Paleomagnetism vs. Geology

The paleomagnetism versus geology controversy, revolves around the geographical position of the Baja California Peninsula from approximately 50 to 130 million years ago. This problem has remained unsolved for the past 25 years due partially to the current political boundaries and the desolate conditions of the Baja region.

The paleomagnetic signature of the Peninsula Ranges batholith (PRB) and geologic relationships is the crux of this controversy. The PRB is mountain range that extends from Southern California to nearly the 28th parallal in Baja California. The formation of the of the PRB occurred approximately 90 to 130 million years ago during the Cretaceous period. At that time the Farallon plate was being subducted and consumed along the western margin of North America. This resulted in the formation of large quantities of magmatic material that cooled and crystallized kilometers below the surface. As the PRB cooled tiny particles of iron rich minerals aligned themselves with the Cretaceous magnetic field of the earth. With the Cretaceous magnetic field now permently recorded in these granitic rocks a geographic position of the Baja Peninsula could now be determined. The techniques for determining these data are highly sophisticated, but these are the same techniques that proved the theory of plate tectonics. The conclusion from all paleomagmatic studies obtained from the PRB indicate that the position of the Baja Peninsula was 900 km south of its pre-batholithic location. This locates the Baja Peninsula near the states of Michoacan and Guerrero, it is also thought that the Baja Peninsula was an island arc very near the coastline of Mexico during the Cretaceous.

Geologic data suggest a different scenario to the evolution of Baja. It has been shown that miogeoclinal facies of latest Proterozoic and Paleozoic rocks, 900 to 254 million years old, in northeastern portions of Baja California, correlate to rocks of the same age in Sonora Mexico. This suggests that closing of the Gulf of California would restore the Baja Peninsula to its origonal location. During the Eocene epoch (58 to 37 million years ago) large amounts of volcanic rocks were deposited in north San Diego County from a major fluvial system that flowed in an east to west direction. This was also a time when San Diego averaged 40 to 60 inches of rain annually. These volcanic rocks are known as Poway Rhyolites and have been geochemicaly and isotopicaly matched to rhyolite outcrops in northwestern Sonora, Mexico. This requires closing of the Gulf of California ~300 km southeast of its present position in order to align the Eocene fluvial system with deposits in north San Diego County. The implications from these geologic field relationships are that sometime after the end of the Paleozoic the Baja California Peninsula rifted away from the Sonora region of Mexico. The peninsula then traveled south 900 km where upon the PRB acquired its paleomagnetic signature. Then sometime after 90 million years the Baja Peninsula traveled north another 900 km back to its previous location. Many geologist feel this scenario is highly improbable and that tectonic tilting of the batholith can account for the shallow paleomagnetic results, however undisputable evidence of tilting has not been shown. Paleomagnetic data also show that not only was the PRB at a lower latitude during the Cretaceous, but that it was still south of Sonora during the Eocene which would have prevented the deposition of Poway Rhyolites across the peninsula into San Diego.

My undergraduate thesis,The Provenance of Puddingstones in Peninsular California, was a petrographic comparison of puddingstones from the Cretaceous Valle Group near Punta Abreojos, 600 miles south of San Diego, and puddingstones from the Eocene Las Palmas Gravels, located 30 miles south of Tecate in Northern Baja California. A similar undergraduate thesis by my colleague Kevin Rogers was also done on quartzite clasts from the same conglomerate deposits in the Valle Group. These rocks are unique to their present depositional location due to their ultrastable and supermature composition. These rocks are well rounded covered with percussion marks and are not the type of rocks you would expect to find in fore-arc basin deposit. In other words these are not rocks you would expect to find from the denudation of a magmatic arc. The question is, where did these puddingstones and quartzites come from? The petrographic analysis of these rocks indicate a continental origin and if the Baja Peninsula was an island arc during the Cretaceous how did these rocks end up in a Cretaceous deposit along the western margin of the PRB?

