THE WALKER LANE AND GULF OF CALIFORNIA: RELATED...
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THE WALKER LANE AND GULF OF CALIFORNIA: RELATED EXPRESSIONS OF PACIFIC-NORTH AMERICAN PLATE BOUNDARY DEVELOPMENT Chris Henry and Jim Faulds, University of Nevada, Reno ABSTRACT The Walker Lane (WL) and Gulf of California (GC) have undergone similar and, according to us, related evolution but are currently in different stages of development of the Pacific-North American plate boundary. Nevertheless, the GC illustrates the future of the WL. In both areas, a major episode of extension began ~12-13 Ma (proto-Gulf extension, GC) in what had been a continental volcanic arc. ENE proto-Gulf extension, which represented the orthogonal component of oblique Pacific-North American plate motion along the transform boundary west of Baja California (Stock and Hodges, 1989, Tectonics), was mostly sandwiched between the Cretaceous Peninsular Ranges batholith and the mostly 32-20 Ma Sierra Madre Occidental caldera province (SMO). In the WL, extension may have accommodated minor clockwise rotation of the Sierra Nevada block relative to the central Great Basin as western North America expanded toward the transform boundary. This interpretation implies that the amount of extension pre-dating strike-slip faulting should decrease northward along the WL. Extension in the WL was mostly sandwiched between the Sierra Nevada batholith and ~35-10 Ma calderas of the ignimbrite flare-up in central Nevada. In both areas, extension avoided the mid-Cenozoic caldera belt. Calderas are underlain by large, crustal magma chambers, so a nearly continuous batholith probably underlies both belts. The core SMO remains unextended, but the central Nevada caldera belt has undergone ±10% WNW extension, probably related to WL dextral shear. Also in both areas, extension, and subsequently strike-slip faulting, cut through the Cretaceous batholiths because arc magmatism had heated and weakened the crust. Strike-slip faulting and the plate boundary either totally (GC) or partly (WL) stepped into the area that had undergone arc magmatism followed by extension. Development of the ridge-transform system in the GC ~6 Ma placed the WL in a favorable position to accommodate part of the transform motion. We predict that WL strike-slip faulting will propagate northwestward, following the migrating Mendocino triple junction and arc, and that the WL - GC will eventually accommodate all plate boundary motion. Whether the WL part will be a single, major strike-slip system (e.g., San Andreas) or a “leaky transform” (e.g., GC) is unknown. Atwater, T., and Stock, J., 1998, Pacific-North America plate tectonics of the Neogene southwestern United States - An update: International Geological Review, v. 40, p. 375-402. Faulds, J.E., Henry, C.D., and Hinz, N.H., 2005, Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific - North American plate boundary: Geology, v. 33, p. 505-508. Ferrari, L. Pasquare, G., Venegas-Salgado, S., and Romero-Rios, F., 2000, Geology of the western Mexican volcanic belt and adjacent Sierra Madre Occidental and Jalisco Block: Geological Society of America Special Paper 334, p. 65-83. Ferrari, L., Valencia Moreno, M., and Bryan, S., in press, Magmatismo y tectonica en la Sierra Madre Occidental y su relacion con la evolucion de la margen occidental de Norte America: Boletin de la Sociedad Geologica Mexicana y Geological Society of America Special Paper. Hausback, B.P., 1984, Cenozoic volcanic and tectonic evolution of Baja California Sur, Mexico, in Frizzell, V. A., Jr., ed., Geology of the Baja California Peninsula: Pacific Section S. E. P. M., v. 39, p. 219 236. Henry, C.D., and Aranda-Gomez, J.J., 1992, The real southern Basin and Range: Mid- to late Cenozoic extension in Mexico: Geology, v. 20, p. 701-704. Henry, C.D., and Aranda-Gómez, J.J., 2000, Plate interactions control middle-late Miocene, proto-Gulf and Basin and Range extension in the southern Basin and Range: Tectonophysics, v. 318, p. 1-26. Henry, C.D., and Fredrikson, G., 