listric faulting and continental rifting in western mexico a paleomagnetic and structural study abstract the western trans mexican volcanic belt is characterized by several large scale tectonic depressions which recently have been interpreted in terms of active rifting and independent motion of the jalisco block in this paper we report results for the northwestern part of the jalisco block within the ameca tectonic depression the trend of the central part of the depression changes where it is bounded by the ameca river fault there the fault which is characterized by a steep high scarp and a pronounced surficial curvature bounds the pleistocene amatlan de canas volcanic field which consists of small eroded volcanic structures and mafic lava flows intercalated with conglomeratic talus debris the sinuous pattern of outcrop of the fault indicates that the fault plane dips toward the volcanic field the conglomeratic units are abruptly truncated at the fault contact and generally dip 12 to 20 toward the fault plane paleomagnetic results for the young volcanic units give a mean direction which is interpreted to be the result of 12 to 20 of tilting toward the fault regional listric faulting block sliding of the entire block occupied by the volcanic field is postulated to explain the structural stratigraphic and paleomagnetic patterns observed listric faulting occurs within a transtensional tectonic environment related to the southwesterly motion of the jalisco block as it rifts away from mainland mexico

evaluation of recent tectonomagmatic discrimination diagrams and their application to the origin of basic magmas in southern mexico and central america discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years although old diagrams have been extensively used the recent ones based on discriminant functions of ratio variables with or without log transformation proposed during 2004 2010 for the discrimination of four tectonic settings of island arc continental rift ocean island and mid ocean ridge were newly evaluated to show their high success rates of 57 3 100 and 58 5 100 for major element and immobile element based diagrams respectively for the continental arc of the andes evaluated for its similarity to island arc these four sets of diagrams showed success rates of 62 1 83 8 these four sets of five diagrams per set were therefore used to infer tectonic setting of the mexican volcanic belt mvb los tuxtlas volcanic field ltvf and central american volcanic arc cava using this approach the mvb especially its western central and eastern parts and the ltvf of southern mexico show a dominantly continental rift setting and the cava shows an arc setting the west central part of the mvb is consistent with dual tectonics of arc and rift these results confirm the application of an unusual mantle upwelling rift model for the mexican on land volcanism whereas the conventional plate tectonic subduction model seems to be applicable for the cava from guatemala to north western costa rica

volcan las navajas a pliocene pleistocene trachyte peralkaline rhyolite volcano in the northwestern mexican volcanic belt volcan las navajas a pliocene pleistocene volcano located in the northwestern portion of the mexican volcanic belt erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite and is the only volcano in mainland mexico known to have erupted pantellerites las navajas is located near the northwestern end of the tepic zacoalco rift and covers a 200 m thick pile of alkaline basaltic lavas one of which has been dated at 4 3 ma the eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation during the first stage a broad shield volcano made up of alkali basalts mugearites benmoreites trachytes and peralkaline rhyolites was constructed eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera the second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls this stage ended about 200 000 years ago with the eruption of pumice falls and ash flows which led to the collapse of the southern portion of the volcano to form the second caldera during the third stage two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano finally the calc alkaline volcano sanganguey was built on the southern flank of las lavajas alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along nw trending fractures through the area although other mildly peralkaline rhyolites are found in the rift zones of western mexico only las navajas produced pantellerites greater volumes of basic alkaline magma have erupted in the las navajas region than in the other areas of peralkaline volcanism in mexico a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions

geometry of the rivera cocos subduction zone inferred from local seismicity the rivera microplate and the cocos plate are subducting beneath the north american plate to obtain new insights into the subduction geometry we located earthquakes recorded by seismic stations deployed during the mapping the rivera subduction zone experiment from january 2006 to june 2007 we implemented standard procedures for earthquake location and we also manually corrected phase detections and picks to increase accuracy of the earthquake catalog we located about 2100 earthquakes within the 2 0 5 3 m l magnitude range the average location error is 0 8 km horizontally and 1 0 km vertically and the origin time has a mean error of 0 2 s we observed crustal seismicity over the subducted rivera microplate at no depth greater than 25 km on the other hand seismicity over the subducted cocos plate is present to depths of 40 km the rivera plate shows a gently subduction angle near the trench then dives more steeply into the mantle at an angle of 34 in the north and 37 in the south near the colima graben the distance between the inflection and the trench gradually increases from 40 km in the north of the study area to 60 km in the south near the colima rift cr the cocos plate has a slightly curved subducted slab with oblique geometry dipping toward the cr ranging in dip from 18 in the south to 30 in the north

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle

origin of andesite in the deep crust and eruption rates in the tancitaro nueva italia region of the central mexican arc the tancitaro nueva italia region 4400 km 2 is located within the west central mexican volcanic belt and spans 80 km of arc length along the volcanic front on the basis of 65 40 ar 39 ar ages it is shown that a total volume of 326 57 km 3 of magma has erupted from 200 vents since 1 2 ma nearly a third of this volume 103 5 km 3 was erupted as crystal rich andesite from a large stratovolcano volcan tancitaro whereas the remaining two thirds 223 52 km 3 span the complete range from 51 to 63 wt sio 2 are relatively crystal poor and were erupted from peripheral vents the total proportions of erupted lava types are 6 7 basalt 18 basaltic andesite and 74 76 andesite there is no rhyolite and only a trace of dacite there is no evidence that any significant magma differentiation occurred in the upper crust along this arc segment based on phenocryst abundances and modes major and trace element data as well as phase equilibrium experiments from the literature the following mechanisms for formation of andesite in this region can be ruled out 1 crystal fractionation of basalt basaltic andesite in the upper crust 2 magma mixing of basalt basaltic andesite with dacite rhyolite and 3 assimilation of granitoid by basalt basaltic andesite instead the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting 20 of hornblende rich 40 gabbronorite the basaltic andesites are required owing to their elevated ni concentrations to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two pyroxene fractionation in the deep crust

pleistocene to recent alkalic volcanism in the region of sanganguey volcano nayarit mexico forty five cinder cones and associated lava flows have erupted within the last 300 000 years along five parallel lines through the calc alkaline volcano sanganguey in the northwestern segment of the mexican volcanic belt lavas erupted from these cinder cones include ne and hynormative alkali basalts hawaiites mugearites and benmoreites it is unusual that this suite has erupted in a calc aikaline volcanic belt where volcanoes in the vicinity have been erupting calc alkaline andesites dacites and rhyodacites

evolution of waning subduction related magmatism northern sierra madre occidental mexico the stratigraphic record at the tomochic volcanic center chihuahua documents a change from andesitic to rhyolite dominated volcanism with late extrusion of mafic lavas during the final 10 m y of subduction related activity in the northern sierra madre occidental andesitic volcanism which started before 38 ma and continued until 35 ma was followed by eruption of large volume rhyolite ash flows from the las varas 34 1 ma and tomochic 31 8 31 4 ma calderas basaltic andesite the most mafic rock type in the area was extruded near the end of calcalkalic volcanism at 30 ma the change in style of volcanism at the tomochic volcanic center also occurred in other parts of the northern sierra madre occidental and is interpreted to reflect the evolution of waning subduction related magmatism as convergence between the north american and farallon plates slowed before it eventually ceased slowed plate convergence resulted in a decreased flux of mafic melts from the mantle to the crust leading in turn to the stagnation and coalescence of intermediate composition magmas similar to those that fed early andesitic eruptions subsequent differentiation led to growth of zoned magma reservoirs from which the rhyolite ash flows were extruded slowed convergence was accompanied by the onset of crustal extension which possibly aided the coalescence of andesitic magmas and which allowed the ascent and eruption of mafic melts near the end of silicic volcanic activity although minor differences exist the history of mid tertiary magmatism and tectonism in the northern sierra madre occidental was broadly similar to that in parts of the south western united states

onshore marine cenozoic along southwest pacific coast of mexico reconnaissance exploration along the pacific coast of mexico from the isthmus of tehuantepec to puerto vallarta in the northwest beginning in 1977 has shown the presence of at least seven previously unreported areas with late cenozoic sedimentary deposits in addition marine pleistocene deposits on the coast of oaxaca were previously reported in four of the newly discovered areas marine sediments occur all of these occurrences excepting the two southeastern ones were encountered along the federal coastal highway route f 200 beginning in the northwest these areas are along the rio ameca 10 km north of puerto vallarta 10 km south of puerto vallarta tomatlan basin named herein 79 km south of puerto vallarta with fossil vertebrates campo acosta basin named herein beginning 115 km south of puerto vallarta sediments at least partly marine la mira basin named herein straddling the mouth of the rio balsas with at least two marine incursions san nicolas basin named herein about 200 km southeast of acapulco marine sediments encountered in water well colotepec formation marine along the coast of oaxaca between puerto angel and puerto escondido marine deposits near santa cruz oaxaca about 100 km southwest of salina cruz oaxaca of the seven new areas only the la mira basin has been examined in more than a most cursory manner in the la mira basin four formations are recognized ferrotepec formation named herein marine late early to middle miocene in age barranca del perro muerto formation named herein nonmarine el bordonal conglomerate named herein nonmarine abanico brisa mar formation named herein at least partly marine of undifferentiated plio pleistocene age if the attitudes observed in the ferrotepec formation continue seaward the total thickness of the sediments in the basin might attain 1 500 m the fossils from the ferrotepec formation mostly belong to the tertiary caribbean province of woodring but a few show affinities to north pacific coast miocene faunas the la mira basin sediments indicate that the rio balsas has existed since early miocene time the few fossils from the san nicolas basin suggest a late miocene to pliocene age the few fossils from the beds near santa cruz are suggestive of a plio pleistocene age insofar as is known the sediments in the areas northwest of the la mira basin only attain a few meters in thickness offshore from the san nicolas basin at deep sea drilling project dsdp site 493 about 670 m of early miocene to pleistocene sediments are reported the santa cruz beds appear to be about 50 m thick and palmer reports a 15 m thickness for the colotepec formation

on the origin of equatorial atlantic biases in coupled general circulation models many coupled ocean atmosphere general circulation models gcms suffer serious biases in the tropical atlantic including a southward shift of the intertropical convergence zone itcz in the annual mean a westerly bias in equatorial surface winds and a failure to reproduce the eastern equatorial cold tongue in boreal summer the present study examines an ensemble of coupled gcms and their uncoupled atmospheric component to identify common sources of error it is found that the westerly wind bias also exists in the atmospheric gcms forced with observed sea surface temperature but only in boreal spring during this time sea level pressure is anomalously high low in the western eastern equatorial atlantic which appears to be related to deficient excessive precipitation over tropical south america africa in coupled simulations this westerly bias leads to a deepening of the thermocline in the east which prevents the equatorial cold tongue from developing in boreal summer thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical atlantic climate