Geologic Definitions:

Clast: An individual constituent, grain, or fragment of a detrital sediment or sedimentary rock, produced by the physical disintegration of a larger rock mass. Back

Conglomerate: A coarse-grain clastic sedimentary rock, composed of rounded to subangular fragments larger than 2 mm in diameter, that include, (granules, pebbles, cobbles, or boulders), set in a fine-grain matrix of sand or silt, and commonly cemented by calcium carbonate, iron oxide, silica, or hardened clay. Back

Cretaceous: The final period of the Mesozoic era, after the Jurassic and before the Tertiary period of the Cenozoic era, thought to have covered the span of time between 135 and 65 million years ago. Back

Cryptocrystalline: A textural term describing rocks consisting of crystals that are too small to be recognized and distinguished under a petrographic microscope. Back

Denudation: The sum of the processes that result in the wearing away of the earth's surface by weathering, mass wasting, and transportation; also the combined destructive effects of such processes. The term is wider in its scope than erosion. Back

Eocene: An epoch of the early Tertiary period, after the Paleocene and before the Oligocene, covering a time span from 58 to 37 million years ago. Back

Farallon plate: An oceanic plate that was being subducted along the western margin of the North America during the Mesozoic and most of the Cenozoic. The subduction of the Farallon plate was responsible for batholiths, compressional structures, volcanism, and metamorphism that occurred during Mesozoic and early Tertiary oroginies (mountain building events). Back

Fluvial: Pertaining to rivers; produced by the action of a stream or river. Back

Granitic composition: A descriptive term to describe an intrusive rock type found in the Peninsula Ranges batholith. A true granite is composed contains 10 to 50 percent quartz and a alkali feldspar, to total feldspar ratio of 65 to 90 percent. The PRB is composed of gabbro, granodiorite, tonalite, and some monzogranite. Back

Island arc: A curved chain of islands e.g. the Aleutians. Island arcs are a series of volcanoes that trace the shape of subduction zones that created them. Back

Isotope geology: The study of radioactive and stable isotopes, especially their abundances, to geology. It includes the calculation of geologic time, and the determination of the origin, mechanisms, and conditions of geologic processes by isotopic means.Back

Jurassic The second period of the Mesozoic era, after the Triassic and before the Cretaceous, thought to have covered the span of time between 190 and 135 million years ago. Back

Las Palmas Gravels: The Las Palmas Gravels are a conglomerate deposited during the Middle to Late Eocene by a major long distant stream approximately 200 miles in length, these deposits are now found at Valle de las Palmas roughly 30 miles south of Tecate, Mexico in northern Baja California. Back

Magmatic arc: A term used to describe the active magma stage of a linear or arc type emplacement of a batholith. Back

Magmatic material: Naturally occurring molten rock material generated within the earth and from which intrusive and extrusive rocks are derived from. Back

miogeocline facies: A succession of sedimentary rocks located near a craton without any volcanic rocks. Back

Paleogeography: The physical geographic location of a part of the earth's surface at some time in the geologic past. Back

Paleomagnetic data: Results from the study of natural remanent magnetization of rocks in order to determine the intensity and direction of the earth's magnetic field in the geologic past. A geophysicist is then able to determine the paleolatitude of a particular geographic location. Back

Paleozoic: An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, or from about 570 to 225 million years ago. Back

Peninsula Ranges Batholith: A series of granitic batholiths in southern and Baja California, the northern extent includes the San Jancinto Mountains, Santa Ana Mountains, Laguna Mountains, Sierra De Juarez Mountains, Sierra San Pedro Martir Mountains, and the granitic rocks of the Catavina Boulder Field. The granitic composition is typically granodiorite and tonalite. A batholith is a large, generally discordant granitic mass that has more than 100 square kilometers of surface exposure and no known floor. Back