1987, Geology of southern Sinaloa adjacent to the Gulf of California: Geological Society of America, Map and Chart Series, Map MCH063, 14 p. Karig, D.E., and Jensky, W., 1972, The Proto Gulf of California: Earth and Planetary Science Letters, v. 17, p. 169 174. Kimbrough, D.L., Smith, D.P., Mahoney, J.B., Moore, T.E., Grove, M., Gastil, R.G., Ortega-Rivera, A., Fanning, C.M., 2001, Forearc-basin sedimentary response to rapid Late Cretaceous batholith emplacement in the Peninsular Ranges of Southern and Baja California: Geology, v. 29, p. 491-494. Langenheim, V.E., and Jachens, R.C., 2003, Crustal structure of the Peninsular Ranges batholith from magnetic data: Implications for Gulf of California rifting: Geophysical Research Letters, v. 30, no. 11, 1597, doi:10.1029/2003GL017159. McQuarrie, N., and Wernicke, B.P., 2005, An animated tectonic reconstruction of southwestern North America since 36 Ma: Geosphere, v. 1, p. 147-172. Mora-Alvarez, G., and McDowell, F.W., 2000, Miocene volcanism during late subduction and early rifting in the Sierra Santa Ursula of western Sonora, Mexico, in Delgado-Granados, H., Aguirre-Diaz, G., and Stock, J.M., eds., Cenozoic tectonics and volcanism of Mexico: Geological Society of America Special Paper 334, p. 123- 141. Nieto-Samaniego, A.F., Ferrari, L., Alaniz-Alvarez, S.A., Labarthe-Hernández, G., and Rosas-Elguera, R., 1999, Variation of Cenozoic extension and volcanism across the southern Sierra Madre Occidental volcanic province, México: Geological Society of America Bulletin, v. 111, p. 347-363. Sawlan, M.G., 1991, Magmatic evolution of the Gulf of California rift, in Dauphin, J. P. and Simoneit, B.A.,, eds., The Gulf and Peninsular Province of the Californias: American Association of Petroleum Geologists Memoir, v. 47, p. 301-369. Stewart, J.H., 1992, Paleogeography and tectonic setting of Miocene continental strata in the northern part of the Walker Lane belt, in Craig, S.D., ed., Structure, tectonics, and mineralization of the Walker Lane; Walker Lane Symposium, proceedings volume: Geological Society of Nevada, Reno, p. 53-61. Stewart, J.H., and 17 others, including Henry, C.D., 1998, Map showing Cenozoic tilt domains and associated structural features, western North America; in Accommodation zones and transfer zones: The regional segmentation of the Basin and Range province, Faulds, J. E., and Stewart, J. H., eds.: Geological Society of America Special Paper 323. Stock, J.M., and Hodges, K.V., 1989, Pre Pleistocene extension around the Gulf of California and the transfer of Baja California to the Pacific Plate: Tectonics, v.8, p. 99-115. Stockli, D.F., Farley, K.A., and Dumitru, T.A., 2000, Calibration of the apatite (U-Th)/He thermochronometer on an exhumed fault block, White Mountains, California: Geology, v. 28, p. 983-986. Swanson, E.R., and McDowell, F. W., 1984, Calderas of the Sierra Madre Occidental volcanic field, western Mexico: Journal of Geophysical Research, v. 89, p. 8787 8799. Whitehill, C.S., Miller, E.L., Colgan, J.P., Dumitru, T.A., Lerch, D., and McWilliams, M.O., 2004, Extent, style and age of Basin and Range faulting east of Pyramid Lake: Geological Society of America Abstracts with Programs, v. 36, no. 4, p. 37. The story is nearly identical around the Gulf of California where proto- Gulf extension was focused largely, not entirely, between the Peninsular Ranges batholith and the Sierra Madre Occidental caldera belt, and strike- slip faulting jumped to the area that had undergone extension. But both extension and strike-slip faulting cut across the batholith belt, for example, in Sinaloa, where the batholith is moderately extended, tilted up to 40-50 . l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l + + + + + + + ++ + + + + + + + 25 16 24 32 24 24 22 11 20 17 30 34 23 28 11 23-16 Ma andesite 24-11 Ma andesite Andesite belts (Sawlan, 1991) East edge La Posta belt Peninsular Ranges batholith (Kimbrough et al, 2001) East edge magnetic, mafic, western belt Peninsular Ranges batholith (Langenheim and Jachens, 2003) Dated mid-Cenozoic rocks in Sinaloa