Reconnaissance Geology of the State of Baja California
R. Gordon Gastil, Richard P. Phillips, Edwin C. Allison, 1975. "Reconnaissance Geology of the State of Baja California", Reconnaissance Geology of the State of Baja California, R. Gordon Gastil, Richard P. Phillips, Edwin C. Allison
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The prebatholithic terrane of Baja California is represented by a western belt of rocks like those of the Franciscan assemblage, a belt of Mesozoic volcanic-volcaniclastic rocks, a belt of Mesozoic(?) slate and metasandstone, and, along the Gulf of California, Paleozoic rocks of various types. The volcanic-volcaniclastic rocks are, in part, contemporaneous with batholithic emplacement.
The Mesozoic thermal event that produced regional metamorphism and the emplacement of batholithic rocks can be described in terms of four metamorphic zones: (1) essentially unmetamorphosed rocks, (2) slate to phyllitic rocks, (3) schist and amphibolite, and (4) rocks in which the pre metamorphic fabric has been entirely destroyed. Most of the discrete plutons are found in zone 3; zone 4 includes generalized metamorphic-plutonic rocks that have been deep in the crust for a prolonged time. The granitic exposures are made up of tonalite (73 percent), granodiorite (23 percent), and gabbro (2 percent). The plutons of largest diameters are formed of tonalite; plutons of smallest diameters, of gabbro and granite. Gabbro is common in the western half of the batholithic belt and rare in the eastern half. No systematic increase in (K2O + Na2O)/SiO2 is recognized across the peninsula.
The postbatholithic interval began about 90 m.y. ago. The erosion of the western portion of the metamorphic-plutonic terrane was accomplished within the first 20 m.y. Potassium-argon dating indicates that the basement rock underlying the gulf area cooled somewhat later. Upper Cretaceous deposits were derived from the erosion of the western part of the peninsula. In contrast, the Paleocene-Eocene deposits included detritus derived from areas now within the Gulf of California depression and from areas farther east. During the Eocene Epoch, the northern part of the peninsula was a pediplain not far above sea level.
Although volcanism occurred locally during early Cenozoic time, widespread calc-alkalic volcanism and the downfaulting of the gulf area began during late Oligocene time. This volcanic phase was followed about 9 m.y. ago by the entry of the sea into the northern part of the gulf. About 6 to 8 m.y. ago, the gulf area was subjected to folding, uplift, and erosion that removed the early Cenozoic volcanic rocks from wide areas. During this interval, the peninsula also was uplifted and stripped of much of its Cenozoic cover, and the early Cenozoic pediplain and Late Cretaceous coastline were exhumed. Alkalic basalt and basaltic andesite then poured across the dissected Pliocene-Pleistocene surface.
The topography of the peninsula displays remnants of both the erosional surfaces and coastal configurations that were produced during Late Cretaceous and early Cenozoic time. They were disrupted by volcanism and faulting in Miocene and later time, uplifted in the Pliocene Epoch, and deeply dissected by subsequent erosion.
Structurally, Baja California is divided into the stable western and central peninsula, the unstable continental borderland to the west, and the gulf depression to the east. The borderland has been structurally distinct since at least Late Cretaceous time. The gulf depression has undergone about 50 percent dilation since the Oligocene Epoch, and extensional deformation is probably continuing.
A regional gravity survey of the state of Baja California shows the peninsula to be generally in isostatic adjustment. The Bouguer anomaly map reveals a gravity high that extends down the west coast from the Agua Blanca fault to at least lat 28° N.
Mineral exploration indicates that there are several distinct mineral provinces. Three of these are the volcanic copper-iron province, which corresponds to the Mesozoic volcanic-volcaniclastic belt; the schist-gold province, which corresponds to the Mesozoic(?) slate and metasandstone belt; and the Cenozoic hydrothermal province of the gulf area. The most fascinating and least understood of these is the Cenozoic hydrothermal province, where both base and precious metals are probably being deposited today.