Percussion marks: A crescentic scar produced on hard dense pebble, epically chert or quartzite, by a violent impact of one pebble on another; it may be indicative of high-velocity flow. Back

petrography: A branch of geology dealing with the description and systematic classification of rocks using a polarizing microscope. Back

Phenocrysts: Large and ordinarily conspicuous crystals of the earliest generation in a porphyritc igneous rock. Back

Porphyritic: A textural term describing an igneous rock of any composition that contains conspicuous phenocrysts in a fine-grain groundmass. Back

Poway Rhyolite: An abundant clast that occurs in Eocene alluvial fan deposits exposed in the Poway valley northeast of the city of San Diego. Rhyolite is the extrusive equivalent of granite, a volcanic rock that is porphyritic and commonly exhibits flow texture, with phenocrysts of quartz and alkali feldspar in a cryptocrystalline glassy groundmass. Back

Proterozoic: The more recent of 2 great divisions of the Precambrian. Back

Puddingstone: A popular name applied chiefly in Great Britain to a conglomerate consisting of well-rounded pebbles whose colors are in marked contrast with the fine-grain matrix or cement. Back

Quartzite: A metamorphic rock consisting of at least 90% quartz, formed by recrystallization of sandstone by regional or thermal metamorphism; a metaquartzite. Back

Subduction: The process of one lithospheric plate descending beneath another. Typically, oceanic crust that is cold, dense, and older subducts under younger less dense oceanic crust or subducts under less dense continental crust. Back

ultrastable and supermature: Ultrastable is the ability of a rock to remain physically and chemically stable at the surface of the earth. Rocks that are supermature have survived at least 2 depositional events. Rocks of these type are composed primarily of silica Back

Valle Group: A series of turbidite deposits that are punctuated by inner channel fan deposits. Turbidites are sediment deposited from turbidity currents, these are sediment laden currents that flow down slope along the bottom, typically in submarine canyons. Inner channel deposits are typically composed of boulders cobbles and pebbles that are deposited in the upper channels of the submarine canyons. Back