Additions to volcanic belts from this work (Sinaloa); Mora and McDowell, 2003 (Sonora); and Nieto-Samaniego et al, 1999, and Ferrari et al, in press (Nayarit) Crustal and Magmatic Influences on Rifting Sierra Madre Occidental ? ? ? ? Our interpretation is that ancestral Cascade arc magmatism (i.e., that north of the plate age), as shown here in a compilation of magmatism over the last 13 Ma, heated/hydrated and weakened the lithosphere. The Walker Lane and earlier extension nearly exactly match the ancestral arc. E.g., the western limits of all are approximately identical. Extension and strike- slip faulting were focused into the zone of weakened lithosphere and able to cut through the otherwise resistant batholith belt. The north end of the Walker Lane blends imperceptibly into the active Cascade arc. We suggest that strike-slip faulting is essentially channelized into the arc. l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l 13 - 0 Ma Lassen Shasta C a s c ade a r c a n ce s tral a rc 200 km Volcanic centers Lavas, intrusions Juan de F uca plate edge 12 Ma (Stock and Hodges, 1998) Cretaceous batholith 34-7 Ma calderas The Walker Lane is in the same location as the preceding ~13-12 Ma extensional episode, and both are largely between the Sierra Nevada batholith and the mid-Tertiary caldera belt. Note that the southwest edge of the caldera-batholith belt and the northeast edge of the Walker Lane coincide, and both make a big right step at the Mina deflection. The caldera belt is only slightly extended, ±10%, one of the least extended parts of the Basin and Range Province, as shown by the slight (£10 ) tilt of calderas. The caldera belt is underlain by a nearly continuous batholith, very similar to Cretaceous Sierra Nevada batholith. Large, cold batholiths resist deformation. But the Walker Lane (and extension) cut across the Cretaceous batholith belt to the north. Why? l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l 34-7 Ma calderas 200 km C a s c ade a r c ~13–12 Ma Extension W L – E CS Z Cretaceous batholith The same factors, a combination of Mesozoic batholiths, mid- Cenozoic caldera-batholith belts, and late Cenozoic volcanic arcs, determined where extension and strike-slip occurred in both the Gulf and Walker Lane. In both areas, extension began in what had been a continental volcanic arc. Extension in the Gulf was largely, not entirely, sandwiched between the mostly Cretaceous Peninsular Ranges batholith and the mid-Cenozoic Sierra Madre Occidental caldera province, an essentially unextended block. Extension in the Walker Lane was largely, not entirely, sandwiched between the Sierra Nevada batholith and the mid- Cenozoic central Nevada caldera belt, a little extended part of the Basin and Range province. Strike-slip faulting and the plate boundary jumped into the area that had undergone extension, totally in what is now the Gulf at ~6 Ma, but only partly into the Walker Lane. SMO 0 600 km TMVB W L - E C S Z C a s c ad e A r c ~13-12 Ma extension Active arcs Cretaceous Batholiths Mid-Tertiary calderas Crustal influences on the locus of deformation are essentially identical: Mesozoic batholiths, mid-Cenozoic caldera-batholith belts, and ancestral continental volcanic arcs. About 6 Ma, spreading jumped into the proto-Gulf, and the Gulf became the plate boundary. This plate boundary realignment places the Walker Lane, which was previously far inboard, in a much more favorable location to accommodate part of the plate motion. The timing of initiation of strike-slip faulting in the Walker Lane is a key question but highly debated. We favor initiation in the northern Walker Lane no earlier than 6 Ma and possibly 3 Ma, but interpretations go back to ~25 Ma. The fundamental question is really when did the Sierra Nevada start moving NW. Certainly today we have a significant part of Pacific-North American plate motion along the Walker Lane and eastern California shear zone. This series of figures from McQuarrie and Wernicke illustrate rotation of the Sierra Nevada. Initially, ENE extension