MFTB initiation back to ~97 Ma based on the the age of detrital zircons in collected by Barth et al (2004) in the upper McCoy Mountains Formation. Salem (2009) interprets the upper McCoy Mountains Formation to be retroarc foreland basin and shows that this deposition is indeed syndeformational by comparing these depositional ages to ages of crustal deformation in the MFTB. This deformation may be further bracketed occur before 86 Ma, as it appears to have occurred prior to 86–85 Ma plutonism (Barth et al., 2004), although detrital zircon work by Salem (2009) suggests middle crustal deformation started after ~86 Ma and continued until at least 84 Ma. Exhumation continued at a rate of 5–10C/Ma through latest Cretaceous and early Tertiary time, either representing some continued MFTB activity (Knapp and Heizler, 1987) or erosion of the uplifted MFTB, and ended at around 60–54 Ma (Late Paleocene / Early Eocene time).

Maria Fold and Thrust Belt deformation was accompanied by
syntectonic deposition of much of the McCoy Mountains Formation in an East-West trending basin that lies to the south of the MFTB. As MFTB deformation waned and migrated southward, it created penetrative cleavage and open folds in the McCoy Mountains Formation. The onset of this deformation is constrained to be earlier than 73.5 1.3 million years old by uranium-lead dating of zircon in a granodiorite intrusion. This southward-extending MFTB deformation ultimately formed the Mule Mountains Thrust system between 79 and 70 Ma (Barth et al., 2004).

In a much more recent work (PhD dissertation) by Salem (2009), the author shows evidence for three major phases of deformation that can be linked to the tectonic history of the MFTB. He links the D1 and D2 events (~97–84 Ma) to Sevier-related southeast-directed thrusting and crustal shortening along with transpressional deformation along a ESE-striking fault that lies between the MFTB and the McCoy Basin. This basin-bounding (and likely originally rift-related) fault accommodated both shortening (with the Maria Uplift being thrust over the McCoy Basin) and right-lateral strike-slip motion. D3 fabrics are dated to 74–67 Ma and associated with Laramide shallow-angle subduction that more directly interacts with the base of the North American lithosphere and causes SSW–NNE deformation. These D3 fabrics are temporally correlated with peak metamorphism at ~70 Ma. Towards the end of D3 deformation, overthickening of the cratonal crust in the MFTB resulted in northeast-southwest directed extension that removed supracrustal rocks and exhumed middle crustal rocks. Although this model has yet to be challenged and tested by the scientific community, it provides a new and broader framework in which to view the MFTB.

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Salem's (2009) interpretation of the tectonic evolution of the MFTB From Sevier- to Laramide-style convergence.


The Aftermath: Cenozoic Extension in the Colorado River Extensional Corridor

After the extensive Cretaceous crustal shortening in the MFTB, the region was structurally quiescent until Miocene time. At this point, extension in the Colorado River Extensional Corridor resulted in large amounts of strain being accommodated along basal detachment faults. Crustal blocks slid against these faults. This extensional unloading resulted in the uplift and emplacement of metamorphic core complexes throughout much of the MFTB and the infilling of Tertiary-age synextensional sedimentary basins (Spencer and Reynolds, 1990).

This is important for modern geological investigations because other than the Mesozoic folds and thrusts of the Buckskin and Rawhide Mountains, all of the exposed structures of the MFTB are exposed in the the lower plates of detachment faults and their metamorphic core complexes (Spencer and Reynolds, 1990).

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Examples of extensional styles in the Maria Fold and Thrust Belt. Often, extension is accommodated by small block slip and rotation along a throughgoing basal detachment fault. This detachment faulting is often associated with emplacement of metamorphic core complexes. From Spencer and Reynolds (1990).