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	Clast: An individual constituent, grain, or fragment of a detrital sediment or sedimentary rock, produced by the physical disintegration of a larger rock mass. Back
	Conglomerate: A coarse-grain clastic sedimentary rock, composed of rounded to subangular fragments larger than 2 mm in diameter, that include, (granules, pebbles, cobbles, or boulders), set in a fine-grain matrix of sand or silt, and commonly cemented by calcium carbonate, iron oxide, silica, or hardened clay. Back
	Cretaceous: The final period of the Mesozoic era, after the Jurassic and before the Tertiary period of the Cenozoic era, thought to have covered the span of time between 135 and 65 million years ago. Back
	Cryptocrystalline: A textural term describing rocks consisting of crystals that are too small to be recognized and distinguished under a petrographic microscope. Back
	Denudation: The sum of the processes that result in the wearing away of the earth's surface by weathering, mass wasting, and transportation; also the combined destructive effects of such processes. The term is wider in its scope than erosion. Back
	Eocene: An epoch of the early Tertiary period, after the Paleocene and before the Oligocene, covering a time span from 58 to 37 million years ago. Back
	Farallon plate: An oceanic plate that was being subducted along the western margin of the North America during the Mesozoic and most of the Cenozoic. The subduction of the Farallon plate was responsible for batholiths, compressional structures, volcanism, and metamorphism that occurred during Mesozoic and early Tertiary oroginies (mountain building events). Back
	Fluvial: Pertaining to rivers; produced by the action of a stream or river. Back
	Granitic composition: A descriptive term to describe an intrusive rock type found in the Peninsula Ranges batholith. A true granite is composed contains 10 to 50 percent quartz and a alkali feldspar, to total feldspar ratio of 65 to 90 percent. The PRB is composed of gabbro, granodiorite, tonalite, and some monzogranite. Back
	Island arc: A curved chain of islands e.g. the Aleutians. Island arcs are a series of volcanoes that trace the shape of subduction zones that created them. Back
	Isotope geology: The study of radioactive and stable isotopes, especially their abundances, to geology. It includes the calculation of geologic time, and the determination of the origin, mechanisms, and conditions of geologic processes by isotopic means.Back
	Jurassic The second period of the Mesozoic era, after the Triassic and before the Cretaceous, thought to have covered the span of time between 190 and 135 million years ago. Back
	Las Palmas Gravels: The Las Palmas Gravels are a conglomerate deposited during the Middle to Late Eocene by a major long distant stream approximately 200 miles in length, these deposits are now found at Valle de las Palmas roughly 30 miles south of Tecate, Mexico in northern Baja California. Back
	Magmatic arc: A term used to describe the active magma stage of a linear or arc type emplacement of a batholith. Back
	Magmatic material: Naturally occurring molten rock material generated within the earth and from which intrusive and extrusive rocks are derived from. Back
	miogeocline facies: A succession of sedimentary rocks located near a craton without any volcanic rocks. Back
	Paleogeography: The physical geographic location of a part of the earth's surface at some time in the geologic past. Back
	Paleomagnetic data: Results from the study of natural remanent magnetization of rocks in order to determine the intensity and direction of the earth's magnetic field in the geologic past. A geophysicist is then able to determine the paleolatitude of a particular geographic location. Back
	Paleozoic: An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, or from about 570 to 225 million years ago. Back
	Peninsula Ranges Batholith: A series of granitic batholiths in southern and Baja California, the northern extent includes the San Jancinto Mountains, Santa Ana Mountains, Laguna Mountains, Sierra De Juarez Mountains, Sierra San Pedro Martir Mountains, and the granitic rocks of the Catavina Boulder Field. The granitic composition is typically granodiorite and tonalite. A batholith is a large, generally discordant granitic mass that has more than 100 square kilometers of surface exposure and no known floor. Back
	Percussion marks: A crescentic scar produced on hard dense pebble, epically chert or quartzite, by a violent impact of one pebble on another; it may be indicative of high-velocity flow. Back
	petrography: A branch of geology dealing with the description and systematic classification of rocks using a polarizing microscope. Back
	Phenocrysts: Large and ordinarily conspicuous crystals of the earliest generation in a porphyritc igneous rock. Back
	Porphyritic: A textural term describing an igneous rock of any composition that contains conspicuous phenocrysts in a fine-grain groundmass. Back
	Poway Rhyolite: An abundant clast that occurs in Eocene alluvial fan deposits exposed in the Poway valley northeast of the city of San Diego. Rhyolite is the extrusive equivalent of granite, a volcanic rock that is porphyritic and commonly exhibits flow texture, with phenocrysts of quartz and alkali feldspar in a cryptocrystalline glassy groundmass. Back
	Proterozoic: The more recent of 2 great divisions of the Precambrian. Back
	Puddingstone: A popular name applied chiefly in Great Britain to a conglomerate consisting of well-rounded pebbles whose colors are in marked contrast with the fine-grain matrix or cement. Back
	Quartzite: A metamorphic rock consisting of at least 90% quartz, formed by recrystallization of sandstone by regional or thermal metamorphism; a metaquartzite. Back
	Subduction: The process of one lithospheric plate descending beneath another. Typically, oceanic crust that is cold, dense, and older subducts under younger less dense oceanic crust or subducts under less dense continental crust. Back
	ultrastable and supermature: Ultrastable is the ability of a rock to remain physically and chemically stable at the surface of the earth. Rocks that are supermature have survived at least 2 depositional events. Rocks of these type are composed primarily of silica Back
	Valle Group: A series of turbidite deposits that are punctuated by inner channel fan deposits. Turbidites are sediment deposited from turbidity currents, these are sediment laden currents that flow down slope along the bottom, typically in submarine canyons. Inner channel deposits are typically composed of boulders cobbles and pebbles that are deposited in the upper channels of the submarine canyons. Back