occurred from Southern California to northern Mexico. McQuarrie and Wermicke (2005) 18–24 Ma 14–16 Ma 10–12 Ma 6–8 Ma 0–2 Ma By ~10-12 Ma, the Sierra Nevada had rotated another ~5 , and basins opened to the east in the future Walker Lane. The Sierra Nevada continued to rotate until about 6 Ma, for a total of about 10-11 (my rough measurement from McQuarrie and Wernicke reconstruction). So the oblique motion of the Pacific plate relative to North America, recognized by Stock and Hodges, was accommodated by ENE extension in the proto-Gulf and by similar extension related to clockwise rotation of the Sierra Nevada in what was to become the Walker Lane. After 6 Ma, Sierra Nevada underwent northwest translation but no more rotation. About 14-16 Ma, the Sierra Nevada started to rotate clockwise, initially ~3 and possibly related to earlier extension in the northern Gulf. Although this extension was mostly behind the transform margin, it wasn’t limited to a block bounded by the two triple junctions. What appears to have happened in the Walker Lane is the Sierra Nevada block rotated clockwise so that the North American plate kept in contact with the Pacific and Juan de Fuca plates. l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l S i e r r a N e v a d a W L – E CS Z 200 km Areas of Extension Beginning ~13–12 Ma C a s c ade a r c Walker Lane – eastern California shear zone with areas of extension beginning ~13- 12 Ma. Extension at this time generated a series of large, shallow, sedimentary basins (e.g., Stewart, 1992; Esmeralda, Coal Valley, “Reno”) as well as uplift of the White and Sahwave Mts based on thermochronology by Stockli et al., 2000, and Whitehill et al., 2004, respectively. This extension was in the area that was to become the Walker Lane, mostly close to the eastern edge of the Sierra Nevada. It was also low-magnitude, as evidenced by the broad, shallow basins and only moderate tilt of rocks in and around the basins. Why did this extension occur, and how is it related to the proto-Gulf? Possibly not as appreciated, an episode of extension beginning at ~13-12 Ma occurred far outside the “proto- Gulf”, both east of the Sierra Madre Occidental, and in a band far to the northwest in the future Walker Lane in western Nevada and eastern California. (And this compilation is way out of date.) SMO 0 600 km TMVB W L - E C S Z C a s c a d e A r c Known areas of ~13-12 Ma extension The Walker Lane Underwent Similar Development, with a Major Episode of Extension beginning ~13-12 Ma, followed by Strike-Slip Faulting The change from a convergent to transform margin is key. At ~13-12 Ma, the Rivera triple junction and transform margin jumped southward. The continental volcanic arc, which had been active along most of Baja and mainland Mexico until then, shut off. Pacific plate motion was oblique to the transform (Tosco – Abreojos fault), as it was from the start of Pacific- North American plate contact. Stock and Hodges (1989) recognized that this required two components of motion, one parallel to the transform and one perpendicular to it, the well known ENE extension of the proto-Gulf. Atwater website 13 Ma Atwater website 12.5 Ma SMO 0 600 km TMVB W L - E C S Z C a s c a d e A r c The Walker Lane and Gulf of California underwent similar, related evolution as part of plate boundary development: extension beginning ~13- 12 Ma, followed by strike-slip faulting. They are in different stages of evolution: total plate boundary in Gulf, ~20- 25% of Pacific-North American motion in Walker Lane. Crustal influences on the locus of deformation are essentially identical: Mesozoic batholiths, mid-Cenozoic caldera-batholith belts, and ancestral continental volcanic arcs. Faults of the Basin and Range Province from Stewart (1978), Henry and Aranda-Gomez (1992), and Stewart et al (1998) A Familiar Story: Evolution of the Pacific–North American Plate Boundary, the ~13-12 Ma Jump in the Rivera Triple Junction, and "Proto-Gulf Rifting" through Strain Partitioning
Transcript of THE WALKER LANE AND GULF OF CALIFORNIA: RELATED...