post caldera volcanism in situ measurement of u pb age and oxygen isotope ratio in pleistocene zircons from yellowstone caldera the yellowstone plateau volcanic field the site of some of the largest known silicic volcanic eruptions is the present location of ne migrating hotspot volcanic activity most volcanic rocks in the yellowstone caldera 0 6 ma which formed in response to the climactic eruption of 1000 km 3 of lava creek tuff lct have unusually low oxygen isotope ratios ion microprobe analysis of both u pb age and n 18 o in zircons from these low n 18 o lavas reveals evidence of complex inheritance and remelting a majority of analyzed zircons from low n 18 o lavas erupted inside the yellowstone caldera have cores that range in age from 2 4 to 0 7 ma significantly older than their eruption ages 0 5 0 4 ma these ages and the high n 18 o cores indicate that these lavas are largely derived from nearly total remelting of normal n 18 o huckleberry ridge tuff hrt and other pre lct volcanic rocks a post hrt low n 18 o lava shows similar inheritance of hrt age zircons the recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma this newly proposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of oxygen and u pb isotope ratios of individual crystals fl 2001 elsevier science b v all rights reserved

p wave velocity tomography from local earthquakes in western mexico in western mexico the subduction of the rivera and cocos plates beneath the north america plate has deformed and fragmented the overriding plate forming several structural rifts and crustal blocks to obtain a reliable subsurface image of the continental crust and uppermost mantle in this complex area we used p wave arrivals of local earthquakes along with the fast marching method tomography technique we followed an inversion scheme consisting of 1 the use of a high quality earthquake catalog and corrected phase picks 2 the selection of earthquakes using a maximum location error threshold 3 the estimation of an improved 1 d reference velocity model and 4 the use of checkerboard testing to determine the optimum configuration of the velocity nodes and inversion parameters surprisingly the tomography results show a very simple vp distribution that can be described as being controlled by geologic structures formed during two stages of the separation of the rivera and cocos plates the earlier period represents the initial stages of the separation of the rivera and cocos plates beneath western mexico the later period represents the more advanced stage of rifting where the rivera and cocos plates had separated sufficiently to allow melt to accumulate below the colima volcanic complex during the earlier period 14 or 10 1 6 ma ne sw oriented structures lineaments such as the southern colima rift were formed as the two plates separated during the second period 1 6 ma to the present the deformation is attributed to magma generated within and above the tear zone between the rivera and cocos plates rising beneath the region of the colima volcanic complex the rising magma fractured the overlying crust forming a classic triple rift junction geometry this triple rift system is confined to the mid to lower crust perhaps indicating that this rifting process is still in an early stage this fracturing along with fluid circulation and associated heat advection within the fractures can easily explain the observed distribution of vp as well as many of the results of previous seismological studies also surprisingly we find no evidence at deep crustal depths to support either a trenchward migration of the volcanic arc or toroidal asthenospheric flow through the slab tears bounding the jalisco block to the nw and se

the tuff of san felipe an extensive middle miocene pyroclastic flow deposit in baja california mexico we document the existence of a widespread miocene ash flow tuff sheet in northeastern baja california mexico the tuff of san felipe new name was erupted from a vent east of the sierra san felipe of ne baja california at ca 12 6 ma this is the only widespread middle miocene pyroclastic flow deposit identified in northeastern baja california its distinctive age and widespread distribution make it an important marker horizon for structural reconstruction of this part of the gulf extensional province which is on the pacific plate the vent position near the modern gulf of california coast allows the possibility that exposures of the tuff of san felipe may be preserved east of the gulf on the north america plate in sonora yielding a tie point for the past relative position of the two plates this paper summarizes all known information including petrography geochemistry geochronology paleomagnetics geographic distribution and field appearance of this important tuff it is a densely welded crystal rich lithic lapilli pyroclastic flow deposit with 5 15 alkali feldspar and can be 180 m thick in some locations near the vent the tuff of san felipe is 40 m thick up to 40 km sw of the vent and 10 m thick at least 25 km nnw of the vent a minimum volume estimate for the deposit is 54 km 3 some recent 40 ar 39 ar age determinations suggest that the tuff is about 12 6 ma in age in all locations studied the tuff of san felipe has a unique low inclination reversed magnetization which may record a field transition or a geomagnetic excursion within reversed polarity subchron c5ar 2r 12 401 to 12 678 ma this low inclination magnetization as well as the mineralogy and age is key to correlating the tuff across the region because deposits are highly disrupted by subsequent normal faulting and outcrops are sparse and discontinuous away from the vent the documentation of these characteristics is important because the tuff of san felipe is a key structural marker for the subsequent development of the pacific north america plate boundary in the gulf of california and it will be important to identify this tuff in outcrops elsewhere on the baja california peninsula and on the north america plate in sonora

basaltic volcanism and extension near the intersection of the sierra madre volcanic province and the mexican volcanic belt three groups miocene pliocene and pleistocene off oceanic type basalts associated with normal faulting have been identified in an area surrounding the city of guadalajara mexico although most basalts within these groups have compositional characteristics of an asthenospheric source each group is also associated with lavas that have subduction related traits the first group the san cristobal plateau basalts has a minimum volume of 1 800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre existing extensional basin of unknown configuration 10 m y ago the relatively rapid eruption of these basalts along with their volume structural association and chemistry suggest an upwelling mantle plume beneath the guadalajara region this region may be the earliest indication of the separation of the jalisco block from north america the second basalt group the guadalajara basalts consists of small volumes of porphyritic basalts and basaltic andesites 3 3 5 0 ma in age that outcrop near the northern outskirts of guadalajara and near the town of hostotip aquillo to the northwest the youngest basalts in the guadalajara area are the 0 4 1 4 ma santa rosa suite of basalts hawaiites mugearites close to the contemporary 0 2 ma andesitic central volcano v tequila these either flowed from nearby cinder cones toward the rio santiago canyon or erupted in the canyon to dam the river the flat lying tertiary 23 27 ma ash flows of the sierra madre volcanic province that lie to the north of guadalajara are faulted north northeast into horsts and grabens that in one case can be dated as being older than 21 8 ma this fault trend has been reactivated closer to guadalajara as normal faults 200 to 300 m displacement cut the miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city also hidden by the young volcanic succession there is the north west southeast fault zone that extends to the gulf of california as the tepic zacoalco graben system faulting of this zone is well displayed in the region of the santa rosa dam in the santiago canyon northwest of guadalajara two styles of faulting are found there an older 1 ma 3 has been approximately north northeast for at least 4 m y characteristic of the mexican basin and range province

subsidence and strike slip tectonism of the upper continental slope off manzanillo mexico the direction of convergence between the rivera and north american plates becomes progressively more oblique in a counter clockwise sense as measured relative to the trench normal direction northwestward along the jalisco subduction zone by analogy to other subduction zones the forces resulting from this distribution of convergence directions are expected to produce a nw moving fore arc sliver and a nw se stretching of the fore arc area also a series of roughly arc parallel strike slip faults may form in the fore arc area both onshore and offshore as is observed in the aleutian arc in the jalisco subduction zone the jalisco block has been proposed to represent such a fore arc sliver however this proposal has encountered one major problem namely right lateral strike slip faulting within the fore arc sliver and between the fore arc sliver and the north american plate should be observed however evidence for the expected right lateral strike slip faulting is sparse some evidence for right lateral strike slip faulting along the jalisco block north american plate boundary the tepic zacoalco rift system has been reported although some disagreement exists right lateral strike slip faulting has also been reported within the interior of the jalisco block and in the southern colima rift which forms the se boundary of the jalisco block threefold multi channel seismic reflection data were collected in the offshore area of the jalisco subduction zone off manzanillo in april 2002 during the famex campaign of the n o l atalante these data provide additional evidence for recent strike slip motion within the fore arc region of the jalisco subduction zone this faulting offsets right laterally a prominent horst block within the southern colima rift from which we conclude that the sense of motion along the faulting is dextral these data also provide additional evidence for recent subsidence within the area offshore of manzanillo as has been proposed

tectonic implications of post 30 ma pacific and north american relative plate motions the pacific plate moved northwest relative to north america since 42 ma the rapid half rate of pacific farallon spreading allowed the ridge to approach the continent at about 29 ma extinct spreading ridges that occur offshore along 65 of the margin lonsdale 1991 document that fragments of the subducted farallon slab became captured by the pacific plate and assumed its motion prior to the actual subduction of the spreading ridge this plate capture process can be used to explain much of the post 29 ma cordilleran north america extension strike slip and the inland jump of oceanic spreading in the gulf of california the pacific and north american contact zone lengthened with each successive plate capture event underpinning the parts of western north america directly inland with a strong plate undergoing pacific relative motion we suggest that much of the post 29 ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent the plate capture hypothesis is distinctly different from theories involving shallow slab gaps kinematic problems associated with shallow slab gap models cause us to question them this conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the californian margin and the documented reduction of slab pull forces and density associated with young subducting slabs thermal and rheologic modeling suggests that coastal california was a strong zone at all depths allowing it to be firmly linked to pacific motion our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer

geology of the northern colima and zacoalco grabens southwest mexico late cenozoic rifting in the mexican volcanic belt the south trending northern colima graben intersects two other grabens the northwest trending zacoalco graben and the east trending chapala graben to form a rift rift rift triple junction 50 km south southwest of guadalajara in southwest mexico the northern colima graben represents the northernmost extension of the 190 km long 20 to 65 km wide colima rift which overlies the projection of the subducting boundary of the rivera and cocos plates and represents a rift of probable pliocene pleistocene age of the east trending mexican volcanic belt mvb associated with the northernmost 90 km of the rift there are 4 6 m y old to holocene alkaline volcanic rocks rarely found in continental volcanic arc environments and which are more typically associated with either continental intraplate rifting or oceanic islands these alkaline lavas have erupted side by side with 10 m y old to holocene calc alkaline lavas that represent typical arc volcanism it is proposed that the grabens are associated with the initiation of a rifting of a portion of southwest mexico away from the mexican mainland similar to the rifting of baja california away from the mexican mainland the calc alkaline k ar dates are consistent with post mid miocene development of east trending mvb volcanism

