A paleomagnetic test of the proposed Mojave-Sonora megashear in Northwestern Mexico

Cohen, K. K.; Anderson, Terry; Schmidt, Victor A.

doi:10.1016/0040-1951(86)90266-0

Cited by 23 publications

(12 citation statements)

References 21 publications

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“…As an example, the alpha 95 for the Antimonio Formation is based upon use of N = 3 (number of localities) rather than upon N = 19 (number of sites) or N = 115 (number of specimens). These results (Cohen et al, 1986) are interpreted as a strong suggestion that the postulated displacement along the megashear did occur, but not as convincing proof.…”

Section: Paleomagnetic Studymentioning

confidence: 65%

“…Three Upper Triassic-Lower Jurassic formations in the southwestern United States and northwestern Mexico were investigated paleomagnetically as a means of testing for displacement along the proposed megashear (Cohen et al, 1986). The Luning Formation of west-central Nevada and the Sil Nakya Formation of southern Arizona lie north of the megashear, whereas in northwestern Sonora, the Antimonio Formation lies south of the megashear.…”

Section: Paleomagnetic Studymentioning

confidence: 99%

“…Paleomagnetic studies, initiated in order to further test the hypothesis of displacement along the megashear, were conducted by Cohen et al (1986). They concluded that: (1) the pole from Upper Triassic Antimonio Formation in Sonora and that from the Luning Formation in Nevada may be considered coincident at the 95% confi dence level, but the fi t is recognizably improved if the Antimonio pole is rotated 16° clockwise as required by displacement along the megashear; and (2) Cretaceous strata from Sierra del Alamo, southwest of the Mojave-Sonora megashear, do not record rotation.…”

Section: Paleomagnetic Studies-a Record Of Accretion or Lateral Displmentioning

confidence: 99%

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The Mojave-Sonora megashear—Field and analytical studies leading to the conception and evolution of the hypothesis

Anderson¹,

Silver²

2005

The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives

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The megashear hypothesis is based upon reconnaissance geologic and geochronologic studies conducted principally from 1968 until 1974 in northwestern Sonora, Mexico. Our research incorporated U-Pb isotopic analyses of more than 70 zircon populations separated from 33 Precambrian rock samples with fi eld relations and maps based upon structural and stratigraphic measurements. The results delineate a region known as the Caborca block and further reveal that the block is a principal element of an unexpected, discordant pattern of Proterozoic basement provinces. The Mojave-Sonora megashear was conceived in an effort to explain: (1) the unexpected pattern of two Proterozoic crystalline provinces with distinct chronologic histories of crust formation (1.8-1.7 Ga, Caborca block versus 1.7-1.6 Ga, Pinal Province);(2) the distribution of contrasting cover rocks overlying these basement blocks,the abrupt northeastern limit of the Caborca block (terrane) against which volcanic and plutonic rocks of mid-Jurassic (mainly 180-160 Ma) age are juxtaposed, and (4) the distribution of Jurassic magmatic units that intervene between the provinces of Proterozoic crust. The similarities that exist between crystalline crust and overlying pre-Jurassic cover in northwestern Sonora, Mexico, and units in the Inyo Mountains-Death Valley region are attributed to the offset of correlative units along a Late Jurassic left-lateral strike-slip fault postulated to extend from the Gulf of Mexico to California and beyond. This large fault or megashear is a principal structure that accommodated 800-1000 km of left-lateral displacement among a set of transforms related to the opening of the Gulf of Mexico. The fault is compatible with Late Jurassic plate motion.The inferred trace of the Mojave-Sonora megashear is obscured by contractional and extensional deformation and extensive plutonism. These processes, concentrated along the fault, commonly obfuscate and displace fault zone rocks along the inferred trace as well as the rocks adjacent to it. However, the fault zone is exposed in Sierra de Los Tanques near the international boundary between Mexico and the United States, where mylonitic rocks that comprise three aligned, discontinuous, segments crop out Anderson, T.H., and Silver, L.T., 2005, The Mojave-Sonora megashear-Field and analytical studies leading to the conception and evolution of the hypothesis, in Anderson, T.

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Section: Paleomagnetic Studymentioning

confidence: 65%

Section: Paleomagnetic Studymentioning

confidence: 99%

Section: Paleomagnetic Studies-a Record Of Accretion or Lateral Displmentioning

confidence: 99%

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The Mojave-Sonora megashear—Field and analytical studies leading to the conception and evolution of the hypothesis

Anderson¹,

Silver²

2005

The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives

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“…Paleomagnetic studies do not unequivocally support the existence of the Mojave-Sonora megashear (Urrutia-Fucugauchi, 1984;Kluger Cohen et al, 1986;Geissman, 1996, 1999).…”

Section: Introductionmentioning

confidence: 92%

The crust at northwestern Mexico interpreted from Magsat anomalies: Implications for the existence of the Mojave-Sonora megashear