THE WALKER LANE AND GULF OF CALIFORNIA: RELATED EXPRESSIONS OF PACIFIC-NORTH AMERICAN PLATE BOUNDARY DEVELOPMENTChris Henry and Jim Faulds, University of Nevada, Reno
ABSTRACTThe Walker Lane (WL) and Gulf of California (GC) have undergone similar and, according to us, related evolution but are currently in different stages of development of the Pacific-North American plate boundary. Nevertheless, the GC illustrates the future of the WL. In both areas, a major episode of extension began ~12-13 Ma (proto-Gulf extension, GC) in what had been a continental volcanic arc. ENE proto-Gulf extension, which represented the orthogonal component of oblique Pacific-North American plate motion along the transform boundary west of Baja California (Stock and Hodges, 1989, Tectonics), was mostly sandwiched between the Cretaceous Peninsular Ranges batholith and the mostly 32-20 Ma Sierra Madre Occidental caldera province (SMO). In the WL, extension may have accommodated minor clockwise rotation of the Sierra Nevada block relative to the central Great Basin as western North America expanded toward the transform boundary. This interpretation implies that the amount of extension pre-dating strike-slip faulting should decrease northward along the WL. Extension in the WL was mostly sandwiched between the Sierra Nevada batholith and ~35-10 Ma calderas of the ignimbrite flare-up in central Nevada.
In both areas, extension avoided the mid-Cenozoic caldera belt. Calderas are underlain by large, crustal magma chambers, so a nearly continuous batholith probably underlies both belts. The core SMO remains unextended, but the central Nevada caldera belt has undergone ±10% WNW extension, probably related to WL dextral shear. Also in both areas, extension, and subsequently strike-slip faulting, cut through the Cretaceous batholiths because arc magmatism had heated and weakened the crust.
Strike-slip faulting and the plate boundary either totally (GC) or partly (WL) stepped into the area that had undergone arc magmatism followed by extension. Development of the ridge-transform system in the GC ~6 Ma placed the WL in a favorable position to accommodate part of the transform motion. We predict that WL strike-slip faulting will propagate northwestward, following the migrating Mendocino triple junction and arc, and that the WL - GC will eventually accommodate all plate boundary motion. Whether the WL part will be a single, major strike-slip system (e.g., San Andreas) or a “leaky transform” (e.g., GC) is unknown.
Atwater, T., and Stock, J., 1998, Pacific-North America plate tectonics of the Neogene southwestern United States - An update: International Geological Review, v. 40, p. 375-402.
Faulds, J.E., Henry, C.D., and Hinz, N.H., 2005, Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific - North American plate boundary: Geology, v. 33, p. 505-508.
Ferrari, L. Pasquare, G., Venegas-Salgado, S., and Romero-Rios, F., 2000, Geology of the western Mexican volcanic belt and adjacent Sierra Madre Occidental and Jalisco Block: Geological Society of America Special Paper 334, p. 65-83.
Ferrari, L., Valencia Moreno, M., and Bryan, S., in press, Magmatismo y tectonica en la Sierra Madre Occidental y su relacion con la evolucion de la margen occidental de Norte America: Boletin de la Sociedad Geologica Mexicana y Geological Society of America Special Paper.
Hausback, B.P., 1984, Cenozoic volcanic and tectonic evolution of Baja California Sur, Mexico, in Frizzell, V. A., Jr., ed., Geology of the Baja California Peninsula: Pacific Section S. E. P. M., v. 39, p. 219 236.
Henry, C.D., and Aranda-Gomez, J.J., 1992, The real southern Basin and Range: Mid- to late Cenozoic extension in Mexico: Geology, v. 20, p. 701-704.