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

morphotectonic zones along the coast of the pacific continental margin southern mexico geomorphic analysis employing topographic morphologic geologic and bathymetric maps and field studies show that the morphology of the southern coast of mexico can be linked to lateral variations in the geometry and tectonism of the subduction zone a reconnaissance study based on the regional morphological characteristics and correlation with seismotectonic segments regional tectonics and major bathymetric features allows identification of several morphotectonic zones along the coast of southern mexico 1 jalisco zone 2 colima zone 3 michoacan zone 4 guerrero zone 5 oaxaca zone and 6 tehuantepec zone a range of geomorphological evidence including marine terraces river terraces uplifted notches and elevated wave cut platforms indicates local and regional uplift along the coast of the jalisco michoacan guerrero and oaxaca zones coasts of the colima and tehuantepec zones show morphological evidence of subsidence

crustal imaging of western michoacan and the jalisco block mexico from ambient seismic noise abstract detailed crustal imaging of western michoacan and the jalisco block is obtained from ambient noise tomography results show a deep and well delineated volcanic system below the colima volcano complex rooting up to 22 km depth with a shallow magmatic chamber constrained to the first 7 km a shallow low velocity system to the south of the chapala rift and west of the michoacan guanajuato volcanic field merges underneath the colima rift with the colima volcano system at about 20 km depth honoring the geometry of the trans mexican volcanic belt for depths greater than 30 km low velocity features become parallel to the slab strike right beneath the mascota ayutla and tapalpa volcanic fields suggesting the presence of the mantle wedge above the rivera plate all mentioned low velocity bodies are spatially correlated with the superficial volcanic activity suggesting their magmatic origin so that the shallower these bodies the younger are the associated volcanic deposits along the coast different depths of the uppermost layer of the rivera and the cocos plates suggest that the latter plate subducts with an angle 9 steeper than the former

seismotectonic constraints on the convergence rate between the rivera and north american plates there are two significantly different types of models for the convergence rate between the rivera and north american plates the first type the high rate model bandy 1992 predicts convergence rates of approximately 5 0 cm yr near the southern end of the rivera north america subduction zone and between 2 0 and 3 0 cm yr at its northern end in contrast the second type the low rate model e g demets and stein 1990 predicts convergence rates of between 2 0 and 3 3 cm yr near the southern end of the rivera north america subduction zone and between 0 6 and 1 7 cm yr at its northern end seismotectonic relationships which relate seismic characteristics of subduction zones maximum magnitudes maximum seismic depths etc to plate tectonic parameters convergence rates age of the oceanic lithosphere etc provide a means of distinguishing between the two different models three such relationships suggest that the rivera north american and cocos north american convergence rates should be roughly equal across the rivera cocos plate boundary favoring the high rate model employing the high rate model one can evaluate the magnitude and distribution of the strike slip component of forearc motion vss produced by oblique convergence between the rivera and north american plates the analysis indicates both a progressive increase and clockwise reorientation of vss northwestward along the plate contact zone of the rivera north america subduction zone such a distribution in vss should produce a northwestward movement of and nw se oriented extension within the interior of the jalisco block consistent with previous proposals of jalisco block motions also such a distribution in vss should produce a slight clockwise rotation of the jalisco block in the vicinity of bahia de banderas consistent with paleomagnetic data

laser 40ar39ar ages of tephra from indian ocean deep sea sediments tie points for the astronomical and geomagnetic polarity time scales tephra layers within deep sea sediment cores are valuable stratigraphic markers which can be radiometrically dated with high precision by the laser 40ar39ar technique two neogene ash layers from ocean drilling program odp site 758 have been dated these layers are located within detailed oxygen isotope and paleomagnetic stratigraphies which have previously been assigned ages based on the astronomically derived geomagnetic polarity time scale adgpts 7 10 28 we use the radiometric dates from the ash layers to independently evaluate the ages assigned to oxygen isotope stage 19 1 and the paleomagnetic reversals marking the brunhes matuyama boundary and the termination of the nunivak event ash d 20 21 yields an age of 800 20 ka which is consistent with an adgpts age of 780 ka for the overlying brunhes matuyama transition and with the astronomical age for oxygen isotope stage 19 1 ash i located near the top of the nunivak subchron bears evidence of excess 40ar an error weighted mean of five plateau ages gives an apparent age of 4 519 012 ma but an errorchron through a subset of the data suggests a possible eruption age of 4 43 03 ma the plateau age is in agreement with the adgpts 7 10 28 and the errorchron age is in agreement with the time scale of baksi 29 the laser 40ar39ar ages of both ashes are significantly older than the ages from the radiometrically derived time scale rdgpts of mankinen and dalrymple 3 which is primarily based upon a large dataset of k ar dated paleomagnetic poles it is essential to reconcile the adgpts with the rdgpts because radiometric age tie points are the best hope of extending the adgpts to times before 5 ma an analysis of the expected and actual variance for the astronomical and mankinen and dalrymple 3 time scales suggests that only a handful of low k ar ages may have biased the original gpts it is not necessary to invoke general systematic errors in the ar dating system to explain the discrepancies between the rdgpts and adgpts

variation of cenozoic extension and volcanism across the southern sierra madre occidental volcanic province mexico the middle to late cenozoic tectonic magmatic evolution of the sierra madre occidental volcanic province south of the tropic of cancer is summarized and analyzed for the first time based on new geologic and structural work and published information in the eastern part of the study region mesa central physiographic province silicic volcanism occurred in a short lived episode culminating at ca 30 ma and was followed by crustal scale extension between 30 and 27 ma in the western part of the study area sierra madre occidental physiographic province a voluminous episode of ignimbrite volcanism at 24 21 ma was succeeded by east west extension that produced regularly spaced grabens affecting only the upper crust in the westernmost part of the study region an andesitic to rhyolitic arc formed between 17 and 12 ma was affected by crustal scale north northwest trending extensional faulting leading to the formation of the gulf of california in the mesa central the maximum extension was oriented approximately east west and amounted to 20 in the eastern sierra madre occidental physiographic province extension was only 8 and oriented approximately east west we observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late oligocene early miocene and late miocene time comparison with the offshore tectonics indicates that the first two tectonic magmatic pulses coincide with periods of fast spreading at the pacific farallon boundary south of the shirley fracture zone we propose that increases in the spreading rate are related to periods of high subduction rate which in turn correspond to episodes of retreating subduction a retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate boundary conditions i e plate tectonics ultimately determined timing magnitude and orientation of extension whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism

magma eruption rates constrained by 40ar 39ar chronology and gis for the ceboruco san pedro volcanic field western mexico 40ar 39ar geochronology is coupled with quantitative volume determinations utilizing field mapping digital elevation models orthophotos and geographic information system gis software to constrain magma eruption rates at the ceboruco san pedro volcanic field 1600 km2 in the western trans mexican arc ages are reported for 40 volcanic units including volcan ceboruco an active andesitic stratovolcano peripheral domes shields cinder cones and fissure fed flows after a hiatus of 3 m y volcanic activity recommenced to produce 80 5 3 5 km3 of magma at a rate of 63 m3 km2 per year over the past 0 8 m y however 75 of this volume erupted in the past 100 k y including the 51 2 5 km3 of volcan ceboruco equivalent to an eruption rate of 377 m3 km2 per year there have been at least two stages of cone building activity at volcan ceboruco the main edifice is composed of 38 km3 of precaldera andesites the youngest dated at 45 8 ka their eruption was followed by a hiatus interrupted by a plinian eruption at 1 ka the plinian eruption and subsequent lava flows are andesite to dacite in composition and constitute 13 km3 of the total volume of volcan ceboruco overall the relative proportions of lava types erupted in the past 0 8 m y are 18 19 basaltic andesite 56 60 andesite 18 22 dacite and 3 rhyolite the peripheral lavas are each of small volume geochemically diverse and show little evidence of prior storage in an upper crustal chamber the eruptive sequence proportions of lava types phenocryst assemblages textures and geochemistry imply that the lavas do not reflect the differentiation of a single parental liquid in a long lived magma chamber the distinct geochemical signatures were present prior to magma emplacement in the upper crust whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages

petrogenesis and geodynamic significance of silicic volcanism in the western trans mexican volcanic belt role of gabbroic cumulates in the western trans mexican volcanic belt voluminous silicic volcanism has been associated with the rifting of the jalisco block from mainland mexico rhyolitic volcanism started at 7 5 ma after a major pulse of basaltic volcanism aged 11 8 5 ma associated with slab detachment this was followed by a second period between 4 9 and 2 9 ma associated with rhyolitic domes and ignimbrite coexisting with basaltic volcanism the similarity in rare earth element contents between basalts and rhyolites excludes a simple liquid line of descent the low ba and sr contents and the ferroan character of the rhyolites suggest extensive fractional crystallization late miocene early pliocene rhyolite sr isotope values are only slightly more radiogenic than the basalts whereas nd isotope ratios are indistinguishable we successfully modelled the 7 5 3 ma silicic magmatism as a result of partial melting of crustal gabbroic complexes that we infer to have formed in the mid lower crust due to the high density fe enriched composition of the late miocene basaltic volcanism slab rollback since 7 5 ma favoured decompression melting and arrival of additional mafic magmas that intruded in the lower crust these basalts heated and melted the gabbroic complexes forming the silicic magmas which subsequently underwent assimilation and fractional crystallization processes the first silicic pulse was emplaced during a period of low tectonic activity extensional faulting since the pliocene favours the eruption of both silicic magma and lesser amount of mafic lavas

active rifting in southwestern mexico manifestations of an incipient eastward spreading ridge jump a major ongoing episode of continental rifting has formed a system of three intersecting grabens in the western mexican volcanic belt these structures may mark the initial stage of an eastward spreading ridge jump beneath continental crust eastward ridge segment jumps have been propagating northward along this part of the east pacific rise for the past 12 m y but this is the first plate boundary reorganization event to involve continental crust since the separation of baja california in the miocene since pliocene time small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant subduction related calc alkaline magmas erupted from andesitic stratovolcanoes the alkaline magmas are varieties found in zones of active rifting elsewhere in the world

the subducted rivera cocos plate boundary where is it what is it and what is its relationship to the colima rift gravity data the geometry of the wadati benioff zone beneath western mexico and the seafloor morphology of the rivera cocos plate boundary west of the middle america trench suggest that the subducted part of this boundary lies directly beneath and is oriented parallel to the southern colima rift thus the southern colima rift likely formed in response to divergence between the subducting rivera and cocos plates due to direct coupling between these two plates and the overriding north american plate in contrast the subducted plate boundary lies east of and oblique to the northern colima and central colima grabens east of the central colima graben a low density zone overlies the boundary and underlies surface exposures of cretaceous granitoids and associated thermal springs and shallow focus earthquakes characteristics that are explained by thermal convection induced in the upper mantle by divergence between the subducted rivera and cocos plates these characteristics along with the adjacent locations of the low density upper mantle and the central colima graben are consistent with crustal extension produced by the uniform sense normal simple shear mechanism