Campos-Enríquez¹,

Hernández-Quintero²,

Lozada-Zumaeta³

2005

The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives

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In an effort to characterize the crustal structure of northwestern Mexico (and constrain the Mojave-Sonora megashear) we studied the Magsat magnetic anomalies from that area. Published anomaly maps covering this area include an extensive positive anomaly covering the southern United States, a positive anomaly over the southern half of the Baja California peninsula, and a magnetic low in between. We interpreted a magnetic profi le over these anomalies, focusing on its tectonostratigraphic terrane nature. The profi le was further constrained by crustal thicknesses from seismological studies and heat fl ow data. In our model the Cochimi terrane and the North American craton (Colorado Plateau and southern Basin and Range) are characterized by high magnetic susceptibility in agreement with the mafi c nature of their corresponding crusts. The Yuma and the Seri terranes have lower magnetic susceptibilities as expected from their felsic to basic crustal nature. The oceanic crust from the Gulf of California is modeled with a low magnetic susceptibility value due to the high heat fl ow observed at the extensional basins. Our model satisfi es the presence of a subvertical contact between crusts with contrasting magnetic signatures of the Seri terrane (comprising the Caborca subterrane) and the southwestern sector of the North American craton. The Mojave-Sonora megashear itself is below the resolution of the Magsat data. Nevertheless, our model implies that because of their different magnetic signatures, the crystalline basements from the southern United States and northern Mexico (Seri terrane) are of different nature, which does not support the continuity of the North American craton into northwestern Mexico.

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“…Ambos miembros están separados por una superficie de erosión, poseen discordancia angular y se depositaron en ambiente continental; asimismo se encuentran intrusionadas por diques de dacita y comparten un incipiente metamorfismo. La unidad más antigua o miembro inferior tiene dos dataciones isotópicas, una con 230 ± 20 Ma por el método Larsen (Pantoja-Alor, 1972) y otra obtuvo 195.3 ± 5.5 Ma, con 40 Ar/ 39 Ar en plagioclasa de un flujo riolítico (Bartolini y Spell, 1997;Bartolini et al, 2003), se considera en este artículo que la edad más joven es más confiable por el método usado y por lo tanto, el vulcanismo puede corresponder al Jurásico Inferior, como también por paleomagnetismo se considera (Cohen et al, 1986). La unidad más joven o miembro superior no ha sido datada.…”

Section: Sucesiones Del Triásico Superior Y Jurásico De Méxicounclassified

Las formaciones Gran Tesoro y Nazas: evolución de las secuencias Triásico Superior-Jurásico Inferior en México y su significado tectogenético

Antuñano¹,

García²,

Sánchez³

2014

BSGM

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507 ResumenEste artículo es una integración e interpretación de datos, cuyos objetivos consisten en redefinir formalmente a la Formación Gran Tesoro y revisar la estratigrafía de la Formación Nazas, correlacionar ambos depósitos con otras sucesiones estratigráficas situadas en similar intervalo de tiempo geológico y vincular estos depósitos a un evento geodinámico regional. La Formación Gran Tesoro es una sucesión de pizarra y filita, que conserva texturas del protolito formado por rocas ígneas, sedimentarias clásticas y carbonatadas, con olistolitos que tienen fósiles del Paleozoico, por ello se consideró que esta formación tenía esa edad. La presencia de Classopollis cf. C. classoides Pflug, Classopollis cf. C. kieseri Reyre y Micrhystridium lymensis var. lymensis Wall, revela que la Formación Gran Tesoro es un depósito Mesozoico, más joven que lo previsto anteriormente. Originalmente la Formación Gran Tesoro incluyó una facies esquistosa, pero en este trabajo se segrega al esquisto de la formación por las siguientes razones: 1) Por ser de edades diferentes, el esquisto con edad isotópica de 326 ± 26 Ma (Misisípico) y la pizarra y filita de edad Mesozoica; 2) porque sus grados de metamorfismo revelan historias geológicas diferentes; 3) las relaciones de contacto entre el esquisto y las pizarra y filita son tectónicas. En este artículo se da formalidad mnemónica apropiada al Esquisto Pescaditos y a la Formación Gran Tesoro de acuerdo al Código de Nomenclatura Estratigráfica.La Formación Nazas, en el área de Villa Juárez, Durango, presenta dos miembros con capas depositadas en ambiente continental. En el miembro inferior predominan derrames volcánicos con cuerpos hipabisales intercalados entre limolitas, mientras que el miembro superior se compone de areniscas y limolitas que descansan en discordancia angular sobre el miembro inferior. La cima de la Formación Nazas subyace en discordancia angular a la Formación La Joya, y ésta pasa a su vez en transición hacia la Formación La Gloria (Oxfordiano).Las formaciones Gran Tesoro y Nazas forman parte de una secuencia sedimentaria (strictus sensus), limitada en su cima por una discordancia angular, similar a otras secciones estratigráficas de edad Triásico Superior y Jurásico Inferior observadas en México, secciones que han sido deformadas por un evento tectónico poco estudiado; sin embargo, el análisis regional de las relaciones estratigráficas de estas secciones sedimentarias en dicho intervalo geológico, nos conduce a proponer el nombre de Tectogenia Americana, para una deformación post-paleozoica y pre-Oxfordiano, independiente del lapso temporal de las orogenias Nevadiana y Laramide, ambas más jóvenes en edad.Palabras clave: Gran Tesoro, Nazas, Triásico Superior -Jurásico Inferior, Tectogenia Americana, México. Abstract This article is an integration and interpretation of data that has the objectives to formally redefine the Gran Tesoro

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A paleomagnetic test of the proposed Mojave-Sonora megashear in Northwestern Mexico

Cited by 23 publications

References 21 publications

The Mojave-Sonora megashear—Field and analytical studies leading to the conception and evolution of the hypothesis

The Mojave-Sonora megashear—Field and analytical studies leading to the conception and evolution of the hypothesis

The crust at northwestern Mexico interpreted from Magsat anomalies: Implications for the existence of the Mojave-Sonora megashear

Las formaciones Gran Tesoro y Nazas: evolución de las secuencias Triásico Superior-Jurásico Inferior en México y su significado tectogenético

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