Henry, C.D., and Aranda-Gómez, J.J., 2000, Plate interactions control middle-late Miocene, proto-Gulf and Basin and Range extension in the southern Basin and Range: Tectonophysics, v. 318, p. 1-26.
Henry, C.D., and Fredrikson, G., 1987, Geology of southern Sinaloa adjacent to the Gulf of California: Geological Society of America, Map and Chart Series, Map MCH063, 14 p.
Karig, D.E., and Jensky, W., 1972, The Proto Gulf of California: Earth and Planetary Science Letters, v. 17, p. 169 174.
Kimbrough, D.L., Smith, D.P., Mahoney, J.B., Moore, T.E., Grove, M., Gastil, R.G., Ortega-Rivera, A., Fanning, C.M., 2001, Forearc-basin sedimentary response to rapid Late Cretaceous batholith emplacement in the Peninsular Ranges of Southern and Baja California: Geology, v. 29, p. 491-494.
Langenheim, V.E., and Jachens, R.C., 2003, Crustal structure of the Peninsular Ranges batholith from magnetic data: Implications for Gulf of California rifting: Geophysical Research Letters, v. 30, no. 11, 1597, doi:10.1029/2003GL017159.
McQuarrie, N., and Wernicke, B.P., 2005, An animated tectonic reconstruction of southwestern North America since 36 Ma: Geosphere, v. 1, p. 147-172.
Mora-Alvarez, G., and McDowell, F.W., 2000, Miocene volcanism during late subduction and early rifting in the Sierra Santa Ursula of western Sonora, Mexico, in Delgado-Granados, H., Aguirre-Diaz, G., and Stock, J.M., eds., Cenozoic tectonics and volcanism of Mexico: Geological Society of America Special Paper 334, p. 123-141.
Nieto-Samaniego, A.F., Ferrari, L., Alaniz-Alvarez, S.A., Labarthe-Hernández, G., and Rosas-Elguera, R., 1999, Variation of Cenozoic extension and volcanism across the southern Sierra Madre Occidental volcanic province, México: Geological Society of America Bulletin, v. 111, p. 347-363.
Sawlan, M.G., 1991, Magmatic evolution of the Gulf of California rift, in Dauphin, J. P. and Simoneit, B.A.,, eds., The Gulf and Peninsular Province of the Californias: American Association of Petroleum Geologists Memoir, v. 47, p. 301-369.
Stewart, J.H., 1992, Paleogeography and tectonic setting of Miocene continental strata in the northern part of the Walker Lane belt, in Craig, S.D., ed., Structure, tectonics, and mineralization of the Walker Lane; Walker Lane Symposium, proceedings volume: Geological Society of Nevada, Reno, p. 53-61.
Stewart, J.H., and 17 others, including Henry, C.D., 1998, Map showing Cenozoic tilt domains and associated structural features, western North America; in Accommodation zones and transfer zones: The regional segmentation of the Basin and Range province, Faulds, J. E., and Stewart, J. H., eds.: Geological Society of America Special Paper 323.
Stock, J.M., and Hodges, K.V., 1989, Pre Pleistocene extension around the Gulf of California and the transfer of Baja California to the Pacific Plate: Tectonics, v.8, p. 99-115.
Stockli, D.F., Farley, K.A., and Dumitru, T.A., 2000, Calibration of the apatite (U-Th)/He thermochronometer on an exhumed fault block, White Mountains, California: Geology, v. 28, p. 983-986.
Swanson, E.R., and McDowell, F. W., 1984, Calderas of the Sierra Madre Occidental volcanic field, western Mexico: Journal of Geophysical Research, v. 89, p. 8787 8799.
Whitehill, C.S., Miller, E.L., Colgan, J.P., Dumitru, T.A., Lerch, D., and McWilliams, M.O., 2004, Extent, style and age of Basin and Range faulting east of Pyramid Lake: Geological Society of America Abstracts with Programs, v. 36, no. 4, p. 37.