a potassic volcanic front in western mexico the lamprophyric and related lavas of san sebastian the superposition of two tectonic regimes rifting and subduction in western mexico has led to the development of a lamprophyric volcanic front 70 km closer to the middle america trench than is the main axis of andesitic volcanism the san sebastian cindercone field represents the northernmost extent of lamprophyric activity in western mexico and lies 40 km trenchward of volcan ceboruco a historically active andesitic stratovolcano the predominant country rocks are cretaceous silicic ash flows although marine sedimentary rocks emplaced between 45 and 115 ma also occur minette absarokite and kersantite lavas were erupted between 0 48 and 0 26 ma in the san sebastian field in close association with flows of basaltic andesite and hornblende andesite there is a gradual transition observed between the two intermediate lava types basaltic andesite and kersantite as plagioclase is suppressed and hornblende and phlogopite are stabilized in melts of intermediate composition this transition reflects the increasing influence of water during crystallization and documents a wide range of hydrous conditions examination of the primary lamprophyric lava types absarokite and minette reveals a remarkable range in their oxidation states if these lavas reflect their mantle source conditions then the redox state of western mexico9s sub arc mantle spans the interval between the ni nio and magnetite hematite buffers a variation in fo 2 of five log units the oxidation and hydrous enrichment of mexico9s sub arc mantle probably formed in response to subduction the eruption of lamprophyric lavas in front of the andesitic suite of the main axis however may have depended upon rifting 1 to trigger melting of the modified mantle and 2 to provide an easy passage to the surface through crustal thinning and block faulting

late miocene volcanism and intra arc tectonics during the early development of the trans mexican volcanic belt abstract the early stage of the trans mexican volcanic belt hereafter tmvb is marked by widespread mafic to intermediate volcanism emplaced between 11 and 7 ma from the pacific coast to the longitude of mexico city to the north of the modern volcanic arc petrological and geochronological data support the hypothesis that this volcanism made up a unique late miocenic central mexican comagmatic province mafic lavas at the mouth of the gulf of california and along the northwestern sector of the tmvb made up the nayarit district which includes calc alkaline to transitional varieties the central sector of the tmvb is characterized by two basaltic districts the jalisco guanajuato and the queretaro hidalgo which are distinguished from the westernmost ones by their lower nb la and generally lower hfse lile values as well as by spider diagrams characterized by larger negative spikes at th ta nb and ti the surface occurrence of the late miocene basalts appears to be controlled by pre existing zones of crustal weakness that channeled the mafic magmas field observations suggest that these structures have been reactivated in a transtensional fashion induced by differential tectonic motion of crustal blocks to the south and to the north of the tmvb starting from 12 ma the tmvb separates a northern tectonic domain subject to the developing divergent pacific north america plate boundary from a southern tectonic domain characterized by oblique subduction of the rivera and cocos plates apparently far field stresses related to these complex plate boundaries reactivated older suture zones allowing rapid uprise of mantle derived magmas the subduction related signature shown by miocene mafic lavas of the jalisco guanajuato district argues against the existence of mantle plumes beneath this sector of the north america plate on the other hand the occurrence in the western tmvb and in the guadalajara region of a large volume of mafic magmas which sometimes show characteristics transitional to ocean island basalts could be due to passive upwelling of the subslab asthenosphere that might have interacted with subduction related magmas and continental lithosphere to produce the observed basaltic varieties subslab magmas may have flowed through slab free areas along the northern and eastern edges of the subducting rivera plate which in the late miocene was already detached from the farallon plate remnants and diverging from the cocos plate

high bedrock incision rates in the atenguillo river valley jalisco western mexico lava flows from three basaltic shield volcanoes preserve pliocene pleistocene river levels in the atenguillo river basin western mexico k ar dates of these basalt flows together with present and palaeoriver levels allow calculation of bedrock incision rates at three points along the length of the atenguillo river at volcan la laja dated at 0 65 ma incision rates are 25 cm ka 1 at volcan la cienega 2 2 ma incision rates are 23 cm ka 1 and at volcan el vigia 2 7 ma incision rates are 23 cm ka 1 these high incision rates as well as two distinct knickpoints along the profile of the atenguillo river are related to a base level c hange at the northern end of the basin the dynamics of this river basin are controlled by the ongoing process of continental fragmentation associated with the opening of the gulf of california 1997 by john wiley sons ltd

alkaline and calc alkaline lavas near los volcanes jalisco mexico geochemical diversity and its significance in volcanic arcs a remarkably diverse suite of lavas erupted during the late pliocene at the volcanic front of the western mexican volcanic belt near the town of los volcanes jalisco this region is much closer to the middle america trench than the main axis of quaternary andesite dacite stratovolcanoes and volcanism occurred in a complex tectonic regime involving both subduction of the young rivera plate and transverse crustal extension of the jalisco structural block the variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the mexican volcanic belt other alkaline varieties intermediate between these extremes include absarokites trachybasalts and trachyandesites phlogopite amphibole and apatite are common phenocryst phases whole rock compositions show a wide range of alkali contents e g k2o of 1 0 8 6 wt and typically contain 5 wt mgo mgo ni v and cr show little systematic decrease with increasing sio2 suggesting that these lavas have evolved from primitive mantle derived magmas with a wide range of sio2 contents strong enrichments in incompatible trace elements are observed in all of the lavas sr 700 5100 ppm ba 470 4800 ce 22 325 ppm zr 90 700 ppm as is the relative enrichment of large ion lithophile ba sr rb and light rare earth elements la ce over the high field strength elements ti zr which is typical of magmas in volcanic arcs this enrichment pattern suggests that these magmas come from source regions which contain incompatible element rich phases such as phlogopite amphibole and apatite the petrological and geochemical features of the lavas which occur in the los volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins the complex tectonic regime in western mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub arc mantle

rapid localization of pacific north america plate motion in the gulf of california correlation of late miocene volcaniclastic strata across the northern gulf of california shows that the pacific north america plate boundary localized east of the baja california peninsula ca 6 ma dextral offset of the 12 6 ma tuff of san felipe and a pair of overlying ca 6 3 ma pyroclastic flows indicate at least 255 10 km of displacement along an azimuth of 310 isopach and facies trends of the tuff of san felipe support no more than a few tens of kilometers of additional dextral displacement between 12 6 and 6 3 ma these constraints indicate that nearly all of the dextral displacement between the pacific and north american plates prior to 6 3 ma was accommodated outside of the gulf region and by 4 7 ma the plate boundary motion was localized in the gulf of california although continental extension has accounted for a component of plate boundary motion in northwestern mexico since cessation of subduction offshore of southern baja california at 12 5 ma transfer of baja california to the pacific plate was delayed by at least 6 7 m y

hawaiites and related lavas in the atenguillo graben western mexican volcanic belt hawaiite and mugearite alkali olivine basalt and basaltic andesite have erupted from three shield volcanoes in the atenguillo graben jalisco mexico the youngest shield volcano la laja 0 66 ma is the focus of this study basal pyroclastic surge deposits of basaltic ash lapilli and blocks are overlain by seven to eight 5 to 10 m thick hawaiite flows pillow lavas along a 4 km stretch on the southeast flank of the volcano and lake bed sediments define a shoreline and lake bottom southward to the town of atenguillo eruption of la laja apparently dammed the northward flowing atenguillo river and formed a lake of as much as 100 m depth subsequent fluvial erosion has destroyed the lava dam exposing the shield succession discharge rates for the atenguillo river and rainfall estimates for the atenguillo basin both suggest that this lake formed within 20 to 40 yr eruption of nearly 10 km 3 of material within 10 to 100 yr indicates an output rate of 0 1 to 1 km 3 yr which is larger than any rate calculated for volcanoes in the mexican volcanic belt mvb but similar in magnitude to that of mauna loa and kilauea volcanoes hawaii the older volcanoes el vigia 2 7 ma and tin cleto 3 6 ma are composed of basaltic andesite and alkali olivine basalt respectively the composition of these lavas is unusual for a continental arc setting total feo tio 2 and alkali concentrations are higher than those of basic calc alkaline lavas from the western mvb alkaline earth light rare earth and light rare earth high field strength element ratios in these lavas are low relative to those of calc alkaline lavas but similar to those of oceanic island basalt values such compositional traits coupled with a high magma output rate are characteristic of oceanic island volcanoes both hydrous fluid enriched sub arc mantle and oceanic island type mantle components are present beneath western mexico contemporaneous subduction and extension of this region of the western mvb allow tapping of both sources and thus the eruption of compositionally diverse lavas

pliocene quaternary volcanism and faulting at the intersection of the gulf of california and the mexican volcanic belt the junction of the western mexican volcanic belt and the gulf of california represents a superposition of subduction and continental rifting tectonic regimes in the late cenozoic subduction of the oceanic rivera plate has caused late cenozoic uplift of western mexico forming the jalisco block this paper addresses three questions where is the northern boundary of the jalisco block how is the continental fracture system related to that offshore and what is the spatial distribution and composition of subduction related lavas 40 ar 39 ar dates on cretaceous to paleogene silicic ash flows show that the northern boundary of the jalisco block may be defined by the abrupt change in basement age from cretaceous to miocene 40 ar 39 ar and k ar dates on faulted lavas from the nayarit region indicate that extension at the edge of the jalisco block has occurred since ca 4 2 ma the least principal stress 3 direction associated with these faults has had two different orientations from 4 2 ma to the present several pliocene to holocene 4 20 1 05 and 0 65 ma lava flows and aligned cinder cones have a n45 w associated least principal stress direction whereas three pliocene 3 36 3 38 and 3 11 ma lava flows are cut by faults indicating a north to north northeast least principal stress direction the two different stress directions may arise either from structural features in the basement of the arc or from changes in offshore plate boundaries e g 2 5 ma when the rivera and mathematicians plates were locked together whole rock major and trace element analyses of lavas from the coastal nayarit region reveal three different lava types an alkali basalt series basaltic andesite and andesite the alkali basalts show feo and tio 2 enrichment and have low ba zr and ba la ratios consistent with derivation from an oceanic island type mantle the basaltic andesites and andesites are both alkaline and calc alkaline contain hydrous phenocrysts and have high sr zr and la nb ratios consistent with a subduction related source among these lavas there is a correlation between sr zr oxygen fugacity and h 2 o contents suggesting that an oxidized hydrous fluid is involved in the genesis of the subduction related lavas the distribution of subduction related volcanic centers in the coastal nayarit region and areas southeast within the jalisco block defines front that parallels the middle america trench and is consistent with a 45 dip on the subducted rivera plate

oceanic ridge subduction vs slab break off plate tectonic evolution along the baja california sur continental margin since 15 ma the interaction of the pacific farallon spreading centers with the north american convergent margin off baja california mexico supposedly ceased at 12 ma when plate convergence and seafloor spreading stopped we propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23 n to 27 n famex cruise of the r v l9atalante april 2002 the data unveil a major clockwise rotation of the pacific farallon spreading direction starting ca 14 ma that formed a series of short spreading centers that became extinct ca 8 7 ma we suggest that the transcurrent motion between the pacific and north america along baja california was accommodated by seafloor spreading and oblique convergence along the trench this change in spreading direction was followed by a concomitant progressive demise of both pacific farallon seafloor spreading and farallon north america subduction that are attributed to the break off of the farallon slab this also resulted in the opening of a trench parallel slab window beneath baja california

