Marsaglia

PROVENANCE OF SANDS AND SANDSTONES FROM A RIFTED CONTINENTAL ARC GULF OF CALIFORNIA
KATHLEEN M

MEXICO

MARSAGLIA
at

Department of Geological

Sciences

The

University of Texas

El Paso

El Paso

Texas 79968

Deep Sea Drilling Project recovered syn rift and post rift sediments deposited within the are tip Baja California and within the Guaymas Basin Although detrital modesof Leg 64 sand and sandstone along consistent with the dissected magmatic arc tectonic setting of the Gulf of California compositional differences between southern Baja California sites and Guaymas Basin sites reflect the contrasting plate tectonic histories of the two regions Southern Baja California is a true rifted continental margin in which Pliocene and Pleistocene sands deposited inoffshore basins drift related sands The show a temporal change in the Lower Pliocene from volcaniclastic rift related sands to quartzo feldspathic in texture and probably derived from syn rift volcanism microcrystalline volcanic component in the rift related sands is primarily felsitic the Guaymas Basin is a transtensional basin floored by quasi oceanic crust created by successive sill injections into Incontrast Basin show no temporal changes in detrital composition but do sedimentary basin fill Quaternary sands deposited in the Guaymas indicate multiple sources of sand Volcaniclastic sands enter the basin from rivers draining mainland Mexico and Baja peninsula streams The nature of microcrystalline textures in populations of volcanic lithic fragments helps distinguish these two provenances Thean desitic source terranes of Baja California provide predominantly microlitic volcanic lithic fragments whereas the Sierra Madre Occi dental volcanic province yields a hybrid mixture of volcanic lithic fragments displaying felsitic and microlitic microcrystalline textures similar to those observed in modern sands from the Rio Grande rift Guaymas Basin sands have also been variably affected by hydrothermal alteration which has significantly modified detrital modes of some sands by silicification of vitric volcanic fragments and removal of potassium feldspar
ABSTRACT

Leg

64 of the of

Gulf of California

the southern

INTRODUCTION

The Gulf of California is

a

young transtensional proto

gulf Ingersoll 1988 that has formed as the result of instepping of the EastPacific Rise across western Mex ico Fig I Over the past 10 million years or so Baja
oceanic California has been transferred from the North American plate to the Pacific plate Atwater 1970 Mammerickx and

Curray and Moore 1982 Sites drilled dur 64 recovered syn rift and post rift sediments from ing Leg the southernmost tip of Baja California a pure rifted mar
lantic Ocean

gin
mas

Cabo SanLucas

Basin a pull Gulf of California Fig 10 The Gulf of California provides

10 and from the Guay basin in the central portion of the apart

region Fig

Klitgord
and
a

1982

In Cenozoic time

prior

to

13 to 12 Ma

ting
sand

subduction occurred continental

spanned the Baja California Peninsula The migration of the Rivera tri ple junction south along the coast of BajaCalifornia se quentially transformed the subduction margin trench to a transform margin Fig IB C At approximately 5 Ma when the Rivera triple junction lay in the vicinity of the the plate present Gulf a major plate reorganization occurred eastward transferring Baja California to boundary jumped the Pacific plate and initiating rifting and strike slip motion
arc

along magmatic

all of westernMexico

Fig lA length of the

a unique neotectonic set previous studies relating plate tectonic setting to composition have not characterized detrital modes for

and

sands and sandstones from rifted continental arc systems e g Dickinson and Suczek 1979 Dickinson and others
The main purposes of this paper are to document 1983 detrital modes of sands recovered during Leg 64 to deter...