The story is nearly identical around the Gulf of California where proto-Gulf extension was focused largely, not entirely, between the Peninsular Ranges batholith and the Sierra Madre Occidental caldera belt, and strike-slip faulting jumped to the area that had undergone extension. But both extension and strike-slip faulting cut across the batholith belt, for example, in Sinaloa, where the batholith is moderately extended, tilted up to 40-50° .
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Andesite belts(Sawlan, 1991)
East edge La Posta beltPeninsular Ranges batholith(Kimbrough et al, 2001)
East edge magnetic, mafic, western beltPeninsular Ranges batholith(Langenheim and Jachens, 2003)
Datedmid-Cenozoic
rocks in Sinaloa
Additions to volcanic belts fromthis work (Sinaloa);Mora and McDowell, 2003 (Sonora);and Nieto-Samaniego et al, 1999, and Ferrari et al, in press (Nayarit)
Crustal and Magmatic Influences on Rifting
Sierra Madre Occidental?
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Our interpretation is that ancestral Cascade arc magmatism (i.e., that north of the plate age), as shown here in a compilation of magmatism over the last 13 Ma, heated/hydrated and weakened the lithosphere.
The Walker Lane and earlier extension nearly exactly match the ancestral arc. E.g., the western limits of all are approximately identical. Extension and strike-slip faulting were focused into the zone of weakened lithosphere and able to cut through the otherwise resistant batholith belt. The north end of the Walker Lane blends imperceptibly into the active Cascade arc. We suggest that strike-slip faulting is essentially channelized into the arc.
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Volcanic centersLavas, intrusions
Juan de Fuca plate edge12 Ma
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Cretaceousbatholith
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The Walker Lane is in the same location as the preceding ~13-12 Ma extensional episode, and both are largely between the Sierra Nevada batholith and the mid-Tertiary caldera belt.
Note that the southwest edge of the caldera-batholith belt and the northeast edge of the Walker Lane coincide, and both make a big right step at the Mina deflection.
The caldera belt is only slightly extended, ±10%, one of the least extended parts of the Basin and Range Province, as shown by the slight (£10° ) tilt of calderas. The caldera belt is underlain by a nearly continuous batholith, very similar to Cretaceous Sierra Nevada batholith. Large, cold batholiths resist deformation.
But the Walker Lane (and extension) cut across the Cretaceous batholith belt to the north. Why?
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The same factors, a combination of Mesozoic batholiths, mid-Cenozoic caldera-batholith belts, and late Cenozoic volcanic arcs, determined where extension and strike-slip occurred in both the Gulf and Walker Lane.
In both areas, extension began in what had been a continental volcanic arc. Extension in the Gulf was largely, not entirely, sandwiched between the mostly Cretaceous Peninsular Ranges batholith and the mid-Cenozoic Sierra Madre Occidental caldera province, an essentially unextended block. Extension in the Walker Lane was largely, not entirely, sandwiched between the Sierra Nevada batholith and the mid-Cenozoic central Nevada caldera belt, a little extended part of the Basin and Range province. Strike-slip faulting and the plate boundary jumped into the area that had undergone extension, totally in what is now the Gulf at ~6 Ma, but only partly into the Walker Lane.
SMO
0 600 km TMVB
WL - ECSZ
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Cretaceous BatholithsMid-Tertiary calderas
Crustal influences on the locus of deformation are essentially identical: Mesozoic batholiths, mid-Cenozoic caldera-batholith belts, and ancestral continental volcanic arcs.
About 6 Ma, spreading jumped into the proto-Gulf, and the Gulf became the plate boundary. This plate boundary realignment places the Walker Lane, which was previously far inboard, in a much more favorable location to accommodate part of the plate motion.
The timing of initiation of strike-slip faulting in the Walker Lane is a key question but highly debated. We favor initiation in the northern Walker Lane no earlier than 6 Ma and possibly 3 Ma, but interpretations go back to ~25 Ma. The fundamental question is really when did the Sierra Nevada start moving NW. Certainly today we have a significant part of Pacific-North American plate motion along the Walker Lane and eastern California shear zone.