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

isotopic constraints on the production rates crystallisation histories and residence times of pre caldera silicic magmas long valley california pre caldera high silica rhyolites of glass mountain california erupted episodically from 2 1 ma until the catastrophic eruption of the bishop tuff at 0 74 ma the lavas are extremely evolved with rb sr ratios between 128 to 3640 the latter being the highest recorded from a volcanic rock glass separates from pre l 2 ma lavas define two geographically controlled rb sr isoehrons lavas adjacent to the current caldera rim define an isochron age of 2 047 0 013 ma with an initial ratio of 0 7063 2 and lavas more distant from the caldera define an isochron of 1 894 0 013 ma with the same initial ratio the isochrons are consistent with the magmas forming within 26 ka which implies a minimum magma production rate of 0 75 10 3 km3 yr over this period new 4 ar 39ar ages on sanidine and biotite have established that lavas defining each isochron were erupted over a long time interval the isochron ages being up to 360 ka older than the youngest eruption age rb sr isotope data are reported for minerals from three lavas with eruption ages of 1 990 0 012 1 866 0 014 and 1 686 0 011 ma petrographically early apatite inclusions in biotite and biotite inclusions in feldspar and quartz have glass mineral rb sr ages that are indistinguishable from the relevant regional isochron sr diffusion in feldspar is slow at the magmatie temperatures inferred for glass mountain rhyolites 700 c such that over 0 5 ma the cores of large feldspars 1 ram will retain 99 9 of their original sr the cores of sanidine and plagioclase yield glass mineral ages that are up to 300 ka older than eruption ages feldspar rim ages for two samples are indistinguishable from eruption ages the rims of sanidines and plagioclases from the third sample are 110 and 280 ka older than the eruption age and 180 and 20 ka younger than the cores these mineral age data probably reflect the combination of extended periods of mineral growth and partial isotopic exchange with the host liquid during protracted residence in a magma reservoir however the ar and sr isotopic data for biotite phenocrysts are consistent with the presence of a significant component that is recycled from earlier magmatic pulses due to the extreme rb sr ratios of the rocks and minerals it is possible to very precisely resolve the time difference between the formation of different phases assuming that they crystallised from the same host magmas

textures formed during emplacement and cooling of a palaeoproterozoic small volume rhyolitic sill the 52 m thick rhyolitic sill at onedin in northwestern australia intruded wet unconsolidated sediment of the koongie park formation during the palaeoproterozoic the sill is composed of five main zones a thin 1 2 m thick basal peperitic contact zone with sparse large 61 cm amygdales and abundant spherulites grades up over 1 m into the lower zone the lower zone is 10m thick and made up of radiating feldspar laths surrounded by granophyric quartz and feldspar and has less than 5 amygdales in the overlying middle zone 20m thick amygdales increase in abundance from 5 near the base to 20 near the top this zone is dominated by spherulitic rhyolite the 18 m thick upper zone is complex with alternating 10 cm thick layers containing abundant spherulites lithophysae amygdales and perlite amygdales make up to 40 of the rock a pumiceous 2 3 cm thick layer occurs at the upper contact fluidal and blocky peperite marks the 1 2 m thick upper contact zone the five zones formed in response to different cooling rates within the sill the lower zone experienced the slowest cooling rate whereas the upper and basal contact zones with abundant glass cooled rapidly there is an overall upward increase in the amygdale abundance in many cases early formed bubbles became nucleation sites for lithophysae spherical bubbles formed in melt above the glass transition temperature as crystallisation proceeded irregularly shaped bubbles formed relatively late in crystalline zones textural zones in the rhyolitic sill at onedin are most similar to those in large volume rhyolitic lavas even though it is a small volume intrusion this similarity may be related to efficient heat retention beneath covering sediments for the sill or beneath a thick crust for large volume lavas the presence of the crystalline lower zone immediately above the base and a high proportion of crystalline rhyolite and the lack of autobreccia and sub vertical flow banding near the top distinguish this rhyolite as a sill rather than a lava

ignimbrite flare up and deformation in the southern sierra madre occidental western mexico implications for the late subduction history of the farallon plate geologic mapping and structural data for the southern part of the smo demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare up in oligocene 31 5 28 ma and early miocene 23 5 20 ma time and that extensional deformation occurred mostly before the transfer of baja california to the pacific plate extensive ignimbrite successions with 40 ar 39 ar ages clustering at 23 and 21 ma cover most of the southern smo thus correlating in age with ignimbrites exposed in southern baja california and central mexico grabens with a 020 to n s orientation developed in the east almost concurrently with this volcanic episode half grabens and nnw striking listric normal fault systems formed at the end of middle miocene as far as 150 km from the present coast a belt of left lateral transpressional structures formed along the southern boundary of the smo during the same period we link these magmatic and tectonic events to the evolution and dynamics of the farallon and north america plates during the miocene particularly we propose that a first detachment of the lower part of the farallon plate in early miocene time produced a transient thermal event and partial melting of the crust via mafic underplating middle miocene extension would be related to a second detachment event resulting from the slowing subduction that preceded the final capture of the magdalena microplate by the pacific plate at 12 5 ma transpression at the southernmost end of the smo occurred along the inland projection of the magdalenacocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the magdalena plate index terms 5480 planetology solid surface planets volcanism 8450 8150 tectonophysics evolution of the earth plate boundary general 3040 8109 tectonophysics continental tectonics extensional 0905 keywords ignimbrite flare up sierra madre occidental western mexico extensional tectonics slab detachment

statistical examination of the existence and relative motion of the jalisco and southern mexico blocks the continental lithosphere of mexico south of the trans mexican volcanic belt may consist of two lithospheric blocks the jalisco and southern mexico blocks the existence of these two blocks can be examined employing the statistical f test formulated to test for the presence of significant misclosure around a plate circuit to plate motion data derived from marine magnetic anomaly lineations earthquake slip vectors and transform fault azimuths the result of the f test applied to the pacific cocos north american plate circuit indicates that there is no significant at the 99 confidence level misclosure around this plate circuit therefore if the southern mexico block exists its motion relative to the surrounding lithospheric plates is too small to be resolved from these data in contrast the result of the f test applied to the rivera pacific north american plate circuit indicates the presence of a significant misclosure about the rivera pacific north american plate circuit the cause of which is uncertain one interpretation of the misclosure around the rivera pacific north american plate circuit is that it is due to the presence of an independent jalisco block however it is more likely that this misclosure is instead primarily due to the effects of recent changes in the relative motion between the pacific and rivera plates assuming the second explanation is correct a new rivera north american euler pole location 21 8 n 110 4 w is determinable solely from earthquake slip vectors located along the rivera north american boundary

late cenozoic extension of northeastern baja california mexico the pattern and history of normal faulting in a portion of northeastern baja california strongly suggest that the area developed as a result of east west crustal extension during late cenozoic time geometric and kinematic data strongly suggest that this area developed in concert with the opening of the proto gulf of california and the distension of the basin and range province in addition these data coupled with regional tectonic timing relations preclude the application of an oblique rifting model to this area two intervals of extensional tectonics have affected the region since the early miocene at present it is not known if strain was continuous or discontinous with respect to time the first and most intense period initiated between 17 and 9 m y ago was east west directed as determined from tilt patterns orientation of intra terrane transcurrent faults and fault plane kinematic indicators this deformation was dominated by high angle normal faulting and resulted in the disruption of a region wide low relief erosion surface new patterns of sedimentation were established and controlled by north trending grabens that filled with alluvial fan and marine deposits sporadic silicic eruptions occurred in conjunction with the extension thus allowing resolution of the complex history of tilting that accompanied normal faulting tilting and antithetic faulting resulted from the volume compensation required by the downward flattening of normal faults and by fault related monoclinal flexing and reverse drag effects extension during this interval was complete or substantially reduced by 6 m y b p the second interval of east west extension younger than 6 m y b p has been much less intense than the earlier event yet it seems to have utilized the same structures silicic volcanism also waned during this interval giving way to mafic eruptions related to rifting of the present gulf of california

late miocene to recent transtensional tectonics in the sierra san fermin northeastern baja california mexico basins and ranges within part of the gulf of california extensional province mexico have experienced complex distributed deformation including normal and strike slip faulting and block rotations linked to dextral shear at the pacific north america plate boundary in the sierra san fermin and southern sierra san felipe northeastern baja california normal faulting began between 12 5 and 6 ma although most extension occurred between about 6 and 3 ma strongly influencing thickness and distribution of ash flow tuffs and sedimentary deposits extension is generally 10 in 6 ma rocks and somewhat more in 12 5 ma rocks inversion of kinematic data interpreted together with published paleomagnetic data suggests that the axis of least principal stress was oriented between w e and sw ne in late miocene time our data indicate an important change in the amount of dextral shear but not necessarily the least principal stress direction wsw ene at about 3 ma structural constraints limit significant sinistral strike slip faulting conjugate to the dextral plate boundary to the last 3 my progressive changes in the geometry of faulting through time are consistent with regional strain partitioning within the pacific north america plate boundary zone and are predicted by physical and analytical models of oblique divergence as the orientation of the stretching vector changes to lower and lower values