This series of figures from McQuarrie and Wernicke illustrate rotation of the Sierra Nevada. Initially, ENE extension occurred from Southern California to northern Mexico.
McQuarrie and Wermicke (2005)
18–24 Ma 14–16 Ma 10–12 Ma 6–8 Ma 0–2 Ma
By ~10-12 Ma, the Sierra Nevada had rotated another ~5° , and basins opened to the east in the future Walker Lane.
The Sierra Nevada continued to rotate until about 6 Ma, for a total of about 10-11° (my rough measurement from McQuarrie and Wernicke reconstruction).
So the oblique motion of the Pacific plate relative to North America, recognized by Stock and Hodges, was accommodated by ENE extension in the proto-Gulf and by similar extension related to clockwise rotation of the Sierra Nevada in what was to become the Walker Lane.
After 6 Ma, Sierra Nevada underwent northwest translation but no more rotation.
About 14-16 Ma, the Sierra Nevada started to rotate clockwise, initially ~3° and possibly related to earlier extension in the northern Gulf.
Although this extension was mostly behind the transform margin, it wasn’t limited to a block bounded by the two triple junctions. What appears to have happened in the Walker Lane is the Sierra Nevada block rotated clockwise so that the North American plate kept in contact with the Pacific and Juan de Fuca plates.
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Sierra Nevada
WL – ECSZ
200 km
Areas of ExtensionBeginning ~13–12 Ma
Cascade arc
Walker Lane – eastern California shear zone with areas of extension beginning ~13-12 Ma. Extension at this time generated a series of large, shallow, sedimentary basins (e.g., Stewart, 1992; Esmeralda, Coal Valley, “Reno”) as well as uplift of the White and Sahwave Mts based on thermochronology by Stockli et al., 2000, and Whitehill et al., 2004, respectively. This extension was in the area that was to become the Walker Lane, mostly close to the eastern edge of the Sierra Nevada. It was also low-magnitude, as evidenced by the broad, shallow basins and only moderate tilt of rocks in and around the basins.
Why did this extension occur, and how is it related to the proto-Gulf?Possibly not as appreciated, an episode of extension beginning at ~13-12 Ma occurred far outside the “proto-Gulf”, both east of the Sierra Madre Occidental, and in a band far to the northwest in the future Walker Lane in western Nevada and eastern California.
(And this compilation is way out of date.)
SMO
0 600 km TMVB
WL - ECSZ
Casc
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Arc
Known areas of ~13-12 Ma extension
The Walker Lane Underwent Similar Development, with a Major Episode of Extension beginning ~13-12 Ma, followed by Strike-Slip Faulting
The change from a convergent to transform margin is key. At ~13-12 Ma, the Rivera triple junction and transform margin jumped southward. The continental volcanic arc, which had been active along most of Baja and mainland Mexico until then, shut off. Pacific plate motion was oblique to the transform (Tosco – Abreojos fault), as it was from the start of Pacific-North American plate contact.
Stock and Hodges (1989) recognized that this required two components of motion, one parallel to the transform and one perpendicular to it, the well known ENE extension of the proto-Gulf.Atwater website 13 Ma
Atwater website 12.5 Ma
SMO
0 600 km TMVB
WL - ECSZ
Casc
ade
Arc
The Walker Lane and Gulf of California underwent similar, related evolution as part of plate boundary development: extension beginning ~13-12 Ma, followed by strike-slip faulting.
They are in different stages of evolution: total plate boundary in Gulf, ~20-25% of Pacific-North American motion in Walker Lane.
Crustal influences on the locus of deformation are essentially identical: Mesozoic batholiths, mid-Cenozoic caldera-batholith belts, and ancestral continental volcanic arcs.
Faults of the Basin and Range Province from Stewart (1978), Henry and Aranda-Gomez (1992), and Stewart et al (1998)
A Familiar Story: Evolution of the Pacific–North American Plate Boundary, the ~13-12 Ma Jump in the Rivera Triple Junction,and "Proto-Gulf Rifting" through Strain Partitioning