steep subduction geometry of the rivera plate beneath the jalisco block in western mexico the morphology of the rivera plate subducted beneath the jalisco block in western mexico is determined from accurately located hypocenters of locally recorded microearthquakes and from earthquakes with magnitude mb 4 5 recorded at teleseismic distances the hypocenters of these latter earthquakes are relocated and for five of them the focal depth is constrained by the inversion of long period body waves the wadati benioff zone inferred from these data indicates a steep subduction of the rivera plate that resembles the geometry of subduction of the cocos plate beneath central america it is however very different from the shallower and almost subhorizontal subduction of the cocos plate observed in southern mexico southeast of this region the rivera plate is comparable to the juan de fuca plate in terms of the small areal extent young seafloor age low relative velocity and low teleseismic activity in the subduction zone this study shows that the dip of both the juan de fuca and rivera plates are similar once they are decoupled from the overriding continental crust the downgoing rivera plate initially starts with a dip of 10 down to a depth of 20 km and then increases gradually to a constant dip of 50 below a depth of 40 km intermediate depth seismicity is low in this zone associated with the subduction of the slow 2 cm yr and young 9 m y rivera plate the maximum depth extent of earthquakes observed in the rivera subduction zone is about 130 km the andesitic calc alkaline colima volcano appears to be directly related to the subduction of the rivera plate to the nw of this volcano the observed quaternary volcanism in the jalisco block which is parallel to the trench may also be explained by the subduction of the rivera plate

empirical relationships among the convergence rate of plates rollback rate of trench axis and island arc tectonics laws of convergence rate of plates abstract the data on 27 recent arcs which were subducted by normal oceanic lithosphere were compiled among the compression extension rates for island arcs v a compression is defined as positive and the kinematic components of absolute plate motions parallel to the direction of backarc compression extension v on oceanward velocity of the overriding plate v sn continentward velocity of the subducting plate and v tn rollback rate of the trench axis the equations v a v on v sn 7 2 cm y 1 v tn 7 2 cm y v sn 2 are found to hold as for the convergent zones associated with a wadati benioff zone deeper than 200 km equation 1 suggests that backarc spreading and crustal shortening play the role of compensating the deviation of the convergence rate from the critical value of 7 2 cm y the equation 2 means that any subducting oceanic lithosphere has the potentiality of making the trench axis rollback at the rate of 7 2 cm y and that the virtual rollback rate v tn is determined by the balance between the potential rollback rate 7 2 cm y and v sn the present author calls the kinematic relationships represented by equations 1 and 2 laws of convergence rate of plates the convergent zones of plates associated with the wadati benioff zone shallower than 200 km also show a tendency that the greater and smaller values of v on v sn correspond to the backarc compression and extension however the critical value is 3 4 cm y instead of 7 2 cm y

miocene shearing along the northern boundary of the jalisco block and the opening of the southern gulf of california new geologic and structural mapping in western mexico shows that the tectonic boundary between the eocene early miocene sierra madre occidental volcanic province and the cretaceous paleocene jalisco block was reactivated as a transfer zone accommodating a limited amount of lateral motion in middle and late miocene time transpressional folding left lateral strike slip faulting and minor thrusting occurred between 14 5 and 11 5 ma during initial rifting at the mouth of the gulf of california inferred right lateral transtension between 12 and 9 ma took place concurrently with widespread alkaline volcanism and east west extension at the gulf mouth and east of the jalisco block these events can be explained in light of the long extensional history that led to the separation of baja california from mainland mexico since the birth of the magdalena microplate about 14 5 ma left lateral shearing at the sierra madre occidental jalisco block boundary indicates that initial 14 12 ma rifting at the mouth of the gulf was accomplished mainly by east southeast motion of the block this was caused by differential stresses applied to the north america plate at the two sides of the subducting magdalena cocos plate boundary as a consequence of the waning of subduction of the magdalena plate subsequent right lateral transtension at the sierra madre occidental jalisco block boundary and extension to the east imply a limited west northwest motion of the block which possibly accounts for the discrepancy between slow rifting at the mouth of the gulf and fast northwest motion of baja california

active crustal deformation in the jalisco block mexico evidence for a great historical earthquake in the 16th century abstract on december 27th 1568 a large earthquake occurred to the southwest of guadalajara mexico near the northeastern corner of the jalisco block in an area where no great earthquakes have been reported before it caused heavy damage in the region where the colima and tepic zacoalco grabens intersect many churches houses and convents in the neighboring towns collapsed and severe deformation of the ground was observed in the area landslides apparently dammed the ameca river for several days and the opening of large cracks was reported in the lowlands the flow of natural springs and the level of lake zacoalco changed dramatically after the earthquake all of these reports strongly suggest that a local fault was the source of this large and destructive earthquake based on the intensity data inferred from the historical reports the 1568 event is perhaps the largest earthquake to date in the trans mexican volcanic belt compared to other well documented large earthquakes that occurred in the volcanic belt in 1875 1912 and 1920 the magnitude appears to be greater than 7 0 m w the jalisco block is presumed to be rifting away from the north american plate the colima and tepic zacoalco grabens which bound the jalisco block to the east and north respectively are apparently the boundaries where rifting is taking place in a complex and highly faulted environment based on the data available it is impossible to identify the specific fault ruptured during the earthquake unequivocally however the occurrence of this large event in 1568 confirms that active deformation is still taking place in the jalisco block the apparently long recurrence times of these large events suggest that tectonic deformation is slow

miocene to recent structural development of an extensional accommodation zone northeastern baja california mexico the gulf of california extensional province in northeastern baja california contains a major structural transition from a northern domain of widely spaced basins and ranges to a southern domain of closely spaced nnw striking high angle faults in the northern domain the western boundary of the province the main gulf escarpment is a single listric normal fault the san pedro martir fault in the southern domain it is a 5 km wide zone of numerous e dipping high angle normal faults the western half of this structural transition studied here does not occur by a single transfer fault but rather by 1 transfer of displacement from the san pedro martir fault onto multiple fault zones in the footwall without intervening cross faults and 2 increased southward disruption of the hanging wall by wnw striking strike slip faults oblique to the extension direction and nnw striking normal faults and extension fractures this structural transition involves both the hanging wall and the footwall of the escarpment the lack of a discrete transfer structure is attributed either to insufficient extension 10 or to a gradual change in geometry of the basal detachment

the geology and petrology of volcan san juan nayarit mexico and the compositionally zoned tepic pumice abstract volcan san juan is the westernmost quaternary composite volcano in the mexican volcanic belt its activity is divided into three stages of decreasing eruptive volume during stage 1 activity the main cone of san juan and the adjacent cerro alto grew to a combined volume of 60 km3 through eruption of hornblende bearing andesites dacites and rhyodacites stage 1 included explosive eruptions of hornblende dacite and an andesite dated at 33 750 a years ago and 23 010 a stage 2 activity involved a major plinian eruption 14 770 a that produced the hypersthene hornblende bearing compositionally zoned rhyodacitic andesitic tepic pumice and led to formation of an oval shaped caldera 4 1 km in diameter tepic the capital city of nayarit state with a population of 250 000 lies at the ne foot of volcan san juan this location was buried under 2 9 m of rhyodacitic pumice fall deposits from the tepic pumice eruption erosion has since removed virtually all exposures of tepic pumice 15 km from the caldera the estimated volume of erupted tephra is 5 6 km3 equivalent to 1 2 km3 of magma modeling of the eruption dynamics indicates that as the eruption evolved the column height increased from 15 km to 24 km and maximum mass discharge rate increased from 3 106 kg s to 6 107 kg s during stage 3 activity a hornblende andesitic lava dome partially filled the caldera and sent block lava flows down the north slope of the cone total erupted volume was 0 55 km3 compared to other composite volcanoes of the western mexican volcanic belt the products from san juan have the highest sr contents and the lowest rb y yb and lu contents resulting in high la yb and sr y with low rb sr these observations indicate a strong slab melt component at san juan consistent with its proximity to the middle america trench where young hot lithosphere is subducting the san juan andesites dacites and rhyodacites also have relatively high 87sr 86sr 206pb 204pb 207pb 204pb and 208pb 204pb and low end values compared to these other volcanoes which appear to reflect significant crustal contamination of san juan s magmas

the eruptive history of the tequila volcanic field western mexico ages volumes and relative proportions of lava types the eruptive history of the tequila volcanic field 1600 km 2 in the western trans mexican volcanic belt is based on 40 ar 39 ar chronology and volume esti mates for eruptive units younger than 1 ma ages are reported for 49 volcanic units including volcn tequila an andesitic stratovolcano and peripheral domes flows and scoria cones volumes of volcanic units 1 ma were obtained with the aid of field mapping ortho aerial pho tographs digital elevation models dems and arcgis software between 1120 and 200 kyrs ago a bimodal distribution of rhyolite 35 km 3 and high ti basalt 39 km 3 dominated the volcanic field between 685 and 225 kyrs ago less than 3 km 3 of andesite and dacite erupted from more than 15 isolated vents these lavas are crystal poor and show little evidence of storage in an upper crustal chamber approximately 200 kyr ago 31 km 3 of andesite erupted to form the stratocone of volcn tequila the phenocryst assemblage of these la vas suggests storage within a chamber at 2 3 km depth after a hiatus of 110 kyrs 15 km 3 of andesite erupted along the w and se flanks of volcn tequila at 90 ka most likely from a second discrete magma chamber lo cated at 5 6 km depth the youngest volcanic feature 60 ka is the small andesitic volcano cerro tomasillo 2 km 3 over the last 1 myr a total of 128 22 km 3 of

motion of the rivera plate since 10 ma relative to the pacific and north american plates and the mantle abstract to better understand the influence of rivera plate kinematics on the geodynamic evolution of western mexico we use more than 1400 crossings of seafloor spreading magnetic lineations along the pacific rivera rise and northern mathematician ridge to solve for rotations of the rivera plate relative to the underlying mantle and the pacific and north american plates at 14 times since 9 9 ma our comparison of magnetic anomaly crossings from the undeformed pacific plate to their counterparts on the rivera plate indicates that significant areas of the rivera plate have deformed since 9 9 ma dextral shear along the southern edge of the plate from 3 3 2 2 ma during a regional plate boundary reorganization deformed the rivera plate farther into its interior than previously recognized in addition seafloor located north of two rupture zones within the rivera plate sutured to north america after 1 5 ma anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the rivera plate finite rotations that best reconstruct pacific plate anomaly crossings onto their undeformed counterparts on the rivera plate yield stage spreading rates that decrease gradually by 10 between 10 and 3 6 ma decrease rapidly by 20 after 3 6 ma and recover after 1 ma the slowdown in pacific rivera seafloor spreading at 3 6 ma coincided with the onset of dextral shear across the then incipient southern boundary of the rivera plate with the pacific plate the available evidence indicates that the rivera plate has been an independent microplate since at least 10 ma contrary to published assertions that it fragmented from the cocos plate at 5 ma motion of the rivera plate relative to north america has changed significantly since 10 ma in concert with significant changes in pacific rivera motion a significant and robust feature of rivera north america motion not previously recognized is the cessation of margin normal convergence and thus subduction from 2 6 to 1 0 ma along the entire plate boundary followed by a resumption of trench normal subduction along the southern half of the rivera north america plate boundary after 1 0 ma motion of the rivera plate relative to the underlying mantle since 10 ma has oscillated between periods of landward motion and seaward motion the evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion

middle miocene extension in the gulf extensional province baja california evidence from the southern sierra juarez new geologic mapping structural studies and geochronology of miocene volcanic and sedimentary rocks in the southern sierra juarez baja california shed light on the extensional history of the gulf extensional province prior to sea floor spreading in the gulf of california the southern sierra juarez is underlain by lower middle miocene rocks including fluvial strata intermediate composition volcanic deposits basalt lava flows and cinder cones and dacite pyroclastic deposits and lavas that nonconformably overlie the cretaceous peninsular ranges batholith the 40 ar 39 ar geochronology indicates that basaltic rocks are 16 90 0 05 ma and dacite pyroclastic deposits are between 16 69 0 11 ma and 15 98 0 13 ma these strata were subsequently cut by two generations of faults first generation faults comprise a dominant set of north south striking west dipping normal faults a secondary set of north south striking east dipping normal faults and a lesser set of variably oriented strike slip faults all three fault sets are temporally and spatially related and were produced by east west extension the dominant west dipping faults which are antithetic to and oblique to the east dipping main gulf escarpment may have been a precursor or an early phase accommodation zone along the escarpment west dipping normal faults are cut by a 10 96 0 05 ma dacite hypabyssal intrusion thus bracketing the age of east west extension between 15 98 0 13 ma and 10 96 0 05 ma hence this faulting event clearly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated miocene extension within baja california second generation faults which are comprised of east west striking strike slip faults that cut first generation faults and associated northwest striking northeast dipping normal faults may be related to early development of the transpeninsular strike slip province global plate reconstructions suggest that transtensional motion between the north american and pacific plates along the western margin of baja california began during middle miocene time coeval with east west extension in the southern sierra juarez this observation supports a hypothesis that middle miocene transtensional plate motion was partitioned into two components a strike slip component parallel to active faults along the western offshore margin of baja california and an extensional component normal to the margin but located in what is now the gulf extensional province hence the onset of extension within the circum gulf region was in response to plate boundary processes

k ar dating young rhyolitic rocks a case study of the sierra la primavera jalisco mexico this study demonstrates the feasibility of dating by the k ar method an eruptive sequence of silicic volcanic rocks with ages close to the limits of conventional 14 c dating forty six conventional k ar analyses of sanidine and glass from rhyolites of the sierra la primavera mexico yielded dates of 5 to 150 ka thousand years one sigma errors range from 3 to 39 ka although the samples contain small amounts of extraneous argon most of the dates are consistent with relative ages determined in the field the exceptions are dates on immediately postcaldera eruptive units which are older than the dates for the caldera producing ash flow tuff a similar result has been obtained on dating tuffs and lavas of some other pleistocene calderas this effect is attributed to incomplete outgassing of the larger amount of extraneous argon dissolved in magma that occupied a position deeper in the chamber than the ash flow tuff magma and that erupted immediately after caldera collapse we present two new error equations one a simple extension of the commonly used equation demonstrates the importance of including a term for the uncertainty in the spectrometer mass discrimination factor in the error equation the second equation is a more rigorous formulation of the propagation of errors in the calculation of the age it yields meaningful error estimates for all proportions of radiogenic 40 ar and because it expresses the error in absolute rather than relative terms it can be used to put an upper limit on the age of a sample when no radiogenic 40 ar has been resolved

fault tectonics of the baja california peninsula and the opening of the sea of cortez mexico abstract a detailed field analysis of neogene and quaternary faults in baja california has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula the deformation which is related to the opening of the gulf affects the whole peninsula but decreases from east to west most observed faults normal and or dextral strike nnw sse to wnw ese their mechanisms include both strike slip and dip slip as well as intermediate motions compressional events have occurred since late neogene times but were probably of minor quantitative importance because reverse faults are rare and small the principal fault pattern includes dextral nnw sse riedel shears and n s tension faults induced by dextral strike slip along two main nw se fault zones bordering the peninsula the gulf of california to the east which is the most important and the tosco abreojos fault to the west the resulting pattern of deformation shows that the eastern part has been a complex transform extensional zone since late miocene early pliocene times

the eruptive history of the mascota volcanic field western mexico age and volume constraints on the origin of andesite among a diverse suite of lamprophyric and calc alkaline lavas the mascota volcanic field is located in the jalisco block of western mexico where the rivera plate subducts beneath the north american plate it spans an area of 2000 km2 and contains 87 small cones and lava flows of minette absarokite basic hornblende lamprophyre basaltic andesite and andesite there are no contemporary dacite or rhyolite lavas new 40ar 39ar ages are presented for 35 samples which are combined with nine dates from the literature to document the eruptive history of this volcanic field the oldest lavas 2 4 to 0 5 ma are found in the southern part of the field area whereas the youngest lavas predominantly 0 5 ma are found in the northern portion on the basis of these ages field mapping and the use of ortho aerial photographs and digital elevation models it is estimated that a combined volume of 6 8 3 1 km3 erupted in the last 2 4 myr which leads to an average eruption rate of 0 003 km3 kyr and an average volume per eruptive unit of 0 1 km3 the dominant lava type is andesite 2 1 0 9 km3 followed by absarokite 1 6 0 8 km3 basaltic andesite 1 2 0 5 km3 basic hornblende lamprophyre 1 0 0 4 km3 and minette 0 9 0 5 km3 thus the medium k andesite and basaltic andesite comprise approximately half 49 of the erupted magma with twice as much andesite as basaltic andesite and they occur in close spatial and temporal association with the highly potassic lamprophyric lavas there is no time progression to the type of magma erupted a wide variety of evidence indicate that the high mgo 8 9 wt basaltic andesites 52 53 wt sio2 were formed by h2o flux melting of the asthenopheric arc mantle wedge whereas the mafic minettes and absarokites were formed by partial melting induced by thermal erosion of depleted lithospheric mantle containing phlogopite bearing veins there is only limited differentiation of the potassic magmas with none more evolved than 55 4 wt sio2 and 4 4 wt mgo this may be attributable to rapid crystallization of the mantle derived melts in the deep crust owing to their low volumes thus the andesites 58 63 wt sio2 are notable for being both the most voluminous and the most evolved of all lava types in the mascota volcanic field which is not consistent with their extraction from extensively crystallized 60 70 low volume intrusions instead the evidence supports the origin of the andesites by partial melting of amphibolitized mafic lower crust driven by the emplacement of the minettes absarokites and the high mg basaltic andesites

motion partitioning between the pacific plate baja california and the north america plate the tosco abreojos fault revisited 1 during the famex cruise of the r v l atalante april 2002 swath bathymetry and seismic profiles were recorded along the baja california margin from 23 to 27 n the upper slope of the margin exhibits active faulting of recent sediments along the tosco abreojos fault system tafs flower structures evidence strike slip motion along the tafs right lateral strike slip faulting associated in places with small extensional features are consistent with present day slip and extension rates predicted along the tafs the tafs thus belongs to the western active boundary of a baja california block yet not totally transferred to the pacific plate

continental boundaries of the jalisco block and their influence in the pliocene quaternary kinematics of western mexico extensional faulting observed in southwestern mexico has been related to the incipient rifting of the jalisco block from the mexican mainland since the pliocene on the basis of new structural and geophysical data we propose that 1 the continental boundaries of the jalisco block are ancient structures reactivated since the pliocene at a low 1 mm yr rate of deformation and 2 pliocene quaternary extensional faulting at the edges of jalisco block is a basement controlled intraplate deformation related to plate boundary forces rather than to active continental rifting the jalisco block boundaries first developed in response to the uplift of the puerto vallarta batholith in pre neogene time and underwent a complex contractile deformation before the pliocene during pliocene quaternary times north northeast extension reactivated the northern boundary forming the tepic zacoalco rift whereas east southeast extension formed the northern colima rift south of the colima volcano active extension is found only west of the so called southern colima rift and partly reactivates old northeast trending basement faults the parallelism between the subducted rivera cocos plate boundary zone and the eastern neotectonic boundary of the jalisco block supports east southeastward motion of the southern mexican blocks induced by the differential motion and oblique subduction of the cocos and rivera plates on the other hand we envisage pliocene quaternary extension along the northern boundary as an upper plate response to the low convergence rate and the steep subduction angle of the rivera plate

age and stratigraphic relationships of pre and syn rift volcanic deposits in the northern puertecitos volcanic province baja california mexico geologic mapping of volcanic strata of the northern puertecitos volcanic province pvp in northeastern baja california mexico performed in conjunction with 40 ar 39 ar analysis and petrochemical study documents the miocene geologic history of a well preserved volcanic succession within the northern sierra santa isabel and its relationship to the evolving pacific north america plate boundary subduction related volcanic deposits well exposed in profile along the northern margin of the pvp in the informally named santa isabel wash region span pre 17 to 15 ma minor rift related volcanism occurred at 12 5 and 9 ma prior to voluminous pvp forming volcanism at 6 6 5 ma isochron ages typically exhibit precision 1 for plagioclase of 2 7 and for anorthoclase and sanidine of 2 5 and replicate analysis of an internal anorthoclase standard indicate 1 2 reproducibility within a given irradiation and 2 5 for samples irradiated separately improved local correlations made possible by the rich stratigraphic section preserved in santa isabel wash help constrain the relationships of several widespread pyroclastic flow deposits in northeastern baja california these correlations are important for both paleomagnetic studies within the region and for establishing geologic ties across the gulf of california the combined mapping and age results imply that most extensional deformation in the study area is post 6 ma although some earlier faulting and the development of the pre 6 ma matomi accommodation zone are also documented results support a transitional plate boundary model which implies that much of the pacific north america relative plate motion north of delfin basin i e the northernmost gulf of california is accommodated on n to nnw striking faults developed during late miocene ene directed extension the model predicts a zone of divergence east of the pvp which provides a structural mechanism for the positions and jumps of nearby gulf of california spreading centers upper and lower tiburon and delfin basins since 6 ma and relates major pulses of pvp volcanism at 6 and 3 ma to these offshore spreading center adjustments results also imply that most extensional deformation in santa isabel wash is the result of incorporation of the pvp into the gulf extensional province 2 3 ma due to northwestward propagation of the guaymas fracture zone rotational deformation north of the pvp may have begun contemporaneously with this adjustment along the gulf extensional province rift margin

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

a pliocene ignimbrite flare up along the tepic zacoalco rift evidence for the initial stages of rifting between the jalisco block mexico and north america the tepic zacoalco rift a nw trending corridor 50 250 km is one arm of a triple rift system in western mexico together with the colima rift and the middle america trench it bounds the jalisco block a portion of western mexico that may be moving independently of north america the predominant basement rock types in the tepiczacoalco rift are rhyolitic ash fl ow tuffs and lavas which were previously assumed to be oligocene miocene in age related to the sierra madre occidental volcanic province or older new 40 ar 39 ar dates on 41 volcanic samples reveal a previously unrecognized voluminous fl are up of rhyolitic ignimbrites between 5 and 3 ma throughout the entire corridor of the tepic zacoalco rift they are often associated with pliocene high ti basalts the eruption rate during this pliocene time period was an order of magnitude higher hundreds of m m y than that documented in the tepic zacoalco rift over the last 1 m y the pliocene ash fl ow tuffs have been faulted along nw trending lineaments producing vertical offsets up to at least 500 m the voluminous ignimbrite fl are up in the tepiczacoalco rift at 5 3 ma may refl ect the initial stages of rifting of the jalisco block away from north america analogous to what occurred in the proto gulf region at 12 6 ma prior to the transfer of baja california from north america to the pacifi c plate additionally new 40 ar 39 ar dates show that the sierra madre occidental volcanic province extends across the entire width of the tepic zacoalco rift and terminates abruptly at the northern boundary of the jalisco block near the rio ameca in contrast paleoceneeocene basement from the jalisco block extends northward into the tepic zacoalco rift where it is locally overlain by sierra madre occidental rhyolites

relative motions of the pacific rivera north american and cocos plates since 0 78 ma we use magnetic anomaly and fracture zone crossings from 3 n 24 n along the east pacific rise to define the boundaries and relative velocities of the rigid pacific rivera north american and cocos plates since 0 78 ma and to test several hypotheses regarding the nature of deformation between these plates crossings of the 0 78 ma isochron from the pacific rivera rise north of 22 0 n are significantly better fit by the 0 0 78 ma pacific north america rotation than by a pacific rivera rotation thereby supporting a recently proposed model for accretion of the northern part of the rivera plate to north america a significant misfit of the pacific cocos rotation to crossings of anomaly ln from the east pacific rise north of 16 4 n suggests that seafloor north of 16 n moves relative to the rigid pacific or cocos plates we also find that nine crossings of the eastern rivera fracture zone and eight crossings of anomaly ln flanking the manzanillo spreading segment are fit well by a pacific rivera rotation the rivera cocos angular velocity derived from the best fitting pacific cocos and pacific rivera angular velocities predicts 19 3 mm yr 1 95 confidence limit of n s convergence near the center of the rivera cocos boundary nearby left lateral strike slip earthquakes with north trending fault planes suggest that convergence is accommodated via sinistral shear along a north trending zone the unambiguous kinematic and seismologic evidence for significant rivera cocos motion indicates that the absence of a well defined rivera cocos plate boundary cannot be interpreted as evidence that rivera cocos motion is slow or zero as is postulated in several previous studies the rivera cocos angular velocity predicts oblique rivera cocos convergence at the el gordo graben and beneath the manzanillo trough and southern colima graben any extension across these features since 0 78 ma thus cannot have resulted from divergence between the subducting rivera and cococs plates

characteristics of seismicity in the coast and north of jalisco block mexico abstract several studies of local seismicity were carried out between 1996 and 1998 on the coast and north of the jalisco block jb using a portable seismic network consisting of five lennartz m88 seismographs with le 3d 1 hz seismometers the data from these studies complemented with data from resco colima seismic network is used to characterize three seismogenic zones in the region one seismicity zone located east of the middle america trench mat under the continental plate is associated with the interaction of the rivera and north america plates and can be characterized as a double seismic zone dsz with geometry suggesting a bending plate and probably oblique subduction this pattern is clearer south of the 20 n parallel a second zone of continental intraplate seismicity is located between ameca and amatlan de canas in the northeastern part of the study region where the source depths vary from very shallow to about 35 km the third zone is bahia de banderas where the stress pattern seems to be very complex here we observe different types of seismic events the first type consists of shallow events located in the southern part of the bay and a under continental area that seems to be associated with the interplate rivera north america activity a second event type with depths between 10 and 30 km seems to be related to features that cross the bay in an ew direction in this zone there is also a high shallow continental seismicity which agrees with continental topographic features

petrogenesis of a voluminous quaternary adakitic volcano the case of baru volcano the origin of adakite magmas remains controversial because initially the term adakite had petrogenetic significance implying an origin by direct melting of the eclogitized subducting oceanic crust many models have been produced for their origin and until now there has not been a straightforward method to discriminate between these models in a given adakite suite here we use detailed chronological and geochemical studies of selected adakitic edifices that allows for the determination of the magmatic output rate parameter qe which has been correlated with the rates of magma generation deep within subduction zones by providing temporal and eruption rate estimates we provide constraints on the possible petrogenetic processes involved in the generation of adakite like signatures adakite magmas derived from the melting of the subducting slab should be volumetrically insignificant when compared to the adakite like magmas produced by typical arc magma generation processes in this study we use this observation and the extraordinary stratigraphic exposure from miocene to present in an adakitic volcano in panama and to study the temporal and chemical variation in erupted magmas to estimate rates of magma generation detailed chemical and geochronological analyses of baru volcano indicate that the volcanic edifice was constructed in its entirety during the quaternary and magmas display adakite like features such as steep rare earth elements patterns pronounced depletions in the heavy rare earth elements low y high sr and high sr y the magmatic output rates qe that we have calculated show that compared to other typical adakitic volcanoes most of the volcanic edifice of baru volcano was constructed extremely rapidly 213 k a and in time frames that are similar to typical arc volcanoes the observed chemical and mineralogical variation coupled with the high magma production rates indicate that baru volcano is more representative of a typical arc volcano than a small volume melt of the subducting oceanic crust the technique we outline may have broader application in determining the petrogenetic conditions of other adakite suites

calderas of the sierra madre occidental volcanic field western mexico approximately 296 000 km2 of western mexico is covered by a volcanic sequence that has an average thickness of 1 km and is mostly ash flow tuff although some 350 calderas are suggested by this tremendous volume only a few have been located or described in the state of durango the only well documented examples are in the 30 ma old chupaderos caldera complex near durango city although the complex is mostly covered by quaternary basalt and sediment the exposed portion reveals nested calderas that produced two major rhyolitic ash flow tuffs a third major tuff appears to have been erupted from the complex but no caldera has yet been found the caldera formed by the first eruption contains a resurgent dome and a thick sequence of moat sedimentary rocks and lava flows that are hosts for significant iron and tin deposits other possible durango calderas exist at el salto west of durango city at temoaya south of durango city and in the sierra de la silla to the north calderas in chihuahua have beep described from mountain ranges in the central part of the state and from the sierra madre occidental to the west a composite resurgent caldefa 35 ma old and approximately 22 km in diameter has been de scribed from the sierra pastorias immediately south of chihuahua city the main erupted unit is well exposed in the caldera s resurgent dome and a second tuff found outside the caldera was emplaced from a trap door structure nested in the caldera s northwest corner the area also contains a younger smaller resurgentiy domed caldera other possible calderas include a relatively large 35 km in diameter structure in the sierra del nido 75 km north of chihuahua city and a poorly defined structure related to a sequence f older 45 ma silicic volcanic rocks at cerro jesus maria 25 km northwest of chihuahua city calderas in the sierra madre plateau have been described from the tomochic and ocampo areas of western chihuahua the tomochic area records volcanic activity from 34 5 to 28 9 ma and contains at least three calderas these include the tomochic caldera a strikingly well preserved resurgent caldera approximately 20 by 25 km in diameter the north structural margin of the tomochic caldera cuts the intracaldera facies of an ash flow tuff related to the older las varas caldera epithermal precious metal fissure veins are located where the tomochic structural margin cuts this older unit a third structure the corallitos caldera is located only a few kilometers to the northeast reconnaissance studies in far western chihuahua indicate the existence of a large 40 km resurgent caldera centered on the village of ocampo landsat imagery shows a well developed arcuate drainage pattern and maps of the area show a distribution of lithologies that is consistent with the caldera interpretation gold and silver deposits hosted almost exclusively in precaldera rocks are best developed where the older rocks are cut by faults of the caldera s ring fracture zone and resurgent dome in general the calderas thus far mapped in western mexico follow the valles type resurgent caldera model variations however of the ideal caldera cycle are common as are repetitions of the cycle to form caldera complexes all of the better known calderas have associated mineral deposits although they may be best developed in precaldera rocks

slip kinematics and dynamics during and after the 1995 october 9 mw 8 0 colima jalisco earthquake mexico from gps geodetic constraints we use horizontal and vertical crustal displacements derived from gps measurements at 26 sites in western mexico to study the coseismic and post seismic kinematics and dynamics of the 1995 october 9 m w 8 0 colima jalisco earthquake along the middle america trench the measurements bracket the entire landward edge of the approximately 150 km long rupture zone and span a 4 yr period for most sites we solve for the temporal evolution of slip along the subduction interface by inverting gps displacements for the coseismic and four post seismic intervals march 1995 march 1999 subject to the assumption that the crust responds elastically to slip along a shallow dipping curved subduction interface coseismic rupture of up to 5 m was largely focused above depths of 20 km and was limited to a 120 140 km long segment of the subduction zone within one week of the earthquake post seismic slip migrated downdip to depths of 16 35 km where it has since decayed logarithmically we also find evidence for shallow aseismic slip during 1996 or early 1997 northwest of the coseismic rupture zone and increasingly widespread relocking of shallow regions of the subduction interface after early 1997 the relative lack of afterslip in shallow regions of the subduction interface suggests that the interface lies in the unstable frictional regime and hence is strongly coupled between earthquakes by 1999 the cumulative slip moment associated with post seismic slip equaled 70 per cent of the coseismic moment with nearly all of this slip occurring downdip from the coseismic rupture zone the migration of slip after the earthquake to a deeper and presumably velocity strengthening area of the subduction interface and the logarithmic decay of afterslip conform to the qualitative and quantitative predictions of a model in which the fault kinematics are prescribed by rate and state variable frictional laws however misfits to the geodetic displacements exceed the average displacement uncertainties for all epochs implying one or more of the following 1 the elastic response is heterogeneous due to slip along unmodelled upper crustal faults or variations in the elastic properties of the crust 2 other post seismic mechanisms such as viscoelastic or poroelastic effects contribute to or possibly dominate the post seismic response 3 we have underestimated the uncertainties in the gps displacements